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Abstract. Leaf senescence is a highly regulated physiological process that leads to leaf death and is, as such, the last developmental stage of the leaf.
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Functional Plant Biology, 2004, 31, 203–216

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Review:

Die and let live: leaf senescence contributes to plant survival under drought stress Sergi Munné-BoschA,B and Leonor AlegreA A

Departament de Biologia Vegetal, Facultat de Biologia, Universitat de Barcelona, Avinguda Diagonal 645, E-08028 Barcelona, Spain. BCorresponding author; email: [email protected]

Abstract. Leaf senescence is a highly regulated physiological process that leads to leaf death and is, as such, the last developmental stage of the leaf. Plant aging and environmental stresses may induce the process of senescence. Here we will focus on the role of leaf senescence in field-grown plants as a response to adverse climatic conditions and, more specifically, on how it contributes to plant survival under drought stress. Drought induces several responses in plants including leaf senescence, which plays a major role in the survival of several species. Droughtinduced leaf senescence contributes to nutrient remobilisation during stress, thus allowing the rest of the plant (i.e. the youngest leaves, fruits or flowers) to benefit from the nutrients accumulated during the life span of the leaf. In addition, drought-induced leaf senescence, especially when accompanied by leaf abscission, avoids large losses through transpiration, thus contributing to the maintenance of a favourable water balance of the whole plant. Drought-induced leaf senescence occurs gradually and is characterised by specific macroscopic, cellular, biochemical and molecular changes. Leaf yellowing (i.e. chlorophyll degradation) and specific changes in cell ultrastructure (e.g. chromatin condensation, thylakoid swelling, plastoglobuli accumulation), metabolism (e.g. protein degradation, lipid peroxidation) and gene expression occur during leaf senescence in drought-stressed plants. Cytokinins and ABA have been shown to be involved in the regulation of drought-induced leaf senescence, although the possible role of other plant hormones should not be excluded. Reactive oxygen species, whose concentrations increase during drought-induced leaf senescence, are also known to be regulators of this process. The complex mechanisms of regulation of leaf senescence in drought-stressed plants are discussed, and attention is drawn to those aspects that still require investigation. Keywords: cell death, drought, hormones, leaf senescence, oxidative stress. Introduction Die and let live. Although this statement may seem contradictory at first glance, cell death is essential for survival in several organisms, including plants. Programmed cell death (PCD) occurs throughout the life of a plant and resembles, to some extent, apoptosis in animals. PCD is a broad term that refers to a process by which cells promote their own death. It is now generally accepted that many plant developmental processes and stress responses are achieved through the activation of PCD, including, among others, formation of xylem tracheary elements, development of sieve tubes in the phloem, aerenchyma formation in response to waterlogging, death of root cap cells, pollen tube germination, megaspore abortion, degeneration of suspensor and endosperm, the hypersensitive response to pathogens, and senescence of leaves and other plant parts (reviewed by

Buckner et al. 1998; Samuilov et al. 2000). Leaf senescence as a form of PCD has been recognised for many years (Noodén and Leopold 1978). It differs from other types of PCD, in that it occurs more slowly and is associated, at least in part, with the efficient recycling of nutrients that are translocated from the senescing cells to other parts of the plant such as young leaves, developing flowers and fruits, and storage tissues. Leaf senescence is the result of a programmed process that is highly coordinated at the molecular, cellular, biochemical and physiological levels. Distinct from other major developmental events in plants that involve mainly cell division, differentiation and growth, leaf senescence is accompanied by an organ-wide operation of PCD. Therefore this process requires a coordinated integration of events at the cellular, organ and whole-plant levels.

Abbreviations used: AOX, antioxidants; CKs, cytokinins; ETH, ethylene; JA, jasmonic acid; PCD, programmed cell death; ROS, reactive oxygen species; SA, salicylic acid; SAG, senescence-associated genes. © CSIRO 2004

10.1071/FP03236

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Like many other physiologically programmed processes, leaf senescence is subject to regulation by both environmental and endogenous factors. The environmental cues include stresses such as drought, waterlogging, high or low solar radiation, extreme temperatures, phytotoxic compounds such as ozone, excessive soil salinity and inadequate mineral nutrients in the soil; the endogenous factors include age, reproductive development, and levels of plant regulators such as hormones and reactive oxygen species (Fig. 1). Environmental and endogenous factors interact in the onset and progression of leaf senescence. Environmental cues may accelerate leaf senescence by affecting physiological aging, reproductive development and hormone levels, while these endogenous factors may, in turn, affect the capacity of the plant to induce leaf senescence under stress. Among the environmental cues, limited water and nutrient availability (especially nitrogen) are major factors that adversely affect plant life in many ecosystems (di Castri 1981). Plants have evolved mechanisms by which leaf senescence can be induced by these stresses to reallocate nutrients to reproductive organs and to eliminate water consumption by older, less productive leaves. This regulation of leaf senescence has an obvious adaptive value, allowing the plant to complete its life cycle even under stressful conditions (Gan and Amasino 1997; Ono et al. 2001) The molecular regulation of leaf senescence has been reviewed recently (Quirino et al. 2000; Chandlee 2001; Lim

et al. 2003; Yoshida 2003), and we will focus here on the physiological aspects of leaf senescence with an emphasis on the role of leaf senescence in plant survival under drought stress. The study of leaf senescence contributes to our understanding of plant development and plant responses to stress. Additionally, given that leaf senescence may limit yield in certain crops, its study may also lead to ways of manipulating senescence for agricultural applications. In this review, we will focus on (i) the concepts of drought stress and senescence in plants, (ii) the biological significance of drought-induced leaf senescence, (iii) symptoms indicative of leaf senescence under drought stress, (iv) metabolic alterations, and (v) the mechanisms by which plants regulate leaf senescence under drought stress. Concepts of drought stress and senescence in plants Plants are frequently exposed to stress factors, which are generally defined as external conditions that may adversely affect growth, development, or productivity. More specifically, any unfavourable condition or substance that affects or blocks a plant’s metabolism, growth or development can be regarded as stress (Levitt 1972; Lichtenthaler 1996; Bray et al. 2000). Stress can be biotic (imposed by other organisms) or abiotic (arising from an excess or deficit in the physical or chemical environment). Among the environmental conditions that may be considered stress factors are drought, waterlogging, high or low solar radiation, extreme

Endogenous factors

Age Reproductive state CKs ABA, ETH SA, JA, ROS

Senescing leaf

Non-senescing leaf High light, drought, nutrient deficiency, other stresses

Red light

Environmental conditions Fig. 1. Effect of environmental cues and endogenous factors on leaf senescence. CKs, cytokinins; ETH, ethylene; JA, jasmonic acid; ROS, reactive oxygen species; SA, salicylic acid.

Drought-induced leaf senescence

temperatures, ozone, salinity, and inadequate mineral nutrients in the soil (Levitt 1972; Lichtenthaler 1996; Bray et al. 2000). ‘Drought’ is a meteorological term that denotes a period without rain during which plants suffer from lack of water owing to reduced soil water content. Frequently, soil dryness is coupled with strong evaporation caused by the dryness of the air and high levels of solar radiation (Larcher 1995). Accordingly, the term ‘drought stress’ is used to describe stress that arises from the combination of water deficit, high temperatures and high solar radiation. Transient or prolonged drought reduces the amount of water available for plant growth and leads to water deficit. The term ‘water deficit’ refers to the situation in which the rate of transpiration exceeds the rate of water uptake. In contrast to many other stressful events, stress caused by drought does not occur suddenly, but rather develops slowly and increases in intensity with time. Thus, as we will discuss later, time plays a key role in the progression of leaf senescence, and therefore, in the survival of several plant species under drought stress. Drought stress triggers responses ranging from altered gene expression to changes in plant metabolism and growth. These responses may occur within a few seconds (e.g. a change in the phosphorylation state of a protein), minutes and hours (e.g. a change in gene expression) or several days (e.g. leaf senescence). Many factors affect plant responses to drought, including the duration and magnitude of the stress. According to Lichtenthaler (1996), plants may be exposed to (i) short-term and (ii) long-term stress effects, as well as to (iii) low stress events, which can be partially compensated for by acclimation, adaptation and repair mechanisms, and (iv) strong stress or chronic stress events, which cause considerable damage and may eventually lead to plant death. Plant responses to drought stress may also be influenced by plant genotype, possible acclimation to previous exposure to stress, phase of growth, and the part of the plant that is exposed to the stress (Kozlowski and Pallardy 2001). In recent reviews of the response of plants to drought stress (Chaves et al. 2002, 2003), there has been little attention paid to the role of leaf senescence in drought stress resistance, despite this process having been reported in several species growing in the field including, among others, crops of economic interest such as wheat, rice or sorghum (Ali et al. 1999; Borrell et al. 2000; Yang et al. 2002; Hall et al. 2003), and plants with an ecophysiological interest such as North-American riparian cottonwoods (Rood et al. 2000) and Mediterranean plants (Munné-Bosch et al. 2001). Leaf senescence may be defined as a highly regulated physiological process that leads to death and is, therefore, the last developmental stage of the leaf. Although it is tightly associated with plant aging, both processes should be distinguished. Medawar (1957) provided a convenient distinction between senescence and aging by defining aging as

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referring to all those changes that occur with time, without reference to death as a consequence, while senescence ends in death. Senescence is therefore part of the aging process of leaves and, more specifically, the events that comprise the final stage of leaf development. As stated by Smart (1994) leaf senescence may be considered ‘the last will and testament of a leaf, in which bequests of nutrients are made to the rest of the plant’. Leaf senescence therefore implies an active process with a function, and the execution of a death program once this function has been accomplished. Although leaf senescence leads to cell and organ death, it contributes to plant survival under stress, thus playing a role in stress resistance. Senescence proceeds gradually in leaves and is characterised by three phases: the initiation phase, the re-organisation phase and the terminal phase. In Fig. 2 we propose a model depicting some of the major changes occurring during drought-induced leaf senescence. The initiation phase is the result of early signalling cascades that lead to changes in gene expression and trigger the induction of the senescence process. These signalling cascades will determine whether leaf senescence or other processes are induced in response to stress. Leaf senescence is induced in response to these environmental stress factors in some species, but not in others, and its induction also depends on the magnitude and severity of the stress and the growth phase in which the stress imposed. Thus, it may be considered as one more mechanism that some plants have evolved to withstand stress, and needs to be investigated in relation to other stress responses that also afford protection to plants. Although some characterisation of drought stress perception and components of the signal transduction pathways leading to gene expression in drought has been reported (Ingram and Bartels 1996; Ramanjulu and Bartels 2002), the specific signalling events leading to leaf senescence in droughtstressed plants have yet to be elucidated. These changes in gene expression lead to alterations in the endogenous concentrations of plant regulators such as cytokinins, ABA, and other regulators, such as reactive oxygen species (ROS), which depending on their endogenous cellular concentrations and localization will regulate the expression of senescence-associated genes (SAG) and this in turn determines the progression of leaf senescence during the re-organisation phase. It is during the re-organisation (also termed transdifferentiation or degeneration) phase that major metabolic and cell ultrastructural changes associated with leaf senescence occur; these changes include chlorophyll degradation, decreases in photosynthetic activity, and disassembly of cellular integrity. All these changes are linked to nutrient remobilisation. Source-to-sink transition therefore plays a significant role during the re-organisation phase, and it is in the senescing leaves where major changes occur. Events during this phase appear to be degenerative in nature.

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However, it is now known that they are the result of a re-organisation (or transdifferentiation) phenomenon, rather than a degenerative process, which results from a tight regulation of specific genes (Quirino et al. 2000; Lim et al. 2003; Thomas et al. 2003). The re-organisation phase is therefore an active process, regulated by a complex signalling cascade that is activated during the initiation phase. As we will discuss later, the endogenous cellular concentrations of regulators such as cytokinins, ABA, ethylene, salicylates, jasmonates and ROS, will determine the progression of leaf senescence in this phase. While some processes, such as a decrease in photosynthesis and chlorophyll degradation are activated concomitantly with the first alterations in the levels of hormones and ROS, other processes, such as antioxidant loss, lipid peroxidation and loss of cellular integrity, occur when levels of these regulators have reached a certain threshold (Fig. 2). The terminal phase of leaf senescence is thought to be the result of the accumulation of cell-death-inducing factors that lead to a complete loss of cell integrity and finally to death. It is therefore the last step in leaf senescence and may occur only when the re-organisation phase is finished and nutrient

remobilisation has been accomplished. It has been suggested that conversion of chloroplasts to gerontoplasts, and therefore leaf senescence, may be reversible in several species if the senescing tissue is exposed to conditions favouring re-greening (e.g. exogenous application of cytokinins). However, this reversal is only possible before the terminal phase has been initiated and if the ‘point of no return’ is not surpassed (Karagiannis and Pappelis 1994; Bhattacharya et al. 1996; Olah and Masarovicova 1998; Zavaleta-Mancera et al. 1999a, b). The ‘point of no return’ is that point at which, once passed, the leaf is committed to death. Karagiannis and Pappelis (1994) proposed that it coincides with the irreversible degeneration of the nuclear matrix, which results in the inability of the cell to synthesise mRNAs and ribosomal subunits to support the protein synthesis required for maintenance and growth. The terminal phase may therefore be considered as the events that lead to the execution of cell death, and allow the ‘point of no return’ to be surpassed, ultimately leading to death of leaf cells, thus completing leaf senescence. The terminal phase may therefore also be under tight control so that the senescing leaves die only once they have accomplished their

Drought stress perception

Initiation phase

Signal transduction pathway

Changes in gene expression

Re-organisation phase

Decline in photosynthesis, antioxidant increase

Antioxidant loss, lipid peroxidation, changes in cell ultrastructure

CKs decrease, ABA and ROS increase

CKs decrease, and ABA and ROS increase further

Chlorophyll degradation

NUTRIENT REMOBILISATION

Loss of cellular Changes in gene Cell starvation integrity expression

Terminal phase

CELL DEATH

Fig. 2. Proposed model depicting some of the changes occurring during the initiation, re-organisation and terminal phases of drought-induced leaf senescence. CKs, cytokinins; ROS, reactive oxygen species.

Drought-induced leaf senescence

function. Leaf abscission, which has been reviewed elsewhere (Osborne 1989; Taylor and Whitelaw 2001; Roberts et al. 2002), marks the end of leaf senescence in some species and requires very precisely localised and timed cell death in the petiole. In an attempt to simplify the study of leaf senescence, model systems such as detached leaves or leaf segments and dark treatment have often been used to induce senescence. Although this experimental approach offers many practical advantages, several difficulties arise when results are to be extrapolated to intact plants, because mechanical damage and light/dark transitions may themselves alter leaf senescence. Also, sinks for the export of breakdown products are removed in these experiments and the translocation of hormones from the roots to the leaves cannot occur (Woolhouse 1987; Smart 1994). Most studies on leaf senescence, and more specifically, on stress-induced leaf senescence carried out in intact plants, have been performed in potted plants grown in glasshouses or in growth chambers. Despite the important information provided by these experimental approaches, it is clear that in most cases such studies are not predictors of what occurs in nature, and studies in the field are necessary to fully understand the significance of leaf senescence within the complex network of responses that plants have evolved to withstand stress under natural field conditions (Valladares and Pearcy 1997). In the following sections we will describe, with a particular emphasis on information provided by studies in the field during the last two decades, the symptoms, changes in plant metabolism and regulatory mechanisms that characterise leaf senescence induced by drought stress.

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Leaf senescence may be sequential or synchronic depending on its time-course within the canopy. Sequential senescence occurs gradually, from oldest to youngest leaves (Fig. 3). This type of senescence occurs in perennials and is especially relevant in plant responses to drought stress. Senescence in the oldest leaves contributes to plant survival under drought stress by supplying nutrients to the youngest leaves (Diamantoglou and Kull 1988). Depending on the duration and severity of the stress, this response may allow the plant to keep young leaves alive, and allow them to grow further once stressful conditions have passed. Leaf senescence usually occurs concomitantly with other mechanisms for stress resistance in young leaves, so that the plant can reinitiate growth and reproductive development during the following season. It is generally assumed that the main reason for leaf senescence to occur is to take advantage of the nutrients contained in old leaves before they die, so that nutrient utilisation is optimised at the whole-plant level. Droughtinduced leaf senescence leads to a progressive decline in photosynthesis such that the leaf ultimately reaches the

Biological significance of drought-induced senescence Leaf senescence is a part of the developmental program of plants and its occurrence largely depends on the ecological traits and growth habitat to which different plant species have been adapted. The coupling of whole-plant senescence to reproduction is a life history trait that is common to many annual plant species, and is referred to as monocarpic senescence. Monocarpic leaf senescence is characteristic of plant species classified as ruderals, which are adapted to growth in disturbed environments and have evolved traits that are compatible with the high mortality risks from the environment, namely a propensity for early reproductive development and high fecundity (Stebbins 1950; Grimes 1979). In these species, such as Arabidopsis, monocarpic leaf senescence contributes to the early diversion of resources from vegetative to reproductive development, and is therefore essential for plant reproduction. Drought accelerates the time-course of monocarpic senescence in several species (Olsson 1995; Srivalli and Khanna-Chopra 1998; Pic et al. 2002), so that plant reproduction may be achieved even under stressful conditions.

Fig. 3. Drought-induced leaf senescence in sage (Salvia officinalis L.) plants growing in Mediterranean field conditions. In this species, leaf senescence occurs sequentially from oldest to youngest leaves as drought progresses during the summer. Nutrient remobilisation from senescing leaves (sources) to young growing leaves (sinks) allows plant survival during the summer drought. Growth is re-initiated when optimal conditions are re-established in autumn.

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compensation point, at which carbon assimilation and respiration are equal. At this point, the leaf will no longer contribute as a photosynthetic organ to the assimilatory needs of the rest of the plant. However, senescent leaves still contain a significant pool of nutrients in the form of proteins, lipids and other macromolecules, which the plant may metabolise and export to other plant parts. As we will discuss later, of particular interest are the chloroplasts, which have been estimated to contain as much as 75% of the proteins and lipids of the cell (Forde and Steer 1976; Dean and Leech 1982). During drought-induced leaf senescence, the fabric of the photosynthetic apparatus is dismantled and nutrients are exported from senescing leaves to young growing tissues (leaves, flowers and fruits) or storage organs. Accelerated leaf senescence in response to drought is considered to be of adaptive survival value because it reduces the water demand at the whole-plant level. In fact, one of the early responses during drought is stomatal closure, which is particularly notable in the oldest senescing leaves, and reduces the total amount of water that the plant loses by transpiration. In some species leaf senescence is accompanied by abscission, which terminates water consumption of older leaves. In that way, young leaves can retain more of the limited amount of water absorbed by roots in drought (Kozlowski 1976; Proebsting and Middleton 1980). Symptoms of drought-induced leaf senescence It is generally accepted that leaf yellowing is the first visual symptom of leaf senescence. Leaf yellowing is the result of chlorophyll degradation in senescing leaves, which unmasks the presence of carotenoids. In most species carotenoids are also degraded during leaf senescence, but to a lesser extent than chlorophylls, so the ratio of carotenoids to chlorophylls increases (Biswal 1995). Chlorophyll degradation and increases in the ratio of carotenoids to chlorophylls have been reported during drought-induced leaf senescence in several species, including field-grown plants (Smirnoff 1993; Rood et al. 2000; Munné-Bosch et al. 2001; Yang et al. 2002). Another visual symptom of leaf senescence is the appearance of red colours in leaves of some species, which is caused by the accumulation of anthocyanins (Hoch et al. 2001, 2003; Neill and Gould 2003). To our knowledge, increases in anthocyanins during drought-induced leaf senescence have not been reported to date, although some studies have shown parallel increases in anthocyanins and water loss during autumnal leaf senescence (Boyer et al. 1988). However changes in leaf coloration are not enough to indicate the occurrence of leaf senescence, particularly in leaves under stress. In addition to drought, almost all environmental constraints, including nutrient deficits (Drossopoulos et al. 1998), shading, excessive solar radiation or UV-B light (Burkey and Wells 1991; John et al. 2001; Broetto et al.

S. Munné-Bosch and L. Alegre

2002), extreme temperatures (Benbella and Paulsen 1998; Norén et al. 2003), waterlogging (Leul and Zhou 1998), ozone (Vollenweider et al. 2003) and pollutants (Pasi 2000), have been shown to induce chlorophyll degradation, but may not necessarily accelerate leaf senescence. Thus, chlorophyll degradation is considered a pre-requisite for leaf senescence, since it is associated with nutrient remobilisation (Hörtensteiner and Feller 2002), although it is not a reliable indicator of the process. Several studies suggest that the onset of leaf senescence may occur before chlorophyll degradation is apparent. Pic et al. (2002) have shown that a gene homologous to SAG2 from Arabidopsis, which codes for a cysteine-protease and is thought to be a specific marker of senescence, is expressed in water-stressed potted pea plants before a decline in photosynthesis, chlorophylls or protein levels. Hensel et al. (1993) indicated that leaf senescence is activated as a consequence of age-related declines in photosynthesis in Arabidopsis, which supports the contention that sugars act as regulators of leaf senescence (Dai et al. 1999). Thus, it is likely that the initiation of leaf senescence is triggered by specific signals that induce changes in gene expression, decreases in photosynthesis, and other processes before chlorophyll degradation occurs. Unfortunately, the signals leading to the induction of leaf senescence under drought stress are yet to be discovered. Additionally, drought stress is known to induce changes in gene expression and reduce photosynthesis without necessarily inducing chlorophyll degradation and leaf senescence (Chaves et al. 2003). Thus, a combination of specific indicators at the cellular, biochemical and molecular level should be used, aside from visual observations of leaf yellowing, to unequivocally indicate leaf senescence under drought stress (Table 1). Developmental leaf senescence is characterised by specific changes in cell ultrastructure, including membrane whirling, chromatin condensation in the nuclear matrix and nucleolus, swelling of chloroplasts, accumulation of plastoglobuli in the stroma and distortion of thylakoids (Inada et al. 1998). These changes occur in drought-induced leaf senescence in field-grown plants (Munné-Bosch et al. 2001) and have also been considered good indicators of leaf senescence in response to other stresses, such as ozone (Vollenweider et al. 2003). Other symptoms of developmental leaf senescence include specific metabolic changes, such as a decrease in photosynthesis, degradation of proteins, lipid peroxidation and increases in the levels of ROS (Dangl et al. 2000). All these changes have also been reported to occur during drought-induced leaf senescence (Xu and Zhou 1993; Olsson 1995; Ilami and Contour-Ansel 1997; Srivalli and Khanna-Chopra 1998; Ali et al. 1999; Munné-Bosch et al. 2001; Behera et al. 2002; Pic et al. 2002; Yang et al. 2003). Several genes have been considered specific to developmental leaf senescence (Quirino et al. 2000; Chandlee 2001; Lim et al. 2003; Yoshida 2003).

Drought-induced leaf senescence

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However, only some of these genes are known to be specifically regulated during drought-induced leaf senescence (Weaver et al. 1998; Pic et al. 2002). To our knowledge, studies on gene expression in drought-induced leaf senescence in plants growing in the field are still lacking, making it difficult to ascertain which genes have evolved within particular plants in their habitat to induce leaf senescence under drought stress. In summary, leaf senescence is characterised by specific changes at the macroscopic, cellular, biochemical and molecular levels. It is the combined observation of these changes that allows us to unequivocally demonstrate that leaf senescence is occurring in drought-stressed plants. It should also be taken into account that these changes occur in a coordinated manner to accomplish nutrient remobilisation during the progression of leaf senescence under drought stress. The major changes in plant metabolism that fulfil this function and the regulation of the progression of leaf senescence will be the focus of the following sections. Changes in metabolism during drought-induced leaf senescence Metabolism of several cellular organelles is affected during the progression of developmental leaf senescence, although studies of changes in plant metabolism during the progression of leaf senescence in drought-stressed plants have focused almost exclusively on chloroplasts (see Table 1). This is probably due to the assumption that oxidative stress plays a major role in the progression of leaf senescence, as chloroplasts are the most important intracellular generators of ROS (Asada 1999). Chloroplasts are one of the first organelles to be targeted for breakdown as senescence proceeds, while nuclei and mitochondria maintain their integrity until the latest stages of leaf senescence

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(Smart 1994; Dangl et al. 2000). The conversion of chloroplasts to gerontoplasts characterises the re-organisation phase of leaf senescence. Chloroplasts lose volume and density as a consequence of extensive losses of stromal components and thylakoids, and the number and size of plastoglobules increase, as senescence proceeds. Fully developed gerontoplasts consist of a still intact envelope surrounding several large plastoglobules (Parthier 1988; Matile 1992). The degradation of chlorophylls occurs through the concerted action of several enzymes located in different intracellular compartments, starting in the thylakoids and innerenvelope membrane of chloroplasts, and ending in the vacuole (Matile and Hörtensteiner 1999). Chlorophyll degradation in senescing leaves has been shown to depend largely on the activity of phaeophorbide a oxygenase (Rodoni et al. 1998). However, regulation of this or other enzymes of the chlorophyll catabolic pathway during leaf senescence in drought-stressed plants has not been reported to date. In contrast to processes of chlorophyll degradation associated with a rapid and large accumulation of ROS and cell collapse (Noodén 1988), drought-induced leaf senescence is characterised by a progressive and slow decline in chlorophylls, which is compatible with the role of chlorophyll loss in photoprotection and nutrient remobilisation. Chlorophyll loss may play a role in photoprotection during the first stages of drought-induced leaf senescence, when chloroplasts still retain substantial photosynthetic activity. Chlorophyll loss (i) reduces the potentially harmful effects of singlet oxygen formation in thylakoids, (ii) reduces the amount of light absorbed by leaves, which may dampen the potentially damaging heating effects of high solar radiation in drought-stressed plants whose stomata are closed, and (iii) contributes to increasing the amounts of carotenoids and tocopherols per unit of chlorophylls, thus

Table 1. Cellular changes observed during drought-induced leaf senescence in plants Other changes, which are typical of developmental leaf senescence such as DNA fragmentation, increase of respiration, conversion of peroxisomes to glyoxysomes, protein degradation and lipid peroxidation in cellular compartments other than chloroplasts, as well as nucleic acid degradation, are still to be demonstrated during leaf senescence in drought-stressed plants. Chl, chlorophylls; Car, carotenoids Location Nucleus

Change Changes in gene expression

Chromatin condensation Chloroplasts Chl loss and increased Car /Chl ratio Decrease in photosynthesis Protein degradation and lipid peroxidation

Cell wall

Loss of antioxidants, thylakoid swelling, and plastoglobuli increase Increased H2O2 formation

Growth conditions and plant species Potted pea plants; de-rooted Arabidopsis plants Field-grown sage plants Several species, including field-grown plants Several species, including field-grown plants Several species, including field-grown plants

Reference Pic et al. (2002); Weaver et al. (1998)

Field-grown sage plants

Munné-Bosch et al. (2001) Rood et al. (2000); Munné-Bosch et al. (2001); Yang et al. (2002) Ali et al. (1999); Munné-Bosch et al. (2001); Yang et al. (2003) Xu and Zhou (1993); Olsson (1995); Ilami and Contour-Ansel (1997); Srivalli and Khanna-Chopra (1998); Pic et al. (2002); Behera et al. (2002) Munné-Bosch et al. (2001)

Field-grown sage plants

Munné-Bosch et al. (2001)

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increasing the photoprotective and antioxidant capacity of leaves relative to the amounts of photons absorbed (Kyparissis et al. 1995). This protection may be especially relevant during the first stages of the re-organisation phase of leaf senescence under drought stress to guarantee a progressive dismantling of chloroplasts, which, in the absence of protective mechanisms, could occur too rapidly and threaten nutrient remobilisation. Chlorophyll degradation also plays a role in nutrient remobilisation during the re-organisation phase of leaf senescence in drought-stressed plants. Chlorophylls are degraded not because their products are reusable but primarily because they would otherwise block access to more valuable materials. Chloroplastic proteases, whose synthesis increases during drought-induced leaf senescence even before chlorophyll degradation is apparent (Pic et al. 2002), allow the remobilisation of as much as 75% of the total cellular nitrogen present in leaves (Hörtensteiner and Feller 2002). Once chlorophyll degradation occurs, the pigment–protein complex is disassociated and the enzyme is able to break down the proteins, which may account for a significant amount of the nitrogen present in chloroplasts. Most of the remaining nitrogen present in chloroplasts is taken from Rubisco and other stromal photosynthetic enzymes (Hörtensteiner and Feller 2002). Besides chlorophyll degradation, oxidative metabolism may also play a role in the progression of drought-induced leaf senescence (Fig. 2). Synthesis of low-molecular-weight antioxidants, such as α-tocopherol, has been reported in drought-stressed plants (reviewed by Munné-Bosch and Alegre 2002a). Oxidative stress and jasmonic acid activate the expression of genes responsible for the synthesis of tocopherols in plants (Falk et al. 2002; Sandorf and Holländer-Czytko 2002). In agreement with this, an increase in α-tocopherol during the first stages of leaf senescence has been demonstrated in several species (Peisker et al. 1989; Chrost et al. 1999; Munné-Bosch and Peñuelas 2003). α-Tocopherol, which inhibits the propagation of lipid peroxidation and is an efficient singlet oxygen quencher and scavenger, may therefore contribute to the photoprotection of the photosynthetic apparatus during the first stages of leaf senescence, when chloroplasts still retain photosynthetic activity. However, the same reports have shown significant decreases in α-tocopherol during later stages of leaf senescence, which have been associated with increases in lipid peroxidation. Although synthesis of α-tocopherol may be activated during this stage, enhanced formation of ROS in a second oxidative burst may overwhelm the turnover of α-tocopherol. Other low-molecular-weight antioxidants change in parallel with α-tocopherol during the progression of leaf senescence in drought-stressed plants. Levels of ascorbate and glutathione, which participate in the recycling of α-tocopherol, are kept constant or even increase during the first stages of leaf senescence and decrease later, as

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senescence progresses further and α-tocopherol decreases (Xu and Zhou 1993; Munné-Bosch and Peñuelas 2003). During the decrease of antioxidant defences concomitant increases in lipid peroxidation and protein oxidation are observed. This is indicative of enhanced oxidative stress during the latest stages of the re-organisation phase. It is during this period that protein degradation may occur at the highest rates, since protein oxidation seems to be a prerequisite for subsequent enzymatic protein degradation in senescing leaves (Palma et al. 2002). Thus, it is essential that both oxidative stress and protein remobilisation are tightly controlled during the progression of drought-induced leaf senescence. Despite other changes in plant metabolism, such as an increase in mitochondrial respiration, nucleic acid degradation and conversion of lipids to sugars occurring during developmental leaf senescence (reviewed by Dangl et al. 2000), to our knowledge such changes have not yet been demonstrated during drought-induced leaf senescence. Regulation of drought-induced leaf senescence Leaf senescence is considered to be a genetically controlled physiological process. During the last decade, studies of developmental leaf senescence, which have focused mainly on Arabidopsis, have identified several SAG and cellular mechanisms regulating leaf senescence have begun to be elucidated (Buchanan-Wollaston 1997; Nam 1997; Quirino et al. 2000; Lim et al. 2003). Current studies suggest that the regulation of gene expression during leaf senescence is complex and controlled by multiple pathways that form a regulatory network (reviewed by Lim et al. 2003). Furthermore, blocking a particular pathway may not have a significant effect on the progression of senescence. Nevertheless, little is currently known about the regulation of this process in drought-stressed plants. Characterisation of these processes is crucial if we are to better understand plant responses to environmental stresses. Most work on the regulation of drought-induced leaf senescence in field conditions has focused on the role of plant hormones in the remobilisation of nutrients in crops of economical interest (Ali et al. 1999; Yang et al. 2002, 2003). There is a lack of studies on the regulation of gene expression during leaf senescence in drought-stressed plants, and more specifically on the use of molecular genetic tools in plants growing in field conditions. This may be due to the relative complexity of the regulation of droughtinduced leaf senescence in field conditions, as well as the interaction of external and internal factors in this process. The use of different experimental approaches, such as (i) exogenous applications of plant hormones to promote or inhibit leaf senescence, (ii) determination of endogenous concentrations of plant hormones before and during leaf senescence, and (iii) use of transgenic plants and mutants in the study of leaf senescence, have provided evidence that

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plant hormones regulate leaf senescence (Gan and Amasino 1996, 1997). The five major classes of plant hormones, namely auxins, cytokinins, gibberellins, ABA and ethylene, and other plant growth regulators such as jasmonates, salicylates, brassinosteroids and polyamines, have all been implicated (Thomas and Stoddart 1980; Zacarías and Reid 1990; Jackson 1993; He et al. 2002). It is generally accepted that while the first three classes of hormones, and especially cytokinins, typically inhibit developmental leaf senescence, the remainder contribute to its promotion. Drought stress modifies the endogenous levels of plant hormones, although a direct role in the regulation of drought-induced leaf senescence has only been demonstrated for cytokinins and ABA (Ali et al. 1999, Yang et al. 2002, 2003). These studies revealed that enhanced ABA levels increase carbon remobilisation from senescing leaves to grains in drought-stressed rice and wheat plants. In contrast, cytokinin levels, which decrease under drought stress, show a positive correlation with the photosynthetic rate and chlorophyll content and a negative correlation with sucrose phosphate synthase activity, thus presumably preventing leaf senescence in drought-stressed plants. Cytokinins are known to play a major role in the regulation of source-to-sink transitions (Roitsch 2000). While high levels of cytokinins in leaves may promote the activity of apoplastic invertases and sugar transporters and activate cell division so that the tissues (e.g. young leaves) behave as sinks, low levels prevent growth in senescing leaves. Nutrient deficits, such as those caused by drought stress, may reduce cytokinin production by roots and accelerate senescence in the oldest leaves, which have low potential for cytokinin synthesis (Ambler et al. 1991). As a result, old leaves senesce, nutrients are translocated to young leaves, which consequently display the highest levels of photo- and antioxidative protection throughout the progression of stress, and the whole plant is able to withstand drought stress until more favourable conditions are re-established and growth can resume. Werner et al. (2003) have recently shown that leaves of cytokinin-deficient Arabidopsis plants display signs of delayed rather than accelerated senescence. In this study, chlorophyll was retained longer in cytokinin-deficient than in wild-type leaves, particularly in regions adjacent to major veins. The authors argued against the hypothesis that cytokinins act as triggers of leaf senescence if their concentration is decreased below a threshold level, and suggested instead that altered sink–source relationships in the transgenic plants might interfere with the normal mechanism of senescence. From this and other studies (Gan and Amasino 1996, 1997), it is concluded that decreasing the cytokinin content in leaves is necessary for the progression of senescence but is not a signal that triggers its onset. Similarly, other hormones may play a role in nutrient remobilisation, rather than triggering the onset of leaf

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senescence. Neefs et al. (2002) showed that exogenous applications of ethephon, which liberates ethylene, enhanced remobilisation of nitrogen compounds from senescing chicory leaves. Ethylene accumulation in leaves of different species has also been shown to depend on the speed and intensity of the drought imposed on plants. When drought stress is imposed by slowly decreasing the soil water content, ethylene production is not generally increased in leaves (Morgan et al. 1990; Xu and Qui 1993). Taken together, these data suggest that, although ethylene may promote nutrient remobilisation, e.g. by promoting chlorophyll degradation in senescing leaves of some species (Thomas and Stoddart 1980), it is likely that this hormone is not involved in the induction of leaf senescence in droughtstressed plants. Although gibberellins, auxins and polyamines have been suggested as putative regulators of leaf senescence (Thomas and Stoddart 1980; Zacarías and Reid 1990; He et al. 2002), further studies are necessary to provide direct evidence for a role of these compounds in the regulation of droughtinduced leaf senescence. Studies at the molecular level have demonstrated that salicylates, jasmonates and ethylene regulate expression of genes coding for transcription factors, which in turn regulate SAG expression, when plants are exposed to salt, osmotic and cold stresses (Chen et al. 2002), making it likely that this also occurs in drought-stressed plants. It has been shown that oxidative stress, which is the result of an imbalance between ROS and antioxidant levels (Doke 1997), increases during plant aging (Munné-Bosch and Alegre 2002b) and leaf senescence (Smart 1994; Noodén and Guiamét 1996; Buchanan-Wollaston 1997; Dangl et al. 2000). As it occurs with ozone-induced leaf senescence (Vollenweider et al. 2003), the cellular balance between ROS and antioxidants may play a similar role in droughtstressed plants. As shown in Fig. 4, several physiological processes may potentially arise depending on the cellular redox balance. Plants grow and develop under optimal conditions when the constitutive levels of antioxidants (state 1) control basal production of ROS, which is the result of normal aerobic metabolism in plants. In state 2, induction of antioxidant synthesis counteracts increased production of ROS at low concentrations. This state is typical of plants exposed to stress in which survival is guaranteed by a proper induction of antioxidant defences. Also, it is characteristic of the first stages observed during the re-organisation phase of leaf senescence in drought-stressed plants, in which photo- and antioxidative protection increases. However, at that point, the progression of leaf senescence may revert (e.g. by exogenous application of cytokinins) and, in that case, leaf senescence will not be completed. It may instead progress further as a result of higher ROS production rates, which may be caused by a new oxidative burst (state 3). In that case, leaf senescence will be completed, as cell death

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occurs and it is therefore not reversible. Finally, state 4 (necrotic death) may arise when antioxidant defences cannot accommodate extremely large, rapid and uncontrolled accumulations of ROS. The expression of SAG may be regulated by oxidative stress (Navabpour et al. 2003) and it is known that drought induces oxidative stress in plants (Smirnoff 1993). Although no direct evidence has been provided so far for the regulation of SAG by drought-induced oxidative stress, it is likely that, as occurs with other genes (Langenkämper et al. 2001), drought stress regulates the expression of SAG by an enhanced formation of ROS. Among them, hydrogen peroxide is a candidate to regulate SAG, since it has been shown to increase during drought-induced leaf senescence (Munné-Bosch et al. 2001). ROS signals also interact, directly or indirectly, with other signalling pathways, such as nitric oxide and the stress hormones ABA, salicylic acid, jasmonic acid and ethylene, although the relationships between plant hormones and ROS in drought-induced leaf senescence signal transduction cascades is still not clear. The interaction and balance of these pathways determines whether the cell lives or dies (Overmyer et al. 2003). It has been shown that ABA and

salicylic acid induce hydrogen peroxide production (Pei et al. 2000; Hung and Kao 2003; Jiang and Zhang 2002) and that ROS may participate in the ABA signalling pathway (Neill et al. 2002a). Furthermore, the synthesis and action of hydrogen peroxide appears to be linked to that of nitric oxide and both can mediate the transcription of specific genes (Neill et al. 2002a, 2002b). Although it is relevant in the regulation of SAG expression, the exact mechanism by which this process occurs is still unknown. The possibility that ROS and nitric oxide regulate drought-induced ABA synthesis cannot be excluded (Zhao et al. 2001). Besides, exogenous application of hydrogen peroxide may induce ethylene synthesis (Clements and Atkins 2001) and thus, although it is still to be demonstrated in drought-stressed plants, it could affect ethylene concentrations, and therefore nutrient remobilisation during leaf senescence. Since hormones regulate other cell death processes, mainly by modulating ROS levels in plants (Hoeberichts and Woltering 2002), it is likely that a similar mechanism operates in the regulation of leaf senescence. Although drought stress signalling has remained a mystery for several years, molecular tools are now bringing new insights to the study of signalling events regulating

Concentration of reactive oxygen species (ROS)

Basal

Constitutive antioxidant levels

ROS/AOX balance

State 1

Low

Moderate

High

Antioxidant induction

Loss of antioxidants

Structural/ metabolic damage

ROS/AOX balance

Induction of PCD

Cell collapse and death

State 2

State 3 (Senescence)

State 4 (Necrosis)

Fig. 4. Scheme showing the possible influence of the endogenous concentration of reactive oxygen species on different physiological processes observed in response to drought stress in plants. Senescence may occur as a result of moderate increases in reactive oxygen species that lead to a loss of antioxidant defenses and programmed cell death. AOX, antioxidants; PCD, programmed cell death; ROS, reactive oxygen species.

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gene expression in these conditions. New reports are emerging in which gene expression in drought-stressed plants is extensively studied (Bajaj et al. 1999; Seki et al. 2001; Rabbani et al. 2003; Watkinson et al. 2003). These experimental approaches will undoubtedly provide us in the near future with invaluable information to generate a complete picture of changes in gene expression during drought-induced leaf senescence, especially if studies are performed in field-grown plants in which variability in gene expression is maximised. Conclusions and outlook The study of drought-induced leaf senescence is particularly important, not only to better understand plant development and how plants survive under adverse climatic conditions, but also to improve crop yields, since crops with delayed leaf senescence are used to improve yields in crops growing under adverse climatic conditions (Ismail et al. 2000; Hall et al. 2003). In this review, we have defined the complex concepts of drought stress and senescence underlying the differences between aging and senescence in plants. Leaf senescence has been defined as a highly regulated physiological process that leads to leaf death. In that process, we have highlighted the existence of three phases during leaf senescence: the initiation, re-organisation and terminal phases. It is during the re-organisation phase that major cellular and metabolic changes occur that are presumably triggered by progressive decreases in the levels of cytokinins and increases in the levels of ABA and ROS. However, further research is needed to characterise the threshold of ROS and hormones necessary to induce the specific responses associated with leaf senescence in drought-stressed plants. Also, the mechanisms of action of ROS and hormones, and their putative interaction, should be studied further to better understand the regulation of drought-induced leaf senescence. Drought-induced leaf senescence contributes to plant survival under drought stress in several species, since it allows (i) an early diversion of resources from vegetative to reproductive development, thus contributing to the completion of plant life-cycle in monocarpic species even under stressful conditions, (ii) remobilisation of nutrients from senescing leaves to young leaves, thus contributing to plant survival in perennials, and (iii) reductions in water loss at the whole-plant level, especially when it is accompanied by leaf abscission. Leaf senescence is characterised by specific changes at the macroscopic, cellular, biochemical and molecular levels. It is the combined observation of changes at various levels of organisation that allows us to unequivocally demonstrate that leaf senescence is occurring in drought-stressed plants. To this end, an effort must be made in future studies to provide data not only at the macroscopic level, but also at the cellular, biochemical and molecular levels, to

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conclusively indicate leaf senescence in these conditions. Also, several studies have to be performed to better understand the participation of other subcellular compartments, besides chloroplasts, in the progression of droughtinduced leaf senescence. Finally, we have discussed the complexity of the regulation of leaf senescence in drought-stressed plants. It should be considered that it is not the effect of one plant regulator alone, but the combination of hormones interacting with ROS, antioxidants, nitric oxide, and other components of complex signal transduction pathways that regulates leaf senescence under drought stress. Molecular biology is presently providing new tools to study the complex signalling networks involved in the regulation of drought-induced leaf senescence. The combination of molecular, cellular, biochemical and (eco)physiological approaches will undoubtedly bring, in the near future, a deeper understanding of the regulation of drought-induced leaf senescence in plants growing in their natural habitat. Acknowledgments The research on plant responses to drought stress and leaf senescence in our laboratory is supported by the Ministerio de Ciencia y Tecnología (projects MCYT BOS2000–0560 and BOS2003–01032). References Ali M, Jensen CR, Mogensen VO, Andersen MN, Henson IE (1999) Root signalling and osmotic adjustment during the intermittent soil drying sustain grain yield of field grown wheat. Field Crops Research 62, 35–52. doi:10.1016/S0378-4290(99)00003-9 Ambler JR, Morgan PW, Jordan RW (1991) Amounts of zeatin and zeatin riboside in xylem sap of senescent and non-senescent sorghum. Crop Science 32, 411–419. Asada K (1999) The water-water cycle in chloroplasts: scavenging of active oxygens and dissipation of excess photons. Annual Review of Plant Physiology and Plant Molecular Biology 50, 601–639. doi:10.1146/ANNUREV.ARPLANT.50.1.601 Bajaj S, Targolli J, Liu LF, Ho THD, Wu R (1999) Transgenic approaches to increase dehydration-stress tolerance in plants. Molecular Breeding 5, 493–503. doi:10.1023/A:1009660413133 Behera RK, Mishra PC, Choudhury NK (2002) High irradiance and water stress induce alterations in pigment composition and chloroplast activities of primary wheat leaves. Journal of Plant Physiology 159, 967–973. Benbella M, Paulsen GM (1998) Efficacy of treatments for delaying senescence of wheat leaves. II. Senescence and grain yield under field conditions. Agronomy Journal 90, 332–338. Bhattacharya PK, Pappelis AJ, Lee-Song C-D, Bemiller JN, Karagiannis CS (1996) Nuclear (DNA, RNA, histone and nonhistone protein) and nucleolar changes during growth and senescence of may apple leaves. Mechanisms of Ageing and Development 92, 83–99. doi:10.1016/S0047-6374(96)01804-0 Biswal B (1995) Carotenoid catabolism during leaf senescence and its control by light. Journal of Photochemistry and Photobiology. B, Biology 30, 3–13. doi:10.1016/1011-1344(95)07197-A Borrell AK, Hammer GL, Douglas ACL (2000) Does maintaining green leaf area in sorghum improve yield under drought? I. Leaf growth and senescence. Crop Science 40, 1036–1037.

214

Functional Plant Biology

Boyer M, Miller J, Belanger M, Hare E, Wu J (1988) Senescence and spectral reflectance in leaves of northern pin oak Quercus palustris Münch. Remote Sensing of Environment 25, 71–88. doi:10.1016/0034-4257(88)90042-9 Bray EA, Bailey-Serres J, Weretilnyk E (2000). Responses to abiotic stresses. In ‘Biochemistry and molecular biology of plants’. (Eds BB Buchanan, W Gruissem and RL Jones) pp. 1158–1203. (American Society of Plant Physiologists: Rockville, MD) Broetto F, Lüttge U, Ratajczak R (2002) Influence of light intensity and salt-treatment on mode of photosynthesis and enzymes of the antioxidative response system of Mesembryanthemum crystallinum. Functional Plant Biology 29, 13–23. doi:10.1071/ PP00135 Buchanan-Wollaston V (1997) The molecular biology of leaf senescence. Journal of Experimental Botany 48, 181–199. Buckner B, Janick-Buckner D, Gray J, Johal GS (1998) Cell-death mechanisms in maize. Trends in Plant Science 3, 218–223. doi:10.1016/S1360-1385(98)01254-0 Burkey KO, Wells R (1991) Response of soybean photosynthesis and chloroplast membrane function to canopy development and mutual shading. Plant Physiology 97, 245–252. Chandlee JM (2001) Current molecular understanding of the genetically programmed process of leaf senescence. Physiologia Plantarum 113, 1–8. doi:10.1034/J.1399-3054.2001.1130101.X Chaves MM, Pereira JS, Maroco J, Rodrigues ML, Ricardo CPP, Osório ML, Carvalho I, Faria T, Pinheiro C (2002) How plants cope with water stress in the field. Photosynthesis and growth. Annals of Botany 89, 907–916. doi:10.1093/AOB/MCF105 Chaves MM, Maroco JP, Pereira JS (2003) Understanding plant responses to drought — from genes to the whole plant. Functional Plant Biology 30, 239–264. doi:10.1071/FP02076 Chen W, Provart NJ, Glazebroo JH, Katagiri F, Chang HS, et al. (2002) Expression profile matrix of Arabidopsis transcription factor genes suggest their putative functions in response to environmental stress. The Plant Cell 14, 559–574. doi:10.1105/TPC.010410 Chrost B, Falk J, Kernebeck B, Mölleken H, Krupinska K (1999) Tocopherol biosynthesis in senescing chloroplasts — a mechanism to protect envelope membranes against oxidative stress and a prerequisite for lipid remobilization? In ‘The chloroplast: from molecular biology to biotechnology’. (Eds JH ArgyroudiAkoyunoglou and H Senger) pp. 171–176. (Kluwer Academic Publishers: Dordrecht) Clements J, Atkins C (2001) Characterization of a non-abscission mutant in Lupinus angustifolius. I. Genetic and structural aspects. American Journal of Botany 88, 31–42. Dai N, Schaffer A, Petreikov M, Shahak Y, Giller Y, Ratner K, Levine A, Granot D (1999) Overexpression of Arabidopsis hexokinase in tomato plants inhibits growth, reduces photosynthesis, and induces rapid senescence. The Plant Cell 11, 1253–1266. doi:10.1105/TPC.11.7.1253 Dangl JL, Dietrich RA, Thomas H (2000) Senescence and programmed cell death. In ‘Biochemistry and molecular biology of plants’. (Eds BB Buchanan, W Gruissem and RL Jones) pp. 1044–1100. (American Society of Plant Physiologists: Rockville, MD) Dean C, Leech RM (1982) Genome expression during normal leaf development. Plant Physiology 69, 904–910. Diamantoglou S, Kull U (1988) Seasonal variations of nitrogen components in Mediterranean evergreen schlerophyllous leaves. Flora 180, 377–390. di Castri F (1981). Mediterranean-type shrublands of the world. In ‘Mediterranean-type shrublands’. (Eds F di Castri, DW Goodall and RL Specht) pp. 1–52. (Elsevier Scientific Publishing: Amsterdam)

S. Munné-Bosch and L. Alegre

Doke N (1997) The oxidative burst: role in signal transduction and plant stress. In ‘Oxidative stress and the molecular biology of antioxidant defenses’. (Ed. SG Scandalios) pp. 785–813. (Cold Spring Harbor Laboratory Press: New York, NY) Drossopoulos JB, Bouranis DL, Kintzios S, Aivalakis G, Triposkoufi A (1998) Distribution profiles of selected micronutrients in oriental field-grown tobacco plants as affected by nitrogen fertilization. Journal of Plant Nutrition 21, 1398–1406. Falk J, Krauss N, Dähnhardt D, Krupinska K (2002) The senescenceassociated gene of barley encoding 4-hydroxyphenylpyruvate dioxygenase is expressed during oxidative stress. Journal of Plant Physiology 159, 1245–1253. Forde J, Steer MW (1976) The use of quantitative electron microscopy in the study of lipid composition in membranes. Journal of Experimental Botany 27, 1137–1141. Gan S, Amasino RM (1996) Cytokinins in plant senescence: from spray and pray to clone and play. BioEssays 18, 557–565. Gan S, Amasino RM (1997) Making sense of senescence. Plant Physiology 113, 313–319. Grimes JP (1979) ‘Plant strategies and vegetation processes.’ (John Wiley & Sons: New York, NY) Hall AE, Cisse N, Thiaw S, Elawad HOA, Ehlers JD, et al. (2003) Development of cowpea cultivars and germplasm by the Bean/Cowpea CRSP. Field Crops Research 82, 103–134. doi:10.1016/S0378-4290(03)00033-9 He Y, Fukushige H, Hildebrand DF, Gan S (2002) Evidence supporting a role of jasmonic acid in Arabidopsis leaf senescence. Plant Physiology 128, 876–884. doi:10.1104/PP.010843 Hensel LL, Grbic V, Baumgarten DA, Bleecker AB (1993) Developmental and age-related processes that influence the longevity and senescence of photosynthetic tissues in Arabidopsis. Plant Cell 5, 553–564. doi:10.1105/TPC.5.5.553 Hoch WA, Zeldin EL, McCown BH (2001) Physiological significance of anthocyanins during autumnal leaf senescence. Tree Physiology 21, 1–8. Hoch WA, Zeldin EL, McCown BH (2003) Resorption protection. Anthocyanins facilitate nutrient recovery in autumn by shielding leaves from potentially damaging light levels. Plant Physiology 133, 1296–1305. doi:10.1104/PP.103.027631 Hoeberichts FA, Woltering EJ (2002) Multiple mediators of plant programmed cell death: interplay of conserved cell death mechanisms and plant-specific regulators. BioEssays 25, 47–57. doi:10.1002/BIES.10175 Hörtensteiner S, Feller U (2002) Nitrogen metabolism and remobilization during senescence. Journal of Experimental Botany 53, 927–937. doi:10.1093/JEXBOT/53.370.927 Hung KT, Kao CH (2003) Nitric oxide counteracts the senescence of rice leaves induced by abscisic acid. Journal of Plant Physiology 160, 871–879. Ilami G, Contour-Ansel D (1997) Effect of progressive drought on endoproteolytic activities and water status of Brassica napus leaves. Journal of Agronomy and Crop Science 178, 157–164. Inada N, Sakai A, Kuroiwa H, Kuroiwa T (1998) Three-dimensional analysis of the senescence program in rice (Oryza sativa L.) coleoptiles — investigations by fluorescence microscopy and electron microscopy. Planta 206, 585–597. doi:10.1007/ S004250050436 Ingram J, Bartels D (1996) The molecular basis of dehydration tolerance in plants. Annual Review of Plant Physiology and Plant Molecular Biology 47, 377–403. doi:10.1146/ANNUREV. ARPLANT.47.1.377 Ismail AM, Hall AE, Ehlers JD (2000) Delayed-leaf-senescence and heat-tolerance traits mainly are independently expressed in cowpea. Crop Science 40, 1049–1055.

Drought-induced leaf senescence

Jackson MB (1993) Are plant hormones involved in root to shoot communication? Advances in Botanical Research 19, 103–187. Jiang M, Zhang J (2002) Water stress-induced abscisic acid accumulation triggers the increased generation of reactive oxygen species and up-regulates the activities of antioxidant enzymes in maize leaves. Journal of Experimental Botany 53, 2401–2410. doi:10.1093/JXB/ERF090 John CF, Morris K, Jordan BR, Thomas B, Mackerness SA (2001) Ultraviolet-B exposure leads to up-regulation of senescenceassociated genes in Arabidopsis thaliana. Journal of Experimental Botany 52, 1367–1373. doi:10.1093/JEXBOT/52.359.1367 Karagiannis CS, Pappelis AJ (1994) Effect of ethylene on selective ribosomal cistron regulation in quiescent and senescent onion leaf base tissue. Mechanisms of Ageing and Development 75, 141–149. doi:10.1016/0047-6374(94)90082-5 Kozlowski TT (1976) Water supply and leaf shedding. In ‘Water deficit and plant growth’. (Ed. TT Kozlowski) pp. 191–231. (Academic Press: New York, NY) Kozlowski TT, Pallardy SG (2001) Acclimation and adaptive responses of woody plants to environmental stresses. The Botanical Review 68, 270–334. Kyparissis A, Petropoulou Y, Manetas Y (1995) Summer survival of leaves in a soft-leaved shrub (Phlomis fruticosa L., Labiatae) under Mediterranean field conditions: avoidance of photoinhibitory damage through decreased chlorophyll contents. Journal of Experimental Botany 46, 1825–1831. Langenkämper G, Manac’h N, Broin M, Cuiné S, Becuwe N, Kuntz M, Rey P (2001) Accumulation of plastid lipid-associated proteins (fibrillin/CDSP34) upon oxidative stress, ageing and biotic stress in Solanaceae and in response to drought in other species. Journal of Experimental Botany 52, 1545–1554. Larcher W (1995) ‘Physiological plant ecology.’ (Springer-Verlag: Berlin) Leul M, Zhou W (1998) Alleviation of waterlogging damage in winter rape by application of uniconazole: effects on morphological characteristics, hormones and photosynthesis. Field Crops Research 59, 121–127. doi:10.1016/S0378-4290(98)00112-9 Levitt J (1972) ‘Responses of plants to environmental stresses.’ (Academic Press: San Diego, CA) Lichtenthaler HK (1996) Vegetation stress: an introduction to stress concept in plants. Journal of Plant Physiology 148, 4–14. Lim PO, Woo HR, Nam HG (2003) Molecular genetics of leaf senescence. Trends in Plant Science 8, 272–278. doi:10.1016/S1360-1385(03)00103-1 Matile P (1992) Chloroplast senescence. In ‘Crop photosynthesis: spatial and temporal determinants’. (Eds NR Baker and H Thomas) pp. 413–440. (Elsevier: Amsterdam) Matile P, Hörtensteiner S (1999) Chlorophyll degradation. Annual Review of Plant Physiology and Plant Molecular Biology 50, 67–95. doi:10.1146/ANNUREV.ARPLANT.50.1.67 Medawar PB (1957) ‘The uniqueness of the individual.’ (Basic Books: New York, NY) Morgan PW, He CJ, De Greef JA, Proft MP (1990) Does water stress promote ethylene synthesis by intact plants? Plant Physiology 94, 1616–1624. Munné-Bosch S, Alegre L (2002a) The function of tocopherols and toctrienols in plants. Critical Reviews in Plant Sciences 21, 31–57. doi:10.1016/S0735-2689(02)80037-5 Munné-Bosch S, Alegre L (2002b) Plant aging increases oxidative stress in chloroplasts. Planta 214, 608–615. doi:10.1007/ S004250100646 Munné-Bosch S, Peñuelas J (2003) Photo- and antioxidative protection during summer leaf senescence in Pistacia lentiscus L. grown under Mediterranean field conditions. Annals of Botany 92, 385–391. doi:10.1093/AOB/MCG152

Functional Plant Biology

215

Munné-Bosch S, Jubany-Marí T, Alegre L (2001) Drought-induced senescence is characterized by a loss of antioxidant defences in chloroplasts. Plant, Cell and Environment 24, 1319–1327. doi:10.1046/J.1365-3040.2001.00794.X Nam HG (1997) The molecular genetic analysis of leaf senescence. Current Opinion in Biotechnology 8, 200–207. doi:10.1016/S09581669(97)80103-6 Navabpour S, Morris K, Allen R, Harrison E, Mackerness SA, Buchanan-Wollaston V (2003) Expression of senescence-enhanced genes in response to oxidative stress. Journal of Experimental Botany 54, 2285–2292. doi:10.1093/JXB/ERG267 Neefs V, Maréchal I, Hernández-Martínez R, De Proft MP (2002) The influence of mechanical defoliation and ethephon treatment on the dynamics of nitrogen compounds in chicory (Cichorium intybus L.). Scientia Horticulturae 92, 217–227. doi:10.1016/S03044238(01)00306-5 Neill SO, Gould KS (2003) Anthocyanins in leaves: light attenuators or antioxidants? Functional Plant Biology 30, 865–873. doi:10.1071/FP03118 Neill S, Desikan R, Hancock J (2002a) Hydrogen peroxide signalling. Current Opinion in Plant Biology 5, 388–395. doi:10.1016/S13695266(02)00282-0 Neill SJ, Desikan R, Clarke A, Hurst RD, Hancock JT (2002b) Hydrogen peroxide and nitric oxide as signalling molecules in plants. Journal of Experimental Botany 53, 1237–1247. doi:10.1093/JEXBOT/53.372.1237 Noodén LD (1988) The phenomena of senescence and aging. In ‘Senescence and aging in plants’. (Eds LD Noodén and AC Leopold) pp. 1–50. (Academic Press: San Diego, CA) Noodén LD, Leopold AC (1978) Phytohormones and the endogenous regulation of senescence and abscission. In ‘Phytohormones and related compounds: a comprehensive treatise’. (Eds D Letham, P Goodwin and T Higgins) pp. 329–369. (Elsevier: New York, NY) Noodén LD, Guiamét JJ (1996) Genetic control of senescence and aging in plants. In ‘Handbook of the biology of aging’. (Eds EL Schneider and JW Rowe) pp. 94–118. (Academic Press: San Diego, CA) Norén H, Svensson P, Stegmark R, Funk C, Adamska I, Andersson B (2003) Expression of the early light-induced protein but not the PsbS protein is influenced by low temperature and depends on the developmental stage of the plant in field-grown pea cultivars. Plant, Cell and Environment 26, 245–253. doi:10.1046/J.1365-3040. 2003.00954.X Olah R, Masarovicova E (1998) Photosynthesis, respiration, and chlorophylls in presenescent, regreened, and senescent leaves of forest herb Smyrnium perfoliatum L. (Apiaceae). Acta Physiologiae Plantarum 20, 173–178. Olsson M (1995) Alterations in lipid composition, lipid peroxidation and anti-oxidative protection during senescence in drought stressed and non-drought stressed plants of Pisum sativum. Plant Physiology and Biochemistry 33, 547–553. Ono K, Nishi Y, Watanabe A, Terashima I (2001) Possible mechanisms of adaptive leaf senescence. Current Opinion in Plant Biology 3, 234–243. doi:10.1055/S-2001-15201 Osborne DJ (1989) Abscission. Critical Reviews in Plant Sciences 8, 103–129. Parthier B (1988) Gerontoplasts — the yellow end in the ontogenesis of chloroplasts. Endocytobiosis and Cell Research 5, 163–190. Overmyer K, Brosché M, Kangasjärvi J (2003) Reactive oxygen species and hormonal control of cell death. Trends in Plant Science 8, 335–342. doi:10.1016/S1360-1385(03)00135-3 Palma JM, Sandalio LM, Corpas FJ, Romero-Puertas C, McCarthy I, del Río LA (2002) Plant proteases, protein degradation, and oxidative stress: role of peroxisomes. Plant Physiology and Biochemistry 40, 521–533. doi:10.1016/S0981-9428(02)01404-3

216

Functional Plant Biology

S. Munné-Bosch and L. Alegre

Pasi R (2000) Nutrient alterations in Scots pines (Pinus sylvestris L.) under sulphur and heavy metal pollution. Acta Universitatis Ouluensis 353, 1–48. Pei ZM, Murata Y, Benning G, Thomine S, Klusener B, Allen GJ, Grill E, Schroeder JI (2000) Calcium channels activated by hydrogen peroxide mediate abscisic acid signalling in guard cells. Nature 406, 731–734. doi:10.1038/35021067 Peisker C, Duggelin T, Rentsch D, Matile P (1989) Phytol and the breakdown of chlorophyll in senescent leaves. Journal of Plant Physiology 135, 428–432. Pic E, de la Serve BT, Tardieu F, Turc O (2002) Leaf senescence induced by mild water deficit follows the same sequence of macroscopic, biochemical, and molecular events as monocarpic senescence in pea. Plant Physiology 128, 236–246. doi:10.1104/PP.128.1.236 Proebsting EL Jr, Middleton JE (1980) The behavior of peach and pear trees under extreme drought. Journal of the American Society for Horticultural Science 105, 380–385. Quirino BF, Noh Y, Himielblau E, Amasino RM (2000) Molecular aspects of leaf senescence. Trends in Plant Science 5, 278–282. doi:10.1016/S1360-1385(00)01655-1 Rabbani MA, Maruyama K, Abe H, Khan MA, Katsura K, Ito Y, Yoshiwara K, Seki M, Shinozaki K, Yamaguchi-Shinozaki K (2003) Monitoring expression profiles of rice genes under cold, drought, and high-salinity stresses and abscisic acid application using cDNA microarray and RNA gel-blot analyses. Plant Physiology 133, 1755–1767. doi:10.1104/PP.103.025742 Ramanjulu S, Bartels D (2002) Drought- and desiccation-induced modulation of gene expression in plants. Plant, Cell and Environment 25, 141–151. doi:10.1046/J.0016-8025.2001.00764.X Roberts JA, Elliot KA, González-Carranza ZA (2002) Abscission, dehiscence, and other cell separation processes. Annual Review of Plant Physiology and Plant Molecular Biology 53, 131–158. doi:10.1146/ANNUREV.ARPLANT.53.092701.180236 Rodoni S, Schellenberg M, Matile P (1998) Chloroplast breakdown in senescing barley leaves as correlated with phaeophorbide a oxygenase activity. Journal of Plant Physiology 152, 139–144. Roitsch T (2000) Regulation of source/sink relations by cytokinins. Plant Growth Regulation 32, 359–367. doi:10.1023/A: 1010781500705 Rood SB, Patiño S, Coombs K, Tyree MT (2000) Branch sacrifice: cavitation-associated drought adaptation of riparian cottonwoods. Trees 14, 248–257. doi:10.1007/S004680050010 Samuilov VD, Oleskin AV, Lagunova EM (2000) Programmed cell death. Biochemistry (Moscow) 65, 873–887. Sandorf I, Holländer-Czytko H (2002) Jasmonate is involved in the induction of tyrosine aminotransferase and tocopherol biosynthesis in Arabidopsis thaliana. Planta 216, 173–179. Seki M, Narusaka M, Yamaguchi-Shinozaki K, Carninci P, Kawai J, Hayashizaki Y, Shinozaki K (2001) Arabidopsis encyclopedia using full-length cDNAs and its application. Plant Physiology and Biochemistry 39, 211–220. doi:10.1016/S0981-9428(01)01244-X Smart CM (1994) Gene expression during leaf senescence. New Phytologist 126, 419–448. Smirnoff N (1993) The role of active oxygen in the response of plants to water deficit and desiccation. New Phytologist 125, 27–58. Srivalli B, Khanna-Chopra R (1998) Drought-induced enhancement of protease activity during monocarpic senescence in wheat. Current Science 75, 1174–1176. Stebbins GL (1950) ‘Variation and evolution in plants.’ (Columbia University Press: New York, NY) Taylor JE, Whitelaw CA (2001) Signals in abscission. New Phytologist 151, 323–339. doi:10.1046/J.0028-646X.2001.00194.X Thomas H, Ougham HJ, Wagstaff C, Stead AD (2003) Defining senescence and death. Journal of Experimental Botany 54, 1127–1132. doi:10.1093/JXB/ERG133

Thomas H, Stoddart JL (1980) Leaf senescence. Annual Review of Plant Physiology 31, 83–110. doi:10.1146/ANNUREV.PP.31. 060180.000503 Valladares F, Pearcy RW (1997) Interactions between water stress, sun–shade acclimation, heat tolerance and photoinhibition in the sclerophyll Heteromeles arbustifolia. Plant, Cell and Environment 20, 25–36. doi:10.1046/J.1365-3040.1997.D01-8.X Vollenweider P, Ottiger M, Günthardt-Goerg MS (2003) Validation of leaf ozone symptoms in natural vegetation using microscopical methods. Environmental Pollution 124, 101–118. doi:10.1016/ S0269-7491(02)00412-8 Watkinson JI, Sioson AA, Vasquez-Robinet C, Shukla M, Kumar D, et al. (2003) Photosynthetic acclimation is reflected in specific patterns of gene expression in drought-stressed loblolly pine. Plant Physiology 133, 1702–1716. doi:10.1104/PP.103.026914 Weaver LM, Gan S, Quirino B, Amasino RM (1998) A comparison of the expression patterns of several senescence-associated genes in response to stress and hormone treatment. Plant Molecular Biology 37, 455–469. doi:10.1023/A:1005934428906 Werner T, Motyka V, Laucou V, Smets R, van Onckelen H, Schmülling T (2003) Cytokinin-deficient transgenic Arabidopsis plants show multiple developmental alterations indicating opposite functions of cytokinins in the regulation of shoot and root meristem activity. The Plant Cell 15, 2532–2550. doi:10.1105/TPC.014928 Woolhouse HW (1987) Regulation of senescence in the chloroplast. In ‘Plant senescence: its biochemistry and physiology’. (Eds WW Thomson, EA Nothnagel and RC Huffaker) pp. 132–145. (American Society of Plant Physiology: Rockville, MD) Xu CC, Qui Z (1993) Effect of drought on lipoxygenase activity, ethylene and ethane production in leaves of soybean plants. Acta Botanica Sinica 35, 31–37. Xu CC, Zhou Q (1993) The acceleration of senescence of soybean leaves induced by drought and its relation to membrane lipid peroxidation. Acta Agronomica Sinica 19, 359–364. Yang JC, Wang ZQ, Zhu QS (2002) Carbon remobilization and grain filling in Japonica/Indica hybrid rice subjected to postanthesis water deficits. Agronomy Journal 94, 102–109. Yang JC, Zhang JH, Wang ZQ, Zhu QS, Liu LJ (2003) Involvement of abscisic acid and cytokinins in the senescence and remobilization of carbon reserves in wheat subjected to water stress during grain filling. Plant, Cell and Environment 26, 1621–1631. doi:10.1046/ J.1365-3040.2003.01081.X Yoshida S (2003) Molecular regulation of leaf senescence. Current Opinion in Plant Biology 6, 79–84. doi:10.1016/ S1369526602000092 Zacarías L, Reid MS (1990) Role of growth regulators in the senescence of Arabidopsis thaliana leaves. Physiologia Plantarum 80, 549–554. doi:10.1034/J.1399-3054.1990.800409.X Zavaleta-Mancera HA, Franklin KA, Ougham HJ, Thomas H, Scott IM (1999a) Regreening of Nicotiana leaves. I. Reappearance of NADH-protochlorophyllide oxidoreductase and light-harvesting chlorophyll a/b-binding protein. Journal of Experimental Botany 50, 1677–1682. doi:10.1093/JEXBOT/50.340.1677 Zavaleta-Mancera HA, Thomas BJ, Thomas H, Scott IM (1999b) Regreening of Nicotiana leaves. II. Redifferentiation of plastids. Journal of Experimental Botany 50, 1683–1689. doi:10.1093/ JEXBOT/50.340.1683 Zhao Z, Chen G, Zhang C (2001) Interaction between reactive oxygen species and nitric oxide in drought-induced abscisic acid synthesis in root tips wheat seedlings. Australian Journal of Plant Physiology 28, 1055–1061. doi:10.1071/PP00143

Manuscript received 26 November 2003, accepted 7 January 2004

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