Plant Molecular Biology 49: 357–372, 2002. Perrot-Rechenmann and Hagen (Eds.), Auxin Molecular Biology. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.
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Secondary messengers and phospholipase A2 in auxin signal transduction Günther F.E. Scherer Universität Hannover, Institut für Zierpflanzenbau, Baumschule und Pflanzenzüchtung, Abt. Spezielle Ertragsphysiologie, Herrenhäuser Strasse 2, 30419 Hannover, Germany (e-mail
[email protected]) Received 20 April 2001; accepted in revised form 6 August 2001
Key words: auxin, fatty acid, phospholipase A2 , second messenger, signal transduction
Abstract Despite recent progress auxin signal transduction remains largely scetchy and enigmatic. A good body of evidence supports the notion that the ABP1 could be a functional receptor or part of a receptor, respectively, but this is not generally accepted. Evidence for other functional receptors is lacking, as is any clearcut evidence for a function of G proteins. Protons may serve as second messengers in guard cells but the existing evidence for a role of calcium remains to be clearified. Phospholipases C and D seem not to have a function in auxin signal transduction whereas the indications for a role of phospholipase A2 in auxin signal transduction accumulated recently. Mitogenactivated protein kinase (MAPK) is modulated by auxin and the protein kinase PINOID has a role in auxin transport modulation even though their functional linkage to other signalling molecules is ill-defined. It is hypothesized that signal transduction precedes activation of early genes such as IAA genes and that ubiquitination and the proteasome are a mechanism to integrate signal duration and signal strength in plants and act as major regulators of hormone sensitivity. Abbreviations: AACOCF3 , arachidonyltrifluoromethyl carbon; ETYA, 5,8,11,14-eicosatetraynoic acid; NDGA, nordihydroguajaretic acid Introduction: What is cellular signal transduction? When looking into textbooks signal transduction is defined as the steps and reactions leading from a receptor to gene regulation. Usually, gene regulation by transcription factors already is described in a separate chapter. Hence, the border line between signal transduction reactions and gene regulation is the modification of transcription factors. However, many authors, especially in plant biology, include gene regulation into signalling so that the border line rather is a border zone.
The auxin receptor – or auxin receptors? There is much literature on diverse hypothetical auxin receptors and another review in this issue on ABP1, the protein highly suspicious to have an auxin receptor
function (Napier, 1995; Napier and Venis, 1995; Venis and Napier, 1995; Macdonald, 1997; Lüthen et al., 1999). It is philosophical to ask whether there could be several genetically non-homologous types of receptors for a given hormone – not a gene family as the phytochromes are. There could be, but that does not seems to be, a concept of wide distribution in nature. To evoke the complexity of auxin responses it seems to be sufficient to postulate several trans-membrane proteins as interaction partners for the ABP1. These could be more receptor-like in the classical sense, transducing the message across the membrane to a protein which carries out the next step of action (Klämbt, 1990). Different cell types can have different downstream signal transduction components to generate further complexity. The enigma is the relatively high concentration of ABP1 in the ER as compared to the very low amount in the plasma membrane. Indeed, there could start a signal transduction pathway from the ER different
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358 from those at the plasma membrane even though auxin was not found to bind to ABP1 in the ER in the cell (although it certainly does so in vitro) (Tian et al., 1995). ABP1 meets the criteria for a receptor in that modulating its amount by over-expressing it modulates hormone responses predictably (Jones et al., 1998; Bauly et al., 2000), knocking it out is lethal (Chen et al., 2001) and antibodies against it can either inhibit or mimick responses, depending on the antibody (Barbier-Brygoo et al., 1989, 1991, Venis et al., 1992; Rück et al., 1993; Thiel et al., 1993; Leblanc et al., 1999). None of the many other postulated additional receptors for auxin is more than a postulate at this stage, inspired by the fascination for auxin but not supported by such a number of facts as is the receptor function for ABP1. What is important to recall is that auxin is outstanding in that it is polarly transported (Palme and Gälweiler, 1999) which leads to unequal concentrations of auxin in different tissues in close proximity (Jones, 1980). This in itself would be expected to lead to unequal responses to this hormone in those different tissues, even without the possibilities that different tissues could respond differentially to the same hormone concentration. Creating concentration differences by transport proteins thus will always look like influencing function even though the receptor could be a different entity. Another reason why transport proteins look like recepors is that the transport proteins certainly are hormone-binding proteins. A good example is the ‘making’ of the pattern of vascularization by ‘draining’ auxin from the surrounding tissue (Berleth et al., 2000), root tip development (Sabatini et al., 1999) and embryo development (Steinmann et al., 1999). The next set of hormone-binding proteins are the enzymes of auxin metabolism which also change hormone concentrations by their actions. Since we cannot yet analyse all of them in enough detail, the philosophical question of whether there are one or several genetically unrelated auxin receptors will still take a while to be answered.
G proteins and auxin The main problem in writing down opinions on auxin and G proteins is that the receptor for auxin is not unequivocally identified. Even if one accepts the concept that ABP1 has receptor function, and probably then by binding to a transmembrane protein (Klämbt, 1990), one has to realize that the only plant receptor group
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where we have clear speculations about coupling to G proteins is the seven-transmembrane receptor type. All the subunits of trimeric G proteins (Weiss et al., 1993; 1994; Mason et al., 2000) have been found in plants. However, the known corresponding seventransmembrane receptors have been only tentatively identified (Plackidou-Dymock et al., 1998; Devoto et al., 1999) and the coupling to G proteins is then – reasonably – assumed to be homologous to animal or yeast systems (Bockaert and Pin, 1999). G protein subunits might then be coupled to ion channel regulation as in animal systems (Li and Assmann, 1993; Wu and Assmann, 1994; Armstrong and Blatt, 1995). As for the other types of known plant receptors, the soluble cryptochromes and phytochromes (Quail et al., 1995; Lin, 2000), leucine-rich repeat receptors and receptor kinases (Lease et al., 1998; Becraft, 1998), and two-component receptors (Urao et al., 2000), we do not understand the coupling to the protein carrying out the next step of signal transduction. Therefore, models for plants analogous to ras-coupled signalling (Marshall, 1996), i.e. receptor coupling to small G proteins, are attractive but pure speculation at this point (Zheng and Yang, 2000; Fu et al., 2001). Thus, the weak earlier evidence of G protein involvement in auxin signal transduction (Zaina et al., 1990; Scherer and André, 1993; Millner et al., 1996) has to be viewed together with this lack of knowledge on the supposed or expected transmembrane receptor, the hypothetical binding partner to the extracytosolic ABP1 and to cytosolic (small or trimeric) G proteins, or yet another receptor constellation for an auxin receptor. Stronger evidence is needed to support the case for small or trimeric G proteins in auxin signalling. Strong evidence for trimeric G proteins in plant signal transduction has been provided in several papers on mutants of Gα in rice and in Arabidopsis but the characterization of the phenotypes supported a function of Gα subunit in several pathways, positive in gibberellin signalling, negative in abscisic acid signalling, and positive in auxin-induced cell division (Ashikari et al., 1999; Ueguchi-Tanaka et al., 2000; Ullah et al., 2001; Wang et al., 2001). The only knockout plant for the one Gα identified in Arabidopsis, so far, showed decreased cell division as a major trait in its phenotype which is part of the function of auxin – but auxin is not the only player in cell division (den Boer and Murray, 2000). It remains also to be seen whether the known G protein subunits in the Arabidopsis genome are really the only ones and whether perhaps the underlying theme is multi-signal trigger-
359 ing of the cell cycle affected by auxin, gibberellin, abscisic acid, and basic signals like the nutritional status (Bögre et al., 2000; Ullah et al., 2001). There is evidence for a larger type of Gα protein in plants which could be a second type of α subunit and more might still be discovered (Lee and Assman, 1999; Kaydamov et al., 2000). Altogether, this leaves the function of either trimeric or small G proteins in auxin signalling rather open. Undoubtedly, reverse genetics and the complete knowledge of the Arabidopsis genome will allow rapid progress in this area soon. Second messengers Ionic second messengers: pH and Ca2+ Regulation of pH which may both act as a cytosolic second messenger and in the regulation of cell wall pH is described in detail by Becker and Hedrich (2002) in this issue and a number of previous reviews (Lüthen et al., 1999; Roos, 2000; and references therein). What it boils down to is that either the proton pump might be down-regulated to cause cytosolic acidification (and then perhaps up-regulated after a few minutes to cause cell wall acidification) or ion channels in the plasma membrane (or even in the tonoplast) might be blocked or opened and, depending on the direction and charge of ion flow, cytosolic acidification would be the consequence, according to the strong ion difference theory (Stewart et al., 1983). In the case of low auxin concentrations rapid cytosolic acidification was found, whereas alkalinization occurred in the case of very high auxin or of physiological abscisic acid concentrations (Blatt and Thiel, 1994). Rapid stimulation of potassium influx (
