Genetic basis for seed dormancy

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Genetic basis for seed dormancy Michael E. Foley and Steven A. Fennimore Seed Science Research / Volume 8 / Issue 02 / June 1998, pp 173 - 182 DOI: 10.1017/S0960258500004086, Published online: 19 September 2008

Link to this article: http://journals.cambridge.org/abstract_S0960258500004086 How to cite this article: Michael E. Foley and Steven A. Fennimore (1998). Genetic basis for seed dormancy. Seed Science Research, 8, pp 173-182 doi:10.1017/S0960258500004086 Request Permissions : Click here

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Seed Science Research (1998) 8,173-182

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Genetic basis for seed dormancy Michael E. Foley1* and Steven A. Fennimore2 'Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907-1155, USA department of Vegetable Crops, University of California, Davis, CA 95616-8746, USA

Abstract Seed dormancy is an important physiological stage in the life cycle of many seed-bearing plants In an everchanging environment, dormancy increases survival by distributing germination over time In general, seed dormancy is a quantitative trait that is influenced by environmental factors Depending on the species and accession, dormancy is controlled by nuclear factors, maternal factors or both, and dormancy can be genetically the dominant or recessive form of the trait Wild oat {Avena fatua) is a well developed system for dormancy investigations Dormancy in wild oat is regulated by three major genes as well as environmental factors, and dormancy is conditionally the recessive form of the trait This paper summarizes current knowledge on genetic and environmental factors, and the interaction of these factors, on seed dormancy in several cereal grain and dicotyledonous species Some applications of this knowledge to improve crops and manage weeds are outlined Keywords afternpemng, dormancy, environment, genetics, germination, seed, Avena fatua, Hordeum, Tnticum, Oryza

Introduction

Seed dormancy is the absence of germination in a mature intact seed under conditions of light, temperature, water and O2 which would normally favour germination withm a specific time period (Hilhorst, 1995) Seed dormancy enhances survival of seed-bearing plants by optimizing the distribution of germination over time Freshly harvested dormant seeds are in a state of primary dormancy Nondormant or partially afternpened seeds that encounter unfavourable germination conditions are often induced into a state of secondary dormancy There are "•Correspondence Fax +1765 494 0363 E-mail foley@btny purdue edu

two common ways by which primary dormancy is imposed coat-imposed dormancy enforced by restrictive seed coverings, and embryo dormancy where control of dormancy resides within the embryo itself (Bewley and Black, 1982) Dormancy is normally overcome by afterriperung Afternpemng occurs when dormant seeds are exposed for some period of time to a set of environmental conditions after maturation (Simpson, 1990) As seeds in a population afternpen, they become more responsive to a range of conditions that promote germination, and less responsive to conditions that restrict germination For example, as seeds in a population afternpen, they can germinate over a wider range of temperatures than without afternpening Environmental conditions that facilitate afternpening vary with species Many seeds require warm-dry or cool-moist conditions for afternpening (Bewley and Black, 1982) Lack of fundamental knowledge about mechanisms underlying dormancy and afternpening anses largely because traditional physiological approaches confound germination and dormancy-breaking processes (Bradford, 1996, Cohn, 1996) To circumvent this problem, we are taking a genetic approach and investigating factors that affect germinabihty, l e , the predisposition for immediate, intermediate or much delayed germination Here we review current knowledge on the genetics of dormancy and descnbe potential uses for knowledge gained through genetic approaches For recent reviews on the molecular aspects of seed dormancy refer to Bewley (1997) and Li and Foley (1997)

Genetic basis for dormancy The most extensive research on the genetic and environmental basis for seed dormancy has been done with cereal gram species that afternpen under warmdry conditions In particular, oat (Avena spp) is a well developed system, followed by barley (.Hordeum vulgare L ), wheat (Tnticum aestivum L ) and rice (Oryza satwa L), respectively Maize (Zea mays L) does not

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exhibit seed dormancy (Simpson, 1990) However, the study of viviparous maize mutants has advanced our understanding of seed maturation processes and of carotenoid and abscisic acid biosynthesis (McCarty, 1995, Schwartz et al, 1997) Vivipary is a condition of uninterrupted progression from embryogenesis to germination of the seed on the parent plant without developmental arrest in the form of quiescence or dormancy

Genetic models for dormancy in Avena spp The genus Avena contains oat (A satwa) and wild oat (A fatua), an important cereal crop and one of the world's worst weeds, respectively Oat and wild oat are allohexaploid (2rc=6r=42) containing approximately 13 700 Mb DNA (Arumuganathan and Earle, 1991) Genetic and environmental factors affect germmability in both oat and wild oat (Simpson, 1990, Strand, 1991c) Genetic variability affecting germinabihty of wild oat in the field has been well documented (Naylor and Jana, 1976, Jana and Naylor, 1980, Somody et al, 1984) Genetic models for dormancy in oats have been developed on the basis of inter- and mtra-specific crosses Garber and Quisenberry (1923) determined that nondormancy was dominant in Avena spp In a more thorough investigation, Johnson (1935) made reciprocal cross-pollinations between oat and wild oat, and classified the germination phenotypes from multiple populations and their progeny at 20°C Reciprocal crosses are important to determine maternal effects Non-nuclear or maternally transmitted genes, e g in mitochondria, and chloroplasts, as well as seed tissues of maternal origin, e g hull, testa and pericarp, can affect germmability Germination phenotype classification of seed from several populations segregating for dormancy facilitates development of a genetic model Genetic models are often proposed based on one population, e g , F2, and verified with another Backcross (BC) populations to the dormant (BC^F,) and nondormant (BC1NDF,) parent are typically used for verification A critical aspect of model development for a continuous trait like seed dormancy is progeny testing Progeny testing involves verification of the parental phenotype, and in some cases the genotype, by the classification of the progeny phenotype In the first model of seed dormancy for oat, Johnson (1935) concluded that (1) nondormancy is genetically dominant over dormancy, (2) three genes of more or less equal strength control germmability, and (3) the frequency of dominant alleles (A, A2 A3) governs germmability Unfortunately, Johnson, (1935) allowed seed populations and progeny to afternpen for random periods, thus, his model for dormancy must be viewed as hypothetical

Naylor and his colleagues developed a three-gene model for dormancy based on reciprocal crosspolhnations among dormant and nondormant lines of wild oat (Jana et al, 1979) Data from reciprocal crosses were pooled, as maternal effects were not detected In their model, the dominant allele at locus E promotes early germination within the first four weeks of imbibition However, ungermmated seeds remain dormant due to slow afternpenmg imposed by partially dominant loci L, and L., Their model was based on relative germination rates at 20°C observed in F, seeds and F, populations Unfortunately, they did not verify the germination phenotype of F, seeds through progeny testing (Jana et al, 1988), therefore, their model remains hypothetical Nevertheless, Naylor and his colleagues verified that dormancy in wild oat is a quantitative trait Quantitative traits are governed by multiple nuclear encoded genes, exhibit continuous phenotypic variation, and are usually influenced by genotype by environmental (G X E) interactions (Falconer, 1989) We cross-pollinated a dormant and nondormant line of wild oat and developed several populations F, (M73/SH430), F2(F,/self), BC1NDF, ((SH430/F,)/self), BC1DF, ((M73/F ] )/selt), and F,-recombinant inbred lines (RI) Parental lines M73 and SH430 are dormant and nondormant, respectively and are highly inbred and well characterized with respect to dormancy (Adkins et al, 1986) Each of the 127 RI lines was derived by single-seed descent from an individual F, seed The frequency of heterozygous individuals tor a single locus is one-half in the F2 and decreases by 50% each generation of inbreeding (Fehr, 1987) Thus, by the F7-generation more than 98% of the loci will be homozygous The advantages to the use of RI and doubled haploid populations over F^ and backcrossed populations for genetic analysis are that they constitute a set ot fixed genotypes that can be maintained indefinitely We did not make reciprocal crosses because previous research with M73, SH430 and other lines of wild oat and oat determined that maternal effects were not significant (Johnson, 1935, Jana et al, 1979, Jana et al, 1988) Based on quantitative differences in germmability at 15°C in the F, population and F, progeny, we propose a three-gene model We verified our model with data from BC1DF, and BC^pF, populations, their progeny (BC1DF, and BC1NDF2), and RI lines (Fenmmore, 1997) The three genes in our model are represented by G,, G2 and D The letters G and D stand tor germination and dormancy, respectively We determined that nondormancy is conditionally the dominant form of the trait in agreement with the findings of Jana et al (1979) Loci G, and G, promote early germination, while locus D promotes late germination The expression of D is dependent upon the alleles present at G, and G, According to the

Genetic basis for seed dormancy model an embryo is nondormant if it has at least two copies of the dominant G, and G, alleles, regardless of the allele at the D locus, 1 e , at least one copy each of the G, and G2 alleles at the G, and G : loci (G,- G, ), or only two at the G, locus (G, G, g2 g2 - -), or only two at the G, locus (g, g, G, G, — ) with any genotype at the D locus giving a nondormant embryo If only one allele ot either G, or G, or none is present, and the genotype is dd, then the phenotype will also be nondormant If less than two copies of G, or G, are present and the dominant allele D is present, then the phenotype will be dormant The genotype of the SH430 and M73 parents are GjG^G^dd and g,g,g,g2DD, respectively The F, genotype would be GjgjG^Dd, and as expected, the F, phenotype is nondormant The BC^F, genotypes are all G,- G2 and have a nondormant phenotype The BC1DF, population has a range ot phenotypes because of segregation at three loci The number of nondormant and dormant F v BCIDF2 families, and RI lines as classified by cluster analysis were consistent with the expectations of the model (Fennimore, 1997) Genetic models for dormancy are subjective m that quantitative differences in a polygenic character at the phenotypic level are used to predict unknown genetic properties of a population Thus, like the previous genetic models, our model is tentative and may be disproved or revised based on future evidence Other reasons that genetic models for dormancy m wild oat differ is because genetic lines used to develop segregating populations and germination temperature were different Jana et al (1979) used nondormant line CS40 and classified germination ot their populations at 20°C, whereas we used line SH430 and classified germination at 15°C Our investigations revealed a G X E interaction for germination temperature m an F, population Mean germination after ten days of imbibition is greater at 15 than at 20°C The expression of G, or G, is repressed at higher temperatures presumably due to the increased expression of another gene, thus, nondormancy is conditioned by temperature Clearly, more research needs to be done on the G X E interaction with germination temperature and on gene action (I e , effect of gene and allehc dosage) to refine our genetic model Finally, we speculate that failure to control afternpenmg may explain why Johnson (1935) did not detect a dominant dormancy-conferring allele at one or more loci as did Jana et al (1979) and Fennimore (1997) Environmental influences on germmability of wild oat

Hentabihty (/;2) for seed dormancy in wild oat is approximately 50% (Jana and Naylor, 1980) This indicates that the variability for seed dormancy for wild oat populations growing in Canadian prairie

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farmland is about 50% genetic and 50% environmental in origin Temperature is a major environmental factor governing germinabihty of wild oat Temperature can effect dormancy in wild oat and other small grain seeds m four important ways (Simpson, 1990, Strand, 1991c) First, the level of dormancy is influenced by temperature during seed development because exposure of developing seeds to high temperature results m increased germmability, l e , lower level of dormancy (Sexsmith, 1969, Sawhney and Naylor, 1980, Peters, 1982) A G X E interaction with temperature during seed development for wild oat has been proposed, but not verified (Sawhney and Naylor, 1979) Second, persistence of dormancy in dry seeds is determined by temperature (Roberts, 1965, Strand, 1991c, Bnggs et al, 1994) and in wild oat for example, the atternpening process is accelerated as temperature increases from 20-40°C (Foley, 1994) Third, the depth of secondary dormancy induced by anaerobiosis is increased by high temperatures (Tilsner and Upadhyaya, 1985) Finally, temperature can determine the onset and rate of germination after seeds imbibe water (Simpson 1990, Strand, 1991b) As the level of dormancy in a population decreases, the temperature range providing for a rapid onset and rate of germination increases For example, nondormant wild oat seeds germinate readily over a wide range of temperatures, I e , 4-28°C, while the temperature optimum for unafternpened dormant seeds is 4-12°C with little or much delayed germination occurring above that range (Naylor and Fedec, 1978) A G X E interaction with germination temperature has been confirmed for wild oat (Fennimore, 1997, Foley unpublished work) Population genetics of wild oat

The population dynamics of wild oat were investigated to determine the genetic variability of populations m California (Imam and Allard, 1965) Seeds collected from a variety of ecosystems were all reared in one location, and scored for two monogenic traits and measured for five quantitative traits The level of outcrossing based on the monogenic traits was estimated at 1-12% Because of inbreeding, most individuals in any given population were homozygous, thus ensuring that future generations would be well adapted to the local ecosystem However, variability for outcrossing makes wild oat a tenacious colonizer, since most wild oat communities were heterogeneous for this trait Those individuals with high rates of outcrossing were favoured in situations where it was necessary to evolve new and better adapted genotypes, such as when colonizing a new site After a site was colonized and the ecosystem was more static, those inbreeding individuals adapted to that site became the dominant biotype, and

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individuals with high rates of outcrossmg dropped to low levels Wild oat has evolved a flexible genetic system in which appropriate compromises have been reached between the high recombination potential of outbreeders and the ability of inbreeders to hold together desirable complexes of genes (Imam and Allard, 1965, Darmency and Aujas, 1987) Agricultural practices influence the dormancy behaviour of wild oat populations Synthetic populations of wild oat consisting of 50% dormant and 50% nondormant biotypes were established to determine if continuous wheat cropping or summer fallowing, I e , alternate year cropping, place selection pressure on the germination phenotypes of wild oat (Jana and Thai, 1987) Dormant and nondormant biotypes both persisted m continuously cropped fields, but summer fallowmg selected heavily against the nondormant biotypes, while the dormant biotypes persisted It is also known that wild oat populations adapt to herbicide application programmes (Jana and Naylor, 1982, Heap et al, 1993) Genetics of seed dormancy in other cereal grain species

The genetics of dormancy in barley, rice and wheat have been investigated to impart resistance to preharvest sprouting (Buraas and Skmnes, 1984, Tomar, 1984, Mares, 1993) Untimely rains before harvest, but after maturation of these crops, leads to premature seed germination called pre-harvest sprouting Resistance to pre-harvest sprouting is correlated with the level of dormancy in mature seeds The development of pre-harvest sprouting resistance involves a delicate balance Resistance to pre-harvest sprouting is desirable in the field during the period between grain maturity and harvest However, after harvest the persistence of dormancy is undesirable because dormancy creates problems in stand establishment and in the malting process when seeds are used within a short period of time after harvest Moreover, dormancy results in persistence of crop seeds m the soil seed bank which can carry over as a weed in subsequent cropping seasons (Ullrich et al, 1993) Molecular markers for dormancy quantitative trait loci (QTL) have been identified in barley and wheat (Anderson et al, 1993, Ullrich et al, 1993, Oberthur el al, 1995, Han et al, 1996, Larson et al, 1996, Sorrells and Anderson, 1996) Individual loci controlling a quantitative trait like dormancy are referred to as QTL Barley Barley, a diploid (2n=14) species, is the fourth most important cereal gram Dormancy m barley is a quantitative trait with nearly complete dominance for early germination Estimates of hentabihty for

dormancy in barley were 0 69-0 80 (Buraas and Skinnes, 1984) Classical genetic approaches found that cytoplasmic factors did not affect dormancy in several cultivars (Buraas and Skinnes, 1984) However, a molecular approach using 150 doubled haploid lines derived from the Steptoe/Morex (intermediately dormant/nondormant) cross, determined that cytoplasmic effects on dormancy may affect the penetrance of the dormancy phenotype (Han et al, 1996, Larson et al, 1996) Oberthur et al (1995) detected 2 QTL on barley chromosome 7 with significant dormancy effects These two QTL, marked by restriction fragment length polymorphisms (RFLP) PSR128 and ABG391, account for 50 and 15%, respectively, of the variability for dormancy in all environments tested (Oberthur et al, 1995, Han et al, 1996) Quantitative trait loci on chromosome 1 (Amy2) and 4 (BCD402) had lesser effects on dormancy (-5%) and were detected only in specific environments (Oberthur et al, 1995, Han et al, 1996) The detection of minor genes with effects on seed germination in only some environments emphasizes that the germination phenotype of barley is the product of a complex set of interactions between germination-related genes and their environment (Strand, 1991a) Wheat

Wheat, an allohexaploid (2n=6t=42), is the world's most important cereal crop As in barley, a predictable and adequate level of seed dormancy in some wheat varieties is beneficial to prevent pre-harvest sprouting Pre-harvest sprouting is an especially severe problem in white-kemelled wheat (Paterson and Sorrells, 1990) Seed dormancy in wheat is a quantitative trait controlled by one or two major genes and an unknown number of minor genes Estimates of hentabihty for dormancy in white-kemelled wheat were 0 56-0 84 (Anderson et al, 1993) Genotype x E interactions on the expression of dormancy have been observed and indicate that environmental factors during seed development condition seed dormancy in wheat (Reddy et al, 1985, Upadhyay and Paulsen, 1988, Paterson and Sorrells, 1990) Paterson and Sorrells (1990) identified two QTL with dominance for late germination They detected dominant gene action among four crosses, a significant dominance X dominance epistasis in the Houser/Clark's Cream cross, and found no cytoplasmic effects on dormancy Bhatt et al (1983) crossed two nondormant, and two dormant wheat varieties and determined that two genes controlled seed dormancy with dominance for early germination Conflicting results as to whether dormancy or nondormancy is genetically the dominant form of the trait may indicate that multiple loci or alleles with contrasting effects on germinabihty are present in the wheat germplasm Quantitative trait analysis of two F.-denved Rl line

Genetic basis for seed dormancy populations of white-kernelled wheat were performed to identify QTL associated with resistance to preharvest sprouting (Anderson et al, 1993) The sources of the two RI populations were NY18/Clark's Cream and NY18/NY10 NY10 is nondormant and Clark's Cream and NY18 are intermediately dormant lines The RI lines were probed with 27-37 polymorphic RFLP markers Four QTL from each population were significantly associated with resistance to pre-harvest sprouting, and accounted for 44% (NY18/Clark's Cream), and 51% (NY18/NY10) of the genetic variance Marker CDO64 was associated with significant epistatic effects in both populations In a later study the NY18/Clark's Cream RI population was probed with 70 additional RFLP markers, and six different chromosomal regions associated with preharvest sprouting were identified Marker BCD1874 corresponds to the major dormancy QTL marked by PSR128 on barley chromosome 7 (Sorrells and Anderson, 1996) The importance of red-kernel colour genes as markers for resistance to pre-harvest sprouting in wheat is widely accepted The general characteristics of dormancy and the probable location of dormancy genes appears to be similar in red- and whitekernelled cultivars, though there appear to be differences in the mode of inheritance (Mares and Ellison, 1990) In general, dormancy tends to be dominant in red-kernelled cultivars, and recessive in white-kernelled cultivars The most dormant redkernelled wheat cultivars are more dormant than the most dormant white-kernelled cultivars While it has been possible to distinguish red seed-coat colour genes from dormancy genes in wheat, l e , they are not at the same locus, red- colour genes or a factor linked to red- colour genes appears to be acting in concert with dormancy genes Whether this association is due to a pleiotrophic effect of red colour is not clear (Mares, 1993) In maize, the VP1 protein regulates the Cl gene which regulates the expression of anthocyanin genes coding for seed-coat pigments, and VP1 also inhibits viviparous germination (McCarty et al, 1991) The relationship between seed-coat colour and seed dormancy in wheat, or vivipary in maize remains unknown Rice

Rice is the world's second most important cereal grain Rice will likely be the species of choice for cloning dormancy genes from cereal grain species because it displays a dormancy phenotype, is a diploid (2«=24), has a relatively small genome size (450 Mb), and gene content and orders are highly conserved between different species within the grass family (McCouch and Doerge, 1995, Devos and Gale, 1997) Dormancy in nee is a quantitative trait with dormancy bemg the dominant form of the trait (Chang and Tagumpay,

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1973, Tomar, 1984, Shenoy, 1993) Germination results from reciprocal cross-pollinations indicate that cytoplasmic factors are not involved in the control of dormancy (Seshu and Sorrells, 1986) Dormancy genes in rice have cumulative and unequal effects (Chang and Tagumpay, 1973) Both the hull and pericarp contribute to seed dormancy, with the hull factor being predominant (Seshu and Sorrells, 1986, Seshu and Dadlani, 1991) Hull and pericarp-imposed dormancy are controlled by independent factors Hullimposed dormancy is controlled by a single dominant gene along with some modifier genes (Seshu and Sorrells, 1986) There are conflicting reports whether embryo dormancy occurs in rice and whether seed-coat colour affects dormancy (Takahashi, 1961, Wu, 1978, Seshu and Dadlani, 1991) In general, wild species of nee (O satwa) and the cultivated species of African nee (O glabernma) exhibit stronger dormancy than the subspecies indica (Roberts, 1965) Cultivars belonging to tropical indica sub-species show greater dormancy than those belonging to the japonica and javanica subspecies (Seshu and Dadlani, 1991) Estimates of hentabihty for dormancy in nee are lacking, however, it is known that dormancy in rice is responsive to the temperature dunng seed development (Rao, 1994) Genetics of seed dormancy in dicotyledons

The mhentance of dormancy in dicotyledonous species is not well understood In particular, limited research has been done to determine the genetic basis for dormancy in Ambidopsis thahana (Rehwaldt, 1965, Van der Schaar et al, 1997) Reciprocal matings between several individuals of slightly dormant snapdragon {Antirrhinum majus L) subspecies majus and strongly dormant subspecies latifohum lines were made, and several populations and their progeny were evaluated for germinabihty Dormancy in snapdragon is a quantitative trait with nuclear and maternal factors (seed coat) influencing the degree of dormancy (Gunther and Bornss, 1975) Reciprocal matings between two inbred nondormant cucumber (Cucumis satwus L) vaneties satwus and two dormant hardwicku accessions were made, and several populations and their progeny were evaluated for germinabihty F, seeds were nondormant indicating that dormancy was the recessive form of the trait Reciprocal effects were not detected which suggested a lack of cytoplasmic or maternal effects Three to seven genes were estimated to control seed dormancy in cucumber, and hentabihty estimates ranged from 78 to 95% (Staub et al, 1989) Reciprocal matings between lines of hsianthus (Eustoma grandiflorum L) with nondormant and dormant seeds were made and several populations

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were evaluated for germinabihty Progeny testing was not conducted Dormancy in hsianthus seed is recessive, and the trait is controlled by multiple nuclear genes (Ecker et al, 1994) Reciprocal interspecific matings between poppies (Papaver spp) with different levels of seed dormancy indicated both nuclear and maternal factors influence the degree of seed dormancy (Harper and McNaughton, 1960) Reciprocal matings between a wild-type and domesticated variety (Shirley) of corn poppy (Papaver rhoeas L) indicated nondormancy is the dominant form of the trait, and dormancy is mainly controlled by nuclear genes (Lane and Lawrence, 1995) In the aforementioned study, progeny testing was not conducted, and germination and afterripening were applied inconsistently Reciprocal matmgs between petunia (Petunia hybnda L) hybrids with nondormant and slightly dormant seeds indicated a strong maternal control ot the dormancy trait (Girard, 1990) In this study, germinabihty was determined only for parental and F1 seeds, thus, limiting genetic insights into seed dormancy in petunia Reciprocal matings between a nondormant and dormant line of charlock (Smapsis arvensis L ) revealed a combination of embryo and coat-imposed dormancy A single gene was found to control coat-imposed dormancy, but the nature of the nuclear factors was not clear (Garbutt and Witcombe, 1986)

The benefits of genetic research on seed dormancy Seed dormancy is an important physiological stage of arrested development that occurs in most flowering plants Fundamental insights gained through genetic and molecular genetic approaches into mechanisms underlying dormancy and afterripening may bring about serendipitous discoveries related to germination, seed longevity, and plant growth and development, to name just a few For example, a contemporary question in plant developmental biology is How does desiccation or imbibition of seeds facilitate their transition from seed development to germination (McCarty, 1995)7 If dormancy is the result of a mutation in a gene associated with this transition, a great deal could be learned about the relationship of pathways for seed development and germination by investigating factors that regulate germinabihty of dormant seeds Dormancy in crop seeds

Identifying and cloning genes that regulate dormancy could improve agronomic characteristics of cereal gram crops Developing lines of barley, rice and wheat

with predictable levels of resistance to pre-harvest sprouting is a high priority for some cereal grain breeders Molecular markers for dormancy QTL could be used in marker-assisted selection for resistance to pre-harvest sprouting (Sorrells and Anderson, 1996) Discovering different QTL for dormancy in nondomesticated germplasm, perhaps QTL less affected by the environment, and transferring them into adapted lines could be useful for fine-tuning levels of resistance to pre-harvest sprouting Genetic knowledge and molecular markers tightly linked with dormancy QTL in Arena spp could be used for developing lines of spring oats with enhanced yield and disease-escape potential (Burrows, 1970) Winter oats do not possess sufficient winter hardiness to be grown successfully in Canada and the North Central U S One alternative to this limitation has been the development of 'dormoats' which are sown m the autumn, left to overwinter as ungerminated seeds in the soil, and germinate the following spring Dormoats are a cross between oats and dormant wild oats Limitations of time and weather make it more efficient to sow oats in the autumn, since overwintering seeds can normally germinate earlier than fields can be prepared and sown in the spring Theoretically a variety which has the correct level of dormancy should make this project feasible However, problems with insufficient dormancy after sowing that allows some seeds to germinate and be winter killed, as well as uneven germination ot dormoats the following spring due to lingering dormancy, have hindered this project (Andrews and Burrows, 1972) Dormant seeds could be useful in other situations, for example, to protect investments in biotechnology The use of biotechnology has greatly expanded opportunities to develop pest- and herbicide-resistant crops Herbicides have traditionally been developed for selective use m crops that are naturally resistant to the herbicide This selectivity also prevents herbicides from controlling all troublesome weeds in a particular crop Recently, crops have been genetically modified to make them resistant to certain herbicides and thus enhance their value to farmers (Padgette et a\, 1995) The development of herbicide-resistant crops through plant breeding and biotechnology is expensive and time consuming Ten or more years and millions of dollars are required to introduce a new herbicide-resistant variety to the market Biotechnology and seed companies can recover their investment if the resistant crop is, for example, a proprietary maize hybrid which growers must purchase each season However, seeds from selfpollinated crops such as wheat and soybean can be saved and sown the following season To protect their investment Monsanto requires growers who buy glyphosate-tolerant soybean seed, l e , the Roundup Ready system, to sign Monsanto's grower agreement

Genetic basis for seed dormancy in which the grower agrees not to save seed for sowing (Marshall, 1996) Another method to ensure recovery of biotechnology investments by companies might be to deliberately make crop seeds dormant For example, a herbicide resistant crop with seed dormancy could be bred or engineered to require special treatment in order for the crop to germinate Specific afterripenmg conditions might be required to remove the dormant condition at the seed-processing plant Most producers would not have access to such facilities and would be forced to buy new seeds each year since their own seeds would not germinate However, our present knowledge of dormancy and afterripenmg is inadequate to support development ot such a system Predictive models for weed control

Decision-support systems for weed management are being developed (O'Donovan, 1996) These systems are important because they can be used to predict appropriate levels of weed control with less herbicide without sacrificing profit (Buhler et al, 1996, Forcella et al, 1996) In these decision-support systems are models that take into account information on seed population dynamics to predict seedling emergence (Forcella et al, 1992) Dormancy is a primary factor that influences population dynamics of seeds Quantitative traits like seed dormancy can be described by the formula P = G + E, where P is the phenotypic value, G is the genotypic value, and E is the environmental deviation (Falconer, 1989) In the case of a weed species, the time of emergence for a particular seed will be dictated by genes that contribute to the phenotype (dormancy genes) and environmental conditions Predictive models of weed seedling emergence have thus far focused on the impact of the environment on the time to germination, e g, factors such as April to June rainfall and temperature (Forcella et al, 1992, Forcella et al, 1997) In wild oat, approximately one-half of the variation in germination is due to genetic factors, while the remainder is due to the influence of the environment and G X E interactions (Jana and Nay lor, 1980, Fennimore, 1997) This means that a seedling emergence model for wild oat based on weather data alone would only address 50% of the factors that influence germination The precision of models that predict seedling emergence based on soil moisture, rainfall or temperature could be increased by incorporating genetic and environmental information related to seed dormancy (Oryokot et al, 1997) It molecular markers for dormancy QTL were available for an inbreeding species like wild oat, populations in a field could be mapped A seedling emergence model would include a genetic index based on the proportion of dormant

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and nondormant alleles present in a field, as well as the stability of the population within that field Genetic indices could be entered as part of the weed map data on global positioning system field maps Baseline information could be used to evaluate and eventually predict the influence of management practices on the dynamics of seed dormancy in the field (Thai et al, 1985, Jana and Thai, 1987) While no such model yet exists, these ideas provide a framework for future weed seed biology research

Conclusion Genetic information on dormancy in seeds is rudimentary even for the relatively well characterized cereal grains species Dormancy is a quantitative trait controlled by nuclear and sometimes by maternal factors depending on the species and genotype Environmental factors can have significant effects on the germination phenotype, and these factors are known to interact with the genotype Some molecular markers for dormancy QTL have been identified in barley, wheat and rice (Anderson et al, 1993 Oberthur et al, 1995, Han et al, 1996, Wan et al, 1997), however, corresponding genes have not been identified and cloned Understanding the expression of seed dormancy genes, examining G X E interactions, cloning dormancy genes and identifying their products are critical to understanding the mechanisms underlying dormancy and afterripenmg Fundamental knowledge of dormancy will have several practical applications in agriculture

Acknowledgements Research in Dr Foley's lab is supported by a grant from the National Science Foundation (IBN-9318055) This is journal paper series No 15493 of the Purdue University Office of Agricultural Research Programs

References Adkins, S W , Loewen, M and Symons, SJ (1986) Variation within pure lines of wild oats (Avena fatua) in relation to degree of primary dormancy Weed Science 34, 859-864 Anderson, J A , Sorrells, M E and Tanksley, S D (1993) RFLP analysis of genomic regions associated with resistance to pre-harvest sprouting in wheat Crop Science 33,453-459 Andrews, CJ and Burrows, VD (1972) Germination response of dormoat to low temperature and gibberelhn Canadian Journal of Plant Science 52, 295-303

Arumuganathan, K and Earle, E D (1991) Nuclear DNA content of some important plant species Plant Molecular Biology Reporter 9, 208-218

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