Marc Alan Cohn

Plant hormone interactions during seed dormancy release and germination

This review focuses mainly on eudicot seeds, and on the interactions between abscisic acid (ABA), gibberellins (GA), ethylene, brassinosteroids (BR), auxin and cytokinins in regulating the interconnected molecular processes that control dormancy release and germination. Signal transduction pathways, mediated by environmental and hormonal signals, regulate gene expression in seeds. Seed dormancy release and germination of species with coat dormancy is determined by the balance of forces between the growth potential of the embryo and the constraint exerted by the covering layers, e.g. testa and endosperm. Recent progress in the field of seed biology has been greatly aided by molecular approaches utilizing mutant and transgenic seeds of Arabidopsis thaliana and the Solanaceae model systems, tomato and tobacco, which are altered in hormone biology. ABA is a positive regulator of dormancy induction and most likely also maintenance, while it is a negative regulator of germination. GA releases dormancy, promotes germination and counteracts ABA effects. Ethylene and BR promote seed germination and also counteract ABA effects. We present an integrated view of the molecular genetics, physiology and biochemistry used to unravel how hormones control seed dormancy release and germination.

Field evaluation of seed production, shattering, and dormancy in hybrid populations of transgenic rice (Oryza sativa) and the weed, red rice (Oryza sativa).

The genetic and agronomic consequences of transferring glufosinate (Liberty) herbicide resistance from transgenic rice (Oryza sativa L.) lines to the noxious weed red rice (Oryza sativa L.) were evaluated under field conditions. Replicated field trials in Louisiana (LA) and Arkansas (AR) were conducted in 1997 to evaluate ten vegetative and reproductive traits of eight F(2) populations produced from controlled crosses of two transgenic, glufosinate-resistant rice lines and four red rice biotypes. Plant vigor and plant density at both locations were similar among populations derived from either transgenic or non-transgenic parents. Significant differences in plant height and maturity were observed among LA populations produced from transgenic lines when compared to corresponding populations developed from non-transgenic material. However, values for these traits were not greater than those detected in the red rice biotypes. Seed dormancy and seed production were not significantly different at either location among transgenic and non-transgenic populations. Dominant Mendelian segregation of glufosinate resistance was detected in 40% of the populations evaluated. Results of this study indicated that those populations segregating for glufosinate resistance responded in a location-specific manner with respect to life history and fecundity traits.

Seed dormancy in red rice (Oryza sativa). XI. Commercial liquid smoke elicits germination.

A commercial liquid smoke (LS) flavoring product (Reese Hickory brand) in aqueous dilutions of 4–5% (v/v) broke dormancy (40–100% germination) of both intact and dehulled red rice ( Oryza sativa ). To elicit this response in intact grains, the glumes had to be pricked with a dissecting needle following contact with LS and water rinsing. Germination percentages rose with increasing contact time (up to 7 d), and grains first imbibed with water before transfer to LS showed similar germination percentages as those first incubated with LS before transfer to water. LS dilutions were highly acidic (about pH 3). Dormancy-breaking activity of LS was pH-dependent and was reduced at pH values of 5–7. Buffer controls remained dormant irrespective of pH value. Maximum activity varied primarily among different bottles of LS and only to a minor extent between red rice seed lots. Dry-afterripening at 30°C for up to 14 d increased the sensitivity of unpricked, intact grains to LS compared with water controls. LS concentrations that broke dormancy of intact grains partially inhibited germination of nondormant, fully afterripened grains. Storing grains in containers of flooded soil increased sensitivity to both LS and pricking. Nitrite levels in LS were negligible and insufficient to account for the bioactivity of LS.

Chemical mechanisms of breaking seed dormancy

Features of chemicals which break seed dormancy and the mechanisms of their action are discussed with reference to results obtained with red rice (Oryza sativa) and other relevant research. The significance of chemical structure, including the nature and location of functional groups, is considered, and the fact that similar structures can elicit different biochemical responses is illustrated with examples. As yet it is unclear whether changes in cell pH, highly correlated with the uptake of, and metabolism to, weak acids, and which some consider a possible trigger or marker in dormancy breaking, are causative, consequential or merely correlative. Uptake and metabolism of dormancy-breaking compounds is, or can be, very rapid and the importance of establishing the time course of dormancy-breaking, as opposed to germination events, is emphasized.

Developmental arrest: from sea urchins to seeds

The phenomenon of dormancy extends beyond the boundaries of the plant kingdom. While plant biologists typically associate dormancy-breaking treatments only with seeds, buds or tubers, these chemicals and environmental stimuli have much broader activity as general terminators of developmental arrest in other, non-plant species. The activation of growth by these treatments is associated with signal transduction processes, metabolic upregulation and changes in gene expression, in addition to other events that may or may not be species specific. The study of both the classic and current developmental arrest literature beyond the boundaries of plant biology may be helpful in generating useful ideas and analogies for meaningful experimental progress towards understanding seed dormancy.

Mapping of Seed Shattering Loci Provides Insights into Origin of Weedy Rice and Rice Domestication

Seed shattering is an important trait that distinguishes crop cultivars from the wild and weedy species. The genetics of seed shattering was investigated in this study to provide insights into rice domestication and the evolution of weedy rice. Quantitative trait locus (QTL) analysis, conducted in 2 recombinant inbred populations involving 2 rice cultivars and a weedy rice accession of the southern United States, revealed 3-5 QTLs that controlled seed shattering with 38-45% of the total phenotypic variation. Two QTLs on chromosomes 4 and 10 were consistent in both populations. Both cultivar and weedy rice contributed alleles for increased seed shattering. Genetic backgrounds affected both QTL number and the magnitude of QTL effects. The major QTL qSH4 and a minor QTL qSH3 were validated in near-isogenic lines, with the former conferring a significantly higher degree of seed shattering than the latter. Although the major QTL qSH4 overlapped with the sh4, the presence of the nonshattering single nucleotide polymorphism allele in the weedy rice accession suggested involvement of a linked locus or an alternative molecular genetic mechanism. Overlapping of several QTLs with those from earlier studies indicated that weedy rice may have been derived from the wild species Oryza rufipogon. Natural hybridization of rice cultivars with the highly variable O. rufipogon present in different geographic regions might be responsible for the evolution of a wide range of phenotypic and genotypic variabilities seen in weedy rice populations worldwide.

Factors Influencing the Efficacy of Dormancy-Breaking Chemicals

Seed dormancy is advantageous for the survival of wild species but presents serious obstacles for the evaluation of seed performance, stand establishment, and efficient control of weeds necessary for cost-efficient agricultural practices. Significant time and effort have been invested in attempts to derive simple and uniform chemical dormancy- breaking treatments with only limited success.

Seed dormancy in red rice. XIII: Interaction of dry-afterripening and hydration temperature

Department of Plant Pathology and Crop Physiology, Louisiana State University Agricultural Center,302 Life Sciences Building, Baton Rouge, LA 70803, USAAbstractWhile red rice (Oryza sativa L.) can remain dormantand viable for many years when fully imbibed, theenvironmental factors that stimulate germination orinducesecondarydormancyinthefieldhavenotbeencharacterized. In this study, the interactions betweenthe extent of dry-afterripening and germinationtemperature have been evaluated as possible factors.Red rice dispersal units (florets) were afterripenedfor 0–10 weeks at 308C and incubated in water at 1, 5,15, 20, 25, 30 and 358C for 2 weeks; then, all theungerminated florets were transferred to 308C for twoadditional weeks. Germination at the end of each ofthe two sequential treatments was compared to defineboth the effect of differing temperatures on germina-tion (first treatment), and the effect of these tempera-tures on subsequent germination at the optimumtemperature(308C,secondtreatment).Inafterripeningred rice, the opening of the temperature window forgermination begins at 25–358C. Fully dormant floretsacquired the ability to germinate

A Chromosome Segment Substitution Library of Weedy Rice for Genetic Dissection of Complex Agronomic and Domestication Traits

90% at 308C after4 weeks of dry-afterripening. However, imbibingflorets for 2 weeks at 158C followed by 2 weeks at308C, yielded suboptimal germination and induced adegree of secondary dormancy, dependent upon theextent of previous dry-afterripening. Cold stratification(18C) had a consistent promotive effect on thesubsequent germination, particularly when precededby 1–2 weeks of dry-afterripening at 308C. To monitorthe effects of germination temperature, median after-ripening time was utilized as a relative dormancyindex, and changes in this index have been inter-preted as an overlapping of germination and thetemperature-inducedchangesinthedormancystatus.Field weather data suggested that low-temperaturestratification may be a germination trigger in the field,even in southern Louisiana, and this merits furtherinvestigation in studies of soil-buried seeds.Keywords: cold stratification, dry-afterripening, Oryzasativa, seed dormancy, temperatureIntroductionRed rice, a noxious weed when present in crop fields,is widespread in the USA, Latin America and Europe.Redriceisconsideredtobethesamespeciesas cultivated rice (Oryza sativa L.), but possessesunfavourable agronomic characters such as shatter-ing, red pericarp and seed dormancy (Noldin et al.,1999). The shattered dispersal units remain in the soiland perpetuate the weed in subsequent years (Gossand Brown, 1939). Environmental factors triggeringits field germination are not known, but inductionof secondary dormancy and dormancy cycling havebeen reported (Teekachunhatean, 1985), and themagnitude of the red rice soil seed bank suggeststhat such mechanisms should be important forfield survival. A better comprehension of seedbank ecology and dormancy cycling may be a keyto improved forecast modelling for red rice weedmanagement, as has been demonstrated for otherweeds (Forcella et al., 1997; Forcella, 1998).In Polygonum persicaria, temperature exerts a dualroleinregulatinggermination,firstcontrollingchangesin dormancy status, and thereafter modulatinggermination of the non-dormant seeds (Bouwmeesterand Karssen, 1992). In rice cv. Kasalath, completelyafterripened florets were induced into secondarydormancy after imbibition and incubation at tempera-tures between 8 and 198C (particularly at 10 and 158C)when subsequently tested for germination at 258C(Miura and Araki, 1996). With imbibition at 58C, nodormancy developed, and florets germinated 99%when transferred to 258C. However, secondary

Reflections on investigating dormancy-breaking chemicals—a balance of logic and luck

Chromosome segment substitution lines (CSSLs) are a powerful alternative for locating quantitative trait loci (QTL), analyzing gene interactions, and providing starting materials for map-based cloning projects. We report the development and characterization of a CSSL library of a U.S. weedy rice accession ‘PSRR-1’ with genome-wide coverage in an adapted rice cultivar ‘Bengal’ background. The majority of the CSSLs carried a single defined weedy rice segment with an average introgression segment of 2.8 % of the donor genome. QTL mapping results for several agronomic and domestication traits from the CSSL population were compared with those obtained from two recombinant inbred line (RIL) populations involving the same weedy rice accession. There was congruence of major effect QTLs between both types of populations, but new and additional QTLs were detected in the CSSL population. Although, three major effect QTLs for plant height were detected on chromosomes 1, 4, and 8 in the CSSL population, the latter two escaped detection in both RIL populations. Since this was observed for many traits, epistasis may play a major role for the phenotypic variation observed in weedy rice. High levels of shattering and seed dormancy in weedy rice might result from an accumulation of many small effect QTLs. Several CSSLs with desirable agronomic traits (e.g. longer panicles, longer grains, and higher seed weight) identified in this study could be useful for rice breeding. Since weedy rice is a reservoir of genes for many weedy and agronomic attributes, the CSSL library will serve as a valuable resource to discover latent genetic diversity for improving crop productivity and understanding the plant domestication process through cloning and characterization of the underlying genes.