Molecular Genetics of Seed Dormancy in Rice.

Cereal crops differentiated from wild/weed relatives in seed dormancy. The differentiation between cultivated and weedy rice (Oryza sativa) occurred at >10 quantitative trait loci, with ~80% of them having the dormancy-enhancing alleles from the weed. The SD7-1, SD1-2, SD7-2 and SD12 loci were map-based cloned and characterized for molecular functions. SD7-1 controls maternal tissue-imposed dormancy and its underlying gene encoding a bHLH family transcription factor, which promoted the abscisic acid (ABA) biosynthesis and accumulation in early developing seeds and also activated the flavonoid biosynthesis pathway to produce red pigments in the lower epidermal cells of the pericarp tissue. SD1-2 controls endosperm-imposed dormancy and its underlying gene encoding a gibberellin (GA) biosynthesis enzyme, with the loss-of-functional mutations reduced the GA level in early developing seeds and delayed the ABA accumulation, dehydration and germination. SD7-2 encodes a predicted protein kinase presumably phosphorylating the DELLA protein SLR1, a core negative regulator of GA signaling; the functional alleles delay dormancy release and germination. SD12 is a major locus controlling embryo dormancy and its effect on the delay of germination was enhanced by high temperatures during seed development. Underlying SD12 are three tightly linked genes encoding bHLH family transcription factors or hypothetic protein; the linked genes regulate the dormancy development presumably by promoting the expression of heat-shock and late embryogenesis abundant proteins to influence the acquisition of desiccation tolerance. Identities of the cloned genes demonstrated that natural variation in seed dormancy is largely regulated at a transcriptional level or by modifying GA or ABA signaling pathways in component tissues of developing or after-ripening seeds. Some of these genes can be used to improve crop resistance to pre-harvest sprouting or to develop a new technique to mitigate transgene flow into weedy rice.