Molecular Basis Of Early Seed Development In Arabidopsis Regulated By Abscisic Acid

Seed is a reproductive organ in angiosperm. In the agricultural production, seed is the basis of crop yield formation. Clarifying the molecular mechanism of seed size determination might provide important information for the mechanism of crop yield formation.ABA, an important sesquiterpenoids phytohormone, plays critical roles in many biological processes, such as root development, leaf senescence, dehydration response, seed dormancy. However, its molecular mechanism in regulating early seed development remains unclear. In this study, the ABA synthesis mutant aba2-1 was used to analyze the molecular mechanism of ABA for the regulation of early seed development. ABA2 encodes a short-chain dehydrogenase/reductase in ABA biosynthesis. Our previous study showed that the seed size of aba2-1 mutant was larger than those of wild type, the one-hundred-seed weight of aba2-1 muant was also heavier than that of wild type, and cell number of embryo increased. Further analysis showed that embryo development and endosperm cellularization time delayed. Expression analysis of seed development related genes SHB1, MINI3, IKU2, AP2 and DA1 indicated that expression of SHB1, MINI3 and IKU2 was significantly enhanced in the mutant, but reduced in overexpression plants, comparied to those in wild type. The expression level of SHB1 decreased obviously after applying exogenous ABA. GUS staining analysis also indicated that ABA2 expression pattern is similar with SHB1. Therefore we suggest that ABA regulate seed development through negative regulation of SHB1.ABI5, AREB1, AREB2 and ABF3 are transcription factors responding to ABA signal, play important roles in ABA signal transduction. Their expression levels in aba2-1 mutant were lower than those in wild type, of which ABI5 was most significantly decreased. Further analysis showed that seed of abi5 mutant was larger than that of wild type, and SHB1 expression level is increased in abi5 mutant, suggesting that ABA regulate SHB1 expression through ABI5. ChIP experiment and yeast-one-hybrid analysis all indicated that ABI5 can bind to two distinct regions of SHB1 gene promoter(S3, S5), and EMSA assay verified that ABI5 can directly bind to S3 region. Taken together, these experiments demonstrated that ABA negatively regulates SHB1 expression, at least in part, through the action of its downstream signaling component ABI5.In this study, our results clearly revealed the molecular mechanisms of ABA regulating early seed development. ABA biosynthesis genes are responsible for the accumulation of ABA, and then ABA regulates the expression of ABA response gene ABI5, which binds to SHB1 promoter to repress its expression, SHB1 further regulate MINI3 and IKU2 expression, which determine the early seed development.