Molecular and Physiological Characterization of Rice (Oryza sativa) Seed Development Under Heat Stress

Climatic variations are a serious threat to global food security. Amongst climatic variations, rising temperature is a critical issue as yields of major staple crops (such as rice, wheat and maize) are reduced by heat, especially during early seed development. Rice seed is comprised of embryo, endosperm and maternal tissues, where endosperm occupies ~88% of seed volume. After fertilization, endosperm development proceeds rapidly with repeated acytokinetic mitotic divisions resulting in a nuclear/coenocytic endosperm, followed by transition to the cellularized state. The duration and rate of nuclear divisions during coenocytic stage are key determinants of yield since the number of coenocytic nuclei typically correlates with mature seed size. Further, endosperm cellularization exhibits high heat sensitivity. Given the potential role of endosperm cellularization in determining rice yield under heat stress, my research focused on deciphering the underlying molecular mechanisms controlling heat sensitivity at this critical stage of development. To accomplish this I have used high temporal-resolution phenotypic and molecular approaches. First, a temporal-phenotyping experiment determined that seeds are most heat-sensitive during the first two days after flowering. An hourly transcriptome analysis of heat-stressed seeds during this sensitive stage found endoplasmic reticulum stress and jasmonic acid pathways to be core components of earliest heat-induced transcriptional responses. Since DNA methylation is a critical component of endosperm development, I conducted a high-resolution DNA methylation analysis of heat-stressed seeds and found that transcriptional responses to heat stress are controlled by DNA methylation levels in gene body and promoter regions. Finally, I used the seed transcriptome to develop a differential co-expression network that identified OsMYB3R-3 and several other regulators of early seed development. Characterization of OsMYB3R-3, a three MYB repeat protein, showed that it is implicated in cytokinin-mediated regulation of early seed growth under heat stress. OsMYB3R-3 deficiency increased seed size and weight under control conditions but reduced heat resilience of seeds. Collectively, this study enhances our understanding of the molecular mechanisms controlling the interactions between temperature and developmental dynamics in developing seeds