The molecular program leading to seed development in most angiosperms initiates as a result of a plant-specific mating event, known as double fertilization. In this process, two sperms simultaneously enter the 7-celled embryo-sac. One sperm fuses with the haploid (1n) egg cell resulting in a diploid zygote that eventually develops into embryo. Whereas, the other sperm fuses with the central cell that contains two polar (1n) nuclei. This second mating leads to the development of a triploid (3n) nutritive tissue, the endosperm. In most dicotyledonous plants, the endosperm is consumed by the developing embryo, whereas in monocots (especially cereals) the endosperm persists and constitutes major portion of the mature seed. The endosperm cells in rice and other cereals, like wheat, maize, sorghum and pennisetum, are tightly packed with starch granules, which are degraded into soluble sugars at the time of seed germination, providing essential nutrition to the developing seedling.
By using high-throughput microarray-based transcriptomics, our group has identified several candidate genes that could play important regulatory roles during seed development. We are currently involved in functional characterization of these genes by following reverse genetics approach.
One such gene is OsMADS29 that plays role in embryo development and grain filling by affecting hormone homeostasis during seed development in rice. MADS29 protein accumulates mainly in the embryo and aleurone and subaleurone layers of the endosperm. Ectopic expression of MADS29 results in severe dwarfing of the transgenic rice plants, exhibiting elevated levels of cytokinin. Suppression of MADS29 expression by RNAi severely affects seed set. The surviving seeds are smaller in size, with developmental abnormalities in the embryo and reduced size of endosperm cells. The packaging of starch in the endosperm is also not as compact as in the wild type. Transcriptome analysis of overexpression and knockdown lines have indicated genes involved in plastid biogenesis, starch biosynthesis, cytokinin signaling and biosynthesis pathways as probable targets of MADS29. Rice MADS29 has also been found to interact with several seed-expressed transcription factors and its capability to localize in the nucleus is affected by these interactions. Taken together, these data suggest that MADS29 is monocot-specific transcription factor that plays important role during seed development in rice.
Department of Plant Molecular Biology, University of Delhi South Campus, New Delhi-110021, India