Scientist IV National Institute of Plant Genome Research
Rice is a staple food, with variable consumer preferences and grain is the commercially important part. SUPER STARCHY1 (SS1) is a NAC transcription factor (TF), which was seed-specific in our earlier microarray analysis covering multiple stages of rice development. We have characterized the role of SS1 by determining its properties as a TF, its interacting partners and downstream genes. Rice transgenic plants with altered expression of the gene have been analyzed to determine its functional role. Rice transgenic plants harboring the promoter::GUS construct show high expression in both endosperm and embryo during seed development stages. It acts as a transcriptional repressor in yeast. SS1 localizes to endoplasmic reticulum and nucleus in both onion-peel and tobacco-leaf cells. It interacts with other seed-specific TFs, and binds to GLUTELIN promoter. Rice transgenic plants ectopically expressing SS1 exhibit a plantlet lethal phenotype and multiple tillers. Their histological sections show an elongated SAM, and vascular strands to each tiller. Leaf sections have prominent starch granules deeply stained with iodine. RNAseq of plantlets shows up regulation of genes controlling reproduction and embryonic development, and suppression of vegetative pathways, including photosynthesis; supporting plantlet lethality. An overlay of this data with previous microarray analysis, shows up/down regulation of genes differentially expressed/specific during seed development, including those for amylopection biosynthesis, seed size and seed storage proteins. Seed-preferential over expression results in rice plants with increased tiller number, and bigger panicles, leading to an increased seed number. Though seed size and weight are significantly decreased, there is an overall increase in yield per plant. Additionally, SS1 shows much higher expression in japonica rice, compared to indica rice by q-RTPCR. The study concludes that SS1 regulates multiple aspects of rice grain development and favors sticky rice.