A zinc finger transcriptional repressor ZOS5-09 regulates rice grain size, starch and protein biosynthesis

Grain size is one of the key determinants of grain yield. Transcription factors form an important class of regulatory proteins that play crucial roles in regulating developmental responses. Our study focuses on a novel seed-preferential C2H2 zinc finger transcription factor, ZOS5-09 (LOC_Os05g38600) that plays an important role in regulating grain-related traits in rice. Rice plants with ZOS5-09 promoter::GUS construct showed its high expression in rice endosperm. In planta reporter effector assays and localization studies showed that ZOS5-09 is a nuclear-localized transcriptional repressor. It has a C-terminal NoRS (nucleolar retention signal). Ectopic and seed-preferential overexpression of ZOS5-09 resulted in lethality. Seed-preferential overexpression without NoRS was detrimental to grain filling. Rice plants with knock-down or CRISPR based knock-out of ZOS5-09 displayed reduced grain length and weight but increased grain width. Plant height, second leaf width, and panicle number were reduced and second leaf length were increased. The change in grain size was due to lower cell proliferation and increased cell size in the transverse direction due to down regulation of cell cycle related genes and increased expression of expansins. Decreased expression of ZOS5-09 also resulted in reduced total starch and protein content and higher endosperm chalkiness, thus, negatively affecting grain quality. ZOS5-09 directly regulated a seed storage protein encoding gene, GLU6 by binding to the zinc finger binding site in its promoter. It acted as a repressor by promoting deacetylation upon interaction with a histone deacetylase. In summary, our results indicate that optimum expression of ZOS5-09 is essential for proper rice grain development. Our study highlights the role of a transcriptional repressor in regulating rice grain traits and improves our understanding of the transcriptional regulatory networks affecting grain size.