OsSET7, a homologue of ARABIDOPSIS TRITHORAX-RELATED protein that plays a role in grain elongation regulation in rice

Grain shape is a determinate trait of rice quality and largely controlled by spikelet hull size. Although a series of genes controlling grain size have been cloned, the mechanism in epigenetic regulation remained largely unknown. Here, we report that knockdown of OsSET7, which encodes a putative H3K27 monomethyltransferase belonging to the SET-domain family in rice, results in enhanced cell proliferation of the spikelet hull, leading to a significant increase in grain length. The transcript of OsSET7 was accumulated both in vegetative and reproductive tissues, with stronger expression level in the young panicle, implying the crucial roles that OsSET7 plays in panicle development. The expression of cell cycle-related genes, such as E2Fs, CYCA2;2, CDKs, and MCMs was up-regulated in the osset7 mutant. In addition, both yeast two-hybrid system and bimolecular fluorescence complementation (BiFC) assay validated that OsSET7 could combine with proliferating cell nuclear antigen (PCNA). Taken together, our results demonstrate that OsSET7 plays an important role in rice spikelet hull elongation by regulating cell division, which provides a new insight into the functions of SET-domain genes in plants.