Regulates Seed Development and Plant Growth and Participates in the Brassinosteroid Signaling Pathway.

Grain size and the endosperm starch content determine grain yield and quality in rice. Although these yield components have been intensively studied, their regulatory mechanisms are still largely unknown. In this study, we show that loss-of-function of , a member of the NAC transcription factor gene family that has its highest expression in the immature seed, greatly increased grain length, grain weight, apparent amylose content (AAC), and plant height. Overexpression of had the opposite effect, significantly decreasing grain width, grain weight, AAC, and plant height. Cytological observation of the outer epidermal cells of the lemma using a scanning electron microscope (SEM) revealed that increased grain length in the mutant was due to increased cell length compared with wild-type (WT) plants. The expression of and , two positive grain-size regulators that control cell elongation, was consistently upregulated in mutant plants but downregulated in overexpression plants. Furthermore, we also found that several starch synthase-encoding genes, including , were upregulated in the mutant and downregulated in the overexpression plants compared with WT plants, implying a negative regulatory role for both in grain size and starch biosynthesis. Additionally, we found that the expression of was induced exclusively by abscisic acid (ABA) in seedlings, but -overexpressing plants displayed reduced sensitivity to exogenous brassinolide (BR). Therefore, the results of our study demonstrate that negatively regulates seed development and plant growth, and further suggest that participates in the BR signaling pathway.