RGB1, a subunit of heterotrimeric G protein, plays important roles in
regulating grain size and weight of rice. However, the molecular
mechanisms underlying controlling grain filling process by G protein are
still largely unclear. In the present study, we show that, RGB1 controls
not only the grain size, but the grain filling process. Knock-down of
RGB1 significantly delayed grain development and reduced starch
accumulation and grain weight, which was closely related to the delayed
and lower expression of genes encoding sucrose metabolism and starch
biosynthesis related enzymes during grain filling stage. Suppression of
RGB1 expression also resulted in the lower auxin content in grains,
which was correlated with the lower expression of OsNF-YB1 and OsYUC11
during grain filling stage. Further biochemical evidence showed that
OsYUC11 expression was under control of OsNF-YB1 by its interaction with
promoter of OsYUC11. Taken together, we propose that RGB1 controls rice
grain development and grain filling process by changing auxin
homeostasis in endosperm cells, and OsNF-YB1, which acts as a key
downstream effector of RGB1, interacts directly with the promoter of
OsYUC11 and promotes the transactivation of OsYUC11, thereby regulating
auxin biosynthesis and starch accumulation and grain size.