Acts as positive regulator of grain size by controlling grain width, filling and weight. High expression of GS5 in the grain is correlated with large grain size. Induced by light. Circadian regulation with a peak of expression at dusk (PubMed:25711711). Down-regulated by abscisic acid (ABA) and brassinosteroid (BR) (PubMed:25711711). Belongs to the peptidase S10 family.