OsGS5 controls grain size by regulating grain width, filling and weight. GS5 encodes a putative serine carboxypeptidase and functions as a positive regulator of grain size. Higher expression of GS5 is correlated with larger grain size (Li et al., 2011; Xu et al., 2015). GS5 has minor role in controlling grain size as overexpression of GS5 increases the grain width by only ~6% (Xu et al., 2015). This is probably because the effect of competitive interaction of the proteins is dependent on the quantity of the other two proteins (OsBAK1 and OsMSBP1) and is also subject to feedback regulation of BR signalling (Xu et al., 2015). Excess GS5 proteins may get stuck in the ER.