The mitochondria-localized protein OsNDB2 negatively regulates grain size and weight in rice

Grain size is a determinant of rice grain yield. In plants, mitochondria supply energy for cellular metabolism via the mitochondrial electron transport chain. Here we report that OsNDB2, which encodes a putative rotenone-insensitive type II NAD(P)H dehydrogenase (ND), negatively regulates grain size and weight in rice. Six ND members representing three major types of rice were identified, and the predicted OsNDB2 protein was localized to mitochondria. Contents of OsNDB2 transcripts were higher in young panicles and leaf blades. OsNDB2 overexpression reduced grain length, grain width, and 1000-grain weight and moderately influenced plant height, while knockout of OsNDB2 increased grain size and 1000-grain weight. Allelic mutations of OsNDB2 were associated with diverse grain appearances. Cellular observations revealed that variations in grain size of transgenic lines were caused by change in cell expansion but not cell proliferation in spikelet hulls. Our study sheds light on OsNDB2 function and provides a new potential breeding approach for increasing rice grain size and weight.