The grain weight trait is influenced by the size (length and width) of the grain. Total yield in rice is mainly determined by different traits like number of panicles per plant, number of grains per panicle and grain size/weight. According to few surveys, by 2030, rice production should be almost doubled to fulfil the demand of growing human population. Rice is a staple food for more than half of the world population and nearly 3.5 billion people depend on it for their daily calorific intake. We propose that the integrated molecular-genetics approach used here could help identify networks of functional alleles of other regulator and co-regulator genes and thereby inform efforts for marker-assisted introgression of useful alleles in rice crop improvement. We conclude that the transactivation domains of OsNAC024 and OsNAC025 target the KIX domain of OsMED15a in the regulation of grain size/weight–associated genes such as GW2, GW5, and D11. Of note, both OsNAC024 and OsNAC025 bound to the promoters of these three genes. RNAi-mediated repression of OsMED15a expression down-regulated the expression of the grain size/weight regulating genes GW2, GW5 and DR11 and reduced grain length, weight, and yield. Moreover, OsMED15a, OsNAC024, and OsNAC025 all exhibited increased expression during seed development, and we identified several grain size/weight-associated SNPs in these genes in 509 low- and high-grain-weight rice genotypes. Two of these helices, α1 and α3, and three amino acids, 76LRC78, within OsMED15a helix α3 were important for its interaction with several proteins, including interactions with the transactivation domains of two NAC-type TFs, OsNAC024 and OsNAC025.

OsMED15a has an N-terminal, three-helical KIX domain. Expression analysis and high-resolution quantitative trait loci (QTL) mapping suggested that OsMED15a is involved in rice seed development. Here, we report that a Mediator subunit, OsMED15a, links rice grain size/weight-regulating TFs to their target genes. Although several transcription factors (TFs) that regulate seed size/weight in plants are known, the molecular landscape regulating this important trait is unclear.

Gw2 chaperone