An underutilized genetic component in the regulation of soybean seed weight

Seed weight is a key determinant of soybean yield; however, its underlying genetic regulation remains poorly understood. Here, besides the previously reported positive seed weight regulator GmST05/GmSW5, which encodes a protein homologue of Arabidopsis Mother of TFL1 and FT (MFT), we identify GmSW19 that exert negative effect on seed weight. GmSW19 belongs to the basic leucine zipper (bZIP) transcription factor family; it encoded bZIP transcription factor can represses GmSW5 expression. We further discover that the glycogen synthase kinase 3 (GSK3)-like kinase GmSK21 physically interacts with and phosphorylates GmSW19. A single nucleotide polymorphism (A to C), resulting in a T to P substitution in GmSW19, which alters its phosphorylation by GmSK21 and consequently its protein abundance. Functional analyses reveal that the GmSW19^A variant represses GmSW5 and seed weight stronger than that of GmSW19^C. Population analyses show that the heavy-seed allele GmSW19^C has not been fully utilized in modern soybean breeding. These findings elucidate the genetic components and their possible interaction in determining seed weight and highlight the potential for enhancing yield in soybean.