The F box protein <scp>INHIBITOR FOR BROWN FURROWS</scp> 1 ( <scp>IBF</scp> 1) regulates flavonoid accumulation in rice hull by promoting degradation of chalcone synthase 1

Flavonoids are secondary metabolites of plants that play various roles in plants. The transcriptional level regulation of flavonoid synthesis in plants has been extensively studied, but research on the protein level of flavonoid synthesis in plants is still limited. In the present study, a brown hull mutant, bh2, was screened from an ethane methyl sulfonate (EMS)-induced bank from the seeds of the indica cultivar RH2B. The bh2 mutant exhibited a brown hull phenotype and higher levels of total flavonoids and anthocyanins compared with wild-type plants. We identified the gene INHIBITOR FOR BROWN FURROWS 1 (IBF1) in the bh2 mutant through MutMap analysis and subsequently cloned it. IBF1 encodes an F-box protein and is involved in the formation of an SCF (S-phase kinase-associated protein 1 [SKP1], Cullin, and F-box) complex with the Oryza sativa SKP1-like proteins OSK1/OSK20. Through yeast two-hybrid, bimolecular fluorescence complementation, and pull-down assays, the interaction of IBF1 with chalcone synthase 1 (CHS1) was confirmed. This interaction facilitated the degradation of CHS1 through the ubiquitin-26S proteasome system. The ibf1 chs1 double mutants exhibited normal hull color, restoring the phenotype of ibf1. Genetic analysis suggested that IBF1 regulates hull color in a CHS1-dependent manner. Collectively, our study suggests that IBF1 serves as a crucial negative regulator that controls flavonoid biosynthesis by mediating CHS1 degradation, thereby regulating hull color.