The B-box protein BBX13/COL15 suppresses photoperiodic flowering by attenuating the action of CONSTANS in Arabidopsis

SummaryThe optimal timing of transition from vegetative to floral reproductive phase is critical for plant productivity and agricultural yields. Light plays a decisive role in regulating this transition. The B-box (BBX) family of transcription factors regulates several light-mediated developmental processes in plants, including flowering. Here, we identify a previously uncharacterized group II member of the BBX family, BBX13/COL15, as a negative regulator of flowering under long-day conditions.BBX13is primarily expressed in the leaf vasculature, buds, and flowers. Its spatial expression pattern is similar to the major flowering time regulatorsCOandFT. bbx13mutants flower early, whileBBX13-overexpressors exhibit delayed flowering under long days. Genetic analyses showed thatBBX13acts upstream toCOandFTin the flowering pathway and negatively regulates their expression. We found that BBX13 physically interacts with CO and inhibits the CO-mediated transcriptional activation ofFT. In addition, BBX13 directly binds to theCORE2motif on theFTpromoter, a site where CO also binds. Furthermore, our ChIP data indicates that BBX13 reduces the in vivo binding of CO on theFTpromoter. Through luciferase assay, we found that BBX13 inhibits the CO-mediated transcriptional activation ofFT.All these data together suggest that BBX13/COL15 represses flowering in Arabidopsis by attenuating CO activity.Summary statementBBX13, a previously uncharacterized protein in the B-Box family of transcription factors, negatively regulates flowering time inArabidopsis thaliana. BBX13 physically and genetically interacts with CO and inhibits the CO-mediated transcriptional activation ofFT.