Phosphorylation of OsFD1 by OsCIPK3 promotes the formation of RFT1-containing florigen activation complex for long-day flowering in rice

Heading date is a critical trait that determines the regional adaptability and grain productivity of many crops.Although rice is a facultative short-day plant,its domestication led to the Ghd7-Ehd1-Hd3a/RFT1 pathway for adaptation to long-day conditions(LDs).The formation of the"florigen activation complex"(FAC)containing florigen Hd3a has been characterized.However,the molecular composition of the FAC that contains RFT1 for long-day flowering is unclear.We show here that RFT1 forms a ternary FAC with 14-3-3 proteins and OsFD1 to promote flowering under LDs.We identified a calcineurin B-like-interacting protein kinase,OsCIPK3,which directly interacts with and phosphorylates OsFD1,thereby facilitating the localization of the FAC to the nucleus.Mutation in OsCIPK3 results in a late heading date under LDs but a normal heading date under short-day conditions.Collectively,our results suggest that OsCIPK3 phosphorylates OsFD1 to promote RFT1-containing FAC formation and consequently induce flowering in rice under LDs.