SOC1 translocated to the nucleus by interaction with AGL24 directly regulates LEAFY

SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) is one of the flowering pathway integrators and regulates the expression of LEAFY (LFY), which links floral induction and floral development. However, the mechanism by which SOC1, a MADS box protein, regulates LFY has proved elusive. Here, we show that SOC1 directly binds to the distal and proximal region of the LFY promoter where critical cis-elements are located. Intragenic suppressor mutant analysis shows that a missense mutation in the MADS box of SOC1 causes loss of binding to the LFY promoter as well as suppression of the flowering promotion function. The full-length SOC1 protein locates in the cytoplasm if expressed alone in protoplast transient expression assay, but relocates to the nucleus if expressed with AGAMOUS-LIKE 24 (AGL24), another flowering pathway integrator and a MADS box protein. The domain analysis shows that co-localization of SOC1 and AGL24 is mediated by the MADS box and the intervening region of SOC1. Finally, we show that LFY is expressed only in those tissues where SOC1 and AGL24 expressions overlap. Thus, we propose that heterodimerization of SOC1 and AGL24 is a key mechanism in activating LFY expression.