Specification ofArabidopsisfloral meristem identity by repression of flowering time genes

Flowering plants produce floral meristems in response to intrinsic and extrinsic flowering inductive signals. In Arabidopsis , the floral meristem identity genes LEAFY ( LFY ) and APETALA1 ( AP1 ) are activated to play a pivotal role in specifying floral meristems during floral transition. We show here that the emerging floral meristems require AP1 to partly specify their floral identities by directly repressing a group of flowering time genes, including SHORT VEGETATIVE PHASE ( SVP ), AGAMOUS-LIKE 24 ( AGL24 ) and SUPPRESSOR OF OVEREXPRESSION OF CO1 ( SOC1 ). In wild-type plants, these flowering time genes are normally downregulated in emerging floral meristems. In the absence of AP1 , these genes are ectopically expressed, transforming floral meristems into shoot meristems. By post-translational activation of an AP1-GR fusion protein and chromatin immunoprecipitation assays, we further demonstrate the repression of these flowering time genes by induced AP1 activity and in vivo AP1 binding to the cis -regulatory regions of these genes. These findings indicate that once AP1 is activated during the floral transition, it acts partly as a master repressor in floral meristems by directly suppressing the expression of flowering time genes, thus preventing the continuation of the shoot developmental program.