FD and FDP bZIP transcription factors and FT florigen regulate floral development and control homeotic gene expression in <i>Arabidopsis</i> floral meristems

Arabidopsis florigen activation complex (FAC), formed by the interaction of the transcription factor FD and the florigen protein FT, activates gene expression in the shoot apical meristem to induce flowering. We show that FD and its paralog FDP are also expressed in partially overlapping patterns in the floral meristem and floral organs, and that FT is present in floral meristems. The flowers of mutants for FT and its paralog TSF (ft tsf), and of fd fdp mutants show variable numbers of sepals and petals, and larger floral meristems than wild type. In the floral meristem, fd fdp and ft tsf mutants show a significant reduction in the expression of SEP and AG genes, which encode MADS-domain transcription factors, as well as increased expression of the homeobox gene WUS. Binding of FD to SEP genes suggests that diminished SEP gene expression is a primary defect in the mutants. We conclude that, beyond their role in floral transition, FAC components regulate floral homeotic gene expression to control floral meristem size, and influence floral organ development and identity.