The MADS Box Gene FBP2 Is Required for SEPALLATA Function in Petunia

The ABC model, which was accepted for almost a decade as a paradigm for flower development in angiosperms, has been subjected recently to a significant modification with the introduction of the new class of E-function genes. This function is required for the proper action of the B- and C-class homeotic proteins and is provided in Arabidopsis by the SEPALLATA1/2/3 MADS box transcription factors. A triple mutant in these partially redundant genes displays homeotic conversion of petals, stamens, and carpels into sepaloid organs and loss of determinacy in the center of the flower. A similar phenotype was obtained by cosuppression of the MADS box gene FBP2 in petunia. Here, we provide evidence that this phenotype is caused by the downregulation of both FBP2 and the paralog FBP5. Functional complementation of the sepallata mutant by FBP2 and our finding that the FBP2 protein forms multimeric complexes with other floral homeotic MADS box proteins indicate that FBP2 represents the same E function as SEP3 in Arabidopsis.