Divergent Functional Diversification Patterns in the SEP/AGL6/AP1 MADS-box Transcription Factor Superclade

Members of SEPALLATA(SEP) and APETALA1(AP1)/SQUAMOSA(SQUA) MADS-box transcription factor subfamilies were shown to play key roles in floral organ identity determination and floral meristem determinacy in the Rosid species Arabidopsis. Here we present a functional characterization of the seven SEP/AGL6 and four AP1/SQUA genes in the distant Asterid species petunia. Based on the analysis of single and higher order mutants, we found that the petunia SEP½/3 orthologs together with AGL6 encode classical SEP floral organ identity and floral termination functions, with a master role for the petunia SEP3 ortholog FLORAL BINDING PROTEIN 2 (FBP2). In contrast, we show that the FBP9 subclade members FBP9 and FBP23, for which no clear ortholog is present in Arabidopsis, play a major role in determining floral meristem identity together with FBP4, while contributing only moderately to floral organ identity. In turn, we show that the four members of the petunia AP1/SQUA subfamily redundantly are required for inflorescence meristem identity, and act as B-function repressors in the first floral whorl, together with BEN/ROB genes. Overall, these data together with studies in other species suggest major differences in the functional diversification of the SEP/AGL6 and AP1/SQUA MADS-box subfamilies during angiosperm evolution.