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

Abstract

Members of SEPALLATA (SEP) and APETALA1 (AP1)/SQUAMOSA (SQUA) MADS-box transcription factor subfamilies play key\nroles in floral organ identity determination and floral meristem determinacy in the rosid species Arabidopsis (Arabidopsis\nthaliana). Here, we present a functional characterization of the seven SEP/AGL6 and four AP1/SQUA genes in the distant\nasterid species petunia (Petunia 3 hybrida). Based on the analysis of single and higher order mutants, we report that the\npetunia SEP1/SEP2/SEP3 orthologs together with AGL6 encode classical SEP floral organ identity and floral termination\nfunctions, with a master role for the petunia SEP3 ortholog FLORAL BINDING PROTEIN2 (FBP2). By contrast, the FBP9\nsubclade members FBP9 and FBP23, for which no clear ortholog is present in Arabidopsis, play a major role in determining\nfloral meristem identity together with FBP4, while contributing only moderately to floral organ identity. In turn, the four\nmembers of the petunia AP1/SQUA subfamily redundantly are required for inflorescence meristem identity and act as\nB-function repressors in the first floral whorl, together with BEN/ROB genes. Overall, these data together with studies in other\nspecies suggest major differences in the functional diversification of the SEP/AGL6 and AP1/SQUA MADS-box subfamilies\nduring angiosperm evolution.