SEPALLATA-driven MADS transcription factor tetramerization is required for inner whorl floral organ development

MADS genes encode transcription factors that act as master regulators of plant reproduction and flower development. The SEPALLATA (SEP) subfamily is required for the development of floral organs and plays roles in inflorescence architecture and development of the floral meristem. The SEPALLTAs act as organizers of MADS complexes, forming both heterodimers and heterotetramers in vitro. To date, the MADS complexes characterized in angiosperm floral organ development contain at least one SEPALLATA protein. Whether DNA-binding by SEPALLATA-containing dimeric MADS complexes are sufficient for launching floral organ identity programs, however, is not clear as only defects in floral meristem determinacy were observed in tetramerization impaired SEPALLATA mutants. Here, we used a combination of genome-wide binding studies, high resolution structural studies of the SEP3/AGAMOUS tetramerization domain, structure-based mutagenesis and complementation experiments in sep1 sep2 sep3 and sep1 sep2 sep3 ag-4 plants transformed with versions of SEP3 encoding tetramerization mutants. We demonstrate that while SEP3 heterodimers are able to bind DNA both in vitro and in vivo and recognize the majority of SEP3 wild type binding sites genome-wide, tetramerization is not only required for floral meristem determinacy, but also absolutely required for floral organ identity in the second, third and fourth whorls.