SUPERMAN, The Guardian of Floral Organ Gene Expression in Arabidopsis thaliana

Arabidopsis thaliana is a model plant species in floral development. When the Shoot Apical Meristem (SAM), an indeterminate stem cell population, reaches the reproductive stage it initiates the development of floral meristems that bud off on the periphery. The floral meristems give rise to the four different floral organs. Each of the floral organs is determined through expression of Homeotic Floral Organ Identity genes; APETALA1 (AP1), APETALA3 (AP3), AGAMOUS (AG), PISTILATA (PI), and the SEPALLATA (SEP) genes. These genes represent the ABCE class genes that determine specific floral organ fate. Boundaries between each whorl are established through the differential gene expression of the Homeotic Floral Organ Identity genes. The boundary between stamen and carpel is established by the repression of B-class genes in the fourth whorl. This repression of B-class genes is initiated by a transcription factor known as SUPERMAN (SUP). SUP represses B-class genes in the fourth whorl. This study presents evidence that SUP uses a protein complex to repress B-class genes which consists of corepressor TOPLESS (TPL) and Histone Deacetylase 19 (HDA19). The interaction between SUP and TPL was established through yeast-2-hybrid assays. The interaction between proteins was mediated by an intact EAR motif. Single and double mutant analysis of stamen number and carpel morphology suggested that SUP, TPL, and HDA19 work together in a complex to repress B-class genes. CRISPR-Cas9 experiments showed that the complex is functionally dependent on an intact EAR motif to properly repress B-class genes. Overall, these results indicated that SUP represses B-class genes in the fourth whorl in a complex consisting of TPL and HDA19, which requires an intact EAR motif.