The Myb-domain protein ULTRAPETALA1 INTERACTING FACTOR 1 controls floral meristem activities in Arabidopsis

Higher plants continuously and iteratively produce new above-ground organs as leaves, stems and flowers. These organs arise from shoot apical meristem structures which homeostasis depends on coordination between self-renewal of stem cells and their differentiation into organ founder cells. This coordination is stringently controlled by the central WUSCHEL (WUS) transcription factor that is both necessary and sufficient for stem cell specification in Arabidopsis thaliana . ULTRAPETALA1 (ULT1) was previously identified as a plant-specific, negative regulator of WUS expression. However molecular mechanisms underlying this regulation remain unknown. ULT1 protein contains a SAND putative DNA-binding domain and a B-box, previously proposed as protein interaction domain in eukaryotes. Here we characterise a novel partner of ULT1, named ULT1 INTERACTING FACTOR 1 (UIF1) that contains a Myb domain and an EAR motif. UIF1 and ULT1 function in the same pathway for regulation of organ number in the flower. Moreover, UIF1 displays DNA binding activity and specifically binds to WUS regulatory elements. We thus provide genetic and molecular evidences that UIF1 and ULT1 work together in floral meristem homeostasis, likely via direct repression of WUS expression.