MADS Domain Transcription Factors Mediate Short-Range DNA Looping That Is Essential for Target Gene Expression in Arabidopsis

MADS domain transcription factors are key regulators of eukaryotic development. In plants, the homeotic MIKC MADS factors that regulate floral organ identity have been studied in great detail. Based on genetic and protein-protein interaction studies, a floral quartet model was proposed that describes how these MADS domain proteins assemble into higher order complexes to regulate their target genes. However, despite the attractiveness of this model and its general acceptance in the literature, solid in vivo proof has never been provided. To gain deeper insight into the mechanisms of transcriptional regulation by MADS domain factors, we studied how SEEDSTICK (STK) and SEPALLATA3 (SEP3) directly regulate the expression of the reproductive meristem gene family transcription factor-encoding gene VERDANDI (VDD). Our data show that STK-SEP3 dimers can induce loop formation in the VDD promoter by binding to two nearby CC(A/T) 6GG (CArG) boxes and that this is essential for promoter activity. Our in vivo data show that the size and position of this loop, determined by the choice of CArG element usage, is essential for correct expression. Our studies provide solid in vivo evidence for the floral quartet model.