WOX family transcription factors regulate multiple developmental programs. The intermediate clade transcriptional activator WOX9 functions together with the modern clade transcriptional repressor WOX genes in embryogenesis and meristems maintenance, but the mechanism of this interaction is unclear. STFandLAM1are WOX1 orthologs required for leaf blade outgrowth inMedicago truncatulaandNicotiana sylvestris, respectively. Using biochemical methods and genome editing technology, here we show thatWOX9is an abaxial factor and functions antagonistically toSTFandLAM1to regulate leaf blade development. WhileNsWOX9ectopic expression enhances thelam1mutant phenotype, and antisense expression partially rescues thelam1mutant, both overexpression and knockout ofNsWOX9inN. sylvestrisresulted in a range of severe leaf blade distortions, indicating important role in blade development. Our results indicate that direct repression ofWOX9by WUS clade repressor STF/LAM1 is required for correct blade architecture and patterning inM. truncatulaandN. sylvestris. These findings suggest that controlling transcriptional activation and repression mechanisms by direct interaction of activator and repressor WOX genes may be required for cell proliferation and differentiation homeostasis, and could be an evolutionarily conserved mechanism for the development of complex and diverse morphology in flowering plants.