Floral asymmetry involves an interplay between TO and MYB transcription factors in Antirrhinum

To understand how genes control floral asymmetry, we have isolated and analyzed the role of the RADIALIS (RAD) gene in Antirrhinum. We show that the RAD gene encodes a small MYB-like protein that is specifically expressed in the dorsal region of developing flowers. RAID has a single MYB-like domain that is closely related to one of the two MYB-like domains of DIV, a protein that has an antagonistic effect to RAD on floral development. Interactions between RAD and othergenes indicate that floral asymmetry depends on the interplay between two pairs of transcription factors. First, a pair of TO proteins is expressed in dorsal regions of the floral meristem, leading to the activation of RAD in the dorsal domain. The RAID MYB-like protein then antagonizes the related DIV MYB-like protein, preventing DIV activity in dorsal regions. In addition to its role in dorsal regions, RAD acts nonautonomously on lateral regions either directly, through RAD protein movement, or indirectly, through a signaling molecule.