The Handlebars gene is required with Phantastica for dorsoventral asymmetry of organs and for stem cell activity in Antirrhinum

In angiosperms individual lateral organs and whole flowers may develop asym metrically along their dorsoventral axes. Dorsoventral asymmetry of Antirrh inum leaves requires activity of the Phantastica gene and other factors act ing redundantly with it. We describe the effects of a mutation in the Handl ebars gene, identified as an enhancer of the phantastica mutant phenotype. Genetic analysis suggests that Handlebars functions redundantly with Phanta stica to promote dorsal fate in lateral organs and to maintain activity of stem cells within shoot apical meristems. Handlebars appears dispensable in vegetative development but is needed for asymmetry of petals along the dor soventral axis of the flower as a whole. This suggests that common mechanis ms may control dorsoventral asymmetry in lateral organ primordia and in flo ral meristems.