Epidermal control of floral organ identity by class B homeotic genes in Antirrhinum and Arabidopsis

To assess the contribution of the epidermis to the control of petal and sta men organ identity, we have used transgenic Antirrhinum and Arabidopsis pla nts that expressed the Antirrhinum class B homeotic transcription factors D EFICIENS (DEF) and GLOBOSA (GLO) in the epidermis. Transgene expression was controlled by the ANTIRRHINUM FIDDLEHEAD (AFI) promoter, which directs gen e expression to the Ll meristematic layer and, later, to the epidermis of d ifferentiating organs. Transgenic epidermal DEF and GLO chimeras display si milar phenotypes, suggesting similar epidermal contributions by the two cla ss B genes in Antirrhinum. Epidermal B function autonomously controls the d ifferentiation of Antirrhinum petal epidermal cell types, but cannot fully control the pattern of cell divisions and the specification of subepidermal petal cell-identity by epidermal signalling. This non-autonomous control i s enhanced if the endogenous class B genes can be activated from the epider mis. The developmental influence of epidermal B function in Antirrhinum sta men development is very limited. In contrast, epidermal B function in Arabi dopsis can control most if not all epidermal and sub-epidermal differentiat ion events in petals and stamens, without any contribution from the endogen ous class B genes. Possible reasons for differences in the efficacy of B-fu nction-mediated cell communication between the two species are discussed. I nterestingly, our experiments uncovered partial incompatibility between cla ss B functional homologues. Although the DEFICIENS/PISTILLATA heterodimer i s functional in transgenic Arabidopsis plants, the APETALA3/GLOBOSA heterod imer is not.