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 stamen organ identity, we have used transgenic Antirrhinum and Arabidopsis plants that expressed the Antirrhinum class B homeotic transcription factors DEFICIENS (DEF) and GLOBOSA (GLO) in the epidermis. Transgene expression was controlled by the ANTIRRHINUM FIDDLEHEAD (AFI) promoter, which directs gene expression to the L1 meristematic layer and, later, to the epidermis of differentiating organs. Transgenic epidermal DEF and GLO chimeras display similar phenotypes, suggesting similar epidermal contributions by the two class B genes in ANTIRRHINUM: Epidermal B function autonomously controls the differentiation of Antirrhinum petal epidermal cell types, but cannot fully control the pattern of cell divisions and the specification of sub-epidermal petal cell-identity by epidermal signalling. This non-autonomous control is enhanced if the endogenous class B genes can be activated from the epidermis. The developmental influence of epidermal B function in Antirrhinum stamen development is very limited. In contrast, epidermal B function in Arabidopsis can control most if not all epidermal and sub-epidermal differentiation events in petals and stamens, without any contribution from the endogenous class B genes. Possible reasons for differences in the efficacy of B-function-mediated cell communication between the two species are discussed. Interestingly, our experiments uncovered partial incompatibility between class B functional homologues. Although the DEFICIENS/PISTILLATA heterodimer is functional in transgenic Arabidopsis plants, the APETALA3/GLOBOSA heterodimer is not.