Flower Development in Petunia.

Recent genetic studies on both Arabidopsis and Antirrhinum have led to a common genetic model (reviewed in Coen and Meyerowitz, 1991; see also Coen and Carpenter, 1993, this issue). This model for flower pattern formation proposes three gene functions, a, b, and c, that are active in the floral meristem in partly overlapping whorls (a and b or b and c) or complementing whorls (a and c). The a function is active in whorls 1 and 2, b is active in whorls 2 and 3, and c is active in whorls 3 and 4. When only a is active, as in the first whorl, the organs will develop into sepals. In the second whorl, the combination of a and b determines peta1 differentiation; in the third whorl, b and c specify stamens; and in the central fourth whorl, c alone determines carpel formation. This model serves as a useful framework for the more detailed analysis of molecular interactions between the different homeotic genes in Arabidopsis and Antirrhinum. Why then study flower development in petunia or other species? First, the development of many wild-type flowers or putative homeotic mutant flowers cannot be easily reconciled with the model because in these flowers a homeotic conversion is limited to one whorl only (e.g., homeotic mutants in petunia, tulip, and lily). Only by expanding our floral developmental studies to such floral mutants will we be able to determine which part of the model has general validity and which part is species specific. Second, petunia has the advantage that it is susceptible to Agrobacterium-mediated transformation, which allows the evaluation of gene function by transgenic technology. Moreover, the large flowers of petunia render it convenient for the isolation and biochemical characterization of homeotic proteins as they occur in the plant. Similar arguments can be made for tomato, and, indeed, putative homeotic genes have now been cloned from both plant species (Pnueli et al., 1991; Angenent et al., 1992; Kush et al., 1993; Tsuchimoto et al., 1993). In this review, we will focus mainly on homeotic genes from petunia and their roles in floral development.