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Antirrhinum majus DEFICIENS (DEF) and Arabidopsis thaliana APETALA3 (AP3) MADS box proteins are required to specify petal and stamen identity. Sampling of DEF/AP3 homologs revealed two types of DEF/AP3 proteins, euAP3 and TOMATO MADS BOX GENE6 (TM6), within core eudicots, and we show functional divergence in Petunia hybrida euAP3 and TM6 proteins. Petunia DEF (also known as GREEN PETALS [GP]) is expressed mainly in whorls 2 and 3, and its expression pattern remains unchanged in a blind (bl) mutant background, in which the cadastral C-repression function in the perianth is impaired. Petunia TM6 functions as a B-class organ identity protein only in the determination of stamen identity. Atypically, Petunia TM6 is regulated like a C-class rather than a B-class gene, is expressed mainly in whorls 3 and 4, and is repressed by BL in the perianth, thereby preventing involvement in petal development. A promoter comparison between DEF and TM6 indicates an important change in regulatory elements during or after the duplication that resulted in euAP3- and TM6-type genes. Surprisingly, although TM6 normally is not involved in petal development, 35S-driven TM6 expression can restore petal development in a def (gp) mutant background. Finally, we isolated both euAP3 and TM6 genes from seven solanaceous species, suggesting that a dual euAP3/TM6 B-function system might be the rule in the Solanaceae.
INTRODUCTION
Ever since the formulation of the classic ABC model of flower development (Coen and Meyerowitz, 1991), tremendous progress has been made in the understanding of the genetic control of flower development. The ABC model proposes the existence of three types of gene function (A, B, and C) that act in different combinations to specify the identity of the floral organs. A alone yields sepals; A in combination with B yields petals; B with C yields stamens; and C alone yields carpels. Moreover, the model implies an antagonistic relationship between the A and C functions. The analysis of floral developmental mutants in the model species Arabic/apsis thaliana and Antirrhinum majus revealed the central role that MADS box transcription factors play in flower development. It turned out to be mostly members of this gene family that carry out all or part of the A, B, and C functions (Sommer et al., 1990; Yanofsky et al., 1990; Huijser et...