![]() Similarly, the C-function genes of Antirrhinum and Arabidopsis are expressed in the third and fourth whorls and show similar mutant phenotypes. For example, the B-function genes of Antirrhinum and Arabidopsis are expressed mainly in the second and third whorls and show similar mutant phenotypes. Often, genes from different species which fall into one of these sub-families have similar expression patterns and similar roles in development. The >200 known plant MADS-factors, from many different species, have been divided into sub-families based mainly on their predicted amino acid sequences ( Doyle, 1994 Purugganan et al., 1995 Theissen et al., 1996). Although MADS-box genes have been identified and characterized in animals and yeasts ( Shore and Sharrocks, 1995), they are found most extensively in plants ( Theissen et al., 1996, Liljegren et al., 1998). The MADS-box family is defined by the presence of a 58 amino acid domain which is conserved between the founding members MCM1 (yeast), AGAMOUS and DEFICIENS (plants) and SRF (animals) ( Schwarz-Sommer et al., 1990). In both Antirrhinum and Arabidopsis, a single C-function and two B-function genes have been identified and shown to encode MADS-box transcription factors ( Sommer et al., 1990 Yanofsky et al., 1990 Jack et al., 1992 Tröbner et al., 1992 Bradley et al., 1993 Goto and Meyerowitz, 1994). In C-function mutants, perianth organs develop in place of reproductive organs. In B-function mutants, petals and stamens are replaced by sepals and carpels, respectively. Expression of the A-function alone determines sepal development, co-expression of the A- and B-functions or the B- and C-functions specifies petals or stamens, respectively, and expression of the C-function alone results in carpel development. The model proposes that three functions are expressed in adjacent, overlapping whorls such that the A-function is expressed in whorls 1 and 2, the B-function in whorls 2 and 3, and the C-function in whorls 3 and 4. The study of Antirrhinum and Arabidopsis floral homeotic mutants produced a simple combinatorial model to explain how the regulated expression of a few genes could determine the organ composition of the flower ( Coen and Meyerowitz, 1991). The difference in phenotypes of mutants in two highly related genes illustrates the importance of the position within the regulatory network in determining gene function. ![]() This feature of the two Antirrhinum C-function-like genes is markedly different from the control of the inner boundary of the B-function expression domain in Arabidopsis, and we propose and discuss a model to account for these differences. Double mutant analysis reveals an unexpected, redundant negative control over the B-function MADS-box genes. Expression studies of PLE and FAR, in wild-type and mutant backgrounds, show complex interactions between the two genes. Unlike ple mutants, which show homeotic conversion of reproductive organs to perianth organs and a loss of floral determinacy, far mutants have normal flowers which are partially male-sterile. Despite striking similarities between FAR and the class C MADS-box gene PLENA ( PLE), the phenotypes of their respective mutants are dramatically different. ![]() We report the discovery of an Antirrhinum MADS-box gene, FARINELLI ( FAR), and the isolation of far mutants by a reverse genetic screen.
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