CONTROL OF FLORAL HOMEOTIC GENE-EXPRESSION AND ORGAN MORPHOGENESIS INANTIRRHINUM

The development of reproductive organs in Antirrhinum depends on the e xpression of an organ identity gene, plena, in the central domain of t he floral meristem, To investigate the mechanism by which plena is reg ulated, we have characterised three mutants in which the pattern of pl ena expression is altered. In polypetala mutants, expression of plena is greatly reduced, resulting in a proliferation of petals in place of reproductive organs. In addition, polypetala mutants exhibit an alter ed pattern of floral organ initiation, quite unlike that seen in loss- of-function plena mutants. This suggests that polypetala normally has two roles in flower development: regulation of plena and control of or gan primordia formation. In fistulata mutants, plena is ectopically ex pressed in the distal domain of petal primordia, resulting in the prod uction of anther-like tissue in place of petal lobes, Flowers of fistu lata mutants also show a reduced rate of petal lobe growth, even in a plena mutant background. This implies that fistulata normally has two roles in the distal domain of petal primordia: inhibition of plena exp ression and promotion of lobe growth. ii weak allele of the floral mer istem identity gene, floricaula, greatly enhances the effect of fistul ata on plena expression, showing that floricaula also plays a role in repression of plena in outer whorls. Taken together, these results sho w that genes involved in plena regulation have additional roles in the formation of organs, perhaps reflecting underlying mechanisms for cou pling homeotic gene expression to morphogenesis.