Morphological and molecular analysis of a double-flowered mutant of the dioecious plant white campion showing both meristic and homeotic effects

Many double-flowered plants, in which petals replace stamens, are highly va lued by the horticultural industry. These mutants exhibit a homeotic conver sion of floral organs and frequently also a meristic increase in floral org an number. By gamma irradiation we generated a novel double-flowered mutant , Sl-dfl, in a male genetic background of the dioecious plant white campion (Silene latifolia). This mutant shows a homeotic conversion of stamens to petals, together with uncontrolled growth and division of second and third whorl floral organ primordia, causing a proliferation of petal and chimeric petal-stamen organs. We characterize this mutant developmentally by scanni ng electron microscopy and demonstrate that the effects of the mutation com mence following the formation of a correctly partitioned floral meristem wi th a wild-type arrangement of organ primordia. We have commenced a molecula r investigation of the Sl-dfl mutant by testing the expression and genomic organization of the known while campion putative MADS-box floral. homeotic genes. These si;dies indicate Four MADS-box genes to be unlikely to be muta ted in the double-flowered mutant. The possibility that a putative C-functi on MADS-box gene may cause the mutant phenotype has not currently been excl uded, though our morphological studies suggest that a C-funciion mutation i s not involved in this case. We conclude that a number of different classes of double-flowered mutation exist, not all of which are currently known fr om model plant species. This may be indicative of important developmental d ifferences between species and may also emphasize a need for comparative st udies of floral development. (C) 1999 Wiley-Liss, Inc.