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 valued by the horticultural industry. These mutants exhibit a homeotic conversion of floral organs and frequently also a meristic increase n floral organ 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 scanning electron microscopy and demonstrate that the effects of the mutation commence following the formation of a correctly partitioned floral meristem with a wild-type arrangement of organ primordia. We have commenced a molecular investigation of the Sl-dfl mutant by testing the expression and genomic organization of the known white campion putative MADS-box floral homeotic genes. These studies indicate four MADS-box genes to be unlikely to be mutated in the double-flowered mutant. The possibility that a putative C- function MADS-box gene may cause the mutant phenotype has not currently been excluded, though our morphological studies suggest that a C-function mutation is 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 from model plant species. This may be indicative of important developmental differences between species and may also emphasize a need for comparative studies of floral development.