Pistil Mating Type and Morphology Are Mediated by the Brassinosteroid Inactivating Activity of the -Locus Gene in Heterostylous Species.

Heterostyly is a breeding system that promotes outbreeding through a combination of morphological and physiological floral traits. In these traits are governed by a single, hemizygous -locus containing just three genes. We report that the -locus gene, , is mutated and encodes a severely truncated protein in a self-compatible long homostyle species. Further, a self-compatible long homostyle mutant possesses a   allele with a point mutation in a highly conserved domain of BAHD acyl transferases. Wild type and mutant alleles were expressed in to assay for brassinosteroid (BR) inactivating activity. The wild type but not mutant allele caused dwarfism, consistent with the wild type possessing, but the mutant allele having lost, BR inactivating activity. To investigate whether BRs act directly in self-incompatibility, BRs were added to in vitro pollen cultures of the two mating types. A small morph specific stimulatory effect on pollen tube growth was found with 5 µM brassinolide, but no genotype specific inhibition was observed. These results suggest that acts pleiotropically to mediate pistil length and physiological mating type through BR inactivation, and that in regard to self-incompatibility, BR acts by differentially regulating gene expression in pistils, rather than directly on pollen.