The male determinant of self-incompatibility in Brassica oleracea is located in the pollen coating

Abstract

An in vitro bioassay has been developed to explore the role of the pollen coating in the pollen/stigma interaction in Brassica oleracea. In the assay, coating is removed from pollen grains, supplemented with protein fractions isolated from coatings of different S (self incompatibility) haplotypes, and then-using micromanipulation-interposed between individual pollen grains end the stigmatic surface. Normally, the coating used is of the same haplotype as the pollen in the experiment-thus constituting an 'extension' of its own coat-but carrying the supplemented protein fractions. Initial experiments confirmed preliminary data that the pollen coating contained the male determinant of self incompatibility (SI); not only did the addition of 'self' coating (i.e. that with the same S-haplotype as the stigma) prevent the success of a compatible cross pollination, but a 'cross' coating (i.e. that with a different S-haplotype from the stigma) could induce the germination and growth of self pollen. Protein supplementation experiments demonstrated that the pollen-hold determinant is contained within the water soluble component of the pollen coat, while further analysis revealed that the active molecular species possesses an M(r) ≤10 kDa. More extensive fractionation by gel filtration and reverse phase HPLC was used to isolate a family of basic, cysteine-rich proteins (PCP-A: Pollen Coat Proteins-class A)-one of which is known to bind to stigmatically-expressed components of the S-locus in Brassica. Introduction of the PCP-A protein fraction into the bioassay confirmed the male determinant of SI as a protein, and probably a member of the PCP-A protein family.