Pollen—stigma interaction during sporophytic self-incompatibility in Brassica oleracea

Abstract

The consequence of self-incompatibility (SI), defined as the ‘inability of a fertile hermaphrodite seed plant to produce zygotes after self pollination’ by de Nettancourt (1977), is obligate outbreeding which ensures continual gene flow within the populations of a species. SI may be either gametophytically or sporophytically controlled, di-allelic or multi-allelic. Gametophytic systems, where the incompatibility pollen phenotype is determined by the haploid pollen genome, are always multi-allelic, since the minimum number of alleles permissible at a gametophytic locus is three (Richards 1986). Sporophytic systems, where the compatibility of the pollen phenotype is determined by the male parent genotype, can be either di-or multi-allelic. Di-allelic sporophytic systems occur in thirteen plant families, usually in conjunction with floral dimorphy (Richards 1986). It has been suggested that the two systems of SI differ fundamentally in their operation on the basis of a number of observations. Gametophytic systems possess binucleate pollen grains, which germinate rapidly and easily in vitro, in contrast to the trinucleate grains of plants with sporophytic SI (SSI) which do not. The stigmatic surface of plants with gametophytic SI (GSI) is usually ‘wet’, while that of plants expressing SSI is ‘dry’. It has been considered for some time that these differences must provide clues to mechanisms of self-pollen rejection in each system (Brewbaker 1959; Heslop-Harrison 1975). However, in both gametophytic and sporophytic multi-allelic systems, it is clear that SI involves a complex biochemical signalling system between the pollen grain (and/or the tube) and the pistil.