Cysteine-rich Pollen Coat Proteins (PCPs) and their Interactions with StigmaticS (Incompatibility) and S -Related Proteins in Brassica: Putative Roles in SI and Pollination

Abstract

In Brassica, recognition and rejection of ‘self’ pollen is rapid, occurring at the stigmatic surface usually prior to pollen germination and is mediated by products of theS (incompatibility)-locus. Two polymorphic stigmatically-expressed S -locus genes, thought to participate in the perception of ‘self’ pollen have been characterized to date. One, the SRK(S -receptor kinase) encodes a transmembrane receptor kinase likely to be activated by the binding of an S -specific pollen-borne ligand. The other, SLG(S -locus glycoprotein), encodes a secreted cell wall glycoprotein whose role in self-incompatibility (SI) is less clear and increasingly questioned. The pollen coating inBrassica is known to carry the male determinant of SI as well as factors crucial for competent pollen–stigma interactions. Characterization of the coating has revealed the presence of several families of gametophytically expressed small cysteine-rich proteins (PCPs—pollen coat proteins). PCPs are strongly implicated in playing a central role in SI and other aspects of the pollen–stigma interaction. PCP-A (for PCP, class ‘A’) class proteins have specific affinities for stigmatic S - and S -related proteins and these, along with other recently characterized groups of PCPs, are abundant components of fractions having S -specific activity in pollination bioassays. The role of the pollen coating, and particularly PCPs and their expression patterns, will be discussed in the perspective of the pollen–stigma interaction and SI.