Studies on pollen coat proteins involved in pollen-stigma interactions in Brassica sp.

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Plant sexual reproduction depends on highly specific interactions between pollen and pistil. Pistils can recognize to select from the genetically diverse range of pollen arriving on the stigma so that only appropriate pollen effects fertilization. These systems include barriers to cross-species fertilization and intraspecific barriers such as self-incompatibility (SI) that prevent self-fertilization.In Brassica, stigmaticaly expressed members of a protein family, SLG family, are expected to play central roles in these recognition systems. SRK (S-receptor kinase) and SLG (S-locus glycoprotein), are shown to be involved in the recognition of self/non-self pollen in SI.SLR1 is suggested to be involved in the initial stages of pollen-stigma adhesion. Although these familial proteins are expected to interact with some components on pollen grains, the molecular nature of these components remained elusive.In this study, we searched for these interacting molecules in PCP (pollen coat protein) family of proteins and succeeded to identify two key component named SP11 and SLR1-BP.SP11, which is encoded in the self-incompatibility (S) locus and belongs to a novel class of PCP family, is tumed out to be the pollen determinant of SI.We presented experimental evidence showing that SP11 is working as a self/non-self recognition marker which is perceived by SRK on stigmatic papillar cells. SLR1-BP, which is a new member of PCP class A proteins, is interacting with SLR1 with high affinity. Although the biological signficance of SLR1-BP is as yet to be exprimentally demonstrated, the interaction between SLR1 and SLR1 -BP is expected to be involved in the pollen-stigma adhesion process. These two pollen components identified in this study should be key molecules in understanding the pollen-stigma recognition process.