The S receptor kinase determines self-incompatibility in Brassica stigma

The self-incompatibility possessed by Brassica is an intraspecific reproductive barrier by which the stigma rejects self-pollen but accepts non-self-pollen for fertilization. The molecular/biochemical bases of recognition and rejection have been intensively studied. Self-incompatibility in Brassica is sporophytically controlled by the polymorphic S locus(1). Two tightly linked polymorphic genes at the S locus, S receptor kinase gene (SRK) and S locus glycoprotein gene (SLG), are specifically expressed in the papillar cells of the stigma(2-4), and analyses of self-compatible lines(5-7) of Brassica have suggested that together they control stigma function in self-incompatibility interactions. Here we show, by transforming self-incompatible plants of Brassica rapa with an SRK28 and an SLG(28) transgene separately, that expression of SRK28 alone, but not SLG(28) alone, conferred the ability to reject self (S-28)-pollen on the transgenic plants. We also show that the ability of SRK28 to reject S-28 pollen was enhanced by SLG(28). We conclude that SRK alone determines S haplotype specificity of the stigma, and that SLG acts to promote a full manifestation of the self-incompatibility response.