The S receptor kinase determines self-incompatibility in Brassica stigma

The self-incompatibility possessed by Brassica is an intraspecific reproduc tive barrier by which the stigma rejects self-pollen but accepts non-self-p ollen 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 li nked polymorphic genes at the S locus, S receptor kinase gene (SRK) and S l ocus glycoprotein gene (SLG), are specifically expressed in the papillar ce lls of the stigma(2-4), and analyses of self-compatible lines(5-7) of Brass ica have suggested that together they control stigma function in self-incom patibility interactions. Here we show, by transforming self-incompatible pl ants of Brassica rapa with an SRK28 and an SLG(28) transgene separately, th at 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 th e ability of SRK28 to reject S-28 pollen was enhanced by SLG(28). We conclu de that SRK alone determines S haplotype specificity of the stigma, and tha t SLG acts to promote a full manifestation of the self-incompatibility resp onse.