The pollen determinant of self-incompatibility in Brassica campestris

Many flowering plants possess self-incompatibility (SI) systems that preven t inbreeding. In Brassica, SI is controlled by a single polymorphic locus, the S locus. Two highly polymorphic S locus genes, SLC (S locus glycoprotei n) and SRK (S receptor kinase), have been identified, both of which are exp ressed predominantly in the stigmatic papillar cell. We have shown recently that: SRK is the determinant of the S haplotype specificity of the stigma. SRK is thought to serve as a receptor for a pollen ligand, which presumabl y is encoded by another polymorphic gene at the S locus. We previously have identified an S locus gene, SP11 (S locus protein 11), of the S-9 haplotyp e of Brassica campestris and proposed that it potentially encodes the polle n ligand. SP11 is a novel member of the PCP (pollen coat protein) family of proteins, some members of which have been shown to interact with SLG. In t his work, we identified the SP11 gene from three additional S haplotypes an d further characterized the gene. We found that (i) SP11 showed an S haplot ype-specific sequence polymorphism; (ii) SP11 was located in the immediate flanking region of the SRK gene of the four S haplotypes examined; (iii) SP 11 was expressed in the tapetum of the anther, a site consistent with sporo phytic control of Brassica SI; and (iv) recombinant SP11 of the S-9 haploty pe applied to papillar cells of S-9 stigmas, but not of S-8 stigmas, elicit ed SI response, resulting in inhibition of hydration of cross-pollen. All t hese results taken together strongly suggest that SP11 is the pollen S dete rminant in SI.