Background : Self-incompatibility (SI) in the Solanaceae, Rosaceae and Scrophulariaceae is gametophytically controlled by a single polymorphic locus, termed the S -locus. To date, the only known S -locus product is a polymorphic ribonuclease, termed SRNase, which is secreted by stylar tissue and thought to act as a cytotoxin that degrades the RNA of incompatible pollen tubes. However, understanding how S -RNase causes S -haplotype specific inhibition of pollen tubes has been hampered by the lack of a cloned pollen S -determinant gene.

Many flowering plants possess self-incompatibility (SI) systems that prevent inbreeding. In Brassica, SI is controlled by a single polymorphic locus, the S locus. Two highly polymorphic S locus genes, SLG (S locus glycoprotein) and SRK (S receptor kinase), have been identified, both of which are expressed 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 presumably 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 S9 haplotype of Brassica campestris and proposed that it potentially encodes the pollen 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 this work, we identified the SP11 gene from three additional S haplotypes and further characterized the gene. We found that (i) SP11 showed an S haplotype-specific sequence polymorphism; (ii) SP11 was located in the immediate flanking region of the SRK gene of the four S haplotypes examined; (iii) SP11 was expressed in the tapetum of the anther, a site consistent with sporophytic control of Brassica SI; and (iv) recombinant SP11 of the S9 haplotype applied to papillar cells of S9 stigmas, but not of S8 stigmas, elicited SI response, resulting in inhibition of hydration of cross-pollen. All these results taken together strongly suggest that SP11 is the pollen S determinant in SI.I n Brassica, SI is sporophytically controlled by a highly polymorphic locus, termed the S locus, with more than 100 haplotypes identified so far (1, 2). To date, two stigmatically expressed highly polymorphic genes have been identified at the S locus. One is the S locus glycoprotein (SLG) gene, which encodes a secreted glycoprotein abundantly present in the papillar cell of the stigma surface (3, 4), and the other is the S locus receptor kinase (SRK) gene, which encodes a putative receptor-like serine͞threonine protein kinase presumed to span the plasma membrane of the papillar cell (5). The predicted extracellular domain of SRK shares extensive sequence similarity with SLG. Results from our recent gain-of-function experiments have shown that SRK is the sole determinant of the S haplotype specificity of the stigma (6). SRK is thought to function as a receptor for the S determinant of pollen with the same S haplotype. Binding of SRK to the pollen S determinant then would elicit a signaling cascade in the papillar cell, leading to the rejection of self-pollen. Our recent study also has shown that the role of SLG is probably to enhance this recognition process, although how this is accomplished is not yet known (6).In contrast to the S determinant of the stigma, the pollen S determinant had long remained elusive. Doughty et al. (7,8) analyzed the pollen coat protein (PCP) of Brassica oleracea and identified a basic 7-kDa protein, termed PCP-A1 (protein 1 of cla...