Molecular and biochemical bases of gametophytic self-incompatibility in solanaceae

Gametophytic self-incompatibility is a self/nonself recognition mechanism in flowering plants that allows the female reproductive organ to recognize and reject self pollen or pollen from genetically related individuals. For solanaceous species that display gametophytic self-incompatibility, a single multiallelic locus, called the S-locus, controls the outcome of pollination. S-allele-associated proteins (S-proteins) in the pistil have been identified based on their cosegregation with S-alleles. All the characteristics of S-proteins determined are consistent with their playing a role in self-incompatibility. Further, both gain-of-function and loss-of-function approaches have been used to show that S-proteins indeed control the ability of the pistil to recognize and reject self pollen. Since S-proteins have RNase activity and are glycoproteins, site-directed mutagenesis has been carried out to show that the RNase activity, but not the carbohydrate moiety, is essential for the function of S-proteins. Thus, the biochemical mechanism of S-protein-mediated rejection of self pollen involves the cytotoxic action of the RNase activity, and the S-allele specificity determinant of S-proteins lies in their amino acid sequence. Chimeric S-alleles between S₁- and S₃-alleles of Petunia inflata have been constructed and are being used to identify the S-allele specificity determinant of S-proteins. Models for how S-proteins specifically inhibit self pollen are discussed.