Recent advances in our understanding of the S-RNase-based gametophytic self-incompatibility system in Prunus

The S-RNase-based gametophytic self-incompatibility (GSI) system found in most Prunus species hinders the efficient breeding and cultivation of fruit trees such as sweet cherry (P. avium) and sour cherry (P. cerasus). This system is also present in Solanaceae and Plantaginaceae families and subtribe Malinae of Rosaceae. While working to clarify the molecular basis of GSI in Prunus, we have identified the pistil S determinant (S-RNase) and the pollen S determinant (S haplotype-specific F-box protein; SFB). Although similar molecules are involved in specificity determination of the SI recognition reaction across different taxa, accumulated data suggest the presence of distinct taxon-specific SI recognition mechanisms. Although the pistil-part S determinant S-RNase is believed to generally act as a cytotoxin against pollen tube RNA in incompatible pollen tubes, pollen S gene mutations between Prunus and other plant species indicate that pollen S functions differently. Prunus SFB is assumed to facilitate S-RNase cytotoxic effects during self-recognition, while pollen S in Solanaceae, Plantaginaceae and subtribe Malinae are thought to function to avoid S-RNase cytotoxicity during non-self-recognition. This report summarizes recent advances in our understanding of the S-RNase-based GSI system with special reference to evolutionary paths generating the Prunus-specific SI recognition mechanism and a novel SC mutant in Prunus.