A mutation near the active site of S-RNase causes self-compatibility in S-RNase-based self-incompatible plants

The structurally simplest amino acid glycine could make contribution to nuclease activity of S-RNase and self-incompatibility in S-RNase-based plants. S-RNase is regarded as inhibitor of self-pollen tube in S-RNase-based self-incompatibility plants. Certain residues like histidine are necessary for RNase activity and self-incompatibility; however, it is unknown whether any other residues contribute to this. Previously, we identified an association between the self-compatible Chinese pear (Pyrus × bretschneideri) cultivar 'Yanzhuang' (YZ) and a mutation causing a residue shift (glycine-to-valine) in the 2nd conserved region (C2) of S₂₁-RNase; however, it was unclear how this nonpolar aliphatic amino acid substitution caused self-compatibility. In this study, we observed that 'YZ' offspring were self-compatible when S₂₁-RNases were all mutated. In vitro pollen tube (S₂₁S₂₁) growth was not completely arrested by the mutated S₂₁-RNase. Residue frequency analysis showed that the glycine residue is highly conserved in diverse S-RNases across many plant species. We therefore generated a mutated petunia S_V'-RNase (glycine to valine) and transformed it into S_3LS_3L petunia. The transformed pistil could not inhibit S_V pollen tubes. Three-dimensional protein prediction suggested that the glycine-to-valine mutation alters the spatial structure near the active site, and RNase activity of mutated S-RNase was reducing. Thus, the glycine residue in the C2 is essential for RNase activity, substitution of this residue leads to a failure of self-incompatibility.