Comparison of an S-protein expression between self-compatible and -incompatible Japanese pear cultivars

Abstract

 An S₄-allele-associated protein (S₄-protein) was identified by both isoelectricfocusing (IEF) polyacrylamide-gel electrophoresis and N-terminal amino-acid sequences in self-incompatible Nijisseiki (self-in- compatibility genotype=S₂S₄) and self-compatible Osa-Nijisseiki (S₂S₄ ^SM, SM=stylar-part mutant) styles of the Japanese pear, Pyrus serotina Rehd. var. culta Rehd. Expression of the protein in the cultivars was compared during flower bud development and at post-transcriptional levels.

In mature styles, S₄-protein could be detected on IEF gel in both cultivars and the N-terminal amino acid sequences were identical, although Osa-Nijisseiki contained only one-third the amount contained in Nijisseiki. No difference was observed in S₂-protien amounts. In Nijisseiki styles, S₄-protein was already detectable 8 days before anthesis (DBA) and it was synthesized consistently until 2 days after anthesis (DAA); the amount increased 4.7-fold during these 10 days. In contrast, S₄-protein in Osa-Nijisseiki was not detected earlier than 6 DBA; a small amount was found at 4 DBA, and it increased gradually as flowers developed. Thus, expression of Osa-Nijisseiki S₄-protein is developmentally controlled in the same way as that of Nijisseiki S₄-protein, but with a time lag of several days; the protein level at 2 DAA corresponded to that of Nijisseiki earlier than 4 DBA. S₄-proteins from both Nijisseiki and Osa- Nijisseiki showed RNase activity and the activity was also developmentally controlled; it increased about fourfold during the interval from 8 DBA to 2 DAA in Nijisseiki, and 3.3-fold during the interval from 4 DBA to 2 DAA in Osa-Nijisseiki. Activity at 2 DAA, however, was twice as high in Nijisseiki. In vitro protein synthesis showed that poly(A)⁺ from mature Osa-Nijisseiki styles could form S₄-like protein in a manner similar to that of Nijisseiki. These results suggest that the self-compatibility of Osa-Nijisseiki is due to a low level of S₄-protein expression, a mechanism very similar to the low level of S-protein and weak incompatibility in immature styles of self-incompatible Nijisseiki. Part of this protein repression may be regulated during post-transcriptional events.