Expression and Possible Role of S4(SSM4) Genes in the Styles of Self-incompatible Pear Cultivar and Its Self-compatible Mutant

Stylar soluble proteins in self-incompatible “Nijisseiki” (S2S4), self-compatible “Osa-Nijisseiki” ( S2SSM4, SM means stylar-part mutant) and its progeny were analyzed by isoelectric focusing polyacrylamide gel electrophoresis (IEF-PAGE).SSM4-allele associated protein, SSM4-protein, existed in the style of “Osa-Nijisseiki” and its progeny. The SSM4-protein expressed only in the stigma of “Osa-Nijisseiki”, whereas in its original variety “Nijisseiki”, S4-protein expressed in the upper and lower parts of the style as well as in the stigma, and its expression amount decreased from the upper part to the lower part. The protein bands analyzed by IEF-PAGE were subjected to RNase activity staining. The results showed that the S4- and the SSM4-proteins have the similar molecular weights (approximately 30 kD) and RNase activity. The specific-activities measured with yeast RNA were similar, equivalent to approximately 275 U·min-1·mg-1 protein. The S SM4-protein showed almost the same inhibitory effects as the S4-protein on the pollen germination and the pollen tube growth with S4- and SSM4-alleles in vitro . From the above results, the reasons of the self-compatibility of “Osa-Nijisseiki” are considered as (1) low expression of the SSM4-gene and (2) the SSM4-gene expression only in stigma.