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The S-allele-associated proteins (S-proteins) in the styles of the Japanese pear (Pyrus serotina Rehd. var. culta Rehd.) were purified by cation exchange chromatography. Their inhibitory action on the growth of incompatible pollen tubes (pollen tubes bearing the same Sallele as in the style from which the S-proteins were prepared) was characterized in vitro. Germination and tube growth of self-pollen (pollen from the same cultivar from which the S-proteins were prepared) decreased dose-dependently when the S-protein was added to the medium. Tube length was reduced to 10% that of compatible pollen tubes (pollen tubes bearing the S-allele different from that in the style from which the S-proteins were prepared) at 1.5 µg µl 1 . S-proteins from Shinsui (S 4 S 5 ) also inhibited growth of cross-incompatible Kosui (S 4 S 5 ) pollen tubes, but not of compatible Chojuro (S 2 S 3 ) pollen tubes. After inactivation of RNase of the Sprotein, the inhibitory action of the S-protein disappeared. These results indicate that the S-protein acts directly to inhibit growth of incompatible pollen tubes in Japanese pear styles, and that the RNase activity of the protein is essential for the biological function. However, small amounts of proteins that co-migrated with the Sprotein may also play some roles in the inhibition. This is the first report on the selective inhibitory action of Sproteins in Rosaceae. Materials and methods Plant materialsAdult trees of the Japanese pear Pyrus serotina Rehd. var. culta Rehd. planted in the orchards of Mie University, Tsu, Japan, were used. The cultivars and their S-genotypes were: Chojuro (S 2 S 3 ), Osa-Nijisseiki (S s S 4 SM ), Nijisseiki (S 2 S 4 ), Shinsui (S 4 S 5 ) and Kosui (S 4 S 5 ). Osa-Nijisseiki is a self-compatible mutant and its S 4 SM allele is designated by progeny analysis of the cultivar (Sato et al. 1988). SM means stylar-part mutant, and S 4 SM -protein is the same as S 4 -protein . Flowers of each cultivar were collected immediately before anthesis, and styles were detached, weighed, and stored in liquid nitrogen until use. Anthers were also collected, dehisced, dried in a bottle containing desiccants, and stored at -30°C.

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