Selective inhibition of the growth of incompatible pollen tubes by S-protein in the Japanese pear

The S-allele-associated proteins (S-proteins) in the styles of the Japanese pear (Pyrus serotina Rehd. var. culta Rehd.) were purified by cation excha nge chromatography. Their inhibitory action on the growth of incompatible p ollen tubes (pollen tubes bearing the same S-allele as in the style from wh ich the S-proteins were prepared) was characterized in vitro. Germination a nd tube growth of self-pollen (pollen from the same cultivar from which the S-proteins were prepared) decreased dose-dependently when the S-protein wa s added to the medium. Tube length was reduced to 10% that of compatible po llen tubes (pollen tubes bearing the S-allele different from that in the st yle from which the S-proteins were prepared) at 1.5 mug mul(1). S-proteins from Shinsui (S4S5) also inhibited growth of cross-incompatible Kosui (S4S5 ) pollen tubes, but not of compatible Chojuro (S2S3) pollen tubes. After in activation of RNase of the S-protein, the inhibitory action of the S-protei n disappeared. These results indicate that the S-protein acts directly to i nhibit growth of incompatible pollen tubes in Japanese pear styles, and tha t the RNase activity of the protein is essential for the biological functio n. However, small amounts of proteins that co-migrated with the S-protein m ay also play some roles in the inhibition. This is the first report on the selective inhibitory action of S-proteins in Rosaceae.