Analyses of pollen-tube growth and biological action of S-RNase in the style of self-compatible Japanese pear

The Japanese pear 'Osa-Nijisseiki' (S2S4SM) (SM = stylar-part mutant) is a self-compatible bud mutant that originated from self-incompatible 'Nijisseiki' (S2S4). The S4-allele of the pear is deleted and it shows unilateral incompatibility to cultivar with an S2S4 genotype. However, when pollen-tube growth was compared between cross-compatible ['Osa-Nijisseiki' × 'Okusankichi' (S5S7)], unilateral-compatible ['Osa-Nijisseiki' × 'Kikusui' (S2S4)], self-compatible ('Osa-Nijisseiki' × 'Osa-Nijisseiki'), and unilateral-incompatible pollination ('Kikusui' × 'Osa-Nijisseiki'), pollen-tube growth clearly showed the following order: cross-compatible > unilateral-compatible > self-compatible > unilateral-incompatible. This indicates that the 'Osa-Nijisseiki' style produces specific inhibitor(s) not only to S2- and S4-pollen but also “self-pollen”, because the phenotype of S4SM-pollen is the same as S4-pollen. Stylar protein analysis demonstrated that 'Osa-Nijisseiki' produces S2-RNase (RNase associated with S2-allele) together with a small amount of S4-RNase. The purified S4-RNase possessed almost the same inhibitory action on the growth of S4-pollen-tubes in vitro at 1 μg μl−1 as that from original 'Nijisseiki'. These results suggest that the depressed growth of unilateral-compatible and self-pollen-tubes in 'Osa-Nijisseiki' is due to this biologically active S4-RNase. Growth of self-pollen-tubes may also be depressed by inhibitor(s) specific to “self-pollen” unrelated to S-alleles.