Self S-RNase restricts ABF-LRX signaling cascade to arrest pollen tube during pear self-incompatibility reaction.

Gametophytic self-incompatibility (GSI) has been widely studied in flowering plants, but the study on mechanism of arresting pollen tube growth by self S-RNase in the GSI species is limited. In this study, two leucine-rich repeat extensin genes, PbLRXA2.1 and PbLRXA2.2, were identified based on transcriptome and quantitative real-time PCR analyses. The expression levels of these two LRX genes were significantly higher in the pollen grains and pollen tubes of self-compatible cultivar 'Jinzhui' (a spontaneous bud mutation of the self-incompatible cultivar 'Yali') than in 'Yali'. Both PbLRXA2.1 and PbLRXA2.2 positively mediated pollen tube growth and attenuated self S-RNase toxicity on pollen tube growth by stabilizing actin cytoskeleton and cell wall integrity. This result indicates that the abnormal expression of PbLRXA2.1 and PbLRXA2.2 involve in the breakdown of self-incompatibility in 'Jinzhui'. The PbLRXA2.1 and PbLRXA2.2 promoters were directly bound by an ABRE-binding factor PbABF.D.2. Knockdown of PbABF.D.2 expression decreased PbLRXA2.1 and PbLRXA2.2 expression and inhibited pollen tube growth. Notably, the expressions of PbLRXA2.1, PbLRXA2.2, and PbABF.D.2 were repressed by self S-RNase, suggesting that self S-RNase can arrest pollen tube growth by restricting the PbABF.D.2-PbLRXA2.1/PbLRXA2.2 signal cascade. These results provide a novel insight of the pollen tube growth arrested by self S-RNase.