Self S-RNase suppresses PbrWHY2 and downstream target genes to inhibit pollen tube growth in pear

Abstract

S-RNase-based gametophytic self-incompatibility (GSI) in flowering plants has been extensively studied, yet its underlying mechanisms remain incompletely understood. Here, we show that overexpression of PbrWHY2, a WHIRLY transcription factor, promotes pear pollen tube growth, sustains reactive oxygen species (ROS) levels, and enhances cellulose synthesis in the tube wall. Conversely, silencing PbrWHY2 yields the opposite phenotype. PbrWHY2 expression is downregulated in incompatible pollen tubes challenged with self S-RNase compared to compatible tubes exposed to non-self S-RNase, and its overexpression mitigates the growth inhibition induced by incompatibility. Furthermore, PbrWHY2 directly binds the promoters of three genes: PbrAGP16 (encoding Arabinogalactan protein 16), PbrRALF6 (encoding Rapid alkalization factor 6), and PbrGIS1. Overexpression of PbrAGP16 promotes pollen tube growth and cellulose accumulation, whereas overexpression of PbrRALF6 or PbrGIS1 enhances growth and elevates tip-localized ROS. Silencing any of these genes (PbrAGP16, PbrRALF6, or PbrGIS1) yields opposing effects. During GSI, self S-RNase reduces expression of PbrWHY2 and its downstream targets, thereby inhibiting pollen tube growth, lowering tip ROS levels, and impairing cellulose synthesis. Notably, WHY2 regulation of AGP16 expression in pollen tubes is conserved across gametophytic self-incompatible species including apple, plum, and pomelo, although WHY2 responsiveness to self S-RNases in plum differs from that in other species. These findings uncover a molecular module in the S-RNase-based self-incompatibility pathway.

Keywords: Pyrus; Pollen tube growth; S-RNase; Whirly transcription factor; self-incompatibility.