The apple MdPTI1L kinase is phosphorylated by MdOXI1 during S-RNase-induced reactive oxygen species signaling in pollen tubes.

Apple (Malus domestica) exhibits classic S-RNase-mediated gametophytic self-incompatibility. Previous studies have shown that the S-RNase secreted from style cells could trigger signal transduction and defense responses mediated by Ca2+ and reactive oxygen species (ROS) after entering into the pollen tube. In this study, we investigated the downstream genes activated by ROS during S-RNase-mediated gametophytic self-incompatibility in pollen tubes. A substantial increase in ROS, as well as up-regulated expression of a serine-threonine protein kinase gene, OXIDATIVE SIGNAL-INDUCIBLE1 (MdOXI1), was detected in apple pollen tubes treated with self-S-RNase. A kinase assay-linked phosphoproteomics (KALIP) analysis suggested that MdOXI1 could bind and phosphorylate the downstream protein kinase Pto-interacting protein 1-like (MdPTI1L). The phosphorylation level of MdPTI1L was significantly reduced after silencing MdOXI1 with antisense oligonucleotides in the pollen tube. Silencing of either MdOXI1 or MdPTI1L alleviated the inhibitory effect of self-S-RNase on pollen tube growth. Our results thus indicate that MdPTI1L is phosphorylated by MdOXI1 in the pollen tube and participates in the ROS signaling pathway triggered by S-RNase.