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.

苹果（Malus domestica）表现出经典的S-RNase介导的配子体自我不相容性。先前的研究表明，花粉管中分泌的S-RNase可能触发由Ca ²⁺和活性氧（ROS）介导的信号转导和防御反应。在这项研究中，我们调查了在花粉管中S-RNase介导的配子体自交不亲和过程中由ROS激活的下游基因。在用自我S-RNase处理过的苹果花粉管中检测到ROS的大量增加，以及丝氨酸-苏氨酸蛋白激酶基因OXIDATIVE SIGNAL-INDUCIBLE1（MdOXI1）的表达上调。激酶分析相关的磷酸化蛋白质组学（KALIP）分析表明MdOXI1可以结合和磷酸化下游蛋白激酶Pto相互作用蛋白1样（MdPTI1L）。在花粉管中用反义寡核苷酸沉默MdOXI1后，MdPTI1L的磷酸化水平显着降低。MdOXI1或MdPTI1L的沉默可减轻自我S-RNase对花粉管生长的抑制作用。因此，我们的结果表明，MdPTI1L在花粉管中被MdOXI1磷酸化，并参与S-RNase触发的ROS信号通路。