Rapid alkalinization factors (RALFs) are cysteine-rich peptides that play important roles in a variety of biological processes, such as cell elongation and immune signaling. Recent studies in Arabidopsis have shown that RALFs regulate pollen tube growth via plasma membrane receptor-like kinases (RLKs). However, the downstream signal transduction mechanisms of RLKs in pollen tubes are unknown. Here, we identified PbrRALF2, a pear (Pyrus bretschneideri) pollen RALF peptide that inhibits pollen tube growth. We found that PbrRALF2 interacts with a malectin-like domain-containing RLK, PbrCrRLK1L13. The relative affinity between PbrRALF2 and PbrCrRLK1L13 was at the submicromolar level, which is consistent with the values of ligand–receptor kinase pairs and the physiological concentration for PbrRALF2-mediated inhibition of pollen tube growth. After binding to its extracellular domain, PbrRALF2 activated the phosphorylation of PbrCrRLK1L13 in a dose-dependent manner. We further showed that the MAP kinase PbrMPK18 is a downstream target of PbrCrRLK1L13 that mediates PbrRALF2-elicited reactive oxygen species (ROS) production. The excessive accumulation of ROS inhibits pollen tube growth. We show that MPK acts as a mediator for CrRLK1L to stimulate ROS production, which might represent a general mechanism by which RALF and CrRLK1L function in signaling pathways.

中文翻译：

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PbrRALF2引发的活性氧信号由梨花粉管中的PbrCrRLK1L13-PbrMPK18模块介导

快速碱化因子 (RALF) 是富含半胱氨酸的肽，在细胞伸长和免疫信号传导等多种生物过程中发挥重要作用。最近的研究拟南芥已经表明RALFs通过质膜受体样激酶（RLKs）调节花粉管的生长。然而，花粉管中RLKs的下游信号转导机制尚不清楚。在这里，我们确定了 PbrRALF2，一个梨（梨树) 抑制花粉管生长的花粉RALF肽。我们发现 PbrRALF2 与含有类似马来蛋白的结构域 RLK PbrCrRLK1L13 相互作用。PbrRALF2和PbrCrRLK1L13之间的相对亲和力处于亚微摩尔水平，这与配体-受体激酶对的值和PbrRALF2介导的花粉管生长抑制的生理浓度一致。在与其细胞外结构域结合后，PbrRALF2 以剂量依赖性方式激活 PbrCrRLK1L13 的磷酸化。我们进一步表明 MAP 激酶 PbrMPK18 是 PbrCrRLK1L13 的下游靶标，它介导 PbrRALF2 引发的活性氧 (ROS) 产生。ROS的过度积累抑制了花粉管的生长。我们表明 MPK 作为 CrRLK1L 的介质来刺激 ROS 的产生，