Mitogen-activated protein kinase (MAPK) cascades play pivotal roles in regulating plant immunity. MAPKs usually transduce signals and regulate plant immunity by phosphorylating the downstream defence-related components. Our previous study indicates that StMPK7 positively regulates plant defence to Phytophthora pathogens via SA signalling pathway. However, the downstream component of StMPK7 remains unknown. In this study, we employed GFP-StMPK7 transgenic potato and performed immunoprecipitation-mass spectrometry (IP-MS) to identify the downstream component of StMPK7. We found that an RNA binding protein StUBA2a/b interacted with StMPK7, as revealed by luciferase complementation imaging (LCI) and coimmunoprecipitation (co-IP) assays. Transient expression of StUBA2a/b in N. benthamiana enhanced plant resistance to Phytophthora pathogens, while silencing of UBA2a/b decreased the resistance, suggesting a positive regulator role of UBA2a/b in plant immunity. Similar to StMPK7, StUBA2a/b was also involved in SA signalling pathway and induced SGT1-dependent cell death as constitutively activated (CA)-StMPK7 did. Immune blotting indicated that StMPK7 phosphorylates StUBA2a/b at thr248 and thr408 (T248/408) sites and stabilizes StUBA2a/b. Silencing of MPK7 in N. benthamiana suppressed StUBA2a/b-induced cell death, while co-expression with StMPK7 enhanced the cell death. Besides, StUBA2a/bT248/408A mutant showed decreased ability to trigger cell death and elevate the expression of PR genes, indicating the phosphorylation by StMPK7 enhances the functions of StUBA2a/b. Moreover, CA-StMPK7-induced cell death was largely suppressed by silencing of NbUBA2a/b, genetically implying UBA2a/b acts as the downstream component of StMPK7. Collectively, our results reveal that StMPK7 phosphorylates and stabilizes its downstream substrate StUBA2a/b to enhance plant immunity via the SA signalling pathway.

中文翻译：

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StMPK7 磷酸化并稳定马铃薯 RNA 结合蛋白 StUBA2a/b 以增强植物防御反应

丝裂原活化蛋白激酶 (MAPK) 级联在调节植物免疫中起着关键作用。MAPK 通常通过磷酸化下游防御相关成分来转导信号和调节植物免疫。我们之前的研究表明，StMPK7 通过 SA 信号通路正向调节植物对疫霉病原体的防御。然而，StMPK7 的下游组件仍然未知。在这项研究中，我们使用 GFP-StMPK7 转基因马铃薯并进行免疫沉淀-质谱法 (IP-MS) 来鉴定 StMPK7 的下游成分。我们发现 RNA 结合蛋白 StUBA2a/b 与 StMPK7 相互作用，如荧光素酶互补成像 (LCI) 和共免疫沉淀 (co-IP) 测定所揭示的那样。StUBA2a/b 在本塞姆氏烟草中的瞬时表达增强了植物对疫霉病原体的抗性，而UBA2a/b的沉默降低了抗性，表明UBA2a/b在植物免疫中具有正调节作用。与 StMPK7 类似，StUBA2a/b 也参与 SA 信号通路并诱导 SGT1 依赖性细胞死亡，就像组成型激活的 (CA)-StMPK7 所做的那样。免疫印迹表明 StMPK7 在 thr248 和 thr408 (T248/408) 位点磷酸化 StUBA2a/b 并稳定 StUBA2a/b。本塞姆氏烟草中 MPK7 的沉默抑制了 StUBA2a/b 诱导的细胞死亡，而与 StMPK7 的共表达增强了细胞死亡。此外，StUBA2a/bT248/408A 突变体显示出触发细胞死亡和提高 PR 基因表达的能力降低，表明 StMPK7 的磷酸化增强了 StUBA2a/b 的功能。此外，CA-StMPK7 诱导的细胞死亡在很大程度上被 NbUBA2a/b 的沉默所抑制，遗传暗示 UBA2a/b 作为 StMPK7 的下游组分。总的来说，我们的结果表明 StMPK7 磷酸化并稳定其下游底物 StUBA2a/b，以通过 SA 信号通路增强植物免疫力。