MOB proteins as novel regulators of pathogen-associated molecular pattern-triggered immunity in Arabidopsis thaliana

植物和动物一样，利用表面定位的受体来感知质外体中的微生物蛋白质，并触发一种称为病原体相关分子模式(PAMP)触发的免疫(PTI)的早期免疫反应。这类受体的原型例子是受体激酶Fls2和Efr，它们分别与细菌PAMPs鞭毛蛋白和延伸因子Tu结合。由Fls2和EFR介导的免疫依赖于与配体结合后立即与主要受体异构化的辅助受体BAK1。尽管最近取得了进展，但发生在受体激活下游的信号事件仍然难以捉摸。这项拟议的研究利用宿主实验室可用的独特工具，使用最近在模式植物拟南芥的一个新的正向遗传筛选中发现的突变体来揭示PTI的遗传机制。这个筛选利用了新的突变体bak1-5，它在PTI中特别受损，但没有表现出通常在bak1缺失突变体中观察到的其他多效性表型。该建议旨在鉴定bak1-5(Mob)突变体的修饰物和相应的蛋白质。暴徒突变体恢复了PAMP在bak1-5突变背景中引发的反应，包括对细菌的免疫。初步数据表明，一个突变体影响了枯草杆菌素样丝氨酸蛋白酶S1P，该酶以前被证明以不同的生物途径处理前肽。一个重要的例子是前肽的切割导致RALF23肽的释放。值得注意的是，宿主实验室的生物信息学分析发现，编码RALF23的基因是依赖flg22的基因表达网络的关键枢纽，因此表明该肽在PTI信号转导中具有功能。RALF23属于一个由RALF33和根特异的RALF1组成的3元基因簇。有趣的是，RALF1最近被发现被RK Feronia(FER)感知，值得注意的是，FER突变体显示出由flg22触发的ROS爆发增加，并增强了对丁香假单胞菌感染的抗病能力。因此，我们的工作模型是S1P裂解PRORALF23和/或PRORALF33释放活性多肽，进而与FER(或相关蛋白)结合，从而抑制PTI信号转导，暗示了一种先前未知的PTI调节机制。这项研究提案的主要目的是挑战这一新颖而令人兴奋的假说。

Plants, like animals, utilize surface-localized receptors to sense microbial proteins in the apoplast and trigger an early immune response known as pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI). Archetypal examples of such receptors are the receptor kinases FLS2 and EFR, which bind the bacterial PAMPs flagellin and elongation factor-Tu, respectively. Immunity mediated by FLS2 and EFR depends on the co-receptor BAK1 that heteromerizes with the main receptors immediately following ligand binding. Despite recent advances, the signalling events that occur downstream of receptor activation remain elusive. The proposed research takes advantage of unique tools available in the host laboratory to uncover the genetic mechanisms underlying PTI using mutants recently identified in a novel forward-genetic screen in the model plant Arabidopsis thaliana. This screen utilized the novel mutant bak1-5, which is specifically impaired in PTI but does not exhibit other pleiotropic phenotypes normally observed in bak1 null mutants. This proposal aims at characterizing modifier of bak1-5 (mob) mutants and corresponding proteins. The mob mutants restore PAMP-triggered responses in the bak1-5 mutant background, including immunity to bacteria. Preliminary data suggest that one mutant affects the subtilisin-like serine protease S1P, which was previously shown to process propeptides in different biological pathways. An important example is the cleavage of the pro-peptide leading to the release of the RALF23 peptide. Notably, bioinformatics analysis in the host lab identified the gene encoding RALF23 as a key hub in an flg22-dependent gene expression network and therefore indicating a function of this peptide in PTI signalling. RALF23 belongs to a 3-member gene cluster including RALF33 and the root-specific RALF1. Interestingly, RALF1 was recently shown to be perceived by the RK FERONIA (FER) and, notably, fer mutants show an increased flg22-triggered ROS burst and enhanced disease resistance to Pseudomonas syringae infections. Therefore, our working model is that S1P cleaves PRORALF23 and/or PRORALF33 to release the active peptide, which in turn binds to FER (or a related protein) resulting in the inhibition of PTI signalling, suggesting a previously unknown mechanism for the regulation of PTI. The main goal of this research proposal is to challenge this novel and exciting hypothesis.