Cellular dynamics between PAMP receptors and pathogen-derived effectors

植物免疫的一个重要主动防御层是检测由细胞表面受体介导的病原相关分子模式（pathogenassociated molecular patterns, PAMPs）。对于疾病的建立，病原体依赖于克服PAMP感知和禁用响应PAMP激活的植物信号通路的能力。因此，模式识别受体（PRRs）是病原体效应物的主要目标。FLS2及其共受体BAK1和EFR编码受体样激酶（RLKs），它们在对抗细菌病原体的免疫中发挥作用，并分别负责感知细菌鞭毛蛋白（flg22）和EF-Tu （elf18）。在SPP1212的第一个资助期内，我们发现拟南芥中丁香假单胞菌pv tomato DC3000 （PtoDC3000）的毒力通过效应物AvrPtoB的作用而增强，这取决于其E3连接酶的活性。我们可以证明AvrPtoB与FLS2的物理关联，这种关联在受体激活后得到增强。我们可以证明AvrPtoB泛素化FLS2并促进FLS2的降解，但不促进BAK1的降解（Göhre和Spallek等人，在修订中）。它仍然是开放的，在植物途径中，AvrPtoB利用它来实现FLS2的降解。我们希望确定FLS2泛素化的负责残基，降解如何与FLS2激活和运输联系起来，并解决AvrPtoB对PRRs的特异性。似乎对PRRs信号的急性抑制对病原体的毒力很重要，我们建议对病原体触发的细胞运输重编程有更多的了解。越来越多的效应物被描述为抑制pamp触发的防御反应，我们希望确定那些在PRRs水平上起作用的效应物。由于这些效应物也包括小分子，我们之前发现了影响FLS2内吞作用的化合物，并研究了它们在flg22信号传导中的作用（Serrano et al, 2007），我们希望扩展这项研究。BAK1作为FLS2的共受体的鉴定（Chinchilla et al, 2007a）极大地促进了我们对受体机制和PAMP信号传导的理解。由于bak1突变体对flg22不敏感，我们通过遗传筛选鉴定了几个新的fli （flg22不敏感）突变体。突变体fli1、fli3和fli6在flg22引发的胼胝质沉积和对细菌感染的抗病能力中受损。在SPP1212的框架中，我们建议分离fli突变体的相关位点，这些位点可能是FLS2运输的调节因子，也是病原体效应物的潜在靶点。总的来说，我们的研究应该为植物免疫受体和病原体模式的相互作用提供新的见解，无论是PAMPs被认为是植物免疫的重要配体，还是作为靶向受体对抗PAMP信号传导从而促进病原体毒力的效应物。

An important layer of active defense in plant immunity is the detection of pathogenassociated molecular patterns (PAMPs) mediated by cell surface receptors. For the establishment of disease, pathogens depend on the ability to overcome PAMP perception and disable plant signalling pathways activated in response to PAMPs. Pattern recognition receptors (PRRs), therefore, represent prime targets for pathogen effectors. FLS2, its co-receptor BAK1, and EFR encode receptor-like kinases RLKs) that play a role in immunity against bacterial pathogens and are responsible for the perception of bacterial flagellin (flg22) and EF-Tu (elf18), respectively.Within the first funding period of the SPP1212 we discovered that virulence of Pseudomonas syringae pv tomato DC3000 (PtoDC3000) in Arabidopsis was enhanced through the action of the effector AvrPtoB, which was dependent on its E3 ligase activity. We could demonstrate physical association of AvrPtoB with FLS2, which was enhanced upon receptor activation. We could show that AvrPtoB ubiquitinates FLS2 and promotes degradation of FLS2 but not BAK1 (Göhre and Spallek et al, in revision). Still it remains open, which in planta pathway is used by AvrPtoB to achieve degradation of FLS2. We would like to identify the responsible residues for FLS2 ubiquitination, how degradation is connected with FLS2 activation and trafficking and address AvrPtoB specificity to PRRs. It appears that acute inhibition of signalling PRRs is important for pathogen virulence, which we propose to gain more insights into pathogen-triggered reprogramming of cellular trafficking. An increasing number of effectors are described to suppress PAMPtriggered defense responses and we would like to identify those acting at the level of PRRs. Because such effectors include also small molecules, we previously identified compounds that affect FLS2 endocytosis and investigated their role in flg22 signalling (Serrano et al, 2007), and we would like to extend this study.The identification of BAK1 as co-receptor of FLS2 (Chinchilla et al, 2007a) significantly advanced our understanding of receptor mechanisms and PAMP signalling. Since bak1 mutants display insensitivity to flg22, we identified several novel fli (flg22-insensitive) mutants through a genetic screen. The mutants fli1, fli3, and fli6 are impaired in flg22- triggered callose deposition and disease resistance to bacterial infection. In the frame of the SPP1212 we propose to isolate the responsible loci of fli mutants, which could be regulators of FLS2 trafficking and are potential targets of pathogen effectors.Overall, our studies should provide novel insights into the interaction of plant immune receptors and pathogen patterns, either PAMPs perceived as ligands important for plant immunity or as effectors that target the receptors to counter act PAMP signalling and thereby contribute to pathogen virulence.