Defence and defence suppression in plant pathogen interactions: the role of CaMV protein P6

植物拥有一系列防御机制，保护它们免受引起疾病的生物体的侵害，这些生物体包括病毒、真菌和细菌。植物必须首先识别入侵病原体的存在，然后进行防御反应，干扰病原体的繁殖和传播能力。反过来，病原体必须要么逃避这些防御反应，要么抑制它们，并不断进化自己的机制来做到这一点。越来越多的证据表明，抑制宿主防御的能力可能是所有生物营养病原体的基本特征，即那些可以在活组织中生长的病原体。植物使用的许多防御反应涉及水杨酸（SA）作为信号中间体（即触发器）。病原体的存在会刺激SA的合成和积累，这反过来又会激活整个植物的各种防御反应。我们发现，当拟南芥的一种病毒病原体花椰菜花叶病毒（CaMV）感染植物时，它会触发sa依赖性反应的激活。然而，这些措施似乎并不能有效地限制病毒的传播。这是因为由病毒编码并在感染细胞中合成的蛋白质P6非常有效地阻断了通常由SA增加量激活的防御反应。我们发现P6不仅可以阻断对CaMV的防御，还可以阻止细菌病原体，使其在通常具有抗性的拟南芥生态型中生长。在拟议的计划中，我们开始了解P6如何作用于防御信号。已知P6具有对CaMV复制至关重要的几种功能，但似乎与防御抑制无关。我们将研究负责这些复制功能的蛋白质部分是否与负责防御抑制的蛋白质部分分开。这将通过在转基因植物中表达整个或部分P6蛋白来实现。我们还将研究表达P6对一系列真菌和细菌病原体敏感性的影响。例如，P6是否可以扩展通常不能在拟南芥中生长的病原体的宿主范围？P6必须与其他参与防御信号传导的拟南芥蛋白相互作用。我们将使用两种平行的方法来确定相互作用的伙伴，酵母双杂交和免疫下拉。我们将利用这些信息试图确定这些相互作用的蛋白质如何在防御信号中发挥作用，以及P6如何阻止它们的作用。这项工作直接属于BBSRC了解植物与其病原体之间相互作用的优先领域。我们的长期目标是描述病毒和其他生物营养病原体可以逃避植物自卫的防御反应的机制。

Plants possess a battery of defence mechanisms that protect them against the organisms that cause disease, these include viruses, fungi and bacteria. The plant must first recognize the presence of invading pathogens and then engage defence responses that interfere with the ability of the pathogen to multiply and spread. Pathogens in turn must either evade these defence responses or suppress them, and are constantly evolving mechanisms of their own to do so. Increasing evidence suggests that the ability to suppress host defences may be an essential feature of all biotrophic pathogens, i.e. those that can grow in living tissue. Many of the defence responses used by plants involve Salicylic Acid (SA) as a signalling intermediate (i.e. trigger). The presence of a pathogen, stimulates the synthesis and accumulation of SA, this in turn signals the activation of various defence responses throughout the plant. We have found that when a viral pathogen of Arabidopsis, Cauliflower mosaic virus (CaMV), infects the plants, it triggers the activation of SA-dependent responses. However, these do not appear to be effective in restricting the spread of the virus. This is because a protein, P6, which is encoded by the virus and synthesized in infected cells, acts very efficiently to block the defence responses that would ordinarily be activated by the increased amounts of SA. We have found that P6 can not only block defence against CaMV but also against a bacterial pathogen, allowing it to grow in an ecotype of Arabidopsis which is normally resistant. In the proposed programme we have set out to understand how P6 acts on defence-signalling. P6 is known to have several functions which are essential for the replication of CaMV, but which do not appear to be related to defence suppression. We will investigate whether the parts of the protein that are responsible for these replicative functions are separate from those that are responsible for defence suppression. This will be achieved by expressing the whole P6 protein or parts of it in transgenic plants. We will also investigate the effect of expressing P6 on the susceptibility to a range of fungal and bacterial pathogens. For example, can P6 extend the host range of pathogens that ordinarily cannot grow in Arabidopsis? P6 must work by interacting with other Arabidopsis proteins that are involved in defence signalling. We will identify the interacting partners using two parallel approaches, yeast two hybrid and immune-pulldown. We will use this information to try to determine how these interacting proteins might be functioning in defence signalling and how P6 blocks their action. This work falls directly within the BBSRC priority area of understanding the interactions between plants and their pathogens. Our long term aims are to characterize the mechanisms by which viruses and other biotrophic pathogens can evade the defence responses by which plants defend themselves.