Microbial Pathogen Effectors in Plant Disease

Abstract

Microbial plant pathogens use secreted effector proteins for successful infection of their host. This evolved state is rather exceptional as most microbes do not cause disease in the vast majority of plant species. An important primary activity of effectors is to interfere with a range of plant immune processes to evade and suppress pathogen detection, or to block immune signalling and downstream responses. Furthermore, effectors can enhance disease susceptibility by altering cellular processes and modulating host transcription. For most of these activities, effectors specifically target plant proteins that are central in these processes. An advanced virulence strategy is the post‐translational modification by effectors of plant targets to change their activity or stability. The knowledge gathered on the molecular mechanisms underlying effector‐triggered susceptibility of plants provides great potential for novel approaches of resistance breeding.

Key Concepts

  • Pathogens secrete and/or translocate effector proteins to promote plant disease.
  • Besides their primary role in promoting disease, effectors or effector‐modified plant proteins can be recognised by resistance proteins to activate an effector‐triggered immune response.
  • Many effectors block pathogen‐associated molecular pattern (PAMP)‐triggered immunity and/or effector‐triggered immunity.
  • Other effectors rewire signalling pathways and reprogramme the plant cell to promote pathogen growth.
  • Certain effectors can affect the activity or function of host proteins by post‐translational modifications e.g. (de)phosphorylation or targeting for proteosomal degradation.

Keywords: biotic stress; immune signalling; pathogen; disease resistance; effectors; effector‐triggered immunity; plant pathology; resistance protein; protein modification; phosphorylation; ubiquitination

Figure 1. Plant invasion and effector delivery strategies of bacteria and filamentous pathogens. The delivery mechanisms and sites of action of effector proteins is largely determined by the pathogen lifestyle, as shown in this schematic drawing of a cross‐section through an infected plant leaf. (a) Filamentous fungal and oomycete pathogens may directly penetrate the cuticle and cell wall. In order to gain access to the host cytoplasm, extracellular pathogens may form haustoria: specialised feeding structures that remain separated from the host by the host‐derived extrahaustorial membrane (EHM). Effectors are secreted from the haustorium, remain extracellularly (orange), or may be translocated into the host (purple). (b) Other filamentous pathogens enter via stomata. Hyphae can grow between the cells in the apoplast or invade the plant cells and spread intracellularly whilst remaining separated from the plant cytoplasm by the plant‐derived extra‐invasive hyphal matrix. (c) Bacteria frequently gain access through stomata or wounds. Many Gram‐negative species use a type III Secretion System (T3SS) to translocate effectors directly into the host cytoplasm.
Figure 2. Examples of subcellular processes in a plant cell that are targets of pathogen effectors. (a) Plants secrete a range of papain‐like cysteine proteases into the apoplast to degrade non‐self proteins. (b) Recognition of effectors by resistance proteins leads to effector‐triggered immunity. (c) The secretory pathway delivers proteins and cell wall components to the apoplast, constituting a first line of defence. (d) Detection of PAMPs, for example, bacterial flagellin, by PRRs in the plant cell membrane activates an MAPK cascade that results in a PTI response. (e) The hormone salicylic acid (SA) accumulates in response to biotrophic pathogens, and acts together with NPR1 monomers that travel into the nucleus. There, activated NPR1 interacts with TGA transcription factors to promote transcription of SA‐responsive genes. Another important defence hormone, jasmonic acid (JA), mediates the COI‐dependent degradation of JAZ proteins. MYC2 is released in the process and drives the transcription of JA responsive genes.
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Further Reading

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Pelgrom, Alexandra JE, and Van den Ackerveken, Guido(Aug 2016) Microbial Pathogen Effectors in Plant Disease. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023724]