Recognition and Response in Plant PAMP‐Triggered Immunity

Abstract

Pathogen‐associated molecular pattern (PAMP)‐triggered immunity (PTI) describes the first events after pathogen invasion, whereby the plant identifies the presence of an invader and mounts a response. A race ensues in which the pathogen deploys its array of virulence molecules that act in opposition to host defence mechanisms and promote establishment of a pathogenic niche. Since the seminal work identifying the first pathogen receptor, FLAGELLIN‐SENSING 2 (FLS2), the field has worked to define receptor complexes and downstream signalling pathways. This article describes recent progress in this area. In addition, many pathogen effectors target the PTI machinery. Thus, identifying PTI components as effector targets helps to validate those molecules as components of the defence machinery. As such, early molecular interactions are key to the outcome of infection. Indeed it is thought that nonhost resistance, in which most plants species are resistant to most pathogens, is likely to be a result of the PTI system.

Key Concepts:

  • Plants have multilayered recognition systems which detect invading pathogens.

  • The first line of recognition involves detection of PAMPs via host pattern‐recognition receptors (PRRs).

  • PRRs are transmembrane receptor‐like kinases or receptor‐like proteins that perceive PAMPs at the cell surface.

  • PRRs act as a major point of control for PTI.

  • Biogenesis and localisation of PRRs to the plasma membrane requires the proteins are processed by the secretory pathway and individual PRRs have varying requirements for endoplasmic reticulum‐quality control pathways.

  • Following PAMP perception, a series of downstream defence responses results in PTI.

  • Attenuation of PRR signalling is necessary to restrict immune responses.

Keywords: disease resistance; flagellin; FLS2; PAMP‐triggered immunity; PAMP; pattern‐recognition receptor; PRR; PTI; signalling pathways

Figure 1.

The life and times of FLS2. (a) FLS2 biogenesis: (1) FLS2 is a glycosylated transmembrane protein. Transcription of FLS2 is under direct control of ethylene (ET). (2) Localisation of FLS2 to the plasma membrane (PM) requires that FLS2 is processed via the secretory pathway. (3) FLS2 is folded and subjected to quality control (QC) in the endoplasmic reticulum (ER), transported to the Golgi apparatus (GA) for modifications, and eventually transported to the plasma membrane cell wall, CW. (b) FLS2 reception. Prior to flagellin activation (− flagellin), FLS2 resides in a complex with the cytoplasmic protein kinase, BIK1. Following flagellin perception at the cell surface (+ flagellin), the receptor kinase BAK1 is recruited by FLS2 to form the FLS2‐BAK1 complex. Assembly of the FLS2‐BAK1 complex results in further phosphorylation events between the three kinase domains and release of the phosphorylated BIK1 from the complex. (c) FLS2 signalling: (1) Flagellin perception results in activation of mitogen activated protein kinase (MAPK) cascades. Ligand binding also triggers a Ca2+ burst, which activates Ca2+‐dependent protein kinases (CDPKs). (2) Induction of MAPK and CDPK cascades independently induces defence gene expression. (3) FLS2 is the subject of a ubiquitination cascade leading to FLS2 degradation. Following flagellin treatment PUB12 and PUB13, two U‐box E3 ubiquitin ligases, are recruited to the FLS2 complex and phosphorylated by BAK1. PUB12 and PUB13 have auto‐ubiquitination activity and polyubiquitinate FLS2, leading to FLS2 degradation and hence downregulation. (d) Potential ET positive feedback: (1) FLS2 transcription under direct control of ethylene via binding of the transcription factors EIN3 and EIL1 to the FLS2 promoter. (2) FLS2 activation by flagellin results in ET production , thereby allowing for a positive feedback function for ethylene. BIK1 appears to be involved in ET perception.

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Godfrey, Dale, and Rathjen, John P(Oct 2012) Recognition and Response in Plant PAMP‐Triggered Immunity. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023725]