Systemic Signalling in Plant Defence


Systemic signalling involves a complex network of signal transduction and amplification that leads to the activation of defence genes and establishment of systemic resistance throughout the entire plant. On microbial invasion, pathogen‐associated molecular patterns and effectors are frequently detected and recognised by plant receptors. Localised perception at the infection site rapidly triggers a cascade of early signalling events, including protein phosphorylation and production of reactive oxygen species. Subsequently, secondary signal molecules are synthesised and involved in amplification of defence signalling and the establishment of systemic acquired resistance (SAR). An increasing number of long‐distance signalling intermediates has been identified. Most of these act together and specifically promote systemic defence via the regulation of the local release of a set of systemically mobile signals. In the systemic tissue, the induction of SAR depends on synergistic interactions between systemic and ubiquitous salicylic acid‐associated immune signals.

Key Concepts

  • Local resistance to pathogen infection generally results from PAMP (pattern‐associated molecular pattern)‐triggered immunity and microbial effector‐triggered immunity.
  • Microbial infection of local tissues often leads to systemic acquired resistance (SAR) in distal tissues which provides long‐lasting broad‐spectrum resistance to a variety of pathogens.
  • Reactive oxygen species mediate systemic immunity acting as both cell‐to‐cell systemic signals and as local signalling partners upstream of other phloem‐mobile signals.
  • Mobile signals for SAR that are translocated via the vasculature may include methyl salicylate, the C9 lipid peroxidation product azelaic acid, one or more lipid transfer proteins, the diterpene dihydroabietinal and the nonprotein amino acid pipecolic acid.
  • Many of the SAR mobile signals cooperate with each other and with salicylic acid in one or more parallel and synergistic signalling pathways.
  • As a form of priming, SAR is supported by epigenetic regulation of defence gene expression.

Keywords: systemic acquired resistance; plant defence response; salicylic acid; long‐distance signalling; signal transduction

Figure 1. Hypersensitive response and systemic acquired resistance (SAR). Tobacco mosaic virus infection of tobacco cultivars carrying a disease resistance gene (e.g. N gene) leads to the HR (hypersensitive response) and subsequent establishment of SAR. The HR is characterised by host cell death and necrosis at the site of infection (left). Several days after the primary infection, SAR is induced throughout the plant. As a result, secondary infections of the plant with the same virus or other unrelated pathogens lead to much smaller lesions or weaker symptoms (right). The leaves are shown 4 days after viral infection.
Figure 2. A working model depicting local and systemic signalling events that are involved in the establishment of systemic immunity. Solid arrows represent established signalling interactions, broken arrows represent hypothetical signalling interactions, the double‐headed arrow represents a physical protein–protein interaction. Proteins are depicted in circles, metabolites are depicted in squares or non‐shape‐coded. Proteins and metabolites in red are putative phloem‐mobile signals. The broken red arrow depicts putative phloem transport, the blue arrows depict a systemic signalling event operating via cell‐to‐cell communication. See text for details and abbreviations.


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Further Reading

Conrath U, Beckers GJM, Langenbach CJG and Jaskiewicz MR (2015) Priming for enhanced defense. Annual Review of Phytopathology 53: 97–119.

Gao QM, Zhu S, Kachroo P and Kachroo A (2015) Signal regulators of systemic acquired resistance. Frontiers in Plant Science 6: 228.

Martinez‐Medina A, Flors V, Heil M, et al. (2016) Recognizing plant defense priming. Trends in Plant Science 21: 818–822.

Pieterse CMJ, Zamioudis C, Berendsen RL, et al. (2014) Induced systemic resistance by beneficial microbes. Annual Review of Phytopathology 52: 347–375.

Shah J, Chaturvedi R, Chowdhury Z, et al. (2014) Signaling by small metabolites in systemic acquired resistance. Plant Journal 79: 645–658.

Skelly MJ, Frungillo L and Spoel SH (2016) Transcriptional regulation by complex interplay between post‐translational modifications. Current Opinion in Plant Biology 33: 126–132.

Spoel SH and Dong X (2012) How do plants achieve immunity? Defence without specialized immune cells. Nature Reviews Immunology 12: 89–100.

Yan S and Dong X (2014) Perception of the plant immune signal salicylic acid. Current Opinion in Plant Biology 20: 64–68.

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Vlot, A Corina, Pabst, Elisabeth, and Riedlmeier, Marlies(Mar 2017) Systemic Signalling in Plant Defence. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001322.pub3]