Systemic Signalling in Plant Defence

Abstract

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 C 9 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.

Demystifying biotrophs: FISHing for mRNAs to decipher plant and algal pathogen–host interaction at the single cell level

“…The results presented here therefore reinforce the previously established hypothesis that PbBSMT is produced to inactivate SA produced by the host in response to chitin 24,33,48 . However, methylsalicylate (MeSA) is a more volatile and membrane-permeable form of SA and is either excreted via the leaves or an inducer of systemic responses in plants 56 . MeSA was specifically emitted from Arabidopsis leaves infected with P. brassicae 33 .…”

Local Responses and Systemic Induced Resistance Mediated by Ectomycorrhizal Fungi

“…The mobile signals activate MAPK (MITOGEN-ACTIVATED PROTEIN KINASE) cascades ( Conrath et al, 2015 ) and induce the expression of pathogenesis-related (PR) proteins, especially PR1 (PATHOGENESIS-RELATED 1) involving antagonistic key regulators NPR1 and NPR3/4 [NON-EXPRESSER OF PR GENES ( Ding et al, 2018 )]. Other compounds invoked as mobile SAR signals are azaleic acid (a C 9 lipid peroxidation product), lipid transfer proteins, and the diterpene dihydroabietinal ( Vlot et al, 2017 ). Ultimately, an enhanced defense is achieved either through direct defenses (e.g., callose deposition) or trough priming, whereby the plant exhibits stronger defenses toward a secondary infection ( Conrath et al, 2006 ; Jung et al, 2009 , 2012 ; Pieterse et al, 2014 ; Mauch-Mani et al, 2017 ).…”

Unravelling Plant Responses to Stress—The Importance of Targeted and Untargeted Metabolomics

“…The initial stress response of plants is through an integrated network of signaling molecules, inclusive of a wide range of secondary messengers (calcium, reactive oxygen species (ROS), nitric oxide (NO), and lipids) and phytohormones (e.g., salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), and ethylene) [ 7 , 8 , 9 , 10 , 11 ]. The network is primarily mediated by mitogen and calcium-activated protein kinase (MAPK and CAPK) signal transduction cascades [ 12 , 13 , 14 ].…”