Abstract
Systemic acquired resistance (SAR) is a defence mechanism that induces protection against a wide range of pathogens in distant, pathogen-free parts of plants after a primary inoculation. Multiple mobile compounds were identified as putative SAR signals or important factors for influencing movement of SAR signalling elements in Arabidopsis and tobacco. These include compounds with very different chemical structures like lipid transfer protein DIR1 (DEFECTIVE IN INDUCED RESISTANCE1), methyl salicylate (MeSA), dehydroabietinal (DA), azelaic acid (AzA), glycerol-3-phosphate dependent factor (G3P) and the lysine catabolite pipecolic acid (Pip). Genetic studies with different SAR-deficient mutants and silenced lines support the idea that some of these compounds (MeSA, DIR1 and G3P) are activated only when SAR is induced in darkness. In addition, although AzA doubled in phloem exudate of tobacco mosaic virus (TMV) infected tobacco leaves, external AzA treatment could not induce resistance neither to viral nor bacterial pathogens, independent of light conditions. Besides light intensity and timing of light exposition after primary inoculation, spectral distribution of light could also influence the SAR induction capacity. Recent data indicated that TMV and CMV (cucumber mosaic virus) infection in tobacco, like bacteria in Arabidopsis, caused massive accumulation of Pip. Treatment of tobacco leaves with Pip in the light, caused a drastic and significant local and systemic decrease in lesion size of TMV infection. Moreover, two very recent papers, added in proof, demonstrated the role of FMO1 (FLAVIN-DEPENDENT-MONOOXYGENASE1) in conversion of Pip to N-hydroxypipecolic acid (NHP). NHP systemically accumulates after microbial attack and acts as a potent inducer of plant immunity to bacterial and oomycete pathogens in Arabidopsis. These results argue for the pivotal role of Pip and NHP as an important signal compound of SAR response in different plants against different pathogens.
Highlights
Systemic acquired resistance (SAR) is an inducible defence mechanism that provides protection in distant, pathogen-free parts of plants against a broad range of pathogens
Synthetic analogues and viral double stranded RNAs (dsRNAs) itself act as microbe-associated molecular patterns (MAMPs) and can induce SERK-1 dependent specific antiviral resistance signalling in Arabidopsis but this mechanism is independent of RNA silencing [45]
Reciprocal grafting experiments between WT and SAMT1 (SALICYLIC ACID METHYLTRANSFERASE1) silenced tobaccos, suggested that methyl salicylate (MeSA) can serve as a signal for SAR induction inside the plants via transport [12]: (i) functional SAMT1 gene was necessary for SAR induction in primary inoculated leaves; (ii) MeSA accumulated in phloem exudate of infected leaves and MeSA could be the long-distance signal for SAR; (iii) competitive, pharmacological inhibitor of MeSA demethylase, 2,2,2,2-tetrafluoro-acetofenon could attenuate SAR development in the distant, pathogen free leaves and (iv) further grafting experiments between WT and SABP2 (SALICYLIC ACID BINDING PROTEIN2) silenced plants suggested the importance of its demethylase function in distant leaves in salicylic acid (SA) production and SAR induction [12,80]
Summary
Systemic acquired resistance (SAR) is an inducible defence mechanism that provides protection in distant, pathogen-free parts of plants against a broad range of pathogens. This feature indirectly excludes the activation of a virus species-specific mechanism during SAR induction. Synthetic analogues and viral dsRNA itself act as MAMPs and can induce SERK-1 dependent specific antiviral resistance signalling in Arabidopsis but this mechanism is independent of RNA silencing [45]. The significance of this fact in SAR induction is unknown. This review will focus on signal transduction mechanisms between pathogen perception and SAR response, especially on the environmental factors that can influence, qualitatively or quantitatively, the signalling processes
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