Induced systemic resistance (ISR) against pathogens in the context of induced plant defences

Abstract

Induced systemic resistance (ISR) of plants against pathogens is a widespread phenomenon that has been intensively investigated with respect to the underlying signalling pathways as well as to its potential use in plant protection. Elicited by a local infection, plants respond with a salicylic-dependent signalling cascade that leads to the systemic expression of a broad spectrum and long-lasting disease resistance that is efficient against fungi, bacteria and viruses. Changes in cell wall composition, de novo production of pathogenesis-related-proteins such as chitinases and glucanases, and synthesis of phytoalexins are associated with resistance, although further defensive compounds are likely to exist but remain to be identified. In this Botanical Briefing we focus on interactions between ISR and induced resistance against herbivores that is mediated by jasmonic acid as a central signalling molecule. While many studies report cross-resistance, others have found trade-offs, i.e. inhibition of one resistance pathway by the other. Here we propose a framework that explains many of the thus far contradictory results. We regard elicitation separately from signalling and from production, i.e. the synthesis of defensive compounds. Interactions on all three levels can act independently from each other.

Fig. 1. Variable outcomes of cross‐talk between ISR (induced systemic resistance) and IRH (induced resistance against herbivores) signalling. A, Overview of interactions. On the level of elicitation and production several ‘common factors’ (in green boxes) appear in both signalling pathways (necrosis, cell wall fragments and oxidative burst during elicitation; phenolics and PR proteins on the production level) and might represent factors leading to cross‐resistance phenomena. B, Elicitation by a herbivore. While inducing mainly the octadecanoid pathway, the ‘common’ elicitors might lead to partial induction of ISR signalling. Resources are mainly allocated to herbivore resistance, but some resistance against pathogens is expressed, too. C, Elicitation by a pathogen. The partial induction of the octadecanoid pathway by the ‘common’ elicitors might lead to the occurrence of some early metabolites such as OPDA, but later, the pathway is blocked by the inhibitory effects of SA. On the phenotypic level, only resistance against pathogens is expressed. D, Exogenous elicitation bypasses regulatory mechanisms on the elicitation and the signalling level. The competition between both pathways for limiting resources therefore dominates the outcome and leads to phenotypically visible trade‐offs when both pathways are induced at the same time. See text for further details.