Systemic Induced Resistance

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Induced resistance is the phenomenon in which a plant, once appropriately stimulated, exhibits an enhanced resistance upon “challenge” inoculation with a pathogen. Induced resistance can be localized as well as systemic, and can be induced by limited pathogen infection, avirulent pathogens, certain non-pathogenic bacteria, and certain chemicals. Systemic induced resistance has been particularly well studied in tobacco, cucumber and Arabidopsis. Under the influence of the inducing stimulus, a mobile signal is generated and transported to other parts of the plant where it enhances the mechanisms normally functioning to limit infection, growth, multiplication and spread of fungi, bacteria and viruses. In the systemic acquired resistance (SAR) induced by pathogens causing hypersensitive necrosis, resistance mechanisms to challenge inoculation become operative earlier or operate at a higher level than in non-induced plants. The expression of SAR is dependent on the accumulation of salicylic acid (SA) and associated with the induction of pathogenesis-related proteins (PRs) with anti-pathogen activities. In contrast, rhizobacterially-mediated induced systemic resistance (ISR) does not require SA, can occur without production of PRs, and is dependent on ethylene and jasmonic acid (JA) signalling. Mutants in the signal-transduction pathways of SAR and ISR have been identified in Arabidopsis which point to both pathways converging at the final step, while additional antimicrobial peptides are independently regulated. A different type of systemic induced resistance, involving production of proteinase inhibitors against herbivores, also requires JA. Synthesis of JA is initiated on perception of the Polypeptide systemin, which is released on wounding of tomato and potato and acts as the mobile signal. These various types of systemic induced resistance confer an enhanced defensive capacity on plants against a broad spectrum of attackers and offer great potential for exploitation in crop protection.