Priming as a mechanism in induced systemic resistance of plants

Induced systemic resistance is a plant defence state that is associated with an enhanced ability - the so-called priming - to resist pathogen attack by stronger activation of cellular defence responses. So far, however, priming has not been widely appreciated when studying induced plant disease resistance. During the past several years, it has been demonstrated that pre-treatment of cultured parsley cells with inducers of systemic resistance, salicylic acid or a benzothiadiazole, leads to the direct activation of a set of defence-related genes and also primes the cells for stronger elicitation of another set of defence genes including those encoding phenylalanine ammonia-lyase. From these results, it was concluded that the resistance inducers have at least a dual role in plant defence-gene activation. When elucidating whether priming plays a role in induced systemic resistance of Arabidopsis, pre-treating plants with benzothiadiazole was found to augment the subsequent activation of phenylalanine ammonia-lyase genes by Pseudomonas infection, wounding and osmotic stress and also to enhance wound/osmotic stress-induced callose production. The augmentation of phenylalanine ammonia-lyase gene activation or/and callose deposition was not seen in the Arabidopsis non-expresser of pathogenesis-related genes1 mutant which is compromised in induced resistance, while it was present, without benzothiadiazole pre-treatment, in the constitutive expresser of pr genes1 and 5 mutants in which induced resistance is constitutive. Together these studies point to priming as an important cellular mechanism in induced systemic resistance of plants which requires the intact non-expresser of pathogenesis-related genes1 gene.