Pseudomonas aeruginosa 7NSK2-induced systemic resistance in tobacco depends on in planta salicylic acid accumulation but is not associated with PR1a expression

Root colonization by rhizobacteria can induce a systemic resistance in plan ts that is phenotypically similar to systemic acquired resistance induced b y a localized pathogen infection. We used the tobacco-tobacco mosaic virus model to investigate whether the systemic resistance induced by the rhizoba cterium Pseudomonas aeruginosa 7NSK2 is mediated by the systemic acquired r esistance signal transduction pathway. Experiments with nahG-transformed to bacco revealed that Pseudomonas aeruginosa 7NSK2-induced resistance depende d on in planta salicylic acid accumulation for its expression but not for i ts induction and is, in this respect, similar to systemic acquired resistan ce. However, Pseudomonas aeruginosa 7NSK2-induced resistance was, unlike sy stemic acquired resistance, not associated with PR1a expression at the time of challenge with tobacco mosaic virus. This suggests that Pseudomonas aer uginosa 7NSK2 treatment would only potentiate defense gene expression in sy stemic tissue, which would also explain why its level of resistance is lowe r than in case of systemic acquired resistance. Because we demonstrated tha t induced resistance by Pseudomonas aeruginosa 7NSK2 exclusively depends on the production of salicylic acid by this strain our conclusions might also account for other salicylic acid-producing and resistance-inducing rhizoba cteria.