Enhancement of induced disease resistance by simultaneous activation of salicylate- and jasmonate-dependent defense pathways in Arabidopsis thaliana

The plant-signaling molecules salicylic acid (SA) and jasmonic acid (1A) pl ay an important role in induced disease resistance pathways. Cross-talk bet ween sA- and JA-dependent pathways can result in inhibition of JA-mediated defense responses. We investigated possible antagonistic interactions betwe en the SA-dependent systemic acquired resistance (SAR) pathway, which is in duced upon pathogen infection, and the JA-dependent induced systemic resist ance (ISR) pathway, which is triggered by nonpathogenic Pseudomonas rhizoba cteria. In Arabidopsis thaliana, SAR and ISR are effective against a broad spectrum of pathogens, including the foliar pathogen Pseudomonas syringae p v. tomato (Pst). Simultaneous activation of SAR and ISR resulted in an addi tive effect on the level of induced protection against Pst. In Arabidopsis genotypes that are blocked in either SAR or ISR. this additive effect was n ot evident. Moreover. induction of ISR did not affect the expression of the SAR marker gene PR-1 in plants expressing SAR. Together. these observation s demonstrate that the SAR and the ISR pathway are compatible and that ther e is no significant cross-talk between these pathways. SAR and ISR both req uire the key regulatory protein NPR1. Plants expressing both types of induc ed resistance did not show elevated Npr1 transcript levels, indicating that the constitutive level of NPR1 is sufficient to facilitate simultaneous ex pression of SAR and ISR. These results suggest that the enhanced level of p rotection is established through parallel activation of complementary, NPR1 -dependent defense responses that are both active against Pst. Therefore, c ombining SAR and ISR provides an attractive tool for the improvement of dis ease control.