Separate jasmonate-dependent and salicylate-dependent defense-response pathways in Arabidopsis are essential for resistance to distinct microbial pathogens

The endogenous plant hormones salicylic acid (SA) and jasmonic acid (JA), w hose levels increase on pathogen infection, activate separate sets of genes encoding antimicrobial proteins in Arabidopsis thaliana. The pathogen-indu cible genes PR-1, PR-2, and PR-5 require SA signaling for activation, where as the plant defensin gene PDF1.2, along with a PR-3 and PR-4 gene, are ind uced by pathogens via an SA-independent and JA-dependent pathway. An Arabid opsis mutant, coil, that is affected in the JA-response pathway shows enhan ced susceptibility to infection by the fungal pathogens Alternaria brassici cola and Botrytis cinerea but not to Peronospora parasitica, and vice versa for two Arabidopsis genotypes (npr1 and NahG) with a defect in their SA re sponse. Resistance to P. parasitica was boosted by external application of the SA-mimicking compound 2,6-dichloroisonicotinic acid [Delaney, T., et al . (1994) Science 266, 1247-1250] but not by methyl jasmonate (MeJA), wherea s treatment with MeJA but not 2,6-dichloroisonicotinic acid elevated resist ance to Alternaria brassicicola. The protective effect of MeJA against A. b rassicicola was the result of an endogenous defense response activated ira planta and not a direct effect of MeJA on the pathogen, as no protection to A. brassicicola was observed in the coil mutant treated with MeJA. These d ata point to the existence of at least two separate hormone-dependent defen se pathways in Arabidopsis that contribute to resistance against distinct m icrobial pathogens.