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

The endogenous plant hormones salicylic acid (SA) and jasmonic acid (JA), whose levels increase on pathogen infection, activate separate sets of genes encoding antimicrobial proteins in <i>Arabidopsis thaliana</i>. The pathogen-inducible genes <i>PR-1</i>, <i>PR-2</i>, and <i>PR-5</i> require SA signaling for activation, whereas the plant defensin gene <i>PDF1.2</i>, along with a <i>PR-3</i> and <i>PR-4</i> gene, are induced by pathogens via an SA-independent and JA-dependent pathway. An <i>Arabidopsis</i> mutant, <i>coi1</i>, that is affected in the JA-response pathway shows enhanced susceptibility to infection by the fungal pathogens <i>Alternaria brassicicola</i> and <i>Botrytis cinerea</i> but not to <i>Peronospora parasitica</i>, and <i>vice versa</i> for two <i>Arabidopsis</i> genotypes (<i>npr1</i> and <i>NahG</i>) with a defect in their SA response. Resistance to <i>P. parasitica</i> was boosted by external application of the SA-mimicking compound 2,6-dichloroisonicotinic acid [Delaney, T., <i>et al. </i> (1994) Science 266, 1247-1250] but not by methyl jasmonate (MeJA), whereas treatment with MeJA but not 2,6-dichloroisonicotinic acid elevated resistance to <i>Alternaria brassicicola</i>. The protective effect of MeJA against <i>A. brassicicola</i> was the result of an endogenous defense response activated <i>in planta</i> and not a direct effect of MeJA on the pathogen, as no protection to <i>A. brassicicola</i> was observed in the <i>coi1</i> mutant treated with MeJA. These data point to the existence of at least two separate hormone-dependent defense pathways in <i>Arabidopsis</i> that contribute to resistance against distinct microbial pathogens.