Environmentally sensitive, SA-dependent defense responses in the cpr22 mutant of Arabidopsis

To investigate the signaling pathways through which defense responses are a ctivated following pathogen infection, we have isolated and characterized t he cpr22 mutant. This plant carries a semidominant, conditional lethal muta tion that confers constitutive expression of the pathogenesis-related (PR) genes PR-7, PR-2 PR-5 and the defensin gene PDF1.2. cpr22 plants also displ ay spontaneous lesion formation, elevated levels of salicylic acid (SA) and heightened resistance to Peronospora parasitica Emco5. The cpr22 locus was mapped to chromosome 2, approximately 2 cM telomeric to the AthB102 marker . By analyzing the progeny of crosses between cpr22 plants and either NahG transgenic plants or npr1 mutants, all of the cpr22-associated phenotypes e xcept PDF1.2 expression were found to be SA dependent. However, the SA sign al transducer NPR1 was required only for constitutive PR-1 expression. A cr oss between cpr22 and ndr1-1 mutants revealed that enhanced resistance to P . parasitica is mediated by an NDR1-dependent pathway, while the other cpr2 2-induced defenses are not. Crosses between either coi1-1 or etr1-1 mutants further demonstrated that constitutive PDF1.2 expression is mediated by a JA- and ethylene-dependent pathway. Based on these results, the cpr22 mutat ion appears to induce its associated phenotypes by activating NPR1-dependen t and NPR1-independent branches of the SA pathway, as well as an ethylene/J A signaling pathway. Interestingly, the SA-dependent phenotypes, but not th e SA-independent phenotypes, are suppressed when cpr22 mutants are grown un der high humidity.