Salicylic acid induction-deficient mutants of Arabidopsis express PR-2 andPR-5 and accumulate high levels of camalexin after pathogen inoculation

In Arabidopsis, systemic acquired resistance against pathogens has been ass ociated with the accumulation of salicylic acid (SA) and the expression of the pathogenesis-related proteins PR-1, PR-2, and PR-5. We report here the isolation of two nonallelic mutants impaired in the pathway leading to SA b iosynthesis. These SA induction-deficient (sid) mutants do not accumulate S A after pathogen inoculation and are more susceptible to both virulent and avirulent forms of Pseodomonas syringae and Peronospora parasitica. However , sid mutants are not as susceptible to these pathogens as are transgenic p lants expressing the nahG gene encoding an SA hydroxylase that degrades SA to catechol. In contrast to NahG plants, only the expression of PR-1 is str ongly reduced in sid mutants, whereas PR-2 and PR-5 are still expressed aft er pathogen attack. Furthermore, the accumulation of the phytoalexin camale xin is normal. These results indicate that SA-independent compensation path ways that do not operate in NahG plants are active in sid mutants. One of t he mutants is allelic to eds5 (for enhanced disease susceptibility), wherea s the other mutant has not been described previously.