The Arabidopsis ssi1 mutation restores pathogenesis-related gene expression in npr1 plants and renders defensin gene expression salicylic acid dependent

The Arabidopsis NPR1 gene was previously shown to be required for the salicylic acid (SA)- and benzothiadiazole (BTH)-induced expression of pathogenesis-related (PR) genes and systemic acquired resistance. The dominant ssi1 (for suppressor of SA insensitivity) mutation characterized in this study defines a new component of the sA signal transduction pathway that bypasses the requirement of NPR1 for expression of the PR genes and disease resistance. The ssi1 mutation caused PR (PR-1, BGL2 [PR-2], and PR-5) genes to be constitutively expressed and restored resistance to an avirulent strain of Pseudomonas syringae pv tomato in npr1-6 (previously called sail) mutant plants. In addition, ssi1 plants were small, spontaneously developed hypersensitive response-like lesions, accumulated elevated levels of SA, and constitutively expressed the antimicrobial defensin gene PDF1.2. The phenotypes of the ssi1 mutant are SA dependent. When SA accumulation was prevented in ssi1 npr1-5 plants by expressing the SA-degrading salicylate hydroxylase (nahG) gene, all of the phenotypes associated with the ssi1 mutation were suppressed. However, lesion formation and expression of the PR genes were restored in these plants by the application of BTH. Interestingly, expression of PDF1.2, which previously has been shown to be SA independent but jasmonic acid and ethylene dependent, was also suppressed in ssi1 npr1-5 plants by the nahG gene. Furthermore, exogenous application of BTH restored PDF1.2 expression in these plants. Our results suggest that SSI1 may function as a switch modulating cross-talk between the SA- and jasmonic acid/ethylene-mediated defense signal transduction pathways.