A recessive mutation in the Arabidopsis SSI2 gene confers SA- and NPR1-independent expression of PR genes and resistance against bacterial and oomycete pathogens

The Arabidopsis thaliana NPR1 gene is required for salicylic acid (SA)-indu ced expression of pathogenesis-related (PR) genes and systemic acquired res istance. However, loss-of-function mutations in NPR1 do not confer complete loss of PR gene expression or disease resistance. Thus these responses als o can be activated via an NPR1-independent pathway that currently remain to be elucidated. The ssi2-1 mutant, identified in a genetic screen for suppr essors of npr1-5, affects signaling through the NPR1-independent defense pa thway(s). In comparison with the wild-type (SSI2 NPR1) plants and the npr1- 5 mutant (SSI2 npr1-5), the ssi2-1 npr1-5 double mutant and the ssi2-1 NPR1 single mutant constitutively express PR genes [PR-1, BGL2 (PR-2) and PR-5] ; accumulate elevated levels of SA; spontaneously develop lesions; and poss ess enhanced resistance to a virulent strain of Peronospora parasitica. The ssi2-1 mutation also confers enhanced resistance to Pseudomonas syringae p v. tomato (Psf); however, this is accomplished primarily via an NPR1-depend ent pathway. Analysis of ssi2-1 NPR1 nahG and ssi2-1 npr1-5 nahG plants rev ealed that elevated SA levels were not essential for the ssi2-1-conferred p henotypes. However, expression of the nahG transgene did reduce the intensi ty of some ssi2-1-conferred phenotypes, including PR-1 expression, and dise ase resistance. Based on these results, SSI2 or an SSI2-generated signal ap pears to modulate signaling of an SA-dependent, NPR1-independent defense pa thway, or an SA- and NPR1-independent defense pathway.