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

Plant J. 2001 Mar;25(5):563-74. doi: 10.1046/j.1365-313x.2001.00992.x.


The Arabidopsis thaliana NPR1 gene is required for salicylic acid (SA)-induced expression of pathogenesis-related (PR) genes and systemic acquired resistance. However, loss-of-function mutations in NPR1 do not confer complete loss of PR gene expression or disease resistance. Thus these responses also can be activated via an NPR1-independent pathway that currently remain to be elucidated. The ssi2-1 mutant, identified in a genetic screen for suppressors of npr1-5, affects signaling through the NPR1-independent defense pathway(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 possess enhanced resistance to a virulent strain of Peronospora parasitica. The ssi2-1 mutation also confers enhanced resistance to Pseudomonas syringae pv. tomato (Pst); however, this is accomplished primarily via an NPR1-dependent pathway. Analysis of ssi2-1 NPR1 nahG and ssi2-1 npr1-5 nahG plants revealed that elevated SA levels were not essential for the ssi2-1-conferred phenotypes. However, expression of the nahG transgene did reduce the intensity of some ssi2-1-conferred phenotypes, including PR-1 expression, and disease resistance. Based on these results, SSI2 or an SSI2-generated signal appears to modulate signaling of an SA-dependent, NPR1-independent defense pathway, or an SA- and NPR1-independent defense pathway.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Arabidopsis / genetics*
  • Arabidopsis / microbiology
  • Arabidopsis Proteins*
  • Blotting, Northern
  • Gene Expression Regulation, Plant
  • Immunity, Innate / genetics
  • Mutation
  • Oomycetes / pathogenicity*
  • Phenotype
  • Plant Diseases / genetics*
  • Plant Proteins / genetics*
  • Plant Proteins / metabolism
  • Pseudomonas / pathogenicity*
  • RNA, Plant / analysis
  • Salicylic Acid / metabolism*
  • Virulence


  • Arabidopsis Proteins
  • NPR1 protein, Arabidopsis
  • Plant Proteins
  • RNA, Plant
  • Salicylic Acid