Abstract
We employed a chemical biology approach to investigate Arabidopsis R-gene mediated defense responses to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 (avrRpm1). A high-throughput screening method for HR cell death, using suspension cells, was established and seven compounds that enhance HR were identified from a diverse chemical library. CB_6, one of the isolated potentiators, induced PR1 gene expression in Arabidopsis seedlings, independently of salicylic acid (SA) biosynthesis. CB_6 did not affect endogenous SA levels, suggesting that CB_6 is a partial SA agonist which is not involved in positive feedback regulation of SA. Agonists can be powerful tools for the identification of signaling molecules, as was shown recently by the identification of an abscisic acid receptor, and we therefore started genetic and biochemical analyses using CB_6 to explore SA downstream factors. Twenty eight Arabidopsis sgi (SA agonist insensitive) mutants that exhibited insensitivity to CB_6 were isolated and classified into two groups based on their sensitivity. Four of the CB_6 weak insensitive mutants showed hypersensitivity when grown on media containing 400mM SA, similar to npr1.
