The Arabidopsis constitutive induced resistance 1 (cir1) mutant displays salicylic acid (SA)-dependent constitutive expression
of defence genes and enhanced resistance to biotrophic pathogens. To further characterise the role of CIR1 in plant
immunity we conducted epistasis analyses with two key components of the SA-signalling branch of the defence network,
ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) and PHYTOALEXIN DEFICIENT4 (PAD4). We demonstrate that the constitutive
defence phenotypes of cir1 require both EDS1 and PAD4, indicating that CIR1 lies upstream of the EDS1-PAD4 regulatory
node in the immune signalling network. In light of this finding we examined EDS1 expression in cir1 and observed increased
protein, but not mRNA levels in this mutant, suggesting that CIR1 might act as a negative regulator of EDS1 via a posttranscriptional
mechanism. Finally, as environmental temperature is known to influence the outcome of plant-pathogen
interactions, we analysed cir1 plants grown at 18, 22 or 25uC. We found that susceptibility to Pseudomonas syringae pv.
tomato (Pst) DC3000 is modulated by temperature in cir1. Greatest resistance to this pathogen (relative to PR-1:LUC control
plants) was observed at 18uC, while at 25uC no difference in susceptibility between cir1 and control plants was apparent.
The increase in resistance to Pst DC3000 at 18uC correlated with a stunted growth phenotype, suggesting that activation of
defence responses may be enhanced at lower temperatures in the cir1 mutant.