ANALYSIS OF RESISTANCE GENE-MEDIATED DEFENSE RESPONSES IN ARABIDOPSIS-THALIANA PLANTS CARRYING A MUTATION IN CPR5

In resistant plants, pathogen attack often leads to rapid activation o f defense responses that limit multiplication and spread of the pathog en. To investigate the signaling mechanisms underlying this process, m e carried out a screen for mutants in the signaling pathway governing resistance in Arabidopsis thaliana to the bacterial pathogen Pseudomon as syringae. This involved screening for suppressor mutations that res tored resistance to a susceptible line carrying a mutation in the RPS2 resistance gene. A mutant that conferred resistance by activating def ense responses in the absence of pathogens was isolated. This mutant, which carries a mutation at the CPR5 locus and was thus designated cpr 5-2, exhibited resistance to P. syringae, spontaneous development of n ecrotic lesions, elevated PR gene expression in the absence of pathoge ns, and abnormal trichomes. Resistance gene-mediated defenses, includi ng the hypersensitive response, restriction of pathogen growth, and in duction of defense-related gene expression, were functional in cpr5-2 mutant plants. Additionally, in cpr5-2 plants RPS2-mediated induction of PR-1 expression was enhanced, whereas RPM1-mediated induction of EL 13 was not. These findings suggest that CPR5 encodes a negative regula tor of the RPS2 signal transduction pathway.