SYSTEMIC RESPONSES IN ARABIDOPSIS-THALIANA INFECTED AND CHALLENGED WITH PSEUDOMONAS-SYRINGAE PV SYRINGAE

Attack of plants by necrotizing pathogens leads to acquired resistance to the same or other pathogens in tissues adjacent to or remotely loc ated from the site of initial attack. We have used Arabidopsis thalian a inoculated with the incompatible pathogen Pseudomonas syringae pv sy ringae on the lower leaves to test the induction of systemic reactions . When plants were challenged with Pseudomonas syringae pv syringae in the upper leaves, bacterial titers remained stable in those preinfect ed on the lower leaves. However, there was a distinct decrease in symp toms that correlated with a local and systemic increase in salicylic a cid (SA) and in chitinase activity. Peroxidase activity only increased at the site of infection. No changes in catalase activity were observ ed, either at the local or at the systemic level. No inhibition of cat alase could be detected in tissue in which the endogenous levels of SA were elevated either naturally (after infection) or artificially (aft er feeding SA to the roots). The activity of catalase in homogenates o f A. thaliana leaves could not be inhibited in vitro by SA. SA accumul ation was induced by H2O2 in leaves, suggesting a link between H2O2 fr om the oxidative burst commonly observed during the hypersensitive rea ction and the induction of a putative signaling molecule leading to sy stem acquired resistance.