1Laboratoire Interactions Plantes-Pathogènes, UMR 217 INRA/INA P-G/Université Paris 6 and CNRS, 16 rue Claude Bernard, Paris 75005, France; 2Unité de Microbiologie et Génétique, UMR 5122 CNRS/INSA/UCB, Université Lyon 1, F-69622 Villeurbanne cedex, France
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Accepted 7 February 2007.
Many taxonomically diverse plant species are attacked by Erwinia chrysanthemi, a member of the causal agents of soft-rotting diseases. Symptom development is due to the collective action of pectin-degrading enzymes secreted by the bacterium through a type II secretion system (T2SS). Using Arabidopsis thaliana as a susceptible host, we show that plants respond to E. chrysanthemi 3937 by expressing cell-wall reactions, production of an oxidative burst, and activation of salicylic acid (SA) and jasmonic acid (JA) or ethylene (ET) signaling pathways. We found that the oxidative burst is mainly generated via the expression of the AtrbohD gene, constitutes a barrier of resistance to bacterial attack, and acts independently of the SA-mediated response. To determine the importance of T2SS-secreted proteins in elicitation of these defenses, we used a T2SS deficient mutant and purified enzymatic preparations of representative members of strain 3937 pectate lyase activity. The T2SS-secreted proteins were responsible only partially for the activation of SA and JA or ET signaling pathways observed after infection with the wild-type bacterium and were not involved in the expression of other identified defense reactions. Our study shows the differential role played by pectate lyases isoenzymes in this process and highlights the complexity of the host immune network, which is finely controlled by the bacterium.
© 2007 The American Phytopathological Society