1.2.6S
NITRIC OXIDE AS A SIGNAL WITH REACTIVE OXYGEN INTERMEDIATES IN THE ACTIVATION OF DISEASE RESISTANCE MECHANISMS

M Delledonne1, Y Xia1, RA Dixon2 and C LAMB1

1The Salk Institue, La Jolla, California 92037 USA; 2The Noble Foundation, Ardmore, Oklahoma 73402 USA

The rapid generation of reactive oxygen intermediates in response to pathogen avirulence factors drives cell wall cross-linking and signals the activation of sets of defense genes and the initiation of hypersensitive cell death.
However, quantitative analysis of the level of cell death induced by activation of an oxidative burst in the absence of pathogen, e.g. by the synergistic interaction between salicylic acid and cantharidin, relative to the level of H2O2 that accumulates, suggests the operation of a co-signal in the response to avirulent pathogens. Here we show that nitric oxide (NO) is required together with H2O2 in the activation of disease resistance mechanisms. In gain-of-function experiments, defense gene activation and cell death in soybean cell suspension cultures can be induced by NO generators, but only in the presence of reactive oxygen intermediates. In loss-of function experiments, NO scavengers and inhibitors of NO production block the induction of hypersensitive cell death and defense genes by an avirulent strain of Pseudomonas syringae. Moreover, in the absence of NO synthesis, these responses to the avirulent pathogen can be rescued by exogenous NO. NO production is induced by the avriulent strain of P. syringae, whereas the isogenic virulent strain has a much weaker effect. Similar gain- and loss-of-function experiments confirm the function of NO as a signal in the hypersensitive reponse in planta in the interaction between Arabidopsis and P. syringae. Our data disclose a binary signal system involving divergent pathways for the generation of reactive oxygen intermediates and NO, which then function together in defense induction.