Recognition of an avirulent pathogen triggers the rapid production of the reactive oxygen intermediates superoxide (O2−) and hydrogen peroxide (H2O2)1. This oxidative burst drives cross-linking of the cell wall2, induces several plant genes involved in cellular protection and defence3,4, and is necessary for the initiation of host cell death in the hypersensitive disease-resistance response1,3. However, this burst is not enough to support a strong disease-resistance response4,5. Here we show that nitric oxide, which acts as a signal in the immune, nervous and vascular systems6, potentiates the induction of hypersensitive cell death in soybean cells by reactive oxygen intermediates and functions independently of such intermediates to induce genes for the synthesis of protective natural products. Moreover, inhibitors of nitric oxide synthesis compromise the hypersensitive disease-resistance response of Arabidopsis leaves to Pseudomonas syringae, promoting disease and bacterial growth. We conclude that nitric oxide plays a key role in disease resistance in plants.
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