Signal interactions between nitric oxide and reactive oxygen intermediates in the plant hypersensitive disease resistance response

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Abstract
Nitric oxide (NO) and reactive oxygen intermediates (ROIs) play key roles in the activation of disease resistance mechanisms both in animals and plants. In animals NO cooperates with ROIs to kill tumor cells and for macrophage killing of bacteria. Such cytotoxic events occur because unregulated NO levels drive a diffusion-limited reaction with O(2)(-) to generate peroxynitrite (ONOO(-)), a mediator of cellular injury in many biological systems. Here we show that in soybean cells unregulated NO production at the onset of a pathogen-induced hypersensitive response (HR) is not sufficient to activate hypersensitive cell death. The HR is triggered only by balanced production of NO and ROIs. Moreover, hypersensitive cell death is activated after interaction of NO not with O(2)- but with H(2)O(2) generated from O(2)(-) by superoxide dismutase. Increasing the level of O(2)(-) reduces NO-mediated toxicity, and ONOO(-) is not a mediator of hypersensitive cell death. During the HR, superoxide dismutase accelerates O(2)(-) dismutation to H(2)O(2) to minimize the loss of NO by reaction with O(2)(-) and to trigger hypersensitive cell death through NO/H(2)O(2) cooperation. However, O(2)(-) rather than H(2)O(2) is the primary ROI signal for pathogen induction of glutathione S-transferase, and the rates of production and dismutation of O(2)(-) generated during the oxidative burst play a crucial role in the modulation and integration of NO/H(2)O(2) signaling in the HR. Thus although plants and animals use a similar repertoire of signals in disease resistance, ROIs and NO are deployed in strikingly different ways to trigger host cell death.