The functions of nitric oxide-mediated signaling and changes in gene expression during the hypersensitive response

Nitric oxide (NO) is a highly reactive molecule that rapidly diffuses and permeates cell membranes. In animals, NO is implicated in a number of diverse physiological processes, such as neurotransmission, vascular smooth muscle relaxation, and platelet inhibition. It may have beneficial effects, e.g., as a messenger in immune responses, but it is also potentially toxic when the antioxidant system is overwhelmed and reactive oxygen intermediates (ROI) accumulate. During the last few years, NO has been detected in several plant species, and an increasing number of reports on its function have implicated NO as an important effector in plant growth, development, and defense. The broad chemistry of NO involves an array of interrelated redox forms with different chemical reactivities and numerous potential biological targets in plants. NO signaling functions depend on its reactivity. ROI are key modulators of NO in triggering cell death, but the nature of the mechanisms by which this occurs in plants is different from those commonly observed in animals. This review focuses on the signaling functions of NO, when channeled through the cell death pathway by ROI.