Biosynthesis of Nitric Oxide in Plants

Abstract

Nitric oxide (NO) regulates important events in plant physiology, disease resistance and stress tolerance. In plants, distinct enzymatic and chemical processes can generate NO from nitrite (\( {\text{NO}}_{2}^{ - } \)), l-Arginine and possibly other N-compounds. Reduction of \( {\text{NO}}_{2}^{ - } \) to NO is catalyzed by nitrate reductase and the mitochondrial electron transport chain. Deoxygenated heme-proteins also facilitate NO production from \( {\text{NO}}_{2}^{ - } \). NO may also be released in nonenzymatic processes from nitrous acid and S-nitrosoglutathione. Whether plants have a specific enzyme with primary oxidative NO synthesizing activity is an open debate. Although, NO synthase-homolog genes are present in green algae, and a protein (AtNOS1/AtNOA1) with regulatory effects on oxidative NO synthesis is known in vascular plants, integration of the multiple NO producing processes requires a complex regulatory network in the plant cell. However, our insight into the underlying molecular mechanisms is still limited. Plant hormones, stress and injury signals, modulation of intracellular Ca2+ levels are the potential drivers of plant NO synthesis under physiological and stress conditions.