Biosynthesis of nitric oxide in plants: An oxidative pathway orchestrated by the interplay of CYP79s, flavin-containing monooxygenases, and peroxidases.

Nitric oxide (NO) is in the pantheon of plant signal molecules and hormones controlling plant growth, development, and adaptation to environmental challenges. The route of NO biosynthesis in plants has remained enigmatic. Previous studies have shown the ability of peroxidases to utilize oximes for the production of NO. Peroxidases are widely spread and highly expressed in plant tissues. What then is the identity of the pathway signature enzymes determining tight, spatiotemporal regulation of NO production to effectuate its specific signal functions? What are the key selection criteria to be fulfilled for genes and enzymes operating at the global level in an oxidative pathway for NO production in plants? Convergently evolved CYP79s and N-OX flavin-containing monooxygenases (N-OX FMOs) catalyze the conversion of different amino acids into oximes. In this Perspective, we delineate how these oxygenases fine-tune the spatiotemporal formation of the oximes as committed substrates for peroxidase-catalyzed NO production. Based on the spatiotemporal location of CYP79s and N-OX FMOs present in a specific plant species, NO formation in its different meristematic tissues is catalyzed by CYP79s, N-OX FMOs, or by their operation in conjunction. The oxime-based NO production is accompanied by the formation of stoichiometric amounts of a diagnostic specific aldehyde detectable by gas-liquid chromatography/high pressure liquid chromatography-mass spectrometry. When oximes derived from tryptophan, tyrosine, or phenylalanine are substrates for NO production, the different aldehydes formed may be oxidized to auxins. The outlined oxidative route for NO production in plants explains observations that were difficult to interpret in previous plant signal and hormone studies. FMOs may also contribute to NO formation in animals.