Nitrate Reductase Regulates Plant Nitric Oxide Homeostasis
Nitrate reductase (NR) is a key enzyme for nitrogen acquisition by plants, algae, yeasts, and fungi. Nitrate, its main substrate, is required for signaling and is widely distributed in diverse tissues in plants. In addition, NR has been proposed as an important enzymatic source of nitric oxide (NO). Recently, NR has been shown to play a role in NO homeostasis by supplying electrons from NAD(P)H through its diaphorase/dehydrogenase domain both to a truncated hemoglobin THB1, which scavenges NO by its dioxygenase activity, and to the molybdoenzyme NO-forming nitrite reductase (NOFNiR) that is responsible for NO synthesis from nitrite. We review how NR may play a central role in plant biology by controlling the amounts of NO, a key signaling molecule in plant cells. Trends NO synthesis remains a complex picture with many unresolved questions. NR has been assumed to be the main enzymatic source, but a new and more complex picture for the mechanism of NO synthesis is emerging. NR, the first enzyme for nitrate assimilation, is a multi-redox protein able to mediate the donation of electrons from NAD(P)H to artificial acceptors and redox proteins. In Chlamydomonas , two of these redox partners are NOFNiR, which efficiently synthesizes NO, and THB1, a truncated hemoglobin, which eliminates NO by its dioxygenase activity. Homeostasis of the crucial signaling molecule NO in photosynthetic organisms depends on at least two key molybdoenzymes, NR and NOFNiR, as well as on the dioxygenase activity of hemoglobins.
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