Nitrate Reductase Regulates Plant Nitric Oxide Homeostasis

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. 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. 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. a process mediated by microorganisms, at very low oxygen concentrations, in which oxidized forms of nitrogen are used as the terminal electron acceptors (nitrate, nitrite, nitric oxide, nitrous oxide) to produce dinitrogen gas. in enzymology these terms refer to enzymes that are able to reduce acceptor molecules with electrons from reduced pyridine nucleotides, NADH or NADPH. These enzymes bear flavins as prosthetic groups. an enzyme able to activate dioxygen and incorporate both atoms of oxygen into the products. a regulated process that maintains the stability of a system, compensating for changes occurring in its environment through dynamic balances. this domain is present in diverse metal–sulfur cluster biosynthesis proteins including molybdenum cofactor sulfurases. a protein bearing diverse structural or functional units (domains). These domains contribute to the overall function of the protein, and can be found in other proteins with different functions. the introduction of a nitro (-NO2) group into an organic molecule. Proteins are usually nitrated into their tyrosine amino acid residues. the incorporation of nitric oxide (NO) into an organic molecule. Proteins are nitrosylated by reaction of NO and thiol groups of protein cysteines. a covalent and generally enzyme-catalyzed process occurring after protein biosynthesis. The modification can be very diverse with respect to the functional group introduced (phosphate, acetate, …) and the residue modified (amino acid side-chains or the C- or N-termini of the protein. cofactors (organic or metallo-organic) that are tightly or covalently and permanently bound to an enzyme. a conserved part of a protein whose sequence and function can evolve and exist independently of the remainder of the protein.