Nitrate metabolism in plants under hypoxic and anoxic conditions

Plants are frequently exposed to a variety of external conditions able to affect their growth, development and productivity. Their ability to adapt and live in a changing environment relies on tolerance or resistance to adverse growing seasons. Oxygen limitation is the major abiotic stress in flooded soils. In the open field, during winter or the raining season in poorly drained soils, plants can experience waterlogging, an event causing scarce root oxygenation and leading to a hypoxic stress. The metabolism of plants undergoes deep modifications in order to minimize energy losses, the most important changes concerning of course glycolysis. With the reduction of oxygen availability specific anaerobic enzymes become activated and their transcripts are over-expressed. Fermentation is induced and potentially toxic metabolites like ethanol are accumulated into anoxic tissues. Among the metabolic pathways strongly affected by oxygen deficiency stands also nitrate metabolism. The enzymes involved in nitrate assimilation and reduction are up-regulated during oxygen deficiency and some authors showed several correlations between nitrate availability and plant survival to flooding. Moreover nitrites and nitric oxide seem to be directly involved in plant survival at least during the first hours after the onset of hypoxia/anoxia. Intracellular nitrates and nitrites concentrations may play an effective role in maintaining cellular pH homeostasis, limiting cytoplasmic acidosis deriving from fermentation and thus contributing to survival. This mini-review is intended to summarize and discuss the nitrate metabolism under limited oxygen conditions with brief introduction of the other metabolic pathways that interact with nitrate assimilation.