The role of alternative oxidase in the maintenance of cellular redox balance under hypoxia via participation in nitric oxide turnover.

Alternative oxidase (AOX) regulates the level of reactive oxygen species and nitric oxide (NO) in plants. While it alleviates NO formation under normoxic conditions, there are several indications that in conditions of low oxygen, such as during seed germination before radicle protrusion, in meristematic stem cells, and in flooded roots, AOX can be involved in the production of NO from nitrite. Whereas the first reports considered this role as indirect, more evidence has since accumulated that AOX can act as a nitrite:NO reductase. Such activity of the structurally similar di-iron proteins in bacteria has been demonstrated. We review the literature on this topic and show that AOX can be induced under hypoxic conditions and participate in NO turnover via the phytoglobin-NO cycle. This results in the facilitation of glycolytic reactions by reoxidation of the glycolytically formed NADH and diverting the glycolytic carbon toward the formation of alanine and other amino acids. Pyruvate formed in glycolysis can activate AOX and facilitate its operation under these conditions. It is concluded that AOX is an important player in the hypoxic response in plants that regulates the redox level by participating in NO turnover as a nitrite:NO reductase in cooperation with nitrate reductase and phytoglobin.