Submitted to: Book Chapter
Publication Type: Book / chapter
Publication Acceptance Date: 8/1/2010
Publication Date: 2/1/2011
Citation: Huber, S.C., Kaiser, W.M., Jain, V. 2011. Post-translational regulation of nitrate reductase. In: Jain, Vanita, Kumar, P. Ananda, editors. Nitrogen Use Efficiency in Plants. New Delhi, India: New India Publishing Agency. p. 21-44. Interpretive Summary: Most plants receive the majority of the nitrogen needed for growth by taking up nitrate from the soil water. In order to be used in plant growth, the nitrate must first be reduced to nitrite, followed by reduction to ammonium and incorporation into amino acids. The first step in this process involves the enzyme, nitrate reductase. Recent results indicate that nitrate reductase is also a major source of the signaling molecule, nitric oxide that regulates many developmental and physiological processes. Consequently, there is resurgence in interest in mechanisms that regulate the amount and activity of nitrate reductase in plant cells. Control of nitrate reductase by protein phosphorylation has been known for some time, and recent work suggests that this mechanism may be modulated by reactive oxygen species such as peroxide. These novel interactions may explain how nitrate reductase is activated when plants are subjected to root flooding, which is commonly encountered in the field.
Technical Abstract: Nitrate reductase (NR) catalyzes the reduction of nitrate to nitrite, which is the first step in the nitrate assimilation pathway, but can also reduce nitrite to nitric oxide (NO), an important signaling molecule that is thought to mediate a wide array of of developmental and physiological processes in plants. Accordingly, the amount of NR protein and its catalytic activity are closely controlled in the cell. This chapter focuses on the posttranslational regulation of NR activity, which has been known for some time to involve reversible protein phosphorylation. The hinge 1 region of NR contains the serine-534 regulatory site, which when phosphorylated can bind a 14-3-3 protein resulting in inhibition of NR activity. This fundamental mechanism underlies the rapid modulation of NR activity in leaves in response to light/dark transitions, and the activation of NR that occurs in response to hypoxia (e.g., as occurs with soil flooding). Recent results suggest that the basis for hypoxia-induced NR activation, which is thought to be essential for plant survival, may involve reactive oxygen species (ROS), such as H2O2, that are produced in response to many plant stressors including hypoxia. Exogenous ROS can increase total NR protein as well as promoting the dephosphorylation/activation of existing NR. The latter effect may involve oxidation of a methionine residue (Met-538) that is located within the Ser-534 phosphorylation motif (“methionine oxidation redox switch” mechanism). Plants over expressing an enzyme involved in reversing methionine oxidation were previously shown to have increased steady-state levels of NR-Ser534 phosphorylation in the dark, suggesting that some Met-538 oxidation may occur even under ‘normal’ conditions and may attenuate NR phosphorylation. We now show that these transgenic plants also show increased shoot growth, in particular at super optimal temperatures (28°C). Finally, we discuss an indirect link between NR activity in roots and degradation of leaf proteins. It is known that soil nitrate availability directly and indirectly impacts cytokinin production in roots for transport to shoots. We recently demonstrated that supply of cytokinin leaves played an important role in preventing protein degradation; disruption of cytokinin supply resulted in protein degradation and proteins that had been carbonylated (oxidatively modified) were preferentially lost. These results have implications for whole plant responses to changes in soil N-availability and are at least indirectly related to root NR activity. The critical role of NR in NO production in plants has renewed interest in understanding the mechanisms involved in control of NR protein and its catalytic activity.