Post-translational regulation of nitrate reductase: mechanism, physiological relevance and environmental triggers

Assimilatory nitrate reductase (NR) of higher plants is a most interesting enzyme, both from its central function in plant primary metabolism and from the complex regulation of its expression and control of catalytic activity and degradation. Here, present knowledge about the mechanism of post-trans lational regulation of NR is summarized and the properties of the regulator y enzymes involved (protein kinases protein phosphatases and 14-3-3-binding proteins) are described. It is shown that light and oxygen availability ar e the major external triggers for the rapid and reversible modulation of NR activity, and that sugars and/or sugar phosphates are the internal signals which regulate the protein kinase(s) and phosphatase. It is also demonstra ted that stress factors like nitrate deficiency and salinity have remarkabl y little direct influence on the NR activation state. Further, changes in N R activity measured in vitro are not always associated with changes in nitr ate reduction rates in vivo, suggesting that NR can be under strong substra te limitation. The degradation and half-life of the NR protein also appear to be affected by NR phosphorylation and 14-3-3 binding, as NR activation a lways correlates positively with its stability. However, it is not known wh ether the molecular form of NR in vivo affects its susceptibility to proteo lytic degradation, or whether factors that affect the NR activation state a lso independently affect the activity or induction of the NR protease(s). A second and potentially important function of NR, the production of nitric oxide (NO) from nitrite is briefly described, but it remains to be determin ed whether NR produces NO for pathogen/stress signalling in vivo.