Nitrate reductase in higher plants: A case study for transduction of environmental stimuli into control of catalytic activity

In higher plants, cytosolic NAD(P)H-nitrate reductase (NR) is rapidly modul ated by environmental conditions such as Light, CO2, or oxygen availability . In lea,es, NR is activated by photosynthesis, reaching an activation stat e of 60-80%. In the dark, or after stomatal closure, leaf NR is inactivated down to 20 or 40% of its maximum activity. In roots, hypoxia or anoxia act ivate NR, whereas high oxygen supply inactivates IVR. Spinach leaf NR is in activated by phosphorylation of serine 543 and subsequent Mg2+-dependent bi nding of 14-3-3 proteins at, or close to, this phosphorylation site. At lea st three different protein kinases (NR-PK) have been identified in spinach leaves that are able to phosphorylate NR on serine 543. Two of them show up as calmodulin-like domain protein kinases (CDPKs), and one as a SNF1-like protein kinase. Dephosphorylation of serine 543 is catalyzed by a Mg2+-depe ndent protein phosphatase and by a type 2A protein phosphatase (NR-PP), whi ch is regulated by a trimer/dimer interconversion. The NR-PKs, NR-PPs, and 14-3-3s are present el en in NR-depleted plant tissues. Artificial activati on of NR in vivo is achieved by cellular acidification, by respiratory inhi bitors, or by mannose feeding. As for anoxia, these treatments seem to act, at least in part, via cytosolic acidification, mediated by low cytosolic.- ITP levels. Activation is also achieved by ionophore-induced release of div alent cations from the cytosol. In addition, cytosolic AMP and phosphate es ters seem to regulate NR-PK and IVR-PP activities, thereby adapting NR acti vity within minutes to the changing environment.