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

Abstract

In higher plants, cytosolic NAD(P)H‐nitrate reductase (NR) is rapidly modulated by environmental conditions such as light, CO2, or oxygen availability. In leaves, NR is activated by photosynthesis, reaching an activation state 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 activate NR, whereas high oxygen supply inactivates NR. Spinach leaf NR is inactivated by phosphorylation of serine 543 and subsequent Mg2+‐dependent binding of 14‐3‐3 proteins at, or close to, this phosphorylation site. At least 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+‐dependent protein phosphatase and by a type 2A protein phosphatase (NR‐PP), which is regulated by a trimer/dimer interconversion. The NR‐PKs, NR‐PPs, and 14‐3‐3s are present even in NR‐depleted plant tissues. Artificial activation of NR in vivo is achieved by cellular acidification, by respiratory inhibitors, or by mannose feeding. As for anoxia, these treatments seem to act, at least in part, via cytosolic acidification, mediated by low cytosolic ATP levels. Activation is also achieved by ionophore‐induced release of divalent cations from the cytosol. In addition, cytosolic AMP and phosphate esters seem to regulate NR‐PK and NR‐PP activities, thereby adapting NR activity within minutes to the changing environment.

Interaction of Sulfate Assimilation with Carbon and Nitrogen Metabolism in Lemna minor

“…It seems that the carbon sources effective in reactivation of NR must enter the carbohydrate metabolism and be able to replenish the hexose pools (Fig. 3) or to increase the content of sugar phosphates, which are known to protect NR against dark inactivation (De Cires et al, 1993) most probably due to the inactivation of the NR protein kinase (Kaiser et al, 1999). Not all carbon sources were able to restore NR activity after the treatment in air without CO 2 , as demonstrated by lack of effect after addition of OAS (Figs.…”

An analytical model of non‐photorespiratory CO2 release in the light and dark in leaves of C3 species based on stoichiometric flux balance

“…One component, in particular nitrate assimilation, is a major sink for reductant in the light (Bloom et al 1989), and nitrate reductase activity is strongly enhanced by light both at the transcriptional and translational levels (Kaiser, Weiner & Huber 1999;Sherameti et al 2002;Stitt et al 2002). Nitrogen assimilation is also suppressed by a reduction in carbohydrate status (Fritz et al 2006), and therefore by the duration of the dark period.…”

Nitrite–dependent nitric oxide production pathway: implications for involvement of active nitrogen species in photoinhibitionin vivo

“…NR seems to play a role similar to the D 1 protein. Recent studies have indicated that NR is highly regulated by complex transcriptional and posttranslational mechanisms (Campbell 1999;Huber et al 1996;Kaiser et al 1999). The NR enzyme is degraded after several hours in the dark (Kelker & Filner 1971).…”