Photosynthetic post-translational activation of nitrate reductase

Experiments with barley (Hordeum vulgare) leaf segments were undertaken to elucidate the signal transduction chain linking phocosynthesis in the chlor oplasts and the NR activating system in the cytosol where NR (EC 1.6.6.1) i s located. Fructose-1,6-bisphosphate phosphatase (FBAse) was used as a mark er enzyme to reveal the reduction state of the ferredoxin pool, which possi bly could be a link in the signal transduction chain. Photosynthesis in the presence of NaHCO3 or inhibition of the Calvin cycle by DL-glyceraldehyde resulted in high activity of FBAse as well as NR. However, other treatments , i.e. illumination in the absence of NaHCO3, activated FBAse only; therefo re, a reduced ferredoxin pool was not sufficient for triggering the activat ion of NR. Inhibitors known to induce an oxidized or reduced plascoquinone pool, respectively, both induced a low NR activity state, showing that diff erent redox states of the plastoquinone pool did not alter me NR activity s tate. Infiltration of sucrose or glucose into the leaf slices resulted in a ctivation of NR in the dark, indicating that sugars or sugar derivatives co uld be a link between photosynthesis and NR activation. This was also suppo rted by testing a maize (Zea mays) mutant lacking Calvin cycle activity. In this mutant, light activation of NR was absent. On the other hand high Cal vin cycle activity was not always necessary for light activation, because i n the presence of DL-glyceraldhyde CO2-fixation was reduced by 97%; however , NR was still activated. Side effects of DL-glyceraldehyde may mask the ef fect of Calvin cycle inhibition. The ambivalent evidence for involvement of the Calvin cycle in NR activation points to the complexity of the activati ng system and likely existence of multiple signal transduction pathways inf luencing the NR activity state.