Nitrate reductase modulation in response to changes in C/N balance and nitrogen source in Arabidopsis

Environmental cues modulate carbon (C) and nitrogen (N) balance which are essential elements for plant metabolism and growth. In Arabidopsis, photochemical efficiency of PSII, phosphorylation status and localization of many enzymes, and total soluble sugars level resulted affected by unbalanced C/N ratio. Since differences in C/N affect these parameters, here we checked whether different sources of N have different effects when high C/N ratio was imposed. NO3- and NH4+ were separately provided in C/N medium. We investigated the effects on photochemical efficiency of PSII, total soluble sugars level and nitrate reductase activity under stressful C/N condition compared with control condition. We found that treated plants accumulated more total soluble sugars when compared with control. Photochemical efficiency of PSII did not show significant differences between the two sources of nitrogen after 24 h. Nitrate reductase actual activity was the result of a combination among activity, activation state and protein level. This activity constantly decreased starting from time zero in control condition; by contrast, nitrate reductase actual activity showed a peak at 2 h after treatment with NO3- , and at 30 min with NH4+. This, according with the total soluble sugars results, can be explained by the existence of a crosstalk between the sugars in excess and low nitrate in the medium, that blocks the activity of nitrate reductase in stressful sugar condition until the plant is adapted to the stress.