LIGHT REGULATION OF NITRATE REDUCTASE IN GREEN LEAVES OF HIGHER-PLANTS

Light stimulates de novo synthesis, as well as activation of higher plant nitrate reductase at the protein level. In green leaves, the light effects are mediated through photosynthetically active light by products of the Calvin cycle. A daily, light-induced increase in nitrate reductase (NR) mRNA, protein, and activity is observed in various higher plants. Glucose or sucrose can replace light in eliciting the increase in nitrate reductase mRNA accumulation, and some Metabolite from nitrate assimilation exerts a negative feedback on transcription of the nitrate reductase genes. The positive feed-forward induced by sugars, and the negative feedback from nitrogen compound(s) apparently result in the circadian rhythms of NR mRNA observed in various plants. Nitrate reductase shows hysteretic behaviour, converting between a low and a high activity form in response to NAD(P)H. The low activity form is inhibited by calcium/magnesium, as opposed to the high activity form that is not inhibited. Examination of nitrate reductase activity in crude extracts made from plants before and after a light-dark transition suggests the existence of two different forms of nitrate reductase; a 'light form' with a pH optimum of 7.8 in potassium phosphate buffer that is not inhibited by calcium or magnesium, and a 'dark form' with a pH optimum of 7.5 that is strongly inhibited by calcium or magnesium. It is postulated that a transient lack of NADH as a result of decreased illumination can convert nitrate reductase into the less active form in situ.