SHORT-TERM MODULATION OF NITRATE REDUCTASE-ACTIVITY BY EXOGENOUS NITRATE IN NICOTIANA-PLUMBAGINIFOLIA AND ZEA-MAYS LEAVES

Maize (Zea mays L.) grown on low (0.8 mM) NO3-, as well as untransform ed and transformed Nicotiana plumbaginifolia constitutively expressing nitrate reductase (NR), was used to study the effects of NO3- on the NR activation state. The NR activation state was determined from the r elationship of total activity extracted in the presence of ethylenedia minetetracetic acid to that extracted in the presence of Mg2+. Light a ctivation was observed in both maize and tobacco leaves. In the tobacc o lines, NO3- did not influence the NR activation state. In excised ma ize leaves, no correlation was found between the foliar NO3- content a nd the NR activation state. Similarly, the NR activation state did not respond to NO3-. Since the NR activation state determined from the de gree of Mg2+-induced inhibition of NR activity is considered to reflec t the phosphorylation state of the NR protein, the protein phosphatase inhibitor microcystin LR was used to test the importance of protein p hosphorylation in the NO3--induced changes in NR activity. In-vivo inh ibition of endogenous protein phosphatase activity by microcystin-LR d ecreased the level of NR activation in the light. This occurred to the same extent in the presence or absence of exogenous NO3-. We conclude that NO3- does not effect the NR activation state, as modulated by pr otein phosphorylation in either tobacco (a C-3 species) or maize (a C- 4 species). The short-term regulation of NR therefore differs from the NO3--mediated responses observed for phosphoenolpyruvate carboxylase and sucrose phosphate synthase.