Nitrate assimilation in higher plants: the effects of metabolites and light

Current aspects of the enzymes involved in the assimilation of nitrate and ammonium ions are described. The three initial reactions, the uptake and the two reductases (nitrate and nitrite reductase) are turned on in root and leaf tissues by nitrate additions to the medium. In root tissues an isoform of ferredoxin glutamate synthase and of glutamine synthetase are also induced by nitrate. cDNA clones of the enzymes nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase and glutamate dehydrogenase have been reported and regions of the promoter of the nitrite reductase gene which respond to nitrate and light have been defined. Asparagine and glutamine repress the induction of the two reductases and the uptake system but are without effect on protein levels of the other enzymes involved in the assimilation process.