Light-mediated regulation of nitrate reductase in higher plants

Regulation of nitrate reductase (NR) by light is a complex process and is manifested through gene expression, both at the level of transcription and translation, covalent modification of the enzyme and supply of reductant. Light induces nia gene transcription by some still unknown mechanism. Light induction of nia mRNA is also mediated via phytochrome. By using protein synthesis inhibitors it has been observed that light enhances translation of NR mRNA. Light induction of NR synthesis follows the circadian rhythmicity. Cytokinin increases NR activity by regulating NR expression at the transcriptional level. In response to light-dark transition, NR is quickly inactivated/activated by phosphorylation/dephosphorylation. Inactivation of NR involves phosphorylation of the serine-543, located at the hinge 1 region connecting MoCo domain and cyt b domain of NR, by a Mg2+ dependent protein kinase and subsequent binding of an inhibitor protein. A type 2A protein phosphatase dephosphorylates/activates NR in response to light signal. Light also regulates supply of reductant via photosynthesis for activity of NR. Whether light is absolutely necessary for NR activity is also discussed.