Partial Characterization of the Signaling Pathway for the Nitrate-Dependent Expression of Genes for Nitrogen-Assimilatory Enzymes Using Detached Maize Leaves

We have studied the regulation of the nitrate-dependent expression of genes for the nitrogen-assimilatory enzymes, nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS)and glutamate synthase (GOGAT), using detached leaves of maize plants that were grown with a limited supply of nitrate. In the detached leaves, transcripts of genes for NR, NiR, plastidic GS (GS2), ferredoxin- GOGAT (Fd-GOGAT)and NADH-GOGAT accumulated upon treatment of detached leaves with nitrate. The kinetics of accumulation was similar for all transcripts but the amount accumulated was smaller for the transcripts of genes for both GOGATs. Experiments with cordycepin and cycloheximide suggested that, in leaves, the response of the various genes to nitrate is regulated transcriptionally and that the induction does not involve the synthesis of novel protein(s)in the cytosol. The significant repression of the expression of the genes for NR, NiR, GS2, and GOGATs in response to nitrate after pretreatment with EGTA or La3+ ions suggested that Ca2+ ions are involved, at least to some extent, in the signal transduction and that the induction of nitrogen-assimilatory genes by nitrate is regulated in the same or similar manner. Furthermore, the effects of inhibitors of protein kinases and protein phosphatases on the nitrate-dependent accumulation of the transcripts of genes for NR, NiR and GS2, suggested the involvement of protein phosphorylation in the regulation of the levels of transcripts of these genes.