Glutamine Synthetase and Ferredoxin-Dependent Glutamate Synthase Expression in the Maize (Zea mays) Root Primary Response to Nitrate (Evidence for an Organ-Specific Response)

To define further the early, or primary, events that occur in maize (Zea mays) seedlings exposed to NO3-, accumulation of chloroplast glutamine synthetase (GS2; EC 6.3.1.2) and ferredoxin-dependent glutamate synthase (Fd-GOGAT; EC 1.4.7.1), transcripts were examined in roots and leaves. In roots, NO3- treatment caused a rapid (within 30 min), transient, and cycloheximide-independent accumulation of GS2 and Fd-GOGAT transcripts. In addition, 10 [mu]M external NO3- was sufficient to cause transcript accumulation. The induction was NO3- specific, since NH4Cl treatment did not affect mRNA levels. GS2 and Fd-GOGAT mRNA accumulation in roots was similar to that observed for nitrate reductase (NR) mRNA. Therefore, the four genes involved in NO3- assimilation (NR, nitrite reductase, GS2, and Fd-GOGAT) are expressed in the root primary response to NO3-, suggesting that all four genes can respond to the same signal transduction system. In contrast, relatively high levels of GS2 and Fd-GOGAT mRNAs were present in untreated leaf tissue, and NO3- treatment had little or no influence on transcript accumulation. Rapid, transient, and cycloheximide-independent NR mRNA expression was seen in the NO3--treated leaves, demonstrating that NO3- was not limiting. The NO3--independent constitutive expression of GS2 and Fd-GOGAT is likely due to the requirement for reassimilation of photorespiratory NH4+ in these young leaves.