Glutamine synthetase and ferredoxin-dependent glutamate synthase expression in the maize (Zea mays) root primary response to nitrate. Evidence for an organ-specific response
1993
Redinbaugh, M.G. | Campbell, W.H.
To define further the early, or primary, events that occur in maize (Zea mays) seedlings exposed to NO3(-)1 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 micromolar 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.
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