Abstract

Although the nitrate assimilation into amino acids in photosynthetic leaf tissues is active under the light, the studies during 1950s and 1970s in the dark nitrate assimilation provided fragmental and variable activities, and the mechanism of reductant supply to nitrate assimilation in darkness remained unclear. 15N tracing experiments unraveled the assimilatory mechanism of nitrogen from nitrate into amino acids in the light and in darkness by the reactions of nitrate and nitrite reductases, glutamine synthetase, glutamate synthase, aspartate aminotransferase, and asparagine synthetase. Nitrogen assimilation in illuminated leaves and non-photosynthetic roots occurs either in the redundant way or in the specific manner regarding the isoforms of nitrogen assimilatory enzymes in their cellular compartments. The electron supplying systems necessary to the enzymatic reactions share in part a similar electron donor system at the expense of carbohydrates in both leaves and roots, but also distinct reducing systems regarding the reactions of Fd-nitrite reductase and Fd-glutamate synthase in the photosynthetic and non-photosynthetic organs.

Highlights

  • Plants use inorganic nitrogen present in the soil for their growth mainly in the form of nitrate (NO3 − )

  • Plants can assimilate NO3 − to amino acids in both the photosynthetic leaves and non-photosynthetic roots, the major sites of nitrate assimilation are green shoots where energy (ATP), reductant and organic skeletons are produced by photosynthetic reactions using solar energy

  • Leaves represent a major site of primary nitrogen assimilation in concert with roots, photorespiratory NH4 + re-fixation, and translocation of nitrogen within the plant

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Summary

Introduction

Plants use inorganic nitrogen present in the soil for their growth mainly in the form of nitrate (NO3 − ). After the findings of 15 N-labelled amino acids in the roots of rice (Oryza sativa L.) seedlings by feeding with 15 N-ammonium [6] and 15 N-nitrate [7], Ito and Kumazawa [8] conducted light and dark incubation of leaf discs of sunflower (Helianthus annuus L.) with 15 NO3 − , 15 NO2 − , and 15 NH4 +. In the 1970s, a simple in vivo assay of nitrate reductase activity was widely employed using leaf segments without extraction of enzymes In this technique, it was assumed that nitrite, nitrate reductase reaction product, was barely assimilated when the assay was carried out in the dark [10,11,12]. Adapted from Yoneyama [14]

Nitrate Reduction to Ammonia
Glutamine Synthesis and Metabolism to Glutamate and Asparagine in Leaves
Reductant Supply for Nitrate Reductase and Nitrite Reductase
Reductant Supply to the Fd-Dependent GOGAT
Findings
Conclusions
Full Text
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