ABSTRACT
From the metabolome analysis, the concentration ratios of metabolites for the nitrate treatment vs. the control indicated that most of the amino acids, phosphorous-compounds and organic acids in roots were increased about twofold in the roots, whereas in the nodules most of the concentrations of the amino acids, P-compounds and organic acids were decreased while asparagine increased exceptionally.
Some part of the NO3- absorbed in the root cell is reduced to nitrite ( NO2 -) by nitrate reductase (NR) in cytosol, then reduced to ammonia by nitrite reductase (NiR) in plastids followed by assimilation via GS/GOGAT pathway to amino acids. When a high concentration of NO3 - is supplied, a part of NO3 - is temporary stored in vacuoles. Some part of NO3 - is transported cell to cell via symplast pathway and effluxed in the stele and transported via xylem with transpiration stream in the form of NO3 -.
Ozone can interfere with nitrogen metabolism of soybean. Typical environmental stresses faced by the legume nodules and their symbiotic partner (Rhizobium) may include photosynthate deprivation, water stress, salinity, soil nitrate, temperature, heavy metals, and biocides. A given stress may also have more than one effect: e.g., salinity may act as a water stress, which affects the photosynthetic rate, or may affect nodule metabolism directly. The most problematic environments for rhizobia are marginal lands with low rainfall, extremes of temperature, acidic soils of low nutrient status, and poor water-holding capacity.
In the nodulated intact soybean plants, the ammonia produced by nitrogen fixation is initially assimilated into amide group of Gln with Glu by the enzyme glutamine synthetase (GS). Then, Gln and 2-OG produce two moles of Glu by the enzyme glutamate synthase (GOGAT). Some part of Gln is used for purine base synthesis, and uric acid is transported from the infected cells to the adjacent uninfected cells in the central symbiotic region of nodule. Uric acid is catabolized into allantoin and allantoic acid in the uninfected cells and then transported to the shoot through xylem vessels in the roots and stems. A small portion of fixed N was assimilated into alanine and Glu in the bacteroids, but it was not by GS/GOGAT pathway.
Gene expression ratios of nitrate vs. the control were highly enhanced for those probesets related to nitrate transport and assimilation and carbon metabolism in the roots, but much less so in the nodules, except for the nitrate transport and asparagine synthetase.
