Nitrogen transport from roots

Most green plants transport nitrogen from roots to growing shoots as asparagine. However, in peanuts P-methyleneaspartate is the major nitrogen carri-er, °° and in some legumes, including soybeans, allantoin and allantoate (Eq. 24-15) play this role. Allantoin arises from hydrolysis of purines (see Fig. 

Fig. 9. Proposed model for the cellular compartmentalization of the reactions of nitrogen fixation, ammonium assimilation, purine synthesis, and ureide biogenesis in infected and uninfected cells of soybean root nodules. Uncertainty still exists with respect to the nature of the intermediate (e.g., IMP, XMP, xanthine, glutamine ) transported from the infected cell to the uninfected cell as well as the site of purine synthesis. In addition, as discussed in the text the site(s) of PRPP synthesis (plastid and/or cytosolic) and the path and site of synthesis (de novo from the PPP or via salvage) of tibose S-phosphate (R-S-P) are s not defined, lliese uncertainties are indicated with question marks and/or dashed lines. Lb, leghemoglobin.

There is a close relationship between the metabolism of the shoot and the root. It is generally accepted that the xylem forms the main path for upward movement of water and ions from the roots to the leaves. Most of the essential major elements are transported in the xylem as inorganic ions. Nitrogen may be transported along the xylem as N03 if it is present in the external solution as nitrate. However, the plant sap may also contain organic nitrogen compounds such as amino acids. In the xylem, heavy metals will usually only be transported if special chelates are formed, eg, by citrate (Streit and Stumm, 1993). Iron is taken up and transported more readily when supplied as a chelated complex, such as ferric ethylenediamine tetraacetate (FeEDTA) or as ferric diethylenetriamine pentaacetate (FeDTPA) (Wallace and North, 1953). Calcium may also be transported in a chelated form (Jacoby, 1966). 

Fig. 2. Transmission electron micrograph (16,000x enlargement) through an infected root cell of a soybean plant. The subcellular organelles of the host cell [mitochondria and plasts (a)] are present at the periphery of the cell adjacent to the host cell wall (b). The nitrogen-fixing bacteroids (c) are kept apart from the host cell cytoplasm (the location of leghemoglobin) by the peribacteroid space (d) and the peribacteroid membrane (e), which regulates transport of materials to and from the bacteroids.

Molecular mechanisms of nitrate accumulation depend not only on the nitrate reductase system, but also on the ability of roots to take from the soil, nitrate or ammonium ions, and on the plant s capacity for their conversion by assimilation processes to higher products. Besides this, the assimilation depends on the ability of a given genotype to transport substances necessary for the synthesis. It was shown that genotype differences of the nitrate reductase level do not depend on the nitrate content in tissues [25]. Nitrates are accumulated in plant organisms at high concentrations when aU the nitrogen accepted cannot be utilized for the production of amino acids and for subsequent protein synthesis [26]. This occurs when the plant, in the course of its metabolism, is unable to reduce the accepted nitrates into the assimilable ammonia form. 

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