Allantoin nodules

Asparagine and glutamine are the major transport compounds found in plants, with arginine frequently being present particularly in perennials (e.g., trees). Nonprotein amino acids, homoserine, canavanine, and y-methy-leneglutamine are utilized in specific plants. The ureides allantoin and allantoic acid are frequently found in nodulated tropical legumes. 

There appears to be a correlation between nodule production and allantoin synthesis, (Fujiharacr al., 1977). Non-nodulating varieties of soybean grown on 50 ppm nitrate produce very low levels of allantoin, (Matsumoto et al., 1975 Tajima et al., 1977). However, the allantoin concentration increases rapidly if the plants are cultured on higher concentrations of nitrate or urea (Tajima et al., 1977). Thus it would appear that allantoin production is not a direct result of nodule formation or bacterial infection, but a response mechanism to the presence of high concentrations of reduced N. Allantoin represents a convenient compound for the storage of reduced N at a minimum cost of fixed carbon (see Section V). 

Allantoinase which may be considered as involved in the synthesis of allantoic acid or in the breakdown of allantoin, is apparently widespread throughout the plant. Although the enzyme is present in large amounts in the nodules it may also be detected in leaves, pods, and maturing seeds (Tajima et al., 1977 Herridge er a/., 1978). 

Ureides are metabolized by conversion to urea and glyoxylate as shown in Fig. 6. Allantoin may be hydrolyzed to allantoic acid as discussed above. The key enzyme in the breakdown is allantoicase, which is located in the shoots and pods (Tajimaet al., 1977). Such a system allows for the transport of N from the nodule to the shoot and rapid metabolism of the ureides to liberate N for protein synthesis (Herridge et al., 1978). 

Allantoin and ALA, which are synthesized in the nodule, are the primary products of recent Nj fixation in legumes of tropical origin. These ureides are rapidly labeled after exposure of nodulated roots of soybean (Ohyama and Kumazawa, 1978 Schubert era/., 1981 Schubert and Coker, 1982) and cowpea (Herridge et al., 1978) to labeled N2. Allantoic acid accounted for 40% of the radioactivity after exposure of soybean nodules to Nj for 10 min (Schubert and Coker, 1982 Schubert et ai, 1981). Within 30-60 min of exposure of nodulated roots to Nj, labeled ureides were detected in the xylem sap of... 

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