Ureide plants

The so-called ureide plants use allantoin and allantoic acid as a nitrogen store from which ammonia may be liberated by further degradation (E 2.2), In the liver of lungfish a glycine-allantoin cycle (Fig. 182) causes the formation of urea (cf. the formation of urea via L-ornithine derivatives, D 19). 

Alkaline Proteinase Activity in Yams. The release of ammonia at several stages during ureide metabolism suggested a potential for alkaline conditions in yam tubers, rather than the usual neutral or acid conditions generally found in seeds and plants. 

Plants also form the ureides allantoin and allantoic acid, and in some legumes, such as soy beans, these compounds account for 70-80% of the organic nitrogen in the xylem. They appear to function in nitrogen transport.337 As indicated in Fig. 25-18, the hydrolysis to glyoxylate, NH4+, and C02 follows a different pathway than in animals. See also Chapter 24, Section C. 

Ammonium, the primary product of nitrogen fixation, is transported to the host cell cytoplasm where it is assimilated into amides and, in some cases, further converted into ureides before being transported to the shoot. Since the physiological environment within the nodule is apparently different from the other parts of the plant, nodule-specific or nodule-abundant forms of several enzymes of the nitrogen and carbon assimilation pathways have evolved, and are induced to improve the efficiency of nitrogen and carbon metabolism in nodules. 

Stegink, S.J., Vaughan, K.C. Verma, D.P.S. (1987). Antigenic similarity in urate oxidase of major ureide-producing legumes and its correlation with the type of peroxisomes in uninfected cells of nodules. Plant Cell Physiology 28, 387-96. 

In many legumes, transportation of N from root to shoot occurs in the form of ureids, allantoin, and allantoic acid, which are synthesized from uric acid, an oxidation product of purine (xanthine). Poor growth of legumes in the presence of Mo deficiency can be ascribed in part to poor upward transport of N because of disturbed xanthine catabolism. In plants, oxidation of xanthine is mediated by another molybdoenzyme, xanthine dehydrogenase (Mendel and Muller, 1976 Nguyen and Feierabend, 1978). This enzyme has a constitution similar to that of the xanthine oxidase found in animals. It has two identical subunits, and each unit contains one Mo atom, one FAD, and four Fe-S groups. 

A similar classification may be appropriate for N2-fixing actinorhizal species. Asparagine is the major product of N2 fixation in Myiica species (32), w hereas the ureide, citrulline, predominates in Alnus species (20, 33). The structures of the difiFerent nitrogenous components transported from N2-fixing plants are given in Figure 3. 

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. 

The ureides allantoin and allantoic acid are well known as major N containing compounds of plants (Reinbothe and Mothes, 1962 Hegarty and Peterson, 1973). However there has recently been an increase in interest in these compounds as they are major transport compounds in economically important legumes (e.g., soybean and cowpea). 

It has been clearly demonstrated that only a limited number of compounds are utilized for N transport in higher plants. By far the most common compounds are the amides asparagine and glutamine and to a lesser extent the ureides. Canavanine and the various substituted amides are only found in a very small number of species. 

Ureide planis plant families that accumulate al-lantoin and/or all toic add, and use these compounds as nitrogen reserves, U.p. are members of the Aceraceae, Boraginaceae, Hippocastanaceae and Pla-... 

Schutte, M. Luckner, eds.). Deutscher Verlag der Wissenschaften, Berlin 1985 Thomas, R. J., Schrader, L. E. Ureide metabolism in higher plants. Phytochemistry 20, 361-371 (1981)... 

Among plants, urea is found in many fungi and moulds in fairly high concentration. Green plants contain traces, probably as unstable ureides, or carbamide acids, such as citrulline. 

The two- to fourfold variation in sucrose cost per NHj assimilated (Fig. 3B) is not faithfully reflected in respiratory losses of CO2, since dark fixation inputs of CO2 apply to certain compounds (e.g., aspartate and asparagine) but not at all, or to only a minor extent, in the case of others. Net CO2 exchange values therefore range from a net fixation input of 0.58 and 1.08 CO2/NH3 in the case of glutamate and aspartate, respectively, to a net evolution of 0.02 to 0.53 CO2/NH3 for ureide, citrulline, and proline. Based on these calculations the type of products selected by a plant for export of assimilated nitrogen might aJter significantly the apparent respiration status (respiratory quotient) of its assimilatory tissues. 

N2 Fixation The NH3 produced by the bacteroids has first to be exported via bacteroid and host membranes to cytosol of the nodule and then possibly assimilated within various organelles or cell types to form amino acids, amides, or ureides for export in the xylem. Considering the numbers of nitrogen atoms exported per compound, costs of 3, 2, and 1.75 ATP/nitrogen transported would appear to be reasonable estimates of the energy cost for the transport of amino acids, amides, and ureides, respectively. Added to this would be any costs in transmembrane transfers of ammonia from bacteroid to plant. 

Ureides are a class of cyclic or acyclic nitrogenous organic compounds derived from or structurally related to urea. Representatives of this class which have been found in plants include allantoin, allantoic acid, citrulline, uric acid, hypoxanthine, xanthine, caffeine, hydroxycitruUine, and albizziine. The structures of allantoin (ALN), allantoic acid (ALA), and citrulline (CIT), ureides which occur throughout the plant kingdom, are presented in Fig. 1. These ureido compounds play an essential role in the assimilation, metabolism, transport, and storage of nitrogen in plants. In this chapter, attention will be focused on the occurrence, function, synthesis, and metabolism of these key metabolites. 

Fig. 1. Structures of the most common ureides found in plants.

The occurrence of ureides in plants was first noted in 1881 when Schulze and Barbieri (1881) reported that allantoin was present at high levels in roots of Platanus orientalis. Based on herbal folklore, Macalister (1912) subsequently... 

The discovery of allantoic acid and citrulline followed in 1926 and 1930, respectively. During the 70-year period following the discovery of allantoin in plants, most of the research on ureides was descriptive in nature, with the work of German and French scientists being the most notable. 

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