Oxalate accumulating plants

Figure 2. Cleavage of i.-ascorbic acid in plants of the Geraniaceae. Oxalate accumulating plants also cleave i.-ascorbic acid between C2 and C3 but the fragment does not accumulate as tartrate.

Oxalate formation from Ascorbate in Oxalate Accumulating Plants... 

To establish the common origin of oxalate as a C2 fragment from Cl plus C2 of ascorbate, paired experiments were performed with several oxalate accumulating plants and a nominal oxalate former (30) using L-[1- C]- or L-[U- C]ascorbic acid as the source of label. Theoretically, the former will furnish three times as much to oxalic acid as the latter if Cl plus C2 of ascorbic acid corresponds to the C2 donor. All seven oxalate accumulating plants tested had two to three times more C in oxalate when l-[1- C]ascorbic acid rather than L-[U- C]ascorbic acid was fed (Table VIII). A nominal oxalate former, tomato, produced very little [ C] oxalate from either source. 

Table VIII. Comparative Production of Oxalic Acid from L-[l- C]-and L-[U- C]Ascorbic Acid (Asa) in Oxalate Accumulating Plants (Metabolic Period, 24 h)...

Nuss R A, Loewus F A 1978 Further studies on oxalic acid biosynthesis in oxalate accumulating plants. Plant Physiol 61 590-592... 

A synthetic coupled cycle of calcium and carbon through the oxalate-carbonate pathway is shown in Fig. 12.8. Atmospheric CO2 is fixed by the plants through photosynthesis to produce biomass. Inside the plant, oxalate crystals form. In addition, fungal mycelium may also accumulate oxalate. Mainly in the form of calcium oxalate (COM or COD), this carbon pool is used by oxalotrophic bacteria as a carbon, energy and electron source. The transformation of oxalate can occur in the soil... 

Fig. 12.8. Simplified sketch showing main relationships inside the coupled calcium and carbon cycles of the oxalate-carbonate pathway in a hypothetical ecosystem. Plants and fungi are oxalate producers. Oxalotrophic bacteria (in the soil or animal guts) use oxalate as carbon, energy and electron sources, leading to CO2 and calcium carbonate production. Calcium carbonate can accumulate inside the soils. Because the carbon of the carbonate originates from organic carbon, its fossilization in the soil constitutes a carbon sink.

Clarke BL (1992) Stoichiometric network analysis of the oxalate-persulfate-silver osdllator. J Chem Phys 97 2459-2472 Clarke BL (1995) What is stoichiometric network analysis Web site Alberta University at Edmonton (no longer online) Clemens S, Palmgren MG, Kramer U (2002) A long way ahead understanding and engineering plant metal accumulation. Trends Plant Sci 7 309-315... 

It has been known for some time that tolerance towards high levels of both essential and toxic metals in a local soil environment is exhibited by species and clones of plants that colonize such sites. Tolerance is generally achieved by a combination of exclusion and poor uptake and translocation. Some species can accumulate large quantities of metals in their leaves and shoots at potentially toxic levels, but without any harmful effects. These metal-tolerant species have been used in attempts to reclaim and recolonize metal-contaminated wastelands. More recently such species have attracted the attention of inorganic chemists. There is abundant evidence that the high metal levels are associated with carboxylic acids, particularly with nickel-tolerant species such as Allysum bertolonii. The main carboxylic acids implicated are citric, mahc and malonic acids (see refs. 30 and 31 and literature cited therein). Complexation of zinc by malic and oxalic acids has been reported in the zinc-tolerant Agrostis tenuis and oxalic acid complexation of chromium in the chromium-accumulator species Leptospermum scoparium ... 

Phytochemistry Aboveground parts contain 4.35-5.57 % resins, with 1.08-1.37 % resins in roots and 7.91 % in flowers. The whole plant contains organic acids (citric, malic, oxalic, acetic, propionic, and valerianic) and tannins (3.61. 74 % in aboveground parts and 2-2.5 % in the roots). The aboveground part contains essential oil of which the maximum accumulation (0.96 %) happens during the flowering stage (Khodzhimatov 1989). The major components of the essential oils are camphor, 1,8-cineole, and P-caryophyllene (Cha et al. 2005). 

Ascorbic acid is oxidized by plants with ascorbate oxidase to yield dehydroascorbic acid (3,6-anhydro-L-Ay/o-hex-ulono-1,4-lactone hydrate) (76) (Loewus, 1980, 1988). In plants, ascorbic acid is rarely accumulated, but is usually converted into tartaric (77) and oxalic (78) acids (Fig. 15.14). 

Oxalic acid and calcium oxalate, a store for excess Ca + (E 2.2) and a repellent for potential predators (E 5.5.3), accumulate frequently in plants and microbial cultures. Oxalyl derivatives of amino acids occur in different groups of organisms (D 9.1). O-Oxalyl-L-homoserine is involved in the biosynthesis of L-methio-nine in microorganism (D 12). 

Oxalic acid poses a problem to both leafy plants and vertebrates because these organisms cannot catabolize it [108]. Although accumulation of oxalate leads to stress in plants, in vertebrates this molecule can be metaboHzed by bacteria present in the intestinal tract [109]. Oxalate can be catabolized in different ways by oxidation, by decarboxylation of oxalyl-coenzyme A or by direct decarboxylation. Both oxidation and decarboxylation of oxalate are catalyzed by Mn-containing enzymes. Here we will discuss the oxalate decarboxylate reaction that produces formate and CO2. The crystal structure of oxalate oxidase from Bacillus subtilis... 

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