Enzymic synthesis of oxalic acid in Oxalis pes-caprae

Oxalate in Plants: Metabolism, Function, Regulation, and Application

Characterized by strong acidity, chelating ability, and reducing ability, oxalic acid, a low molecular weight dicarboxylic organic acid, plays important roles in the regulation of plant growth and development, the response to both biotic and abiotic stresses such as plant defense and heavy metals detoxification, and food quality. The metabolism of oxalic acid has been well-studied in microorganisms, fungi, and animals but remains less understood in plants. However, excessive accumulation of oxalic acid is detrimental to plants. Therefore, the level of oxalic acid has to be precisely controlled in plant tissues. In this review, we summarize the metabolism, function, and regulation of oxalic acid in plants, and we discuss solutions such as agricultural practices and plant biotechnology to manipulate oxalic acid metabolism to regulate plant responses to both external stimuli and internal developmental cues.

High and low oxalate content in spinach: an investigation of accumulation patterns

BACKGROUND

Oxalic acid is a common antinutrient in the human diet, found in large quantities in spinach. However, spinach is highly regarded by vegetable producers because of its nutritional content and economic value. One of the primary purposes of spinach-breeding programs is to improve the nutritional value of spinach by adjusting oxalate accumulation. Knowledge of the biosynthetic patterns of oxalic acid, and its different forms, is important for a better understanding of this process.

RESULTS

We found three biosynthetic patterns of accumulation and concentration of oxalates. Two of them are related to the maximum type and one is related to the minimum type. We also developed a general model of variations in these compounds in the genotypes that were studied.

CONCLUSION

This study introduced a unique type of spinach with high oxalate accumulation, which could be particularly suitable for consumption. This had the highest ratio of insoluble oxalate to soluble oxalate. It also accumulated more ascorbic acid (AA) than other types. Our findings in this study also indicate a small role for AA as a precursor to oxalate production in spinach, possibly confirming the significant role of glyoxylate as the most critical precursor in this plant. © 2021 Society of Chemical Industry.

Metabolome analysis of rice leaves to obtain low-oxalate strain from ion beam-mutagenised population

INTRODUCTION

Rice leaves and stems, which can be used as rice straw for livestock feed, accumulate soluble oxalate. The oxalate content often reaches 5% of the dry weight leaves. Excess uptake of oxalate-rich plants causes mineral deficiencies in vertebrates, so it is important to reduce the oxalate content in rice leaves to produce high-quality rice straw. However, the mechanism of oxalate accumulation in rice has remained unknown.

OBJECTIVES

To understand metabolic networks relating oxalate accumulation in rice.

METHODS

In this study, we performed metabolome analysis of rice M₂ population generated by ion-beam irradiation using CE-MS.

RESULTS

The result showed wide variation of oxalate contents in M₂ plants compared with those of control plants. Multivariate analyses of metabolome dataset revealed that oxalate accumulation was strongly related with anionic compounds such as 2OG and succinate. For low-oxalate plants, four patterns of metabolic alterations affected oxalate contents in the M₂ leaves were observed. In M₃ plants, we found putative low-oxalate line obtained from low-oxalate M₂ mutant.

CONCLUSIONS

These findings would lead to produce the low-oxalate rice and to understand the oxalate synthesis in plants.These findings would lead to produce the low-oxalate rice and to understand the oxalate synthesis in plants.

Organic Acids: The Pools of Fixed Carbon Involved in Redox Regulation and Energy Balance in Higher Plants

Organic acids are synthesized in plants as a result of the incomplete oxidation of photosynthetic products and represent the stored pools of fixed carbon accumulated due to different transient times of conversion of carbon compounds in metabolic pathways. When redox level in the cell increases, e.g., in conditions of active photosynthesis, the tricarboxylic acid (TCA) cycle in mitochondria is transformed to a partial cycle supplying citrate for the synthesis of 2-oxoglutarate and glutamate (citrate valve), while malate is accumulated and participates in the redox balance in different cell compartments (via malate valve). This results in malate and citrate frequently being the most accumulated acids in plants. However, the intensity of reactions linked to the conversion of these compounds can cause preferential accumulation of other organic acids, e.g., fumarate or isocitrate, in higher concentrations than malate and citrate. The secondary reactions, associated with the central metabolic pathways, in particularly with the TCA cycle, result in accumulation of other organic acids that are derived from the intermediates of the cycle. They form the additional pools of fixed carbon and stabilize the TCA cycle. Trans-aconitate is formed from citrate or cis-aconitate, accumulation of hydroxycitrate can be linked to metabolism of 2-oxoglutarate, while 4-hydroxy-2-oxoglutarate can be formed from pyruvate and glyoxylate. Glyoxylate, a product of either glycolate oxidase or isocitrate lyase, can be converted to oxalate. Malonate is accumulated at high concentrations in legume plants. Organic acids play a role in plants in providing redox equilibrium, supporting ionic gradients on membranes, and acidification of the extracellular medium.

Expression and properties of the glyoxysomal and cytosolic forms of isocitrate lyase in Amaranthus caudatus L

Isocitrate lyase (EC 4.1.3.1) catalyzes the reversible conversion of d-isocitrate to succinate and glyoxylate. It is usually associated with the glyoxylate cycle in glyoxysomes, although the non-glyoxysomal form has been reported and its relation to interconversion of organic acids outside the glyoxylate cycle suggested. We investigated the expression of two isocitrate lyase genes and activities of the glyoxysomal (ICL1) and cytosolic (ICL2) forms of isocitrate lyase in amaranth (Amaranthus caudatus L.) seedlings. Both forms were separated and purified. The cytosolic form had a low optimum pH (6.5) and was activated by Mn(2+) ions, while Mg(2+) was ineffective, and had a lower affinity to d, l-isocitrate (Km 63 μM) as compared to the glyoxysomal form (optimum pH 7.5, K(m) 45 μM), which was activated by Mg(2+). The highest ICL1 activity was observed on the 3rd day of germination; then the activity and expression of the corresponding gene decreased, while the activity of ICL2 and gene expression increased to the 7th day of germination and then remained at the same level. It is concluded that the function of ICL1 is related to the glyoxylate cycle while ICL2 functions independently from the glyoxylate cycle and interconverts organic acids in the cytosol.

Reduction of oxalate levels in tomato fruit and consequent metabolic remodeling following overexpression of a fungal oxalate decarboxylase

The plant metabolite oxalic acid is increasingly recognized as a food toxin with negative effects on human nutrition. Decarboxylative degradation of oxalic acid is catalyzed, in a substrate-specific reaction, by oxalate decarboxylase (OXDC), forming formic acid and carbon dioxide. Attempts to date to reduce oxalic acid levels and to understand the biological significance of OXDC in crop plants have met with little success. To investigate the role of OXDC and the metabolic consequences of oxalate down-regulation in a heterotrophic, oxalic acid-accumulating fruit, we generated transgenic tomato (Solanum lycopersicum) plants expressing an OXDC (FvOXDC) from the fungus Flammulina velutipes specifically in the fruit. These E8.2-OXDC fruit showed up to a 90% reduction in oxalate content, which correlated with concomitant increases in calcium, iron, and citrate. Expression of OXDC affected neither carbon dioxide assimilation rates nor resulted in any detectable morphological differences in the transgenic plants. Comparative proteomic analysis suggested that metabolic remodeling was associated with the decrease in oxalate content in transgenic fruit. Examination of the E8.2-OXDC fruit proteome revealed that OXDC-responsive proteins involved in metabolism and stress responses represented the most substantially up- and down-regulated categories, respectively, in the transgenic fruit, compared with those of wild-type plants. Collectively, our study provides insights into OXDC-regulated metabolic networks and may provide a widely applicable strategy for enhancing crop nutritional value.

Untangling metabolic and spatial interactions of stress tolerance in plants. 1. Patterns of carbon metabolism within leaves

The localization of the key photoreductive and oxidative processes and some stress-protective reactions within leaves of mesophytic C(3) plants were investigated. The role of light in determining the profile of Rubisco, glutamate oxaloacetate transaminase, catalase, fumarase, and cytochrome-c-oxidase across spinach leaves was examined by exposing leaves to illumination on either the adaxial or abaxial leaf surfaces. Oxygen evolution in fresh paradermal leaf sections and CO(2) gas exchange in whole leaves under adaxial or abaxial illumination was also examined. The results showed that the palisade mesophyll is responsible for the midday depression of photosynthesis in spinach leaves. The photosynthetic apparatus was more sensitive to the light environment than the respiratory apparatus. Additionally, examination of the paradermal leaf sections by optical microscopy allowed us to describe two new types of parenchyma in spinach-pirum mesophyll and pillow spongy mesophyll. A hypothesis that oxaloacetate may protect the upper leaf tissue from the destructive influence of active oxygen is presented. The application of mathematical modeling shows that the pattern of enzymatic distribution across leaves abides by the principle of maximal ecological utility. Light regulation of carbon metabolism across leaves is discussed.

The mechanism of oxalate biosynthesis in higher plants: investigations with the stable isotopes 18 O and 13 C

Substantial incorporation of 18 O 2 into photorespiratory carbon oxidation cycle intermediates in illuminated Spinacia oleracea leaves confirms that oxygenase activity of the enzyme ribulose biphosphate carboxylase–oxygenase is a major source of glycollate in illuminated leaves. No 18 O 2 incorporation into oxalate was detected in these experiments, although 13 C incorporation from 13 CO 2 shows that oxalate synthesis is occurring under the experimental conditions. This result tends to minimize the role of a direct oxidation of glyoxylate derived (via phosphoglycollate and glycollate) from ribulose biphosphate oxygenase activity in oxalate synthesis in Spinacia . Measurements of δ 13 C show (in confirmation of earlier reports) that oxalate from Spinacia is less depleted in 13 C than is bulk organic C in the plant; it is possible the phosphoenolpyruvate carboxylase is involved in the production of the oxalate precursor. Of the plants tested, Mercurialis and Pelargonium shared with Spinacia the high δ 13 C value, while Chenopodium (closely related to Spinacia ), Oxalis (more distantly related to Pelargonium ) and two members of the Polygonaceae had oxalate δ 13 C values close to the whole-leaf δ 13 C value, which suggests derivation of both oxalate C atoms from carboxylase activity of the enzyme ribulose biphosphate carboxylase–oxygenase.

Oxalic acid biosynthesis and oxalacetate acetylhydrolase activity in Streptomyces cattleya

In addition to producing the antibiotic thienamycin, Streptomyces cattleya accumulates large amounts of oxalic acid during the course of a fermentation. Washed cell suspensions were utilized to determine the specific incorporation of carbon-14 into oxalate from a number of labeled organic and amino acids. L-[U-14C]aspartate proved to be the best precursor, whereas only a small percentage of label from [1,5-14C]citrate was found in oxalate. Cell-free extracts catalyzed the formation of [14C]oxalate and [14C]acetate from L-[U-14C]aspartate. When L-[4-14C]aspartate was the substrate only [14C]acetate was formed. The cell-free extracts were found to contain oxalacetate acetylhydrolase (EC 3.7.1.1), the enzyme that catalyzes the hydrolysis of oxalacetate to oxalate and acetate. The enzyme is constitutive and is analogous to enzymes in fungi that produce oxalate from oxalacetate. Properties of the crude enzyme were examined.

Synthesis of oxalic Acid by enzymes from lettuce leaves

A rapid purification of lactate dehydrogenase and glycolate oxidase from lettuce (Lactuca sativa) leaves is described. The kinetics of both enzymes are reported in relation to their possible roles in the production of oxalate. Lettuce lactate dehydrogenase behaves like mammalian dehydrogenase, catalyzing the dismutation of glyoxylate to glycolate and oxalate. A model is proposed in which glycolate oxidase in the peroxisomes and lactate dehydrogenase in the cytosol are involved in the production of oxalate. The effect of pH on the balance between oxalate and glycolate produced from glyoxylate suggests that in leaves lactate dehydrogenase may function as part of an oxalate-based biochemical, pH-stat.

Oxalate accumulation from citrate by Aspergillus niger. I. Biosynthesis of oxalate from its ultimate precursor

Carbon-14 was incorporated from citrate-1,5-14C, glyoxylate-14C(U), or glyoxylate-1-14C into oxalate by cultures of Aspergillus niger pregrown on a medium with glucose as the sole source of carbon. Glyoxylate-14C(U) was superior to glyoxylate-1-14C and citrate-1,5-14C as a source of incorporation. By addition of a great amount of citrate the accumulation of oxalate was accelerated and its maximum yield increased. In a cell-free extract from mycelium forming oxalate from citrate the enzyme oxaloacetate hydrolase (EC3.7.1.1) was identified. Its in vitro activity per flask exceeded the rate of in vivo accumulation of oxalate. Glyoxylate oxidizing enzymes (glycolate oxidase, EC1.1.3.1; glyoxylate oxidase, EC1.2.3.5;NAD(P)-dependent glyoxylate dehydrogenase; glyoxylate dehydrogenase, CoA-oxalylating, EC1.2.1.7) could not be detected in cell-free extracts. It is concluded that in cultures accumulating oxalate from citrate after pregrowth on glucose, oxalate arises by hydrolytic cleavage of oxaloacetate but not by oxidation of glyoxylate.

On the formation of the pattern of crystal idioblasts - in Canavalia ensiformis DC : IV. The fine structure of the crystal cells

Light and electron-microscope observations were made of the crystal idioblasts in the leaves of Canavalia. The crystal-containing cells occur as pairs in which the crystals, nuclei, and the majority of the chloroplasts are symmetrically arranged with regard to the common wall. The chloroplasts are found in the cytoplasm along this wall.The crystals originate in a vacuole. The space in which the young crystal develops is delimited by a membrane. One to several additional membranes surround the crystal inside the vacuole. Numerous vesicles are distributed between these vacuolar membranes. Dense groups of tubules or fibrils are oriented toward a portion of the crystal surface, suggesting that the material forming the crystal might be transported to the surface by these structures.The cytoplasm of the young idioblasts contains many mitochondria and dictyosomes with associated vesicles. Concentrations of what is assumed to be protein are present in the cytoplasm. These protein accumulations are not seen in neighboring cells, suggesting that protein synthesis is especially high in the idioblasts.In older crystal cells, layers of wall material are deposited on the wall between the two crystals of the pair and towards the cell wall adjacent to the mesophyll. Not only does the original wall become thickened but a new wall develops at the border of the crystal vacuole. Eventually this wall material becomes continuous and the crystal becomes, on two sides, directly connected with the wall.