Reactive oxygen species (ROS) are produced as a normal product of plant cellular metabolism. Various environmental stresses lead to excessive production of ROS causing progressive oxidative damage and ultimately cell death. Despite their destructive activity, they are well-described second messengers in a variety of cellular processes, including conferment of tolerance to various environmental stresses. Whether ROS would serve as signaling molecules or could cause oxidative damage to the tissues depends on the delicate equilibrium between ROS production, and their scavenging. Efficient scavenging of ROS produced during various environmental stresses requires the action of several nonenzymatic as well as enzymatic antioxidants present in the tissues. In this paper, we describe the generation, sites of production and role of ROS as messenger molecules as well as inducers of oxidative damage. Further, the antioxidative defense mechanisms operating in the cells for scavenging of ROS overproduced under various stressful conditions of the environment have been discussed in detail.

https://downloads.hindawi.com/archive/2012/217037.pdf

Reactive Oxygen Species, Oxidative Damage, and Antioxidative Defense Mechanism in Plants under Stressful Conditions

Humans are unable to synthesise L-ascorbic acid (L-AA, ascorbate, vitamin C), and are thus entirely dependent upon dietary sources to meet needs. In both plant and animal metabolism, the biological functions of L-ascorbic acid are centred around the antioxidant properties of this molecule. Considerable evidence has been accruing in the last two decades of the importance of L-AA in protecting not only the plant from oxidative stress, but also mammals from various chronic diseases that have their origins in oxidative stress. Evidence suggests that the plasma levels of L-AA in large sections of the population are sub-optimal for the health protective effects of this vitamin. Until quite recently, little focus has been given to improving the L-AA content of plant foods, either in terms of the amounts present in commercial crop varieties, or in minimising losses prior to ingestion. Further, while L-AA biosynthesis in animals was elucidated in the 1960s, 1 it is only very recently that a distinct biosynthetic route for plants has been proposed. 2 The characterisation of this new pathway will undoubtedly provide the necessary focus and impetus to enable fundamental questions on plant L-AA metabolism to be resolved. This review focuses on the role of L-AA in metabolism and the latest studies regarding its bio- synthesis, tissue compartmentalisation, turnover and catabolism. These inter-relationships are considered in relation to the potential to improve the L-AA content of crops. Methodology for the reliable analysis of L-AA in plant foods is briefly reviewed. The concentrations found in common food sources and the effects of processing, or storage prior to consumption are discussed. Finally the factors that determine the bioavailability of L-AA and how it may be improved are considered, as well as the most important future research needs. # 2000 Society of Chemical Industry

Plant L‐ascorbic acid: chemistry, function, metabolism, bioavailability and effects of processing

Vitamin C (L-ascorbic acid) has important antioxidant and metabolic functions in both plants and animals, but humans, and a few other animal species, have lost the capacity to synthesize it Plant-derived ascorbate is thus the major source of vitamin C in the human diet Although the biosynthetic pathway of L-ascorbic acid in animals is well understood, the plant pathway has remained unknown-one of the few primary plant metabolic pathways for which this is the case L-ascorbate is abundant in plants (found at concentrations of 1-5 mM in leaves and 25 mM in chloroplasts) and may have roles in photosynthesis and transmembrane electron transport We found that D-mannose and L-galactose are efficient precursors for ascorbate synthesis and are interconverted by GDP-D-mannose-3,5-epimerase We have identified an enzyme in pea and Arabidopsis thaliana, L-galactose dehydrogenase, that catalyses oxidation of L-galactose to L-galactono-1,4-lactone We propose an ascorbate biosynthesis pathway involving GDP-D-mannose, GDP-L-galactose, L-galactose and L-galactono-1,4-lactone, and have synthesized ascorbate from GDP-D-mannose by way of these intermediates in vitro The definition of this biosynthetic pathway should allow engineering of plants for increased ascorbate production, thus increasing their nutritional value and stress tolerance

The biosynthetic pathway of vitamin C in higher plants

Ascorbic acid (vitamin C) is an abundant component of plants. It reaches a concentration of over 20 mM in chloroplasts and occurs in all cell compartments, including the cell wall. It has proposed functions in photosynthesis as an enzyme cofactor (including synthesis of ethylene, gibberellins and anthocyanins) and in control of cell growth. A biosynthetic pathway via GDP-mannose, GDP-L-galactose, L-galactose, and L-galactono-1,4-lactone has been proposed only recently and is supported by molecular genetic evidence from the ascorbate-deficient vtc 1 mutant of Arabidopsis thaliana. Other pathways via uronic acids could provide minor sources of ascorbate. Ascorbate, at least in some species, is a precursor of tartrate and oxalate. It has a major role in photosynthesis, acting in the Mehler peroxidase reaction with ascorbate peroxidase to regulate the redox state of photosynthetic electron carriers and as a cofactor for violaxanthin de-epoxidase, an enzyme involved in xanthophyll cycle-mediated photoprotection. The hypersensitivity of some of the vtc mutants to ozone and UV-B radiation, the rapid response of ascorbate peroxidase expression to (photo)-oxidative stress, and the properties of transgenic plants with altered ascorbate peroxidase activity all support an important antioxidative role for ascorbate. In relation to cell growth, ascorbate is a cofactor for prolyl hydroxylase that posttranslationally hydroxylates proline residues in cell wall hydroxyproline-rich glycoproteins required for cell division and expansion. Additionally, high ascorbate oxidase activity in the cell wall is correlated with areas of rapid cell expansion. It remains to be determined if this is a causal relationship and, if so, what is the mechanism. Identification of the biosynthetic pathway now opens the way to manipulating ascorbate biosynthesis in plants, and, along with the vtc mutants, this should contribute to a deeper understanding of the proposed functions of this multifaceted molecule.

Ascorbic acid in plants: biosynthesis and function.

. Here we present a new approach to studying inhibition that is based on in vivo whole- cell voltage clamping. This technique allows the continuous measurement of conductance dynamics during visual activation. We show, in neurons of cat primary visual cortex, that the response to optimally orientated flashed bars can increase the somatic input conductance to more than three times that of the resting state. The short latency of the visually evoked peak of conductance, and its apparent reversal potential suggest a domi- nant contribution from g-aminobutyric acid ((GABA)A) receptor- mediated synapses. We propose that nonlinear shunting inhibi- tion may act during the initial stage of visual cortical processing, setting the balance between opponent 'On' and 'Off ' responses in different locations of the visual receptive field.

Visual input evokes transient and strong shunting inhibition in visual cortical neurons

STUDIES in this laboratory have yielded evidence with both plants and animals that the conversion of D-glucose to L-ascorbic acid is by way of D-glucuronic acid and L-gulonic acid (or derivatives), and it seems that D-galactose may also pass to L-ascorbic acid through corresponding intermediates. The overall conversion of sugars (D-glucose, D-fructose and D-mannose) into L-ascorbic acid had, of course, been demonstrated previously, by Ray1 and others in plants, and by Jackel, Mosbach, Burns and King2 in animals.

/pdf/synthesis-of-l-ascorbic-acid-in-plants-and-animals-2yeft69isp.pdf

Synthesis of L-ascorbic acid in plants and animals.

/pdf/biological-synthesis-of-l-ascorbic-acid-the-conversion-of-l-5c1np38ogh.pdf

Biological synthesis of l-ascorbic acid: the conversion of l-galactono-γ-lactone into l-ascorbic acid by plant mitochondria

/pdf/synthesis-of-l-ascorbic-acid-in-rat-liver-homogenates-2mmn6dnmwn.pdf

Synthesis of l-ascorbic acid in rat-liver homogenates: conversion of l-gulono- and l-galactono-γ-lactone and the respective acids into l-ascorbic acid

In studies on the synthesis of L-ascorbic acid in plants and animals the need arose for a method permitting separation and quantitative estimation of closely related enediol compounds such as D-araboascorbic and L-ascorbic acids. Simple titration methods using 2:6-dichlorophenolindophenol (Harris & Ray, 1935; Birch, Harris & Ray, 1933; Harris & Olliver, 1942) will not differentiate between closely related enediols, and even more specific methods, based on the different rates of condensation of enediols with formaldehyde (Lugg, 1942; Mapson, 1943; Snow & Zilva, 1943), will not distinguish between those with the same ring structure. It seems clear that a preliminary separation must precede the estimation of the individual enediols. Such a separation has been described by Mapson & Partridge (1949). These authors have shown that L-ascorbic acid, D-araboascorbic acid, the enol form of 3-hydroxytetrahydrofuran-2:4dione (hydroxytetronic acid), reductic acid, and reductone can be separated on a paper chromatogram using n-butanol: acetic acid: water, or phenol: acetic acid: water, as solvents. Their experiments were confined to the qualitative separation ofmadeup mixtures of pure compounds, and no attempts were made to estimate the enediols after separation on the paper chromatograms. The problem of separating L-ascorbic acid from D-araboascorbic acid in extracts from biological material was not touched and it was suggested that the separation might be difficult. This opinion was based on the known instability of the enediols and their low concentration in extracts from biological materials. The method now described uses paper chromatograms not only to effect the final separation of the enediols but also to achieve a preliminary concentration from the bulk of the other compounds present in the extract. Since enediols are known to be most stable in strongly acid solution and when heavy metals are absent, both the initial extract of the biological material andthe two chromatographic solvents used were saturated with oxalic acid. Oxalic acid was used because it readily forms complexes with most heavy metals and because it forms a component of two very stable chromatographic solvents (n-butanol: water: oxalic acid, and phenol: water: oxalic acid) which will separate most of the common enediol compounds. The presence of oxalic acid, however, does not prevent a slow decomposition of the enediols, which is particularly noticeable in weak solutions. This occurs even when the solutions are stored in the dark, but is more pronounced when they are exposed to light. The same is true of solutions in metaphosphoric acid, so that it appears to be independent of the compound used to inactivate the heavy metals. This slow breakdown inevitably makes it difficult to devise a really quantitative method. However, the present method, wnilst not having a very high accuracy, does give an approximately quantitative estimate of the enediols in extracts of biological material. It can usefully be employed to confirm the reliability of the simple titration methods which are commonly used for the estimation of L-ascorbic acid in extracts of biological materials.

/pdf/quantitative-estimation-of-ascorbic-acid-and-related-jyww5w3q2r.pdf

Y. T. Chen

Papers

Synthesis of L-ascorbic acid in plants and animals.

Biological synthesis of l-ascorbic acid: the conversion of l-galactono-γ-lactone into l-ascorbic acid by plant mitochondria

Synthesis of l-ascorbic acid in rat-liver homogenates: conversion of l-gulono- and l-galactono-γ-lactone and the respective acids into l-ascorbic acid

Synthesis of L-ascorbic acid in plants and animals.

Quantitative estimation of ascorbic acid and related substances in biological extracts by separation on a paper chromatogram