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

Inoculation of the first true leaf of cucumber with Colletotrichum lagenarium enhanced peroxidase activity in leaf 2. The increased activity was associated, at least in part, with the fastest moving anodic isozymes during polyacrylamide gel electrophoresis. All plant parts and cellular fractions exhibited enhanced peroxidase activity in protected as compared to control plants. Injury of leaf 1 with dry ice neither induced systemic resistance to C. lagenarium nor enhanced peroxidase activity of leaf 2. Inoculation of leaf 1 with C. lagenarium followed by inoculation of leaf 2, 1 week later, elicited a higher degree of protection and higher peroxidase activity in leaf 3 than did single inoculations. Increasing numbers of C. lagenarium lesions on leaf 1 progressively increased peroxidase activity and resistance in leaf 2. Induced resistance was detected 3 to 4 days after inoculation of leaf 1, whereas peroxidase activity increased after the 4th day.

Association of enhanced peroxidase activity with induced systemic resistance of cucumber to Colletotrichum lagenarium

Reactions involving free radicals are an inherent feature of plant senescence and appear to contribute to a process of oxidative deterioration that leads ultimately to cell death. Radical species derived from molecular oxygen are the primary mediators of this oxidative damage, but non-radical excited states of oxygen, specifically singlet oxygen, may also be involved. Several lines of evidence suggest that degradation of lipids in senescing membranes and the ensuing release of free fatty acids initiate oxidative deterioration by providing substrate for lipoxygenase. In some tissues, lipoxygenase activity increases with advancing senescence in a pattern that is consistent with its putative role in promoting oxidative damage. However, there are important exceptions to this which may be explained by the fact that the timing and extent of peroxidative reactions initiated by lipoxygenase are likely to be determined more by the availability of substrate for the enzyme than by changes in its activity. There are both membranous and cytosolic forms of lipoxygenase in senescing tissues, and peroxidation of membrane lipids appears to be initiated by the membranous enzyme once the appropriate fatty acid substrates, linoleic acid and linolenic acid, become available. Since lipid peroxidation is known to form alkoxy and peroxy radicals as well as singlet oxygen, these reactions in membrane bilayers are probably a major source of activated oxygen species in senescing tissues. Further-more, there are indications that activated oxygen from the lipoxygenase reaction can become substrate for the cytosolic form of the enzyme which, in turn, may raise the titre of activated oxygen during senescence. Additional possible sources of increased free radical production in senescing tissues include peroxidase, which shows greatly increased activity with advancing age, leakage of electrons from electron transport systems to oxygen, in particular from the photosynthetic electron transport system, and decompartmentalization of iron, which would facilitate formation of the highly reactive hydroxyl radical from the less reactive superoxide anion. A variety of macromolecules can be damaged by activated oxygen. Unsaturated fatty acids are especially prone to attack, and this implies that membranes are primary targets of free radical damage. The manifestations of this damage in senescing tissues range from altered membrane fluidity and phase properties to leakiness that can be attributed to a destabilized and highly perturbed membrane bilayer. There is also a progressive breakdown of cellular protein with advancing senescence. Free radicals can inactivate proteins by reacting with specific amino acid residues, and a number of in zitro studies have indicated that such alteration renders the proteins more prone to hydrolysis by proteases. Thus, although there is no direct evidence linking enhanced proteolysis during senescence to free radical damage, there is reason to believe that this may be a contributing factor. Wounding of certain plant tissues also initiates a series of reactions that revolve around the breakdown of membrane lipids and their peroxidation. Indeed, as in the case of senescence, membrane deterioration follokving wounding appears to be facilitated by a self-perpetuating wave of free radical production emanating from peroxidation within the lipid bilayer. There is also recent evidence for activation of an O2- -producing NADPH oxidase in plant tissues following fungal infection that may be analogous to the well-characterized O2- -generating NADPH oxidase associated with the plasma membrane of polymorphonuclear leukocytes. This raises the interesting possibility that plants and animals share a common defence response to invading organisms. Contents Summary 317 I. Introduction 318 II. Species of activated oxygen 319 III. Sites of activated oxygen production 319 IV. Free radical production during senescence 323 V. Targets of free radical damage in senescing tissues 330 VI. The role of free radicals in seed ageing 336 VII. The role of free radicals in wounding 337 VIII. Concluding remarks 338 Acknowledgement 338 References 338.

The role of free radicals in senescence and wounding.

The processes underlying lignification, which for many years have been the near-exclusive purview of chemists and biochemists, have more recently been approached using both classical forward genetic screens and targeted reverse genetic approaches such as antisense suppression, RNAi, and characterization of insertional mutants. In this review, we provide an overview of the current understanding of lignin biosynthesis and structure, with emphasis on mutant and transgenic plants that have contributed to this knowledge. We also discuss ongoing work aimed at elucidating the relationship between lignin structure and function in vivo, as well as the phenotypic consequences arising from genetic manipulation of the lignin biosynthetic pathway.

The Genetics of Lignin Biosynthesis: Connecting Genotype to Phenotype

Lignin biosynthesis via shikimate-cinnamate pathways in plants, and the biosynthetic differences of guaiacyl-and syringyl lignins between gymnosperms and angiosperms have been elucidated by tracer experiments using 14C labeled precursors and the following enzyme reactions. The formation of guaiacyl lignin but not syringyl lignin in gymnosperms was attributed to the following factors; absence of ferulate-5-hydroxylase, poor affinity of O-methyltransferase toward 5-hydroxyferulate, and lack of activation and/or reduction of sinapatc. A mechanism of lignin-carbohydrate complexes formation in wood cell walls was elucidated based on the reaction of the quinone methide of guaiacylglycerol-β-guaiacyl ether with sugars, and the analysis of DHP-polysaccharide complexes. The main cleavage mechanisms of side chains and aromatic rings of lignin model compounds and synthetic lignin (DHP) by white-rot fungi and their enzymes, lignin peroxidase and laccase have been elucidated using 2H, 13C and 18O-labeled lignin substructure dimcrs with 18O2 and H2
18O. Side chains and aromatic rings of these substrates were cleaved via aryl cation radical and phenoxy radical intermediates, in reaction mediated only by lignin peroxidase/H2O2 and laccase/O2.

Lignin biochemistry: Biosynthesis and biodegradation *

Purification and properties of cinnamyl alcohol dehydrogenase from higher plants involved in lignin biosynthesis

From young lignifyin stems of Forsythia an enzyme specifically catalyzing the formation of CoA thiol esters of hydroxylated cinnamic acids was isolated and partially purified This enzyme is most active with o-, m-, p-coumarate, caffeate, ferulate and isoferulate CoA, ATP, Mg2+ and a reduced thiol are required as cofactors; ATP is cleaved in this reaction to AMP and PPi The enzyme has a molecular weight of approx 55000 and shows a pH optimum at pH 73 In the presence only of cinnamic acids, the enzyme catalyzes an exchange reaction between PPi and ATP, which is drastically inhibited by CoA A large number of benzoic acids as well as aliphatic and α-amino acids is inactive in this assay Feruloyl-AMP could be demonstrated to participate as an activated intermediate in the formation of the CoA thiol esters

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1974.tb03359.x

Isolation and Properties of Hydroxycinnamate:CoA Ligase from Lignifying Tissue of Forsthia

Cell wall-bound malate dehydrogenase from horseradish

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2873%2980123-1

Three novel enzymes involved in the reduction of ferulic acid to coniferyl alcohol in higher plants: ferulate: Co a ligase, feruloyl‐Co a reductase and coniferyl alcohol oxidoreductase

After Payen’s fundamental discovery in 1838,103 that wood does not represent a uniform substance but rather consists, in addition to cellulose, of a “true lignous material,” more than a century of intensive research was required to establish the aromatic nature of this natural product and to elaborate the principles of its constitution. As a result of these investigations we know now that lignin is a complex polymer originating from the random oxidative polymerization of hydroxylated cinnamyl alcohols (Fig. 1) as principal constituents, as discussed in the previous chapter. About 20 years ago, when it became increasingly evident that lignin in fact is derived from substituted cinnamyl alcohols, interest was focussed in a variety of laboratories on the reactions leading to lignin and its precursors. The results obtained by a multitude of tracer experiments revealed the principal pathways from CO2 to lignin, 8, 36, 65 thus providing an ample basis for our understanding of the lignification process. However, there remained many questions which could not be answered satisfactorily by these techniques.

Georg G. Gross

Papers

Purification and properties of cinnamyl alcohol dehydrogenase from higher plants involved in lignin biosynthesis

Isolation and Properties of Hydroxycinnamate:CoA Ligase from Lignifying Tissue of Forsthia

Cell wall-bound malate dehydrogenase from horseradish

Three novel enzymes involved in the reduction of ferulic acid to coniferyl alcohol in higher plants: ferulate: Co a ligase, feruloyl‐Co a reductase and coniferyl alcohol oxidoreductase

Biosynthesis of Lignin and Related Monomers