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

The reaction centers of PSI and PSII in chloroplast thylakoids are the major generation site of reactive oxygen species (ROS). Photoreduction of oxygen to hydrogen peroxide (H2O2) in PSI was discovered over 50 years ago by [Mehler (1951)][1]. Subsequently, the primary reduced product was identified

Production and scavenging of reactive oxygen species in chloroplasts and their functions.

The discovery that there is a close relationship between ascorbate and glutathione dates from soon after the characterization of the chemical formulae of the two molecules ([Szent-Gyorgyi, 1931][1]; [Hopkins and Morgan, 1936][2]). Similarly, it has long been known that thylakoids can generate

Ascorbate and glutathione: the heart of the redox hub.

Reactive oxygen species (ROS) have multifaceted roles in the orchestration of plant gene expression and gene-product regulation. Cellular redox homeostasis is considered to be an “integrator” of information from metabolism and the environment controlling plant growth and acclimation responses, as well as cell suicide events. The different ROS forms influence gene expression in specific and sometimes antagonistic ways. Low molecular antioxidants (e.g., ascorbate, glutathione) serve not only to limit the lifetime of the ROS signals but also to participate in an extensive range of other redox signaling and regulatory functions. In contrast to the low molecular weight antioxidants, the “redox” states of components involved in photosynthesis such as plastoquinone show rapid and often transient shifts in response to changes in light and other environmental signals. Whereas both types of “redox regulation” are intimately linked through the thioredoxin, peroxiredoxin, and pyridine nucleotide pools, they ...

Redox Regulation in Photosynthetic Organisms: Signaling, Acclimation, and Practical Implications

Temperature stress can have a devastating effect on plant metabolism, disrupting cellular homeostasis, and uncoupling major physiological processes. A direct result of stress-induced cellular changes is the enhanced accumulation of toxic compounds in cells that include reactive oxygen species (ROS). Although a considerable amount of work has shown a direct link between ROS scavenging and plant tolerance to temperature stress, recent studies have shown that ROS could also play a key role in mediating important signal transduction events. Thus, ROS, such as superoxide (O2–), are produced by NADPH oxidases during abiotic stress to activate stress-response pathways and induce defense mechanisms. The rates and cellular sites of ROS production during temperature stress could play a central role in stress perception and protection. ROS levels, as well as ROS signals, are thought to be controlled by the ROS gene network of plants. It is likely that in plants this network is interlinked with the different networks that control temperature stress acclimation and tolerance. In this review paper, we attempt to summarize some of the recent studies linking ROS and temperature stress in plants and propose a model for the involvement of ROS in temperature stress sensing and defense.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.0031-9317.2005.00582.x

Reactive oxygen species and temperature stresses: A delicate balance between signaling and destruction

To evaluate the physiological importance of thylakoid membrane-bound ascorbate peroxidase (tAPX) in the active oxygen species-scavenging system of chloroplasts, the level of tAPX in tobacco plants was altered by expression of the tAPX cDNA in both sense and antisense orientation. The tobacco plants transformed with constructs of antisense tAPXs from spinach and tobacco could not be obtained, suggesting that the suppression of tAPX in higher plants had a severe effect on the growth even under normal conditions. In contrast, the transgenic tobacco plants (TpTAP-12) overexpressing tAPX, which had approximately 37-fold higher activity than that of the wild-type plants, were generated. The TpTAP-12 plants showed increased tolerance to oxidative stress caused by application of methylviologen (MV, 50 microm) under light intensity (300 and 1600 microE m(-2) sec(-1)) and by chilling stress with high light intensity (4 degrees C, 1000 microE m(-2) sec(-1)). At 24 h after the MV treatment under illumination at 300 microE m-2 sec-1, destruction of chlorophyll was observed in the wild-type plants, but not in the TpTAP-12 plants. The activities of thiol-modulated enzymes in the Calvin cycle, the level and redox status of ascorbate (AsA), and the activity of tAPX in the wild-type plants significantly decreased, while those in the TpTAP-12 plants were hardly changed. These observations suggest that tAPX is a limiting factor of antioxidative systems under photo-oxidative stress in chloroplasts, and that the enhanced activity of tAPX functions to maintain the AsA content and the redox status of AsA under stress conditions.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-313X.2002.01476.x

Thylakoid membrane‐bound ascorbate peroxidase is a limiting factor of antioxidative systems under photo‐oxidative stress

It has been known that leaves exposed to high light contain more L-ascorbic acid (AsA) than those in the shade. However, the mechanism of the light regulation of the AsA pool size in plants is largely unknown. In this work, the relationship between gene expression levels related to AsA biosynthesis and photosynthesis have been studied. When 2-week-old Arabidopsis plants grown under a 16 h daily photoperiod were moved into the dark, the AsA level in the leaves was decreased by 91% in 72 h, whereas it increased by 171% in the leaves of plants exposed to continuous light during the same period. Among the several enzymes of the AsA biosynthesis pathway, the transcript levels of GDP-D-mannose pyrophosphorylase, L-galactose 1-P phosphatase, L-galactono-1,4-lactone dehydrogenase, and the VTC2 gene were down-regulated in the dark. Treatment with inhibitors of photosynthesis, 3-(3,4-dichlorophenyl)-1,1-dimethylurea and atrazine, arrested a rise in the AsA pool size accompanying the decrease in the transcript levels of the genes of the above enzyme in the leaves. When the plants were transferred to a medium containing 0.5% (w/v) sucrose, the photosynthesis activities and the leaf AsA levels were lowered even under exposure to light compared with those in plants on the medium without sucrose. In contrast, the AsA level in leaves of the sugar-insensitive Arabidopsis mutant abi4/sun6 was unaffected by external sucrose. No significant difference in the expression profiles for AsA biosynthesis enzymes was observed between the wild-type and mutant plants by sucrose feeding. The results suggest that photosynthetic electron transport of chloroplasts is closely related to AsA pool size regulation in leaves.

Light regulation of ascorbate biosynthesis is dependent on the photosynthetic electron transport chain but independent of sugars in Arabidopsis

We have studied the enzymological properties of L-galactose dehydrogenase (l-GalDH), a key enzyme in the biosynthetic pathway of l-ascorbate (AsA) in plants. L-GalDH was purified approximately 560-fold from spinach leaves. The enzyme was a homodimer with a subunit mass of 36 kDa. We also cloned the full-length cDNA of spinach L-GalDH, which contained an open reading frame encoding 322 amino acid residues with a calculated molecular mass of 35,261 Da. The deduced amino acid sequence of the cDNA showed 82, 79 and 75% homology to L-GalDH from kiwifruit, apple and Arabidopsis, respectively. Recombinant enzyme expressed from the cDNA in Escherichia coli showed L-GalDH activity. Southern blot analysis revealed that the spinach L-GalDH gene occurs in a single copy. Northern blot analysis suggests that L-GalDH is expressed in different organs of spinach. The purified native L-GalDH showed high specificity for L-galactose with a Km of 116.2+/-3.2 microM. Interestingly, spinach L-GalDH exhibited reversible inhibition by AsA, the end-product of the biosynthetic pathway. The inhibition kinetics indicated a linear-competitive inhibition with a Ki of 133.2+/-7.2 microM, suggesting feedback regulation in AsA synthesis in the plant.

/pdf/feedback-inhibition-of-spinach-l-galactose-dehydrogenase-by-3jxkkhdm4q.pdf

Takashi Motoki

Papers

Thylakoid membrane‐bound ascorbate peroxidase is a limiting factor of antioxidative systems under photo‐oxidative stress

Light regulation of ascorbate biosynthesis is dependent on the photosynthetic electron transport chain but independent of sugars in Arabidopsis

Feedback inhibition of spinach L-galactose dehydrogenase by L-ascorbate.