Several reactive oxygen species (ROS) are continuously produced in plants as byproducts of aerobic metabolism. Depending on the nature of the ROS species, some are highly toxic and rapidly detoxified by various cellular enzymatic and nonenzymatic mechanisms. Whereas plants are surfeited with mechanisms to combat increased ROS levels during abiotic stress conditions, in other circumstances plants appear to purposefully generate ROS as signaling molecules to control various processes including pathogen defense, programmed cell death, and stomatal behavior. This review describes the mechanisms of ROS generation and removal in plants during development and under biotic and abiotic stress conditions. New insights into the complexity and roles that ROS play in plants have come from genetic analyses of ROS detoxifying and signaling mutants. Considering recent ROS-induced genome-wide expression analyses, the possible functions and mechanisms for ROS sensing and signaling in plants are compared with those in animals and yeast.

REACTIVE OXYGEN SPECIES: Metabolism, Oxidative Stress, and Signal Transduction

http://paymanhassibi.persiangig.com/document/mittler%20oxidative.pdf

Oxidative stress, antioxidants and stress tolerance

Reactive oxygen species and antioxidant machinery in abiotic stress tolerance in crop plants

To cope with environmental fluctuations and to prevent invasion by pathogens, plant metabolism must be flexible and dynamic. Active oxygen species, whose formation is accelerated under stress conditions, must be rapidly processed if oxidative damage is to be averted. The lifetime of active oxygen species within the cellular environment is determined by the antioxidative system, which provides crucial protection against oxidative damage. The antioxidative system comprises numerous enzymes and compounds of low molecular weight. While research into the former has benefited greatly from advances in molecular technology, the pathways by which the latter are synthesized have received comparatively little attention. The present review emphasizes the roles of ascorbate and glutathione in plant metabolism and stress tolerance. We provide a detailed account of current knowledge of the biosynthesis, compartmentation, and transport of these two important antioxidants, with emphasis on the unique insights and advances gained by molecular exploration.

ASCORBATE AND GLUTATHIONE: Keeping Active Oxygen Under Control

▪ Abstract Plant responses to salinity stress are reviewed with emphasis on molecular mechanisms of signal transduction and on the physiological consequences of altered gene expression that affect biochemical reactions downstream of stress sensing. We make extensive use of comparisons with model organisms, halophytic plants, and yeast, which provide a paradigm for many responses to salinity exhibited by stress-sensitive plants. Among biochemical responses, we emphasize osmolyte biosynthesis and function, water flux control, and membrane transport of ions for maintenance and re-establishment of homeostasis. The advances in understanding the effectiveness of stress responses, and distinctions between pathology and adaptive advantage, are increasingly based on transgenic plant and mutant analyses, in particular the analysis of Arabidopsis mutants defective in elements of stress signal transduction pathways. We summarize evidence for plant stress signaling systems, some of which have components analogous to t...

Plant cellular and molecular responses to high salinity.

Environmental stress is the major limiting factor in plant productivity. Much of the injury to plants caused by stress exposure is associated with oxidative damage at the cellular level. Widespread losses of forests and crops due to ozone pollution provide a highly visible example of oxidative stress (see Tingey et al., 1993, for a review), but less obvious losses caused by oxidative damage associated with periods of cold or drought also take their toll in the accumulation of incremental setbacks during a growing season. The role of ROIs in plant stress damage is indicated by the increased production of ROIs and the increased oxidative damage in tissues during stress. In plants, the highly energetic reactions of photosynthesis and an abundant oxygen supply make the chloroplast a particularly rich source of ROIs. High light intensity can lead to excess reduction of PSI so that CO2 fixation cannot keep pace and NADP+ pools are reduced. Under these conditions, 02 can compete for electrons from PSI, leading to the generation of ROIs through the Mehler reaction. When CO2 fixation is limited by environmental conditions such as cold temperatures or low CO2 availability (closed stomata), excess PSI reduction and increased ROI production can occur even at moderate light intensities. Efficient removal of ROIs from chloroplasts is critical, since H202 concentrations as low as 10 ptM can inhibit photosynthesis by 50% (Kaiser, 1979). Although the toxicity of *?2and H202 themselves is relatively low, their metal-dependent conversion to the highly toxic -OH via the Haber-Weiss reaction is thought to be responsible for the majority of the biological damage associated with these molecules. Antioxidant systems of plant chloroplasts include enzymes such as SOD and APX, and nonenzymatic components such as ascorbic acid and glutathione. The proposed ROI scavenging pathway of chloroplasts is shown in Figure 1 (Asada, 1994). Superoxide radicals are produced by the reduction of molecular oxygen at PSI via the Mehler reaction. This ?2- is rapidly dismuted to H202 by SOD that is associated with the thylakoid. The H202 produced is

Dissection of Oxidative Stress Tolerance Using Transgenic Plants.

Transgenic tobacco seedlings that overexpress a cDNA encoding an enzyme with both glutathione S-transferase (GST) and glutathione peroxidase (GPX) activity had GST- and GPX-specific activities approximately twofold higher than wild-type seedlings. These GST/GPX overexpressing seedlings grew significantly faster than control seedlings when exposed to chilling or salt stress. During chilling stress, levels of oxidized glutathione (GSSG) were significantly higher in transgenic seedlings than in wild-types. Growth of wild-type seedlings was accelerated by treatment with GSSG, while treatment with reduced glutathione or other sulfhydryl-reducing agents inhibited growth. Therefore, overexpression of GST/GPX can stimulate seedling growth under chilling and salt stress, and this effect could be caused by oxidation of the glutathione pool.

Overexpression of glutathione S-transferase/glutathione peroxidase enhances the growth of transgenic tobacco seedlings during stress.

Transgenic tobacco plants that express a chimeric gene that encodes chloroplast-localized Cu/Zn superoxide dismutase (SOD) from pea have been developed. To investigate whether increased expression of chloroplast-targeted SOD could alter the resistance of photosynthesis to environmental stress, these plants were subjected to chilling temperatures and moderate (500 mumol of quanta per m2 per s) or high (1500 mumol of quanta per m2 per s) light intensity. During exposure to moderate stress, transgenic SOD plants retained rates of photosynthesis approximately 20% higher than untransformed tobacco plants, implicating active oxygen species in the reduction of photosynthesis during chilling. Unlike untransformed plants, transgenic SOD plants were capable of maintaining nearly 90% of their photosynthetic capacity (determined by their photosynthetic rates at 25 degrees C) following exposure to chilling at high light intensity for 4 hr. These plants also showed reduced levels of light-mediated cellular damage from the superoxide-generating herbicide methyl viologen. These results demonstrate that SOD is a critical component of the active-oxygen-scavenging system of plant chloroplasts and indicate that modification of SOD expression in transgenic plants can improve plant stress tolerance.

https://www.pnas.org/content/pnas/90/4/1629.full.pdf

Increased resistance to oxidative stress in transgenic plants that overexpress chloroplastic Cu/Zn superoxide dismutase.

Photosynthesis of leaf discs from transgenic tobacco plants (Nicotiana tabacum) that express a chimeric gene that encodes chloroplast-localized Cu/Zn superoxide dismutase (SOD+) was protected from oxidative stress caused by exposure to high light intensity and low temperature Under the same conditions, leaf discs of plants that did not express the pea SOD isoform (SOD-) had substantially lower photosynthetic rates Young plants of both genotypes were more sensitive to oxidative stress than mature plants, but SOD+ plants retained higher photosynthetic rates than SOD- plants at all developmental stages tested Not surprisingly, SOD+ plants had approximately 3-fold higher SOD specific activity than SOD- plants However, SOD+ plants also exhibited a 3- to 4-fold increase in ascorbate peroxidase (APX) specific activity and had a corresponding increase in levels of APX mRNA Dehydroascorbate reductase and glutathione reductase specific activities were the same in both SOD+ and SOD- plants These results indicate that transgenic tobacco plants that overexpress pea Cu/Zn SOD II can compensate for the increased levels of SOD with increased expression of the H2O2-scavenging enzyme APX Therefore, the enhancement of the active oxygen-scavenging system that leads to increased oxidative stress protection in SOD+ plants could result not only from increased SOD levels but from the combined increases in SOD and APX activity

Overexpression of Superoxide Dismutase Protects Plants from Oxidative Stress (Induction of Ascorbate Peroxidase in Superoxide Dismutase-Overexpressing Plants)

Overexpression of a tobacco glutathione S-transferase with glutathione peroxidase activity (GST/GPX) in transgenic tobacco (Nicotiana tabacum L.) enhanced seedling growth under a variety of stressful conditions. In addition to increased GST and GPX activity, transgenic GST/GPX-expressing (GST+) seedlings had elevated levels of monodehydroascorbate reductase activity. GST+ seedlings also contained higher levels of glutathione and ascorbate than wild-type seedlings and the glutathione pools were more oxidized. Thermal or salt-stress treatments that inhibited the growth of wild-type seedlings also caused increased levels of lipid peroxidation. These treatments had less effect on the growth of GST+ seedling growth and did not lead to increased lipid peroxidation. Stress-induced damage resulted in reduced metabolic activity in wild-type seedlings while GST+ seedlings maintained metabolic activity levels comparable to seedlings grown under control conditions. These results indicate that overexpression of GST/GPX in transgenic tobacco seedlings provides increased glutathione-dependent peroxide scavenging and alterations in glutathione and ascorbate metabolism that lead to reduced oxidative damage. We conclude that this protective effect is primarily responsible for the ability of GST+ seedlings to maintain growth under stressful conditions.

/pdf/stress-tolerance-in-transgenic-tobacco-seedlings-that-40crmdpjnf.pdf

Randy D. Allen

Papers

Dissection of Oxidative Stress Tolerance Using Transgenic Plants.

Overexpression of glutathione S-transferase/glutathione peroxidase enhances the growth of transgenic tobacco seedlings during stress.

Increased resistance to oxidative stress in transgenic plants that overexpress chloroplastic Cu/Zn superoxide dismutase.

Overexpression of Superoxide Dismutase Protects Plants from Oxidative Stress (Induction of Ascorbate Peroxidase in Superoxide Dismutase-Overexpressing Plants)

Stress tolerance in transgenic tobacco seedlings that overexpress glutathione S-transferase/glutathione peroxidase.