The light‐dependent generation of active oxygen species is termed photooxidative stress. This can occur in two ways: (1) the donation of energy or electrons directly to oxygen as a result of photosynthetic activity; (2) exposure of tissues to ultraviolet irradiation. The light‐dependent destruction of catalase compounds the problem. Although generally detrimental to metabolism, superoxide and hydrogen peroxide may serve useful functions if rigorously controlled and compartmentalised. During photosynthesis the formation of active oxygen species is minimised by a number of complex and refined regulatory mechanisms. When produced, active oxygen species are eliminated rapidly by efficient antioxidative systems. The chloroplast is able to use the production and destruction of hydrogen peroxide to regulate the thermal dissipation of excess excitation energy. This is an intrinsic feature of the regulation of photosynthetic electron transport. Photoinhibition and photooxidation only usually occur when plants are exposed to stress. Active oxygen species are part of the alarm‐signalling processes in plants. These serve to modify metabolism and gene expression so that the plant can respond to adverse environmental conditions, invading organisms and ultraviolet irradiation. The capacity of the antioxidative defense system is often increased at such times but if the response is not sufficient, radical production will exceed scavenging and ultimately lead to the disruption of metabolism. Oxidative damage arises in high light principally when the latter is in synergy with additional stress factors such as chilling temperatures or pollution. Environmental stress can modify the photooxidative processes in various ways ranging from direct involvement in light‐induced free radical formation to the inhibition of metabolism that renders previously optimal light levels excessive. It is in just such situations that the capacity for the production of active oxygen species can exceed that for scavenging by the antioxidative defense systems. The advent of plant transformation, however, may have placed within our grasp the possibility of engineering greater stress tolerance in plants by enhancement of the antioxidative defence system.
Free radicals and other active derivatives of oxygen are inevitable by‐products of biological redox reactions. Reduced oxygen species, such as hydrogen peroxide, the superoxide radical anion and hydroxyl radicals, inactivate enzymes and damage important cellular components. In addition, singlet oxygen, produced via formation of triplet state chlorophyll, is highly destructive. This oxygen species initiates lipid peroxidation, and produces lipid peroxy radicals and lipid hydroperoxides that are also very reactive. The increased production of toxic oxygen derivatives is considered to be a universal or common feature of stress conditions. Plants and other organisms have evolved a wide range of mechanisms to contend with this problem. The antioxidant defence system of the plant comprises a variety of antioxidant molecules and enzymes. Considerable interest has been focused on the ascorbate‐glutathione cycle because it has a central role in protecting the chloroplasts and other cellular compartments from oxidative damage. It is clear that the capacity and activity of the antioxidative defence systems are important in limiting photo‐oxidative damage and in destroying active oxygen species that are produced in excess of those normally required for signal transduction or metabolism. In our studies on this system, we became aware that the answers to many unresolved questions concerning the nature and regulation of the antioxidative defence system could not be obtained easily by either a purely physiological or purely biochemical approach. Transgenic plants offered us a means by which to achieve a more complete understanding of the roles of the enzymes involved in protection against stress of many types: environmental and man‐made. The ability to engineer plants which express introduced genes at high levels provides an opportunity to manipulate the levels of these enzymes, and hence metabolism in vivo. Studies on transformed plants expressing increased activities of single enzymes of the antioxidative defence system indicate that it is possible to confer a degree of tolerence to stress by this means. However, attempts to increase stress resistance by simply increasing the activity of one of the antioxidant enzymes have not always been successful presumably because of the need for a balanced interaction of protective enzymes. The study of these transformed plants has allowed a more complete understanding of the roles of individual enzymes in metabolism. Protection against oxidative stress has become a feasible objective through the application of molecular genetic techniques in conjunction with a biochemical and physiological approach.
I. M. 1997. Hydrogen peroxideand glutathione-associated mechanisms of acclimatory stress tolerance and signalling. -Physiol. Plant. 100: 241-254.Plants adapt to environmental stresses through specific genetic responses. The molecular mechanisms associated with signal transduction, leading to changes in gene expression early in the stress response, are largely unknown. It is clear, however, that gene expression associated with acclimatory responses is sensitive to the redox state of the cell. Of the many components which contribute to the redox balance of the cell, two factors have been shown to be crucial in mediating stress responses. Thiol/disulphide exchange reactions, particularly involving the glutathione pool and the generation of the oxidant H2O2, are central components of signal transduction in both environmental and biotic stresses. These molecules are multifunctional triggers, modulating metabolism and gene expression. Both are able to cross biological membranes and diffuse or be transported long distances from their sites of origin. Glutathione and H2O2 may act alone or in unison, in intracellular and systemic signalling systems, to achieve acclimation and tolerance to biotic and abiotic stresses.
Plants adapt to environmental stresses through specific genetic responses. The molecular mechanisms associated with signal transduction, leading to changes in gene expression early in the stress response, are largely unknown. It is clear, however, that gene expression associated with acclimatory responses is sensitive to the redox state of the cell. Of the many components which contribute to the redox balance of the cell, two factors have been shown to be crucial in mediating stress responses. Thiol/disulphide exchange reactions, particularly involving the glutathione pool and the generation of the oxidant H2O2, are central components of signal transduction in both environmental and biotic stresses. These molecules are multifunctional triggers, modulating metabolism and gene expression. Both are able to cross biological membranes and diffuse or be transported long distances from their sites of origin. Glutathione and H2O2 may act alone or in unison, in intracellular and systemic signalling systems, to achieve acclimation and tolerance to biotic and abiotic stresses.
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