When photosynthetic organisms are exposed to abiotic stress, their photosynthetic activity is significantly depressed. In particular, photosystem II (PSII) in the photosynthetic machinery is readily ...inactivated under strong light and this phenomenon is referred to as photoinhibition of PSII. Other types of abiotic stress act synergistically with light stress to accelerate photoinhibition. Recent studies of photoinhibition have revealed that light stress damages PSII directly, whereas other abiotic stresses act exclusively to inhibit the repair of PSII after light-induced damage (photodamage). Such inhibition of repair is associated with suppression, by reactive oxygen species (ROS), of the synthesis of proteins de novo and, in particular, of the D1 protein, and also with the reduced efficiency of repair under stress conditions. Gene-technological improvements in the tolerance of photosynthetic organisms to various abiotic stresses have been achieved via protection of the repair system from ROS and, also, by enhancing the efficiency of repair via facilitation of the turnover of the D1 protein in PSII. In this review, we summarize the current status of research on photoinhibition as it relates to the effects of abiotic stress and we discuss successful strategies that enhance the activity of the repair machinery. In addition, we propose several potential methods for activating the repair system by gene-technological methods.
Repair of photosystem II (PSII) during photoinhibition involves replacement of photodamaged D1 protein by newly synthesized D1 protein. In this review, we summarize evidence for the indispensability ...of ATP in the degradation and synthesis of D1 during the repair of PSII. Synthesis of one molecule of the D1 protein consumes more than 1,300 molecules of ATP equivalents. The degradation of photodamaged D1 by FtsH protease also consumes approximately 240 molecules of ATP. In addition, ATP is required for several other aspects of the repair of PSII, such as transcription of psbA genes. These requirements for ATP during the repair of PSII have been demonstrated by experiments showing that the synthesis of D1 and the repair of PSII are interrupted by inhibitors of ATP synthase and uncouplers of ATP synthesis, as well as by mutation of components of ATP synthase. We discuss the contribution of cyclic electron transport around photosystem I to the repair of PSII. Furthermore, we introduce new terms relevant to the regulation of the PSII repair, namely, “ATP‐dependent regulation” and “redox‐dependent regulation,” and we discuss the possible contribution of the ATP‐dependent regulation of PSII repair under environmental stress.
Here, we review papers published from 1954 to 2016 that are related to the repair of photosystem II (PSII), ATP synthesis, linear and cyclic electron transport, and the environmental stress‐induced stimulation of photoinhibition of PSII. This review is not a simple collection of references but a “real” review, because we have carefully evaluated previous and recent publications and are proposing a new concept for an additional mechanism of regulation in the PSII repair during photoinhibition.
Recent studies of responses of cyanobacterial cells to salt stress have revealed that the NaCl-induced decline in the photosynthetic activities of photosystems II and I involves rapid and slow ...changes. The rapid decreases in the activities of both photosystems, which occur within a few minutes, are reversible and are associated with osmotic effects, which induce the efflux of water from the cytosol through water channels and rapidly increase intracellular concentrations of salts. Slower decreases in activity, which occur within hours, are irreversible and are associated with ionic effects that are due to the influx of Na⁺ and Cl⁻ ions through K⁺(Na⁺) channels and, probably, Cl⁻ channels, with resultant dissociation of extrinsic proteins from photosystems. In combination with light stress, salt stress significantly stimulates photoinhibition by inhibiting repair of photodamaged photosystem II. Tolerance of photosystems to salt stress can be enhanced by genetically engineered increases in the unsaturation of fatty acids in membrane lipids and by intracellular synthesis of compatible solutes, such as glucosylglycerol and glycinebetaine. In this review, we summarize recent progress in research on the effects of salt stress on photosynthesis in cyanobacteria.
Photoinhibition of photosystem II (PSII) occurs when the rate of photodamage to PSII exceeds the rate of the repair of photodamaged PSII. Recent examination of photoinhibition by separate ...determinations of photodamage and repair has revealed that the rate of photodamage to PSII is directly proportional to the intensity of incident light and that the repair of PSII is particularly sensitive to the inactivation by reactive oxygen species (ROS). The ROS‐induced inactivation of repair is attributable to the suppression of the synthesis de novo of proteins, such as the D1 protein, that are required for the repair of PSII at the level of translational elongation. Furthermore, molecular analysis has revealed that the ROS‐induced suppression of protein synthesis is associated with the specific inactivation of elongation factor G via the formation of an intramolecular disulfide bond. Impairment of various mechanisms that protect PSII against photoinhibition, including photorespiration, thermal dissipation of excitation energy, and the cyclic transport of electrons, decreases the rate of repair of PSII via the suppression of protein synthesis. In this review, we present a newly established model of the mechanism and the physiological significance of repair in the regulation of the photoinhibition of PSII.
Various compatible solutes enable plants to tolerate abiotic stress, and glycinebetaine (GB) is one of the most-studied among such solutes. Early research on GB focused on the maintenance of cellular ...osmotic potential in plant cells. Subsequent genetically engineered synthesis of GB-biosynthetic enzymes and studies of transgenic plants demonstrated that accumulation of GB increases tolerance of plants to various abiotic stresses at all stages of their life cycle. Such GB-accumulating plants exhibit various advantageous traits, such as enlarged fruits and flowers and/or increased seed number under non-stress conditions. However, levels of GB in transgenic GB-accumulating plants are relatively low being, generally, in the millimolar range. Nonetheless, these low levels of GB confer considerable tolerance to various stresses, without necessarily contributing significantly to cellular osmotic potential. Moreover, low levels of GB, applied exogenously or generated by transgenes for GB biosynthesis, can induce the expression of certain stress-responsive genes, including those for enzymes that scavenge reactive oxygen species. Thus, transgenic approaches that increase tolerance to abiotic stress have enhanced our understanding of mechanisms that protect plants against such stress.
Inhibition of the activity of photosystem II (PSII) under strong light is referred to as photoinhibition. This phenomenon is due to an imbalance between the rate of photodamage to PSII and the rate ...of the repair of damaged PSII. In the “classical” scheme for the mechanism of photoinhibition, strong light induces the production of reactive oxygen species (ROS), which directly inactivate the photochemical reaction center of PSII. By contrast, in a new scheme, we propose that photodamage is initiated by the direct effect of light on the oxygen-evolving complex and that ROS inhibit the repair of photodamaged PSII by suppressing primarily the synthesis of proteins
de novo. The activity of PSII is restricted by a variety of environmental stresses. The effects of environmental stress on damage to and repair of PSII can be examined separately and it appears that environmental stresses, with the exception of strong light, act primarily by inhibiting the repair of PSII. Studies have demonstrated that repair-inhibitory stresses include CO
2 limitation, moderate heat, high concentrations of NaCl, and low temperature, each of which suppresses the synthesis of proteins
de novo, which is required for the repair of PSII. We postulate that most types of environmental stress inhibit the fixation of CO
2 with the resultant generation of ROS, which, in turn, inhibit protein synthesis.
Glycinebetaine (GB) has been studied extensively as a compatible solute because of the availability of GB-accumulating transgenic plants that harbor a variety of transgenes for GB-biosynthetic ...enzymes. Both the exogenous application of GB and the genetically engineered biosynthesis of GB increase the tolerance of plants to abiotic stress. As reviewed here, studies of such increased tolerance to abiotic stress have led to considerable progress in the characterization of the roles of GB in stress tolerance in plants. In particular, the reproductive organs of GB-accumulating transgenic plants exhibit enhanced tolerance to abiotic stress. Furthermore, accumulation of GB results in increased yield potentials under non-stress conditions.
Inhibition of the activity of photosystem II (PSII) under strong light is referred to as photoinhibition. This phenomenon is due to the imbalance between the rate of photodamage to PSII and the rate ...of the repair of damaged PSII. Photodamage is initiated by the direct effects of light on the oxygen-evolving complex and, thus, photodamage to PSII is unavoidable. Studies of the effects of oxidative stress on photodamage and subsequent repair have revealed that reactive oxygen species (ROS) act primarily by inhibiting the repair of photodamaged PSII and not by damaging PSII directly. Thus, strong light has two distinct effects on PSII; it damages PSII directly and it inhibits the repair of PSII via production of ROS. Investigations of the ROS-induced inhibition of repair have demonstrated that ROS suppress the synthesis de novo of proteins and, in particular, of the D1 protein, that are required for the repair of PSII. Moreover, a primary target for inhibition by ROS appears to be the elongation step of translation. Inhibition of the repair of PSII by ROS is accelerated by the deceleration of the Calvin cycle that occurs when the availability of CO
2 is limited. In this review, we present a new paradigm for the action of ROS in photoinhibition.
Poikilothermic organisms are exposed to frequent changes in environmental conditions and their survival depends on their ability to acclimate to such changes. Changes in ambient temperature and ...osmolarity cause fluctuations in the fluidity of cell membranes. Such fluctuations are considered to be critical to the initiation of the regulatory reactions that ultimately lead to acclimation. The mechanisms responsible for the perception of changes in membrane fluidity have not been fully characterized. However, the analysis of genome-wide gene expression using DNA microarrays has provided a powerful new approach to studies of the contribution of membrane fluidity to gene expression and to the identification of environmental sensors. In this review, we focus on the mechanisms that regulate membrane fluidity, on putative sensors that perceive changes in membrane fluidity, and on the subsequent expression of genes that ensures acclimation to a new set of environmental conditions.
The accumulation of compatible solutes, such as betaines, proline and sugar alcohols, is a widespread response that may protect plants against environmental stress. It is not yet fully understood how ...these compounds are involved in the stress tolerance of whole plants. Some plants have been genetically engineered to express enzymes that catalyze the synthesis of various compatible solutes. Some interventions have increased the tolerance of some crop plants to abiotic stress. Furthermore, analysis of such transgenic plants has begun to clarify the roles of compatible solutes in stress tolerance.
Genetic transformation has allowed the introduction of new pathways for the biosynthesis of various compatible solutes into plants, resulting in the production of transgenic plants with improved tolerance to stress.
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