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Understanding the role of the xanthophyll cycle and elucidating the mechanisms of antenna quenching through the non-photochemical dissipation of excess absorbed energy in the photoprotection of the photochemical apparatus continues to be a major focus of photosynthetic research. In addition to antenna quenching, there is evidence for the non-photochemical dissipation of excess energy through the PS II reaction center. Hence, this photoprotective mechanism is called reaction center quenching. One technique to assess reaction center quenching is photosynthetic thermoluminescence. This technique represents a simple but powerful probe of PS II photochemistry that measures the light emitted due to the reversal of PS II charge separation through the thermally-dependent recombination of the negative charges stabilized on Q^− _A and Q^− _B on the acceptor side of PS II with the positive charges accumulated in the S2- and S3-states of the oxygen evolving complex. Changes in the temperature maxima for photosynthetic thermoluminescence may reflect changes in redox potentials of recombining species within PS II reaction centers. Exposure of Synechococcus sp. PCC 7942, Pinus sylvestris L., Arabidopsis thaliana , and Chlamydomonas reinhardtii to either lowtemperatures or to high light induces a significant downshift in the temperature maxima for S2Q^− _B and S3Q^− _B recombinations relative to S2Q^− _A and S3Q^− _A recombinations. These shifts in recombination temperatures are indicative of lower activation energy for the S2Q^− _B redox pair recombination and a narrowing of the free energy gap betweenQ_AandQ_B electron acceptors. This, in turn, is associated with a decrease in the overall thermoluminescence emission. We propose that environmental factors such as high light and low temperature result in an increased population of reduced QA (Q^− _A), that is, increased excitation pressure, facilitating non-radiative P680+Q^− _A radical pair recombination within the PS II reaction center. The underlying molecular mechanisms regulating reaction center quenching appear to be species dependent. We conclude that reaction center quenching and antenna quenching are complementary mechanisms that may function to photoprotect PS II to different extents in vivo depending on the species as well as the environmental conditions to which the organism is exposed.

This Chapter provides highlights on the mechanism of a photosystem II (PS II) damage and repair cycle in chloroplasts. Photo-oxidative damage to the PS II reaction center is a phenomenon that occurs in every organism of oxygenic photosynthesis. Through the process of evolution, an elaborate repair mechanism was devised, one that rectifies this presumably unavoidable and irreversible photoinhibition and restores the PS II charge separation activity. The repair process entails several enzymatic reactions for the selective removal and replacement of the inactivated D1/32 kD reaction center protein (the chloroplast-encoded psbA gene product) from the massive (>1,000 kD) H_2O-oxidizing and O_2-evolving PS II holocomplex. This repair process is unique in the annals of biology; nothing analogous in complexity and specificity has been reported in other biological systems. Elucidation of the repair mechanism may reveal the occurrence of hitherto unknown regulatory and catalytic reactions for the selective in situ replacement of specific proteins from within multi-protein complexes. This may not only have significant applications in photosynthesis and agriculture but also in medicine and other fields.

The activity of photosystem II (PS II) is severely restricted by a variety of environmental factors and, under environmental stress, is determined by the balance between the rate of damage to PS II and the rate of the repair of damaged PS II. The effects of environmental stress on damage and repair can be examined separately and it appears that, while light can damage PS II directly, most types of environmental stress act primarily by inhibiting the repair of PS II. Studies in cyanobacteria have demonstrated that repair-inhibiting conditions include oxidative stress, salt stress, and low-temperatures stress, each of which suppresses the de novo synthesis of proteins, in particular the D1 protein, which is required for the repair of PS II. The synergistic effects of combinations of different types of environmental stress suggest that it is the repair process that determines the sensitivity of PS II to specific environmental conditions.

Cyclic electron transport around photosystem I has been proposed to play dual roles in the regulation of photosynthetic electron transport: down-regulating PS II and adjusting the ATP/NADPH ratio. Recent molecular genetics revealed that cyclic electron flow is essential for normal photosynthesis and growth. The water-water-cycle would also play a role similar to cyclic electron transport, in addition to the effective scavenging of reactive oxygen species generated in PS I. Though their rates of electron flux are lower than that of linear electron transport at steady state, these alternative electron flows are indispensable for acute responses to environmental changes and stress. Recent biochemical and molecular studies at the protein and gene level have clarified the components participating in the alternative electron transport. These new findings, including the dual functions of cyclic electron flow and the water-water cycle, and their respective roles in stress responses, are discussed in this chapter.

In the last few years, it has become apparent that reactive oxygen species (ROS) have important roles as signaling intermediaries in a large number of cellular processes, especially in relation to plants’ interactions with their environment. A complex network of low molecular weight antioxidants, ROS scavenging enzymes, and enzymes that maintain antioxidant pools are required to control the levels of ROS in all subcellular compartments. The coordinated regulation of this network by ROS themselves and stress-associated hormones such as salicylic acid, abscisic acid, and jasmonic acid reveals that antioxidant metabolism is central to considerations of how signaling networks are regulated. Furthermore, it is becoming apparent that key antioxidants such as glutathione and ascorbate are involved in the regulation of stress hormone-directed signaling pathways without any interaction with ROS. Therefore ROS and antioxidants may be key points at which the coordination of different signaling pathways is achieved. These issues are considered in this chapter.

Ascorbate, glutathione, and tocopherol are the three major low molecular weight antioxidants of plant cells. While tocopherol is hydrophobic and is found only in lipid membranes, ascorbate and glutathione are hydrophilic, accumulating to high concentrations in the chloroplast stroma and other compartments of the plant cell. Ascorbate and glutathione not only limit photo-oxidative damage but can also act independently as signal-transducing molecules regulating defense gene expression. Both metabolites transmit information concerning oxidative load and redoxbuffering capacity. Ascorbate modifies the expression of chloroplast genes. Net glutathione synthesis during stress restores the cellular redox state and allows orchestration of systemic acquired resistance. The degree of redox coupling between these antioxidants has profound implications for regulation, function, and signaling associated with the two major energy-generating systems, i.e. photosynthesis and respiration. Tocopherol fulfills an essential protective function, counter-acting the harmful effects of singlet oxygen production at photosystem II. Ascorbate reduces and thus regenerates oxidized tocopherol, but flux through this reaction is not sufficient to maintain the reduced tocopherol pool under high light stress. This may be because tocopherol regeneration draws on the ascorbate pool of the chloroplast lumen, which may be depleted under stress. Moreover, while glutathione always reduces oxidized ascorbate (dehydroascorbate), the degree of coupling between the ascorbate and glutathione redox couples is variable. The flexibility of coupling between these antioxidant pools is crucial to differential redox signaling, particularly by ascorbate and glutathione.

The chloroplast is the most important biosynthetic compartment of a green plant cell, being the site of photosynthesis and aspects of carbon, sulfur, and nitrogen assimilation as well as other pathways. At the same time, the complex enzymatic machinery of the organelle is a key target for photooxidative stress. The chloroplast contains an evolutionarily conserved set of genes and a specially adaptable gene expression machinery that is in close physical proximity to the photosynthetic apparatus, i.e. the primary source of reactive oxygen species. This adaptability somehow links the rapid gene expression response to the activity status of photosynthetic electron transport and accompanying redox reactions. In this chapter, we address the following questions: (i) which plastid gene products are subject to redox control? (ii) which stage(s) of organellar gene expression are redox-controlled? and (iii) what are the mechanisms and mediators involved?

The chloroplast proteome is encoded by genes that reside in both the chloroplast and the nucleus. This separation of genetic material necessitates a system for coordinating the expression of genes that reside in each compartment. Because the overwhelming majority of genes that encode chloroplast proteins reside in the nucleus, the regulation of nuclear genes by developmental and environmental cues plays a dominant role in chloroplast development and function. However, the chloroplast is not indifferent to its own protein composition. In fact, the chloroplast generates signals that have dramatic effects on the expression of nuclear genes that encode particular chloroplast proteins. Currently it is known that plastids produce at least a few distinct signals during chloroplast development that are required for proper expression of particular nuclear genes that encode components of the photosynthetic machinery. In response to certain environmental signals, mature chloroplasts send additional signals that regulate nuclear gene expression. The molecular nature of most of these plastid-to-nucleus signaling pathways is not well established. However, a number of studies have suggested that accumulation of certain chlorophyll precursors within plastids is a signal that regulates nuclear gene expression during chloroplast development and during the diurnal cycle. Future work in this area should provide detailed molecular information on the influence of chlorophyll synthesis and other plastid-localized metabolism on nuclear gene expression and how plants utilize this formof interorganellar communication during their lifecycles.

Peroxiredoxins (Prx) constitute a group of recently identified peroxidases that detoxify a broad range of peroxides in distinct subcellular compartments, including chloroplasts. They are ubiquitously expressed in all organisms, i.e. bacteria, fungi, and animals, as well as in cyanobacteria and plants, in which they frequently represent a considerable fraction of total cellular and organellar protein. At least seven prx genes are expressed in leaves of Arabidopsis . The gene products of four of them are targeted to chloroplasts. Five genes encoding (putative) Prx are found in Synechocystis sp. PCC 6803. Based on such circumstantial evidence, as well as biochemical analysis and observations on photosynthetic organisms with modified levels of Prx, it has been established that a subset of Prx plays a role in the context of photosynthesis. The conclusion is further strengthened by studies that showed a modulation of prx gene expression in response to photosynthetic activity. This chapter describes the properties of peroxiredoxins in general and focuses on the role of Prx in protecting the photosynthetic apparatus from oxidative damage and, possibly, in redox signaling in photooxygenic cells.

Lipoxygenases are a family of enzymes that dioxygenate unsaturated fatty acids, thus initiating lipoperoxidation of membranes and the synthesis of signaling molecules, or inducing structural and metabolic changes in the cell. This activity is the basis for the critical role of lipoxygenases in a number of pathophysiological conditions, in both animals and plants. In the past few years, a pro-apoptotic effect of lipoxygenase has been reported in different cells and tissues, leading to cell death along unrelated apoptotic pathways. However, anti-apoptotic effects of lipoxygenases have also been reported, often based on the use of enzyme inhibitors. In the present review, the characteristics of the lipoxygenase family and the role of lipoxygenase activation in apoptosis of animal and plant cells are discussed, suggesting a common signal transduction pathway in cell death conserved through the evolution of both kingdoms. In addition, the inhibition of lipoxygenases by antioxidants and its consequences on apoptosis will be presented.