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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) H2O-oxidizing and O2-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. [ABSTRACT FROM AUTHOR]

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 QA− and QB− 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 low temperatures or to high light induces a significant downshift in the temperature maxima for S2QB−B− and S3QB− recombinations relative to S2QA− and S3QA− 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 between QA and QB 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 (QA−), that is, increased excitation pressure, facilitating non-radiative P680+QA− 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. [ABSTRACT FROM AUTHOR]

Photoprotection seems to be an intrinsic property of light-harvesting systems, and an interesting question to address is whether the light-harvesting or the photoprotection function was the “original” function, and which function evolved subsequently. It appears that the cyanobacterial one-helix proteins, the presumed ancestors to the LHC proteins, were not designed as antenna proteins but were involved in photoprotection and /or pigment metabolism. Some intermediate steps (two- and four-helix proteins) also seem to have photoprotective functions. The antenna function appeared later in evolution, and many different LHC proteins with somewhat diversified functions arose. To some extent, this happened before the lineages leading to Chlamydomonas and higher plants separated, but further diversification also took place following the split, and some of the proteins may have evolved in a direction away from optimizing light harvesting. When the evolution of feedback de-excitation is put into this evolutionary scheme, it is likely that xanthophyll conversions, that evolved previously to optimize photoprotection, were starting to be used as indicators of light stress and regulators of antenna function. [ABSTRACT FROM AUTHOR]

Plants have diverse defense mechanisms against high light stress. Plants can reduce absorption of light energy through chloroplast avoidance and antenna size reduction. However, the capacity of the avoidance and the antenna size reduction for protection is limited, so that plants often absorb more energy than they can use. Therefore, plants need mechanisms to deal with this excess absorbed light energy, such as harmless thermal dissipation by feedback de-excitation. The transthylakoid pH gradient, xanthophyll cycle, PsbS, and other light-harvesting complex proteins are required for this thermal dissipation. In addition, alternative electron transport allows electrons to pass to acceptors other than CO2, thereby relieving overreduction of electron transport components in high light conditions. To detoxify reactive oxygen species that are inevitably produced during high light stress, plants have antioxidants including carotenoids, ascorbate, and tocopherols. In spite of these photoprotective mechanisms, photodamage may still occur, and efficient repair of damaged systems could be a photoprotective mechanism. In this chapter, recently published molecular genetics studies on each step of photoprotection have been reviewed. Genes required for each defense mechanism that have been identified thus far are introduced, and cloned genes that can possibly be related to photoprotection are discussed. [ABSTRACT FROM AUTHOR]

Application of novel techniques for the characterization of in vivo protein phosphorylation has revealed sixteen distinct phosphorylation sites in ten integral and two peripheral proteins in photosynthetic thylakoid membranes. In addition to phosphorylation of the photosystem II (PS II) proteins D1, D2, CP43, and PsbH, and the light-harvesting antenna polypeptides LHCII and CP29, phosphorylation has been found in photosystem I (PS I) protein PsaD and in two recently identified proteins TSP9 and TMP14. The accumulated knowledge favors an involvement of reversible phosphorylation in adaptive stress responses and cellular signaling, but not in direct regulation of photosynthetic activities like electron transfer or oxygen evolution. Enhancement of PS II protein phosphorylation by abiotic stress maintains the integrity of PS II before it migrates to the stroma regions of the thylakoids where dephosphorylation and subsequent protein turnover take place. Specific dephosphorylation of the D1, D2, and CP43 polypeptides is performed by a heat shock-inducible protein phosphatase intrinsic to the thylakoid membrane. The phosphatase activity is regulated by the lumenal peptidyl-prolyl isomerase TLP40. This regulation may coordinate the protein folding activity of TLP40 in the lumen with the protein dephosphorylation at the opposite side of the thylakoid membrane. Reversible phosphorylation of LHCII in vivo is under complex redox and metabolic control and is probably involved in regulation of the size of the PS II antennae. Cold- and high light-induced phosphorylation of CP29 may facilitate photoprotective energy dissipation by changing PS II-LHCII interactions under stress conditions. Phosphorylation of PsaD protein could be involved in regulation of PS I stability and ferredoxin reduction by PS I. The light-induced phosphorylation of TSP9, followed by its release from thylakoids, is implicated in plant cell signaling. The exact physiological roles of the protein phosphorylation events in thylakoids should be revealed by studies with appropriate mutants of plants and algae. [ABSTRACT FROM AUTHOR]

This chapter summarizes the effects of excessive solar radiation on aquatic primary producers with an emphasis on macroalgae. The introductory paragraphs deal with the aquatic environment and the specific implications for sessile algae and their vertical distribution on the coast. Macroalgae are exposed to dramatically changing irradiances and complicated light patterns governed by the diel solar cycle, the tidal rhythm, and changing cloud cover. The following sections concentrate on the phenomenon of photoinhibition with specific reference to in situ measurements with as little disturbance of the specimens on site as possible. Despite its low percentage contribution in solar radiation, short wavelength ultraviolet is a major component in photoinhibition of algae in their natural habitat. Fast kinetics of fluorescence parameters demonstrates the rapid adaptation of the organisms to their changing photic environment. Early developmental stages are more prone to inhibitory effects of excessive solar radiation. Pigment bleaching and resynthesis are important consequences of solar exposure. Macroalgae have developed several strategies for protection against excessive light stress. UV-absorbing substances, which they share with cyanobacteria and phytoplankton, limit the amount of UV photons reaching the photosynthetic apparatus and the nucleus. They include carotenoids, mycosporine-like amino acids, as well as several chemically not yet identified substances. In addition, the fast turnover of the D1 protein in photosystem II allows rapid recovery from photoinhibition. [ABSTRACT FROM AUTHOR]

Some of the more frequent abiotic stresses in plants are limited availability of nutrients and water, as well as salinity. All these situations occur both in natural habitats and in crops. Stressed plants often experience decreases in photosynthetic rates, whereas they still harvest sunlight. Environmental stresses such as those may decrease the efficiency with which solar energy is harvested and used by plants in photosynthetic reactions. This feature is what the scientific community has often called photoinhibition. Some researchers tacitly assume that photoinhibition may result from photodamage, whereas others believe that it is more the integration of a series of regulatory and protective adjustments. The aim of this review is to summarize the current knowledge concerning photoinhibition- and photoprotection-related processes under nutrient deficiencies, drought, and salinity stress, and to discuss the role that photoinhibition could play under such environmental stresses. [ABSTRACT FROM AUTHOR]

The photosynthetic apparatus is exquisitely adapted to capture light energy and convert it into reduced carbon compounds while also protecting against the potential deleterious effects of excessive excitation energy. The latter is achieved through fine regulation of thermal energy dissipation over multiple time scales and in response to many different environmental stresses. Over short time scales in the absence of additional stress, control is exerted through pH regulation of the enzymatic conversion of violaxanthin to zeaxanthin (and its return to violaxanthin) and engagement of zeaxanthin in thermal energy dissipation. Under more extreme exposure to excess light (transfer of shade leaves to high light or the imposition of additional stresses in the presence of high light), greater levels of zeaxanthin are retained and may also be maintained in a dissipative configuration even in darkness. Engagement of zeaxanthin in thermal energy dissipation lowers the maximal efficiency of photosystem II (PS II) as the excess excitation energy is diverted away from the reaction centers and harmlessly released as heat. Thus, maximal PS II efficiency exhibits decreases and increases with varying degrees of light absorption. Under prolonged and/or pronounced exposure to excess light, maximal PS II efficiency can furthermore exhibit nocturnally sustained decreases as the potential for photoprotective zeaxanthin-dependent energy dissipation is maintained. Zeaxanthin-dependent energy dissipation that is sustained at moderate temperatures is also typically accompanied by downregulation of photosynthesis, including photosynthetic electron transport. Decreases in photosynthetic electron transport presumably lower the likelihood of electrons reducing molecular oxygen to superoxide, and sustained zeaxanthin-dependent energy dissipation mitigates the formation of singlet excited oxygen. Thus, while sustained decreases in maximal PS II efficiency and photosynthetic capacity are key characteristics of photoinhibition, they are also the features that provide powerful photoprotection against the formation of toxic reactive oxygen species. [ABSTRACT FROM AUTHOR]

The chloroplast-coded D1 protein of Photosystem II (PS II) is the major membrane protein synthesized within the plastid. It is involved in light-dependent electron transport, is a major target for photosynthesis herbicides and is universal to oxygenic phototrophs. The defining feature of D1 is its rapid turnover in spite of its being a structural component of the PS II reaction center core. Processing of nascent D1 precursor (33.5–34 kDa) occurs on unstacked stromal lamellae. The mature protein (32 kDa) then migrates to the grana where an initial scission occurs producing a 23 kDa N-terminal degradation fragment. Post-translational and reversible palmitoylation and phosphorylation accompany the protein along its life cycle. Both anabolism and catabolism of D1 are photoregulated, with synthesis coupled to phosphorylation but degradation coupled to PS II electron transport. Dephosphorylation of D1, in turn, is regulated by PS I excitation. Thus, the phosphorylation state of the protein is sensitive to the relative energy distribution between the two photosystems. Beyond redox regulation of D1 phosphorylation, an internal, circadian clock exerts overriding control. Two photosensitizers are involved in D1 degradation: chlorophyll pigments in the visible and far-red regions of the spectrum, and plastosemiquinone in the UV-B region. D1 degradation in visible light is a process only marginally overlapping with photoinhibition and overwhelmingly associated with fluences limiting for photosynthesis. Mixing physiological levels of visible and UV-B radiances leads to synergistic effects such that above a critical threshold of UV-B, the D1 as well as its sister protein, D2, both are targeted for accelerated degradation. These and other D1 protein studies, mainly carried out with intact Spirodela plants during the past 25 years in the authors’ laboratories, are presented in a historical perspective. [ABSTRACT FROM AUTHOR]

This is an account of my personal and professional life as a student of the violaxanthin-antheraxanthin-zeaxanthin scheme for the xanthophyll cycle in higher plants. I had no early vision of becoming a scientist, but one circumstance led to another, and what began as a random walk ultimately developed into a life-long study of the biochemistry, physiology, and function of the xanthophyll cycle. The circumstances and people with whom I shared this path are described, with special attention given to the early developments. [ABSTRACT FROM AUTHOR]