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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.

This perspective advocates a holistic view of photoinhibition from the molecule to the biosphere; a view that integrates many biophysical and biochemical processes in antennae and reaction centers of the photosystems that, when acting in concert, allow plants to respond to diverse and dynamic light conditions in many different environments. We take the general view that photoinhibition refers to a reduction in the efficiency of light use in the photosynthetic apparatus (Kok, 1956). Since the 1970s, biochemical, ecophysiological, and genetic studies of photosynthetic functions in strong light, in vivo and in situ , and their interactions with biotic and abiotic stresses, have significantly advanced our understanding of photoinhibition. We trace some origins of the idea then, that slow dark reactions, such as growth, CO2 assimilation, photorespiration, and photosynthetic electron transport, ultimately limit light use in photosynthesis, and thus determine whether light is in excess and the magnitude of “excitation pressure” in the photosynthetic apparatus at any moment. This and other ideas are followed through studies of photoacclimation in leaves of plants and algae from diverse terrestrial and marine environments. We highlight two currently interesting possibilities for the photoprotective dissipation of “excitation pressure” that reduce the efficiency of photosynthesis by changes in structure and function of antenna pigment-protein complexes and in the populations of functional and non-functional PS II centers. We conclude by briefly considering challenges presented nowby the discovery of “gain of function”, very high light resistant ( VHLR ) mutants of Chlamydomonas , by the accessory lutein-epoxide cycle, and by technologies for remote sensing of photoinhibition in the field.

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.

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 photoinhibitionand photoprotection-related processes under nutrient deficiencies, drought, and salinity stress, and to discuss the role that photoinhibition could play under such environmental stresses.

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.

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

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.

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.