The water-water cycle is not a major alternative sink in fluctuating light at chilling temperature.

The water-water cycle (WWC) has the potential to alleviate photoinhibition of photosystem I (PSI) in fluctuating light (FL) at room temperature and moderate heat stress. However, it is unclear whether WWC can function as a safety valve for PSI in FL at chilling temperature. In this study, we measured P700 redox state and chlorophyll fluorescence in FL at 25 °C and 4 °C in the high WWC activity plant Dendrobium officinale. At 25 °C, the operation of WWC contributed to the rapid re-oxidation of P700 upon dark-to-light transition. However, such rapid re-oxidation of P700 was not observed at 4 °C. Upon a sudden increase in light intensity, WWC rapidly consumed excess electrons in PSI and thus avoided an over-reduction of PSI at 25 °C. On the contrary, PSI was highly reduced within the first seconds after transition from low to high light at 4 °C. Therefore, in opposite to 25 °C, the WWC is not a major alternative sink in FL at chilling temperature. Upon transition from low to high light, cyclic electron transport was highly stimulated at 4 °C when compared with 25 °C. These results indicate that D. officinale enhances cyclic electron transport to partially compensate for the inactivation of WWC in FL at 4 °C.

The Plasticity of Photosystem I.

Most of life's energy comes from sunlight, and thus, photosynthesis underpins the survival of virtually all life forms. The light-driven electron transfer at photosystem I (PSI) is certainly the most important generator of reducing power at the cellular level and thereby largely determines the global amount of enthalpy in living systems (Nelson 2011). The PSI is a light-driven plastocyanin:ferredoxin oxidoreductase, which is embedded into thylakoid membranes of cyanobacteria and chloroplasts of eukaryotic photosynthetic organism. Structural determination of complexes of the photosynthetic machinery is vital for the understanding of its mode of action. Here, we describe new structural and functional insights into PSI and associated light-harvesting proteins, with a focus on the plasticity of PSI.

Honoring Bacon Ke at 100: a legend among the many luminaries and a highly collaborative scientist in photosynthesis research.

Bacon Ke, who did pioneering research on the primary photochemistry of photosynthesis, was born in China on July 26, 1920, and currently, he is living in a senior home in San Francisco, California, and is a centenarian. To us, this is a very happy and unique occasion to honor him. After providing a brief account of his life, and a glimpse of his research in photosynthesis, we present here "messages" for Bacon Ke@ 100 from: Robert Alfano (USA), Charles Arntzen (USA), Sandor Demeter (Hungary), Richard A. Dilley (USA), John Golbeck (USA), Isamu Ikegami (Japan), Ting-Yun Kuang (China), Richard Malkin (USA), Hualing Mi (China), Teruo Ogawa (Japan), Yasusi Yamamoto (Japan), and Xin-Guang Zhu (China).

Photosystem I is tolerant to fluctuating light under moderate heat stress in two orchids Dendrobium officinale and Bletilla striata.

Under natural field conditions, plants usually experience fluctuating light (FL) under moderate heat stress in summer. However, responses of photosystems I and II (PSI and PSII) to such combined stresses are not well known. Furthermore, the role of water-water cycle (WWC) in photoprotection in FL under moderate heat stress is poorly understood. In this study, we examined chlorophyll fluorescence and P700 redox state in FL at 42 °C in two orchids, Dendrobium officinale (with high WWC activity) and Bletilla striata (with low WWC activity). After FL treatment at 42 °C, PSI activity maintained stable while PSII activity decreased significantly in these two orchids. In D. officinale, the WWC could rapidly consume the excess excitation energy in PSI and thus avoided an over-reduction of PSI upon any increase in illumination. Therefore, in D. officinale, WWC likely protected PSI in FL at 42 °C. In B. striata, heat-induced PSII photoinhibition down-regulated electron flow from PSII and thus prevented an over-reduction of PSI after transition from low to high light. Consequently, in B. striata moderate PSII photoinhibition could protected PSI in FL at 42 °C. We conclude that, in addition to cyclic electron flow, WWC and PSII photoinhibition-repair cycle are two important strategies for preventing PSI photoinhibition in FL under moderate heat stress.

Dynamic thylakoid stacking and state transitions work synergistically to avoid acceptor-side limitation of photosystem I.

TAP38/STN7-dependent (de)phosphorylation of light-harvesting complex II (LHCII) regulates the relative excitation rates of photosystems I and II (PSI, PSII) (state transitions) and the size of the thylakoid grana stacks (dynamic thylakoid stacking). Yet, it remains unclear how changing grana size benefits photosynthesis and whether these two regulatory mechanisms function independently. Here, by comparing Arabidopsis wild-type, stn7 and tap38 plants with the psal mutant, which undergoes dynamic thylakoid stacking but lacks state transitions, we explain their distinct roles. Under low light, smaller grana increase the rate of PSI reduction and photosynthesis by reducing the diffusion distance for plastoquinol; however, this beneficial effect is only apparent when PSI/PSII excitation balance is maintained by state transitions or far-red light. Under high light, the larger grana slow plastoquinol diffusion and lower the equilibrium constant between plastocyanin and PSI, maximizing photosynthesis by avoiding PSI photoinhibition. Loss of state transitions in low light or maintenance of smaller grana in high light also both bring about a decrease in cyclic electron transfer and over-reduction of the PSI acceptor side. These results demonstrate that state transitions and dynamic thylakoid stacking work synergistically to regulate photosynthesis in variable light.

Alterations in leaf photosynthetic electron transport in Welsh onion (Allium fistulosum L.) under different light intensity and soil water conditions.

Welsh onions (Allium fistulosum L.) are often affected by stressful environments, such as high light and drought, during summer cultivation, which hinders their growth. We used CO2 assimilation, OJIP transient and MR curves to analyse the photosynthetic characteristics of Welsh onion. The results showed that single high light stress caused a decrease in the net photosynthesis rate through stomatal limitation, while the single drought treatment and the combined stress induced nonstomatal limitation. FO and FJ increased, Fm decreased, and a distinct K-phase was induced. High light and drought stress blocked MR transients, leading to a gradual decrease in VPSI and VPSII-PSI . In general, photosynthesis of Welsh onion was inhibited by high light and drought, which destroyed the receptor and donor side of PSII and reduced electron transport capacity of PSII and PSI.

SsPsaH, a H subunit of the photosystem I reaction center of Suaeda salsa, confers the capacity of osmotic adjustment in tobacco.

BACKGROUND: Abiotic stress effects agricultural production, so research on improving stress tolerance of crop is important. Suaeda salsa is a halophyte with high salt and drought tolerance and ability to desalinate saline soil and improve soil quality. OBJECTIVE: To discover and utilize of salt and drought tolerance-related genes, we further investigated the mechanisms of salt and drought tolerance. METHODS: Through screening a salt treated Suaeda salsa cDNA library and further cloning a H subunit of the photosystem I reaction center SsPsaH cDNA, and then the protein domain and phylogenetic analyses of PSI genes was conducted with the NCBI Blast, DNAMAN, and MotifScan programs. The S. salsa seedlings were subjected to various stress treatments and analyze expression of SsPsaH under these treatments by real-time RT-PCR. SsPsaH expression construct was introduced into S. pombe cells by electroporation and transformed into N. tabacum plants by the leaf disc transformation method. RESULTS: A member of the H subunit of the Photosystem I reaction center (defined as SsPsaH) was obtained. The expression of SsPsaH was up-regulated by abscisic acid (ABA), salt, and drought stress treatments. Over-expressing SsPsaH in recombinant yeasts enhanced high salinity tolerance and increased tolerance to sorbitol during seed germination and seedling root development in tobacco, respectively. Some stress-related mark genes such as a LEA family gene of NtLEA, a binding protein of a drought response element of NtDREB, the ascorbate peroxidase gene (NtAPX) were also up-regulated in SsPsaH overexpressing transgenic tobacco lines. CONCLUSIONS: These results show that SsPsaH may contribute to the salt and osmotic stress response of plants.

Chloramphenicol enhances Photosystem II photodamage in intact cells of the cyanobacterium Synechocystis PCC 6803.

The effect of chloramphenicol, an often used protein synthesis inhibitor, in photosynthetic systems was studied on the rate of Photosystem II (PSII) photodamage in the cyanobacterium Synechocystis PCC 6803. Light-induced loss of PSII activity was compared in the presence of chloramphenicol and another protein synthesis inhibitor, lincomycin, by measuring the rate of oxygen evolution in Synechocystis 6803 cells. Our data show that the rate of PSII photodamage was significantly enhanced by chloramphenicol, at the usually applied 200 µg mL-1 concentration, relative to that obtained in the presence of lincomycin. Chloramphenicol-induced enhancement of photodamage has been observed earlier in isolated PSII membrane particles, and has been assigned to the damaging effect of chloramphenicol-mediated superoxide production (Rehman et al. 2016, Front Plant Sci 7:479). This effect points to the involvement of superoxide as damaging agent in the presence of chloramphenicol also in Synechocystis cells. The chloramphenicol-induced enhancement of photodamage was observed not only in wild-type Synechocystis 6803, which contains both Photosystem I (PSI) and PSII, but also in a PSI-less mutant which contains only PSII. Importantly, the rate of PSII photodamage was also enhanced by the absence of PSI when compared to that in the wild-type strain under all conditions studied here, i.e., without addition and in the presence of protein synthesis inhibitors. We conclude that chloramphenicol enhances photodamage mostly by its interaction with PSII, leading probably to superoxide production. The presence of PSI is also an important regulatory factor of PSII photodamage most likely via decreasing excitation pressure on PSII.

Far-red absorption and light-use efficiency trade-offs in chlorophyll f photosynthesis.

Plants and cyanobacteria use the chlorophylls embedded in their photosystems to absorb photons and perform charge separation, the first step of converting solar energy to chemical energy. While oxygenic photosynthesis is primarily based on chlorophyll a photochemistry, which is powered by red light, a few cyanobacterial species can harness less energetic photons when growing in far-red light. Acclimatization to far-red light involves the incorporation of a small number of molecules of red-shifted chlorophyll f in the photosystems, whereas the most abundant pigment remains chlorophyll a. Due to its different energetics, chlorophyll f is expected to alter the excited-state dynamics of the photosynthetic units and, ultimately, their performances. Here we combined time-resolved fluorescence measurements on intact cells and isolated complexes to show that chlorophyll f insertion slows down the overall energy trapping in both photosystems. While this marginally affects the efficiency of photosystem I, it substantially decreases that of photosystem II. Nevertheless, we show that despite the lower energy output, the insertion of red-shifted chlorophylls in the photosystems remains advantageous in environments that are enriched in far-red light and therefore represents a viable strategy for extending the photosynthetically active spectrum in other organisms, including plants. However, careful design of the new photosynthetic units will be required to preserve their efficiency.

Consequences of the reduction of the Photosystem II antenna size on the light acclimation capacity of Arabidopsis thaliana.

In several systems, from plant's canopy to algal bioreactors, the decrease of the antenna size has been proposed as a strategy to increase the photosynthetic efficiency. However, still little is known about possible secondary effects of such modifications. This is particularly relevant because the modulation of the antenna size is one of the most important light acclimation responses in photosynthetic organisms. In our study, we used an Arabidopsis thaliana mutant (dLhcb2), which has a 60% decrease of Lhcb1 and Lhcb2, the two main components of the major Photosystem II antenna complex. We show that the mutant maintains the photosynthetic and photoprotective capacity of the Wild Type (WT) and adapts to different light conditions by remodelling its photosynthetic apparatus, but the regulatory mechanism differs from that of the WT. Surprisingly, it does not compensate for the decreased light-harvesting capacity by increasing other pigment-protein complexes. Instead, it lowers the ratio of the cytochrome b6 f and ATP synthase to the photosystems, regulating linear electron flow and maintaining the photosynthetic control at the level of these complexes as in the WT. We show that targeting the reduction of two specific antenna proteins, Lhcb1 and Lhcb2, represents a viable solution to obtain plants with a truncated antenna size, which still maintain the capacity to acclimate to different light conditions.

Non-photochemical quenching in the cells of the carotenogenic chlorophyte Haematococcus lacustris under favorable conditions and under stress.

The microalga Haematococcus lacustris (formerly H. pluvialis) is the richest source of the valuable pigment astaxanthin, accumulated in red aplanospores (haematocysts). In this work, we report on the photoprotective mechanisms in H. lacustris, conveying this microalga its ability to cope with a wide range of adverse conditions, with special emphasis put on non-photochemical quenching (NPQ) of the excited chlorophyll states. We studied the changes in the primary photochemistry of the photosystems (PS) as a function of irradiance and the physiological state. We leveraged the transcriptomic data to gain a deeper insight into possible NPQ mechanisms in this microalga. Peculiar to H. lacustris is a bi-phasic pattern of changes in photoprotection during haematocyst formation. The first phase coincides with a transient rise of photosynthetic activity. Based on transcriptomic data, high NPQ level in the first phase is maintained predominantly by the expression of PsbS and LhcsR proteins. Then, (in mature haematocysts), stress tolerance is achieved by optical shielding by astaxanthin and dramatic reduction of photosynthetic apparatus. In contrast to many microalgae, shielding plays an important role in H. lacistris haematocysts, whereas regulated NPQ is suppressed. Astaxanthin is decoupled from the PS, hence the light energy is not transferred to reaction centers and dissipates as heat. It allows to retain a higher photochemical yield in haematocysts comparing to vegetative cells. The ability of H. lacustris to substitute the "classical" active photoprotective mechanisms such as NPQ with optic shielding and general metabolism quiescence makes this organism a useful model to reveal photoprotection mechanisms.

Photosynthetic regulation under fluctuating light in young and mature leaves of the CAM plant Bryophyllum pinnatum.

Photosystem I (PSI) is the potential target of photodamage under fluctuating light in angiosperms. However, the response of PSI to fluctuating light in young leaves has not yet been clarified. Furthermore, the photosynthetic regulation under fluctuating light in crassulacean acid metabolism (CAM) plants is little known. In this study, we measured PSI redox state and the electrochromic shift signal in the mature and young leaves of a CAM species Bryophyllum pinnatum. The mature leaves showed stronger capacity for photo-reduction of O2 mediated by the alternative electron flow (probably the water-water cycle) when compared with the young leaves. After an increase in light intensity, both the mature and young leaves showed insufficient proton gradient (ΔpH) across the thylakoid membranes within the first seconds. Meanwhile, PSI was highly oxidized in the mature leaves but was in a more reduced state in the young leaves. Furthermore, young leaves were more susceptible to PSI photoinhibition under fluctuating light. Therefore, in the mature leaves, the alternative electron flow significantly optimized the PSI redox state under fluctuating light at relatively low ΔpH. By comparison, in the young leaves, PSI redox state was largely determined by the buildup of ΔpH. Therefore, the major photoprotective mechanism responsible for safeguarding PSI under fluctuating light can be influenced by leaf developmental stages.

Slower development of PSI activity limits photosynthesis during Euonymus japonicus leaf development.

This study primarily explored the limiting factor for photosynthesis during the development of Euonymus japonicus leaves. The analysis of the chlorophyll fluorescence transient, pulse-modulated fluorescence, 820-nm reflection, and expression of core proteins for photosystems demonstrated that photosystem II (PSII) activity developed more rapidly than did photosystem I (PSI) activity. The slower development of the PSI activity restricted linear and cyclic electron transport and thus inhibited the production of ATP and NADPH, which inhibits the activation of Rubisco, resulting in low activity of carboxylation efficiency. The application of exogenous NADPH (50 µM) and ATP (100 µM) to leaves remarkably increased the Pn and CE in the youngest leaf but not in the fully expanded leaf, which indicated that an inadequate supply of the assimilatory power significantly inhibited CE and Pn. We concluded that the slower development of the PSI activity was one of the most important limiting factors for photosynthesis during the development of E. japonicus leaves.

Light-adaptive state transitions in the Ross Sea haptophyte Phaeocystis antarctica and in dinoflagellate cells hosting kleptoplasts derived from it.

Light state transitions (STs) is a reversible physiological process that oxygenic photosynthetic organisms use in order to minimize imbalances in the electronic excitation delivery to the reaction centers of Photosystems I and II, and thus to optimize photosynthesis. STs have been studied extensively in plants, green algae, red algae and cyanobacteria, but sparsely in algae with secondary red algal plastids, such as diatoms and haptophytes, despite their immense ecological significance. In the present work, we examine whether the haptophyte alga Phaeocystis antarctica, and dinoflagellate cells that host kleptoplasts derived from P. antarctica, both endemic in the Ross Sea, Antarctica, are capable of light adaptive STs. In these organisms, Chl a fluorescence can be excited either by direct light absorption, or indirectly by electronic excitation (EE) transfer from ultraviolet light absorbing mycosporine-like amino acids (MAAs) to Chl a (Stamatakis et al., Biochim. Biophys. Acta 1858 [2017] 189-195). Here we show that, on adaptation to PS II-selective light, dark-adapted P. antarctica cells shift from light state 1 (ST1; more EE ending up in PS II) to light state 2 (ST2; more EE ending up in PS I), as revealed by the spectral distribution of directly-excited Chl a fluorescence and by changes in the macro-organization of pigment-protein complexes evidenced by circular dichroism (CD) spectroscopy. In contrast, no STs are clearly detected in the case of the kleptoplast-hosting dinoflagellate cells, and in the case of indirectly excited Chls a, via MAAs, in P. antarctica cells.

Structural adaptations of photosynthetic complex I enable ferredoxin-dependent electron transfer.

Photosynthetic complex I enables cyclic electron flow around photosystem I, a regulatory mechanism for photosynthetic energy conversion. We report a 3.3-angstrom-resolution cryo-electron microscopy structure of photosynthetic complex I from the cyanobacterium Thermosynechococcus elongatus. The model reveals structural adaptations that facilitate binding and electron transfer from the photosynthetic electron carrier ferredoxin. By mimicking cyclic electron flow with isolated components in vitro, we demonstrate that ferredoxin directly mediates electron transfer between photosystem I and complex I, instead of using intermediates such as NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate). A large rate constant for association of ferredoxin to complex I indicates efficient recognition, with the protein subunit NdhS being the key component in this process.

Blue light reduces photosynthetic efficiency of cyanobacteria through an imbalance between photosystems I and II.

Several studies have described that cyanobacteria use blue light less efficiently for photosynthesis than most eukaryotic phototrophs, but comprehensive studies of this phenomenon are lacking. Here, we study the effect of blue (450 nm), orange (625 nm), and red (660 nm) light on growth of the model cyanobacterium Synechocystis sp. PCC 6803, the green alga Chlorella sorokiniana and other cyanobacteria containing phycocyanin or phycoerythrin. Our results demonstrate that specific growth rates of the cyanobacteria were similar in orange and red light, but much lower in blue light. Conversely, specific growth rates of the green alga C. sorokiniana were similar in blue and red light, but lower in orange light. Oxygen production rates of Synechocystis sp. PCC 6803 were five-fold lower in blue than in orange and red light at low light intensities but approached the same saturation level in all three colors at high light intensities. Measurements of 77 K fluorescence emission demonstrated a lower ratio of photosystem I to photosystem II (PSI:PSII ratio) and relatively more phycobilisomes associated with PSII (state 1) in blue light than in orange and red light. These results support the hypothesis that blue light, which is not absorbed by phycobilisomes, creates an imbalance between the two photosystems of cyanobacteria with an energy excess at PSI and a deficiency at the PSII-side of the photosynthetic electron transfer chain. Our results help to explain why phycobilisome-containing cyanobacteria use blue light less efficiently than species with chlorophyll-based light-harvesting antennae such as Prochlorococcus, green algae and terrestrial plants.

In vivo regulation of thylakoid proton motive force in immature leaves.

In chloroplast, proton motive force (pmf) is critical for ATP synthesis and photoprotection. To prevent photoinhibition of photosynthetic apparatus, proton gradient (ΔpH) across the thylakoid membranes needs to be built up to minimize the production of reactive oxygen species (ROS) in thylakoid membranes. However, the regulation of thylakoid pmf in immature leaves is little known. In this study, we compared photosynthetic electron sinks, P700 redox state, non-photochemical quenching (NPQ), and electrochromic shift (ECS) signal in immature and mature leaves of a cultivar of Camellia. The immature leaves displayed lower linear electron flow and cyclic electron flow, but higher levels of NPQ and P700 oxidation ratio under high light. Meanwhile, we found that pmf and ΔpH were higher in the immature leaves. Furthermore, the immature leaves showed significantly lower thylakoid proton conductivity than mature leaves. These results strongly indicated that immature leaves can build up enough ΔpH by modulating proton efflux from the lumenal side to the stromal side of thylakoid membranes, which is essential to prevent photoinhibition via thermal energy dissipation and photosynthetic control of electron transfer. This study highlights that the activity of chloroplast ATP synthase is a key safety valve for photoprotection in immature leaves.

Effect of artificial redox mediators on the photoinduced oxygen reduction by photosystem I complexes.

The peculiarities of interaction of cyanobacterial photosystem I with redox mediators 2,6-dichlorophenolindophenol (DCPIP) and N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) were investigated. The higher donor efficiency of the reduced DCPIP form was demonstrated. The oxidized form of DCPIP was shown to be an efficient electron acceptor for terminal iron-sulfur cluster of photosystem I. Likewise methyl viologen, after one-electron reduction, DCPIP transfers an electron to the molecular oxygen. These results were discussed in terms of influence of these interactions on photosystem I reactions with the molecular oxygen and natural electron acceptors.

Effects of genetic manipulation of the activity of photorespiration on the redox state of photosystem I and its robustness against excess light stress under CO2-limited conditions in rice.

Under CO2-limited conditions such as during stomatal closure, photorespiration is suggested to act as a sink for excess light energy and protect photosystem I (PSI) by oxidizing its reaction center chlorophyll P700. In this study, this issue was directly examined with rice (Oryza sativa L.) plants via genetic manipulation of the amount of Rubisco, which can be a limiting factor for photorespiration. At low [CO2] of 5 Pa that mimicked stomatal closure condition, the activity of photorespiration in transgenic plants with decreased Rubisco content (RBCS-antisense plants) markedly decreased, whereas the activity in transgenic plants with overproduction of Rubisco (RBCS-sense plants) was similar to that in wild-type plants. Oxidation of P700 was enhanced at [CO2] of 5 Pa in wild-type and RBCS-sense plants. PSI was not damaged by excess light stress induced by repetitive saturated pulse-light (rSP) in the presence of strong steady-state light. On the other hand, P700 was strongly reduced in RBCS-antisense plants at [CO2] of 5 Pa. PSI was also damaged by rSP illumination. These results indicate that oxidation of P700 and the robustness of PSI against excess light stress are hampered by the decreased activity of photorespiration as a result of genetic manipulation of Rubisco content. It is also suggested that overproduction of Rubisco does not enhance photorespiration as well as CO2 assimilation probably due to partial deactivation of Rubisco.

Uphill energy transfer in photosystem I from Chlamydomonas reinhardtii. Time-resolved fluorescence measurements at 77 K.

Energetic properties of chlorophylls in photosynthetic complexes are strongly modulated by their interaction with the protein matrix and by inter-pigment coupling. This spectral tuning is especially striking in photosystem I (PSI) complexes that contain low-energy chlorophylls emitting above 700 nm. Such low-energy chlorophylls have been observed in cyanobacterial PSI, algal and plant PSI-LHCI complexes, and individual light-harvesting complex I (LHCI) proteins. However, there has been no direct evidence of their presence in algal PSI core complexes lacking LHCI. In order to determine the lowest-energy states of chlorophylls and their dynamics in algal PSI antenna systems, we performed time-resolved fluorescence measurements at 77 K for PSI core and PSI-LHCI complexes isolated from the green alga Chlamydomonas reinhardtii. The pool of low-energy chlorophylls observed in PSI cores is generally smaller and less red-shifted than that observed in PSI-LHCI complexes. Excitation energy equilibration between bulk and low-energy chlorophylls in the PSI-LHCI complexes at 77 K leads to population of excited states that are less red-shifted (by ~ 12 nm) than at room temperature. On the other hand, analysis of the detection wavelength dependence of the effective trapping time of bulk excitations in the PSI core at 77 K provided evidence for an energy threshold at ~ 675 nm, above which trapping slows down. Based on these observations, we postulate that excitation energy transfer from bulk to low-energy chlorophylls and from bulk to reaction center chlorophylls are thermally activated uphill processes that likely occur via higher excitonic states of energy accepting chlorophylls.