Photoinhibition Of Photosystem II Research Articles

The effects of chilling-light stress on photosystems I and II in three Paphiopedilum species

BackgroundLow temperatures pose a critical limitation to the physiology and survival of chilling-sensitive plants. One example is the genus Paphiopedilum (Orchidaceae), which is mainly native to tropical and subtropical areas from Asia to the Pacific islands. However, little is known about the physiological mechanism(s) underlying its sensitivity to chilling temperature. We examined how chilling-light stress influences the activities of photosystem I (PSI) and photosystem II (PSII) in three species: P. armeniacum, P. micranthum, and P. purpuratum. All originate from different distribution zones that cover a range of temperatures.ResultsPhotosystem II of three Paphiopedilum species was remarkable sensitivity to chilling stress. After 8 h chilling stress, the maximum quantum yield of PSII of three species of Paphiopedilum was significantly decreased, especially in P. purpuratum. The quantity of efficient PSI complex (Pm) value did not significantly differ after 8 h chilling treatment compared to the original value in three species. The stronger PSII photoinhibition and significantly less capacity for cyclic electron flow (CEF) were observed in P. purpuratum.ConclusionsIn conclusion, the three species of Paphiopedilum showed significant PSII photoinhibition when exposed to 4 °C chilling treatment. However, their PSI activities were not susceptible to chilling-light stress during 8 h. The CEF was important for the photoprotection of PSI and PSII in P. armeniacum and P. micranthum under chilling conditions. Our findings suggested that the photosynthetic characteristics of Paphiopedilum were well adapted to their habitat.

Switching off photoprotection of photosystem I-a novel tool for gradual PSI photoinhibition.

Photosystem I (PSI) has evolved in anaerobic atmospheric conditions and until today remains susceptible to oxygen. To minimize the probability of damaging side reactions, plants have evolved sophisticated mechanisms to control electron transfer, and PSI becomes inhibited only when malfunctions of these regulatory mechanisms occur. Because of the complicated induction of PSI photoinhibition, a detailed investigation into the process and following reactions are still largely missing. Here, we introduce the theoretical framework and a novel method for an easy and controlled induction of PSI photoinhibition in vivo. The method mimics the PSI damage mechanisms of fluctuating light-sensitive mutant plants (stn7, pgr5) which cannot control electron donation to PSI. Because PSII and PSI have different light absorption properties, electrons accumulate in the intersystem electron transfer chain (ETC), if PSII is preferentially excited. A saturating light pulse given upon an over-reduced ETC leads to the saturation of PSI electron acceptors, ultimately leading to the production of reactive oxygen species and photoinhibition of PSI. By adjusting the time of the light treatment, PSI can be gradually photoinhibited, providing a novel tool to holistically investigate the PSI photoinhibition phenomenon.

Interaction between photosynthetic electron transport and chloroplast sinks triggers protection and signalling important for plant productivity.

The photosynthetic light reactions provide energy that is consumed and stored in electron sinks, the products of photosynthesis. A balance between light reactions and electron consumption in the chloroplast is vital for plants, and is protected by several photosynthetic regulation mechanisms. Photosystem I (PSI) is particularly susceptible to photoinhibition when these factors become unbalanced, which can occur in low temperatures or in high light. In this study we used the pgr5 Arabidopsis mutant that lacks ΔpH-dependent regulation of photosynthetic electron transport as a model to study the consequences of PSI photoinhibition under high light. We found that PSI damage severely inhibits carbon fixation and starch accumulation, and attenuates enzymatic oxylipin synthesis and chloroplast regulation of nuclear gene expression after high light stress. This work shows that modifications to regulation of photosynthetic light reactions, which may be designed to improve yield in crop plants, can negatively impact metabolism and signalling, and thereby threaten plant growth and stress tolerance.This article is part of the themed issue ‘Enhancing photosynthesis in crop plants: targets for improvement’.

Knock-Down of the IFR1 Protein Perturbs the Homeostasis of Reactive Electrophile Species and Boosts Photosynthetic Hydrogen Production in Chlamydomonas reinhardtii.

The protein superfamily of short-chain dehydrogenases/reductases (SDR), including members of the atypical type (aSDR), covers a huge range of catalyzed reactions and in vivo substrates. This superfamily also comprises isoflavone reductase-like (IRL) proteins, which are aSDRs highly homologous to isoflavone reductases from leguminous plants. The molecular function of IRLs in non-leguminous plants and green microalgae has not been identified as yet, but several lines of evidence point at their implication in reactive oxygen species homeostasis. The Chlamydomonas reinhardtii IRL protein IFR1 was identified in a previous study, analyzing the transcriptomic changes occurring during the acclimation to sulfur deprivation and anaerobiosis, a condition that triggers photobiological hydrogen production in this microalgae. Accumulation of the cytosolic IFR1 protein is induced by sulfur limitation as well as by the exposure of C. reinhardtii cells to reactive electrophile species (RES) such as reactive carbonyls. The latter has not been described for IRL proteins before. Over-accumulation of IFR1 in the singlet oxygen response 1 (sor1) mutant together with the presence of an electrophile response element, known to be required for SOR1-dependent gene activation as a response to RES, in the promoter of IFR1, indicate that IFR1 expression is controlled by the SOR1-dependent pathway. An implication of IFR1 into RES homeostasis, is further implied by a knock-down of IFR1, which results in a diminished tolerance toward RES. Intriguingly, IFR1 knock-down has a positive effect on photosystem II (PSII) stability under sulfur-deprived conditions used to trigger photobiological hydrogen production, by reducing PSII-dependent oxygen evolution, in C. reinhardtii. Reduced PSII photoinhibition in IFR1 knock-down strains prolongs the hydrogen production phase resulting in an almost doubled final hydrogen yield compared to the parental strain. Finally, IFR1 knock-down could be successfully used to further increase hydrogen yields of the high hydrogen-producing mutant stm6, demonstrating that IFR1 is a promising target for genetic engineering approaches aiming at an increased hydrogen production capacity of C. reinhardtii cells.

Diversity of strategies for escaping reactive oxygen species production within photosystem I among land plants: P700 oxidation system is prerequisite for alleviating photoinhibition in photosystem I.

In land plants, photosystem I (PSI) photoinhibition limits carbon fixation and causes growth defects. In addition, recovery from PSI photoinhibition takes much longer than PSII photoinhibition when the PSI core-complex is degraded by oxidative damage. Accordingly, PSI photoinhibition should be avoided in land plants, and land plants should have evolved mechanisms to prevent PSI photoinhibition. However, such protection mechanisms have not yet been identified, and it remains unclear whether all land plants suffer from PSI photoinhibition in the same way. In the present study, we focused on the susceptibility of PSI to photoinhibition and investigated whether mechanisms of preventing PSI photoinhibition varied among land plant species. To assess the susceptibility of PSI to photoinhibition, we used repetitive short-pulse (rSP) illumination, which specifically induces PSI photoinhibition. Subsequently, we found that land plants possess a wide variety of tolerance mechanisms against PSI photoinhibition. In particular, gymnosperms, ferns and mosses/liverworts exhibited higher tolerance to rSP illumination-induced PSI photoinhibition than angiosperms, and detailed analyses indicated that the tolerance of these groups could be partly attributed to flavodiiron proteins, which protected PSI from photoinhibition by oxidizing the PSI reaction center chlorophyll (P700) as an electron acceptor. Furthermore, we demonstrate, for the first time, that gymnosperms, ferns and mosses/liverworts possess a protection mechanism against photoinhibition of PSI that differs from that of angiosperms.

Chloroplastic ATP synthase builds up a proton motive force preventing production of reactive oxygen species in photosystem I.

Over-reduction of the photosynthetic electron transport (PET) chain should be avoided, because the accumulation of reducing electron carriers produces reactive oxygen species (ROS) within photosystem I (PSI) in thylakoid membranes and causes oxidative damage to chloroplasts. To prevent production of ROS in thylakoid membranes the H+ gradient (ΔpH) needs to be built up across the thylakoid membranes to suppress the over-reduction state of the PET chain. In this study, we aimed to identify the critical component that stimulates ΔpH formation under illumination in higher plants. To do this, we screened ethyl methane sulfonate (EMS)-treated Arabidopsis thaliana, in which the formation of ΔpH is impaired and the PET chain caused over-reduction under illumination. Subsequently, we isolated an allelic mutant that carries a missense mutation in the γ-subunit of chloroplastic CF0 CF1 -ATP synthase, named hope2. We found that hope2 suppressed the formation of ΔpH during photosynthesis because of the high H+ efflux activity from the lumenal to stromal side of the thylakoid membranes via CF0 CF1 -ATP synthase. Furthermore, PSI was in a more reduced state in hope2 than in wild-type (WT) plants, and hope2 was more vulnerable to PSI photoinhibition than WT under illumination. These results suggested that chloroplastic CF0 CF1 -ATP synthase adjusts the redox state of the PET chain, especially for PSI, by modulating H+ efflux activity across the thylakoid membranes. Our findings suggest the importance of the buildup of ΔpH depending on CF0 CF1 -ATP synthase to adjust the redox state of the reaction center chlorophyll P700 in PSI and to suppress the production of ROS in PSI during photosynthesis.

Superoxide generated in the chloroplast stroma causes photoinhibition of photosystem I in the shade-establishing tree species Psychotria henryi.

Our previous studies indicated that high light induced significant photoinhibition of photosystem I (PSI) in the shade-establishing tree species Psychotria henryi. However, the underlying mechanism has not been fully clarified. In the present study, in order to investigate the mechanism of PSI photoinhibition in P. henryi, we treated detached leaves with constant high light in the presence of methyl viologen (MV) or a soluble α-tocopherol analog, 2,2,5,7,8-pentamethyl-6-chromanol (PMC). We found that MV significantly depressed photochemical quantum yields in PSI and PSII when compared to PMC. On condition that no PSI photoinhibition happened, although cyclic electron flow (CEF) was abolished in the MV-treated samples, P700 oxidation ratio was maintain at higher levels than the PMC-treated samples. In the presence of PMC, PSI photoinhibition little changed but PSII photoinhibition was significantly alleviated. Importantly, PSI photoinhibition was largely accelerated in the presence of MV, which stimulates the production of superoxide and subsequently other reactive oxygen species at the chloroplast stroma by accepting electrons from PSI. Furthermore, MV largely aggravated PSII photoinhibition when compared to control. These results suggest that high P700 oxidation ratio cannot prevent PSI photoinhibition in P. henryi. Furthermore, the superoxide produced in the chloroplast stroma is critical for PSI photoinhibition in the higher plant P. henryi, which is opposite to the mechanism underlying PSI photoinhibition in Arabidopsis thaliana and spinach. These findings highlight a new mechanism of PSI photoinhibition in higher plants.

Light intensity dependent photosynthetic electron transport in eelgrass (Zostera marina L.)

Responses of electron transport to three levels of irradiation (20, 200, and 1200 μmol photons m−2 s−1 PAR; exposures called LL, ML and HL, respectively) were investigated in eelgrass (Zostera marina L.) utilizing the chlorophyll a fluorescence technique. Exposure to ML and HL reduced the maximum quantum yield of photosystem II (PSII) (Fv/Fm) and the maximum slope decrease of MR/MRO (VPSI), indicating the occurrence of photoinhibition of both PSII and photosystem I (PSI). A comparatively slow recovery rate of Fv/Fm due to longer half-life recovery time of PSII and 40% lower descending amplitude compared to other higher plants implied the poor resilience of the PSII. Comparatively, PSI demonstrated high resilience and cyclic electron transport (CEF) around PSI maintained high activity. With sustained exposure, the amplitudes of the kinetic components (L1 and L2), the probability of electron transfer from PSII to plastoquinone pool (ψET2o), and the connectivity among PSII units decreased, accompanied by an enhancement of energy dissipation. Principle component analysis revealed that both VPSI and Fv/Fm contributed to the same component, which was consistent with high connectivity between PSII and PSI, suggesting close coordination between both photosystems. Such coordination was likely beneficial for the adaption of high light. Exposure to LL significantly increased the activity of both PSI and CEF, which could lead to increased light harvesting. Moreover, smooth electron transport as indicated by the enhancement of L1, L2, ψET2o and the probability of electron transport to the final PSI acceptor sides, could contribute to an increase in light utilization efficiency.

Induction of cyclic electron flow around photosystem I during heat stress in grape leaves

Photosystem II (PSII) in plants is susceptible to high temperatures. The cyclic electron flow (CEF) around PSI is thought to protect both PSII and PSI from photodamage. However, the underlying physiological mechanisms of the photosynthetic electron transport process and the role of CEF in grape at high temperatures remain unclear. To investigate this issue, we examined the responses of PSII energy distribution, the P700 redox state and CEF to high temperatures in grape leaves. After exposing ‘Cabernet Sauvignon’ leaves to various temperatures (25, 30, 35, 40 and 45°C) in the light (600μmol photons m−2s−1) for 4h, the maximum quantum yield of PSII (Fv/Fm) significantly decreased at high temperatures (40 and 45°C), while the maximum photo-oxidizable P700 (Pm) was not affected. As the temperature increased, higher initial rates of increase in post-illumination Chl fluorescence were detected, which were accompanied by an increase in high energy state quenching (qE). The chloroplast NAD(P)H dehydrogenase-dependent CEF (NDH-dependent CEF) activities were different among grape cultivators. ‘Gold Finger’ with greater susceptibility to photoinhibition, exhibited lower NDH-dependent CEF activities under acute heat stress than a more heat tolerant ‘Cabernet Sauvignon’. These results suggest that overclosure of PSII reaction centers at high temperature resulted in the photoinhibition of PSII, while the stimulation of CEF in grape played an important role in the photoprotection of PSII and PSI at high temperatures through contributing to the generation of a proton gradient.

The mechanisms by which phenanthrene affects the photosynthetic apparatus of cucumber leaves

Phenanthrene is a polycyclic aromatic hydrocarbon (PAH) that is widely distributed in the environment and seriously affects the growth and development of plants. To clarify the mechanisms of the direct effects of phenanthrene on the plant photosynthetic apparatus, we measured short-term phenanthrene-treated cucumber leaves. Phenanthrene inhibited Rubisco carboxylation activity, decreasing photosynthesis rates (Pn). And phenanthrene inhibited photosystem II (PSII) activity, thereby blocking photosynthetic electron transport. The inhibition of the light and dark reactions decreased the photosynthetic electron transport rate (ETR) and increased the excitation pressure (1−qP). Under high light, the maximum photochemical efficiency of photosystem II (Fv/Fm) in phenanthrene-treated cucumber leaves decreased significantly, but photosystem I (PSI) activity (Δ I/Io) did not. Phenanthrene also caused a J-point rise in the OJIP curve under high light, which indicated that the acceptor side of PSII QA to QB electron transfer was restricted. This was primarily due to the net degradation of D1 protein, which is caused by the accumulation of reactive oxygen species (ROS) in phenanthrene-treated cucumber leaves under high light.This study demonstrated that phenanthrene could directly inhibit photosynthetic electron transport and Rubisco carboxylation activity to decrease net Pn. Under high light, phenanthrene caused the accumulation of ROS, resulting in net increases in D1 protein degradation and consequently causing PSII photoinhibition.

Responses of Photosystem I Compared with Photosystem II to Fluctuating Light in the Shade-Establishing Tropical Tree Species Psychotria henryi.

Shade-establishing plants growing in the forest understory are exposed to constant high light or fluctuating light when gaps are created by fallen trees. Our previous studies indicate that photosystem I (PSI) is sensitive to constant high light in shade-establishing tree species, however, the effects of fluctuating light on PSI and photosystem II (PSII) in shade-establishing species are little known. In the present study, we examined the responses of PSI and PSII to fluctuating light in comparison to constant high light in the shade-establishing species Psychotria henryi. Accompanying with significant activation of cyclic electron flow (CEF), the P700 oxidation ratio was maintained at high levels when exposed to strong light either under fluctuating light or constant high light. Under moderate fluctuating light, PSI and PSII activities were remained stable in P. henryi. Interestingly, PSI was insusceptible to fluctuating light but sensitive to constant high light in P. henryi. Furthermore, both PSI and PSII were more sensitive to constant high light than fluctuating light. These results suggest that CEF is essential for photoprotection of PSI under fluctuating light in P. henryi. Furthermore, photoinhibition of PSI under high light in P. henryi is more related to the accumulation of reactive oxygen species rather than to P700 redox state, which is different from the mechanisms of PSI photoinhibition in Arabidopsis thaliana and rice. Taking together, PSI is a key determiner of photosynthetic responses to fluctuating light and constant high light in the shade-establishing species P. henryi.

Seasonal variations in photosystem I compared with photosystem II of three alpine evergreen broad-leaf tree species

Low temperature associated with high light can induce photoinhibition of photosystem I (PSI) and photosystem II (PSII). However, the photosynthetic electron flow and specific photoprotective responses in alpine evergreen broad-leaf plants in winter is unclear. We analyzed seasonal changes in PSI and PSII activities, and energy quenching in PSI and PSII in three alpine broad-leaf tree species, Quercus guyavifolia (Fagaceae), Rhododendron decorum (Ericaceae), Euonymus tingens (Celastraceae). In winter, PSII activity remained stable in Q. guyavifolia but decreased significantly in R. decorum and E. tingens. Q. guyavifolia showed much higher capacities of cyclic electron flow (CEF), water-water cycle (WWC), non-photochemical quenching (NPQ) than R. decorum and E. tingens in winter. These results indicated that in alpine evergreen broad-leaf tree species the PSII activity in winter was closely related to these photoprotective mechanisms. Interestingly, unlike PSII, PSI activity was maintained stable in winter in the three species. Meanwhile, photosynthetic electron flow from PSII to PSI (ETRII) was much higher in Q. guyavifolia, suggesting that the mechanisms protecting PSI activity against photoinhibition in winter differed among the three species. A high level of CEF contributed the stability of PSI activity in Q. guyavifolia. By comparison, R. decorum and E. tingens prevented PSI photoinhibition through depression of electron transport to PSI. Taking together, CEF, WWC and NPQ played important roles in coping with excess light energy in winter for alpine evergreen broad-leaf tree species.

Oxidation of P700 in Photosystem I Is Essential for the Growth of Cyanobacteria.

The photoinhibition of photosystem I (PSI) is lethal to oxygenic phototrophs. Nevertheless, it is unclear how photodamage occurs or how oxygenic phototrophs prevent it. Here, we provide evidence that keeping P700 (the reaction center chlorophyll in PSI) oxidized protects PSI. Previous studies have suggested that PSI photoinhibition does not occur in the two model cyanobacteria, Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942, when photosynthetic CO2 fixation was suppressed under low CO2 partial pressure even in mutants deficient in flavodiiron protein (FLV), which mediates alternative electron flow. The lack of FLV in Synechococcus sp. PCC 7002 (S. 7002), however, is linked directly to reduced growth and PSI photodamage under CO2-limiting conditions. Unlike Synechocystis sp. PCC 6803 and S. elongatus PCC 7942, S. 7002 reduced P700 during CO2-limited illumination in the absence of FLV, resulting in decreases in both PSI and photosynthetic activities. Even at normal air CO2 concentration, the growth of S. 7002 mutant was retarded relative to that of the wild type. Therefore, P700 oxidation is essential for protecting PSI against photoinhibition. Here, we present various strategies to alleviate PSI photoinhibition in cyanobacteria.

Moderate Heat Stress Stimulates Repair of Photosystem II During Photoinhibition in Synechocystis sp. PCC 6803.

Examination of the effects of high temperature on the photoinhibition of photosystem II (PSII) in the cyanobacterium Synechocystis sp. PCC 6803 revealed that the extent of photoinhibition of PSII was lower at moderately high temperatures (35-42 °C) than at 30 °C. Photodamage to PSII, as determined in the presence of chloramphenicol, which blocks the repair of PSII, was accelerated at the moderately high temperatures but the effects of repair were greater than those of photodamage. The synthesis de novo of the D1 protein, which is essential for the repair of PSII, was enhanced at 38 °C. Electron transport and the synthesis of ATP were also enhanced at 38 °C, while levels of reactive oxygen species fell. Inhibition of the Calvin-Benson cycle with glycolaldehyde abolished the enhancement of repair of PSII at 38 °C, suggesting that an increase in the activity of the Calvin-Benson cycle might be required for the enhancement of repair at moderately high temperatures. The synthesis de novo of metabolic intermediates of the Calvin-Benson cycle, such as 3-phosphoglycerate, was also enhanced at 38 °C. We propose that moderate heat stress might enhance the repair of PSII by stimulating the synthesis of ATP and depressing the production of reactive oxygen species, via the stimulation of electron transport and suppression of the accumulation of excess electrons on the acceptor side of photosystem I, which might be driven by an increase in the activity of the Calvin-Benson cycle.

Enhancement of photoassembly of the functionally active water-oxidizing complex in Mn-depleted photosystem II membranes upon transition to anaerobic conditions

It has been shown earlier (Khorobrykh and Klimov, 2015) that molecular oxygen is directly involved in the general mechanism of the donor side photoinhibition of photosystem II (PSII) membranes. In the present work the effect of oxygen on photoassembly (“photoactivation”) of the functionally active inorganic core of the water-oxidizing complex (WOC) in Mn-depleted PSII preparations (apo-WOC-PSII) in the presence of exogenous Mn2+, Ca2+ as well as ferricyanide was investigated. It was revealed that the efficiency of the photoassembly of the WOC was considerably increased upon removal of oxygen from the medium during photoactivation procedure using the enzymatic oxygen trap or argon flow. The lowering of O2 concentration from 250μM to 75μM, 10μM and near 0μM results in 29%, 71% and 92%, respectively, stimulation of the rate of O2 evolution measured after the photoactivation. The increase in the intensity of light used during the photoactivation was accompanied by a decrease of both the efficiency of photoassembly of the WOC and the stimulation effect of removal of O2 (that may be due to the enhancement of the processes leading to the photodamage to PSII). It is concluded that the enhancement in photoactivation of oxygen-evolving activity of apo-WOC-PSII induced by oxygen removal from the medium is due to the suppression of the donor side photoinhibition of PSII in which molecular oxygen can be involved.

Effects of Pseudomonas syringae pv. tabaci infection on tobacco photosynthetic apparatus under light or dark conditions.

Pseudomonas syringae pv. tabaci (Pst) is a hemi-biotrophic bacterial pathogen that causes the formation of brown spots named wildfire disease. Pst has received considerable attention in recent years. However, most of the studies focused on the tolerance and defense mechanisms of the host and non-host plants against Pst infection and a toxin originally described as being from Pst named tabtoxin, little information is available on the photosynthetic performance of tobacco leaves after Pst infection. Exploring the effects of Pst on the photosystem Ⅱ (PSⅡ) will not only help in clarifying tobacco-Pst interaction mechanisms, but also deepen the understanding of bacterial pathogen disease from a physiological perspective. By analyzing chlorophyll a fluorescence transient, performing western blot of thylakoid membrane and measuring the content of reactive oxygen species (ROS) and total chlorophyll, the effects of Pst on PS2 in tobacco were studied under light (200 μmol·m-2·s-1) or dark conditions. The results showed that chlorophyll content significantly decreased and significant chlorosis of the infiltrated zone was observed compared to the untreated ones, and tobacco leaves exhibited a visible and overt wildfire symptom at 3 days post Pst infection (dpi) under light and dark conditions. The H2O2 content increased at 3 dpi compared to untreated ones in tobacco leaves under light and dark conditions, and was much higher under light than dark condition. Besides, markedly increase of the normalized relative variable fluorescence at the K step (WK) and the relative variable fluorescence at the J step (VJ), significant decrease of maximal quantum yield of PS2 (Fv/Fm) and density of QA- reducing PS2 reaction centers per cross section (RC/CSm) were observed in tobacco leaves after Pst infection at 3 dpi under light and dark conditions. Moreover, inhibition of the K and J steps was more pronounced in the dark, as indicated by the greater increase of WK and VJ under darkness compared with the light conditions during Pst inoculation. Dramatic (net) degradation of D1 protein and PsaO, the core protein of PS2 reaction center and oxygen evolving complex (OEC) respectively, at 3 dpi after Pst infection was observed in tobacco leaves under both light or dark conditions, and the decline was more exacerbated under dark than light condition. The results indicated that the electron transport from QA to QB of photosynthesis electron transport chain was severely blocked, OEC was damaged on both the donor and acceptor sides, and the reaction center of PS2 was severely damaged by Pst infection in tobacco lea-ves under either light or dark condition. Photoinhibition and photoinhibition-like damage of PSⅡ was observed after Pst infection, and the damage to PS2 under dark condition was much more severe than under light condition in tobacco leaves.

Effect of Reducing Nitric Oxide in Rumex K-1 Leaves on the Photoprotection of Photosystem II Under High Temperature with Strong Light

The purpose of this study was to explore the effect of reducing nitric oxide (NO) in Rumex K-1 leaves on the photoprotection of photosystem II (PSII) under high temperature with strong light. Reducing the content of NO in Rumex K-1 leaves significantly aggravated the PSII photoinhibition and net degradation of D1 protein under high temperature with strong light, but not under high temperature in the darkness. The reduction of NO remarkably inhibited the electron transport of PSII in the leaves under high temperature and strong light, which resulted in an increase in excitation pressure and an over-accumulation of reactive oxygen species (ROS). The over-accumulation of ROS further damaged PSII. However, when the synthesis of D1 protein was inhibited, the D1 protein content and PSII activity were no longer influenced by reducing NO content in the leaves. The reduction of NO in leaves decreased the activities of ROS scavenger enzymes after treatment with high temperature and strong light for 2 h, which enhanced the over-accumulation of ROS to damage photosynthetic apparatus severely. All of these results suggest that NO was involved in the synthesis of D1 protein. Maintaining physiologically appropriate NO content in leaves will alleviate net degradation of D1 protein under high temperature with strong light to keep photosynthetic electrons flowing smoothly, which mitigates the accumulation of ROS in photosystems to avoid damage to the photosynthetic apparatus. Therefore, NO plays an important role in maintaining higher PSII photosynthetic performance under high temperature with strong light.

Photodamage of iron-sulphur clusters in photosystem I induces non-photochemical energy dissipation.

Photosystem I (PSI) uses light energy and electrons supplied by photosystem II (PSII) to reduce NADP(+) to NADPH. PSI is very tolerant of excess light but extremely sensitive to excess electrons from PSII. It has been assumed that PSI is protected from photoinhibition by strict control of the intersystem electron transfer chain (ETC). Here we demonstrate that the iron-sulphur (FeS) clusters of PSI are more sensitive to high light stress than previously anticipated, but PSI with damaged FeS clusters still functions as a non-photochemical photoprotective energy quencher (PSI-NPQ). Upon photoinhibition of PSI, the highly reduced ETC further triggers thylakoid phosphorylation-based mechanisms that increase energy flow towards PSI. It is concluded that the sensitivity of FeS clusters provides an additional photoprotective mechanism that is able to downregulate PSII, based on PSI quenching and protein phosphorylation.

Differential responses of photosystems I and II to seasonal drought in two Ficus species

Hemiepiphytic Ficus species exhibit more conservative water use strategy and are more drought-tolerant compared with their non-hemiepiphytic congeners, but a difference in the response of photosystem I (PSI) and photosystem II (PSII) to drought stress has not been documented to date. The enhancement of non-photochemical quenching (NPQ) and cyclic electron flow (CEF) have been identified as important mechanisms that protect the photosystems under drought conditions. Using the hemiepiphytic Ficus tinctoria and the non-hemiepiphytic Ficus racemosa, we studied the water status and the electron fluxes through PSI and PSII under seasonal water stress. Our results clearly indicated that the decline in the leaf predawn water potential (ψpd), the maximum photosynthetic rate (Amax) and the predawn maximum quantum yield of PSII (Fv/Fm) were more pronounced in F. racemosa than in F. tinctoria at peak drought. The Fv/Fm of F. racemosa was reduced to 0.69, indicating net photoinhibition of PSII. Concomitantly, the maximal photo-oxidizable P700 (Pm) decreased significantly in F. racemosa but remained stable in F. tinctoria. The fraction of non-photochemical quenching [Y(NPQ)] and the ratio of effective quantum yield of PSI to PSII [Y(I)/Y(II)] increased for both Ficus species at peak drought, with a stronger increase in F. racemosa. These results indicated that the enhancement of NPQ and the activation of CEF contributed to the photoprotection of PSI and PSII for both Ficus species under seasonal drought, particularly for F. racemosa.

Superoxide and Singlet Oxygen Produced within the Thylakoid Membranes Both Cause Photosystem I Photoinhibition.

Photosystem I (PSI) photoinhibition suppresses plant photosynthesis and growth. However, the mechanism underlying PSI photoinhibition has not been fully clarified. In this study, in order to investigate the mechanism of PSI photoinhibition in higher plants, we applied repetitive short-pulse (rSP) illumination, which causes PSI-specific photoinhibition in chloroplasts isolated from spinach leaves. We found that rSP treatment caused PSI photoinhibition, but not PSII photoinhibition in isolated chloroplasts in the presence of O2 However, chloroplastic superoxide dismutase and ascorbate peroxidase activities failed to protect PSI from its photoinhibition. Importantly, PSI photoinhibition was largely alleviated in the presence of methyl viologen, which stimulates the production of reactive oxygen species (ROS) at the stromal region by accepting electrons from PSI, even under the conditions where CuZn-superoxide dismutase and ascorbate peroxidase activities were inactivated by KCN. These results suggest that the ROS production site, but not the ROS production rate, is critical for PSI photoinhibition. Furthermore, we found that not only superoxide (O2 (-)) but also singlet oxygen ((1)O2) is involved in PSI photoinhibition induced by rSP treatment. From these results, we suggest that PSI photoinhibition is caused by both O2 (-) and (1)O2 produced within the thylakoid membranes when electron carriers in PSI become highly reduced. Here, we show, to our knowledge, new insight into the PSI photoinhibition in higher plants.