Photosystem II Electron Transport Research Articles

Inhibition of photosynthesis by viral infection: Effect on PSII structure and function

The effect of viral infection on photosynthesis was investigated in Nicotiana benthamiana Gray plants infected with different strains of pepper and paprika mild mottle viruses (PMMoV and PaMMoV) and chimeric viral genomes derived from them. In both symptomatic and asymptomatic leaves of virus‐infected plants, photosynthetic electron transport in photosystem II (PSII) was reduced. In all cases analyzed, viral infection affected the polypeptide pattern of the oxygen‐evolving complex (OEC) in thylakoid membranes. The levels of both the 24 and 16 kDa proteins were reduced to a differing extent when compared with the levels in healthy control. This loss of the OEC extrinsic proteins affected the oxygen evolution rates of thylakoid membranes and leaves from infected plants. Additionally, viral coat protein (CP) was found associated with the chloroplasts and the thylakoid membranes of the infected plants. The CP accumulation level was dependent upon both the post‐infection time and the virus analyzed, but independent of the CP itself since hybrid viruses did not behave as their parental viruses with the same CP, with respect to PSII inhibition, CP accumulation rates and OEC protein levels. Modulated chlorophyll (Chl) fluorescence and oxygen evolution measurements carried out in both types of leaves showed that the quantum yield of PSII electron transport was diminished in infected plants with respect to those of control plants. The decrease in electron transport efficiency was mainly caused by a reduction in the fraction of open reaction centers. The infected plants also showed a reduction in the efficiency of excitation capture in PSII by photoprotective thermal dissipation of excess excitation energy.

Heat-induced Multiple Effects on PSII in Wheat Plants

Summary The effects of heat stress on the various functional aspects of photosystem II (PSII) were investigated by analysis of in vivo chlorophyll fluorescence in wheat ( Tritium aestivum L.) leaves exposed in the dark to a wide range of elevated temperatures (25–45°C) for 10 min. The results revealed that the effects of heat stress on PSII were characterised by two distinct domains of temperatures: moderately elevated temperatures (30–37.5°C) and severely elevated temperatures (higher than 37.5°C). In moderately elevated temperatures, no changes in the maximal efficiency of PSII photochemistry (F 4 /F m ) and photochemical quenching (qp) were observed. The decrease in the quantum yield of PSII electron transport (psn) and the efficiency of excitation energy capture by open PSII reaction centers (Fv'/Fm') was reversible and was due to a significant increase in non-photochemical quenching (qN). In severely elevated temperatures, the further decrease in psn and Fv'/Fm' was irreversible and was associated with a decrease in FJFm, which was a result of the decrease in the oxygen-evolving complex activity and of an inhibition of electron transport at the acceptor side of PSII. Our results suggest that heat stress displayed multiple effects on PSII. The following sequential events leading to an inhibition of PSII electron transport in the descending order of sensitivity to heat stress can be proposed: excitation energy capture by open PSII reaction centers ~ the oxygenevolving complex ofPSII ~ the acceptor side of PSII.

Inhibition of photosynthetic electron transport by UV-A radiation targets the photosystem II complex.

We have studied the inhibition of photosynthetic electron transport by UV-A (320-400 nm) radiation in isolated spinach thylakoids. Measurements of Photosystem II (PSII) and Photosystem I activity by Clark-type oxygen electrode demonstrated that electron flow is impaired primarily in PSII. The site and mechanism of UV-A induced damage within PSII was assessed by flash-induced oxygen and thermoluminescence (TL) measurements. The flash pattern of oxygen evolution showed an increased amount of the S0 state in the dark, which indicate a direct effect of UV-A in the water-oxidizing complex. TL measurements revealed the UV-A induced loss of PSII centers in which charge recombination between the S2 state of the water oxidizing complex and the semireduced Q(A)- and Q(B)- quinone electron acceptors occur. Flash-induced oscillation of the B TL band, originating from the S2Q(B)- recombination, showed a decreased amplitude after the second flash relative to that after the first one, which is consistent with a decrease in the amount of Q(B)- relative to Q(B) in dark adapted samples. The efficiency of UV-A light in inhibiting PSII electron transport exceeds that of visible light 45-fold on the basis of equal energy and 60-fold on the basis of equal photon number, respectively. In conclusion, our data show that UV-A radiation is highly damaging for PSII, whose electron transport is affected both at the water oxidizing complex, and the binding site of the Q(B) quinone electron acceptor in a similar way to that caused by UV-B radiation.

Changes in photosystem II function during senescence of wheat leaves

Analyses of chlorophyll fluorescence were undertaken to investigate the alterations in photosystem II (PSII) function during senescence of wheat (Triticum aestivum L. cv. Shannong 229) leaves. Senescence resulted in a decrease in the apparent quantum yield of photosynthesis and the maximal CO2 assimilation capacity. Analyses of fluorescence quenching under steady‐state photosynthesis showed that senescence also resulted in a significant decrease in the efficiency of excitation energy capture by open PSII reaction centers (F'v/F'm) but only a slight decrease in the maximum efficiency of PSII photochemistry (F'v/F'm). At the same time, a significant increase in non‐photochemical quenching (qN) and a considerable decrease in photochemical quenching (qP) were observed in senescing leaves. Rapid fluorescence induction kinetics indicated a decrease in the rate of QA reduction and an increase in the proportion of QB‐non‐reducing PSII reaction during senescence. The decrease in both F'v/F'm and qP explained the decrease in the actual quantum yield of PSII electron transport ((φPSII). We suggest that the modifications in PSII function, which led to the down‐regulation of photosynthetic electron transport, would be in concert with the lower demand for ATP and NADPH in the Calvin cycle which is often inhibited in senescing leaves.

Photosystem I is an early target of photoinhibition in barley illuminated at chilling temperatures.

Light-induced damage to photosystem I (PSI) was studied during low-light illumination of barley (Hordeum vulgare L.) at chilling temperatures. A 4-h illumination period induced a significant inactivation of PSI electron transport activity. Flash-induced P700 absorption decay measurements revealed progressive damage to (a) the iron-sulfur clusters FA and FB, (b) the iron-sulfur clusters FA, FB, and FX, and (c) the phylloquinone A1 and the chlorophyll AO or P700 of the PSI electron acceptor chain. Light-induced PSI damage was also evidenced by partial degradation of the PSI-A and PSI-B proteins and was correlated with the appearance of smaller proteins. Aggravated photodamage was observed upon illumination of barley leaves infiltrated with KCN, which inhibits Cu,Zn-superoxide dismutase and ascorbate peroxidase. This indicates that the photodamage of PSI in barley observed during low-light illumination at chilling temperatures arises because the defense against active oxygen species by active oxygen-scavenging enzymes is insufficient at these specific conditions. The data obtained demonstrate that photoinhibition of PSI at chilling temperatures is an important phenomenon in a cold-tolerant plant species.

CO2 assimilation, xanthophyll cycle pigments and PSII efficiency in pumpkin plants as affected by ozone fumigation

CO2 assimilation, xanthophyll cycle pigments and PSII efficiency were analyzed in two different ages of pumpkin leaves (Cucurbita pepo L. cv. Ambassador) exposed to 150 nmol mol‐1 of ozone (5 days, 5 h day‐1). Gas‐exchange measurements revealed a reduction in CO2 assimilation and stomatal conductance, accompanied by an increase in the intercellular CO2 concentration both in young and in mature leaves as compared to their respective controls. In both leaves, F0 remained unchanged, while Fm and the Fv/Fm ratio decreased after O3 fumigation, indicating that ozone may induce an alteration in the capability of photosystem II (PSII) to reduce the primary acceptor QA. In the mature leaves the photochemical quenching (qp) was significantly lowered by the pollutant, but this was not the case in the young leaves where qp did not change. In both mature and young ozonated pumpkin leaves, the development of non‐photochemical quenching caused a decrease in the PSII photochemical rate, as shown by the correlation between Fv/Fm and the de‐epoxidation state of dark‐adapted leaves. Decreases in the Fv/Fm ratio are generally attributed to damage to the PSII reaction centre, apart from the down‐regulation of the capacity of PSII electron transport. While in young ozonated leaves the decrease in the Fv/Fm ratio was not associated with damage to the D1 protein, in mature ozonated pumpkin leaves, the decrease in the Fv/Fm was accompanied by a significant decline in the D1 content. In conclusion, ozone exposure induces alterations in the light reactions of photosynthesis in both young and mature leaves. However, in young leaves the engagement of the xanthophyll cycle appears to counteract ozone effects against the photosynthetic apparatus as demonstrated by the absence of damage to the D1 protein. On the other hand, the loss of D1 protein in mature fumigated leaves suggests that the activation of the xanthophyll cycle is not sufficient to prevent photoinhibition, probably because a physiological state of senescence adds to the oxidative stress.

Oxygen-sensitive Differences in the Relationship between Photosynthetic Electron Transport and CO2 Assimilation in C3 and C4 Plants during State Transitions

Wheat (C3) and maize (C4) leaves were exposed to light treatments that were limiting for CO2 assimilation and which excite preferentially photosystem I (PSI) or photosystem II (PSII) and induce State 1 or State 2, respectively. In order to examine the relationships between linear electron transport and CO2 in leaves during State transitions, simultaneous measurements of CO2 assimilation, chlorophyll fluorescence and absorbance at 820 nm were used to estimate the quantum efficiencies of CO2 assimilation and PSII and PSI photochemistry. In wheat leaves with photorespiratory activity, no significant change in quantum efficiency of CO2assimilation was observed during State transitions. This was not the case when photorespiration was inhibited with either 2% O2 or 1000 ppm CO2 and transition from State 2 to State 1 was accompanied by a large decrease (c. 20%) in the quantum efficiency of CO2 assimilation which was not associated with a decrease in the quantum efficiency of electron transport through PSII. Photorespiration appears to buffer the quantum efficiency of CO2 assimilation from changes associated with decreases in the rate of CO2 fixation resulting from imbalances in PPFD absorption by PSI and PSII. When maize leaves were subjected to similar State transitions, no significant change in the quantum efficiency of CO2 assimilation was observed on transition from State 2 to State 1, but on switching back to State 2 a very large decrease (c. 40%) was observed. This decrease could be prevented if leaves were maintained in either 2% O2 or 593 ppm CO2. The possible occurrence of photorespiration in maize leaves on transition from State 1 to State 2, which could result from an inhibition of the CO2 concentrating mechanism, cannot account for the decrease in the quantum efficiency of CO2 assimilation since the relationship between PSII electron transport and CO2 assimilation remained similar throughout the State transitions. Also changes in the phosphorylation status of the light-harvesting chlorophyll a/b protein associated with PSII cannot be implicated in this phenomenon.

Antenna Sizes of Photosystem I and Photosystem II in Chlorophyll b-Deficient Mutants of Rice. Evidence for an Antenna Function of Photosystem II Centers That are Inactive in Electron Transport

Light-harvesting capacities of photosystem I (PSI) and photosystem II (PSII) in a wild-type and three chlorophyll 6-deficient mutant strains of rice were determined by measuring the initial slope of light-respons e curve of PSI and PSII electron transport and kinetics of light-induced redox changes of P-700 and QA, respectively. The light-harvesting capacity of PSI determined by the two methods was only moderately reduced by chlorophyll 6-deficiency. Analysis of the fluorescence induction that monitors time course of QA photoreduction showed that both relative abundance and antenna size of PSIIa decrease with increasing deficiency of chlorophyll b and there is only PSII,, in chlorina 2 which totally lacks chlorophyll b. The numbers of antenna chlorophyll molecules associated with the mutant PSII centers were, therefore, three to five times smaller than that of PSII a in the wild type rice. Rates of PSII electron transport determined on the basis of PSII centers in the three mutants were 60-70% of that in the normal plant at all photon flux densities examined, indicating that substantial portions of the mutant PSII centers are inactive in electron transport. The initial slopes of light-respons e curves of PSII electron transport revealed that the functional antenna sizes of the active populations of PSII centers in the mutants correspond to about half that of PSII a in the wild type rice. Thus, the numbers of chlorophyll molecules that serve as antenna of the oxygen-evolving PSII centers in the mutants are significantly larger than those that are actually associated with each PSII center. It is proposed that the inactive PSII serves as an antenna of the active PSII in the three chlorophyll ^-deficient mutants of rice. In spite of the reduced antenna size of PSII, therefore, the total light-harvesting capacity of PSII approximately matches that of PSI in the mutants.

Persistent effects of low concentrations of SO2 and NO2 on photosynthesis in Scots pine (Pinus sylvestris) needles

In an open‐field experiment, 50‐year‐old trees of Scots pine (Pinus sylvestris L.) were fumigated with low concentrations of SO2 and NO2 (10–15 nl I−1) during the growing season in four consecutive years (1988 to 1991). Results from the autumn and early winter of 1991 and 1992 are presented. The maximum photochemical efficiency of photosystem II (PSII), as indicated by the ratio of variable to maximum fluorescence (Fv/FM) was assessed in current and one‐year‐old needles from the top and the bottom of the canopy. Furthermore, simultaneous measurements of photosynthetic O2 evolution and chlorophyll fluorescence were made in current‐year needles at 20°C. In general, the Fv/FM ratio as well as the gross rate of O2 evolution in needles of fumigated trees was not significantly different from that in needles of control trees during the fumigation period. However, both current and one‐year‐old needles sampled in November and December 1991 from the top of the canopy of fumigated trees had significantly lower Fv/FM values than corresponding needles of control trees. Similar differences in Fv/FM correlated with the treatments were observed in needles from the bottom of the canopy, indicating that the depression of Fv/FM in needles of fumigated trees was not due to an increased susceptibility to photoinhibition. In 1992, when no fumigation occurred, differences in Fv/FM between the treatments were not significant during autumn and early winter. The gross rate of O2 evolution at high irradiances was significantly lower in current‐year needles of fumigated trees sampled in November and December 1991 than in those of control trees. Furthermore, a nearly identical linear relationship between the quantum yield of PSII electron transport determined from chlorophyll fluorescence and the quantum yield of O2 evolution (gross rate of O2 evolution/PPFD) was found during autumn and early winter. This appeared to be largely a result of changes in the thermal energy dissipation within PSII. The observed differences in photosynthetic characteristics correlated with the different treatments after the fumigation period is suggested to be mainly caused by increased sensitivity of the needles of fumigated trees to low and subfreezing temperatures. However, current‐year needles of fumigated trees tended to have a lower N content than those of control trees, which may partly explain the differences in gross photosynthesis between fumigated and control trees.

Increased capacity for photosynthesis in wheat grown at elevated CO2: the relationship between electron transport and carbon metabolism

Spring wheat (Triticum aestivum L.) was grown under optimal nutrition for six weeks at 700 and 350 μmol·mol−1 CO2 and simultaneous measurements of photosystem-II (PSII) chlorophyll fluorescence and gas exchange were conducted on intact attached leaves. Plants grown at elevated CO2 had double the concentration of CO2 at the carboxylation site (Cc) despite a lowered stomatal (gs) and mesophyll (gm) conductance compared with ambient-grown plants. Plants grown at elevated CO2 had a higher relative quantum yield of PSII electron transport (ΦPSII) and a higher relative quantum yield of CO2 fixation (ΦCO2). The higher ΦPSII was due to a larger proportion of open PSII centres, estimated by the coefficient of photochemical quenching of fluorescence (qp), with no change in the efficiency of light harvesting and energy transduction by open PSII centres (F′v/F′m). Analysis of the relationship between ΦPSII and ΦCO2 conducted under various CO2 and O2 concentrations showed that the higher ΦCO2 for a given ΦPSII in leaves developed under elevated CO2 was similar to that obtained in leaves upon a partial reduction in photorespiration. Calculation of the allocation of photosynthetic electron-transport products to CO2 and O2 showed that for leaves developed in elevated CO2, there was an increase in both total linear electron flow and electron flow to CO2 and a decrease in electron flow to O2. Plants developed under elevated CO2 showed positive acclimation manifested by a higher ΦCO2 when measured under ambient CO2 and higher assimilation rates in A/Ci curves. Initial and total activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco EC 4.1.1.39) measured in vitro increased by 16 and 15% respectively in leaves from plants grown in elevated CO2, which was in agreement with a 15% higher in vivo carboxylation efficiency. It is concluded that growth of spring wheat at elevated CO2 enhances photosynthesis due to a change in the balance of component processes manifested as an increased capacity for carbon fixation, total electron transport and Rubisco activity, and a concomitant partial reduction of photorespiration.

Alterations in photosynthesis and pigment distributions in pea leaves following UV‐B exposure

We compared photosynthetic and UV‐B‐absorbing pigment concentrations, gas‐exchange rates and photosystem II (PSII) electron transport rates in leaves of pea (Pisum sativum mutant Argenteum) grown without UV‐B or under an enhanced UV‐B treatment (18 kJ m−2 biologically effective daily dose) in a greenhouse. We also compared the distribution of chlorophyll by depth within leaves of each treatment by using image analysis of chlorophyll autofluorescence. Ultraviolet‐B treatment elicited putative protective responses such as an 80% increase in UV‐B‐absorbing compound concentrations (leaf‐area basis), and a slight increase in mesophyll thickness (178 in controls compared to 191 μm in UV‐B‐treated leaves). However, photosynthetic rates of UV‐B‐treated leaves were only 80% of those of controls. This was paralleled by reductions in leaf conductance to water vapor (50% of controls) and intercellular CO2 concentrations, suggesting that stomatal limitations were at least partly responsible for lower photosynthetic rates under the UV‐B treatment. Total chlorophyll concentrations (leaf‐area basis) in UV‐B‐treated leaves were only 70% of controls, and there was a shift in the relative distribution of chlorophyll with depth in UV‐B‐treated leaves. In control leaves chlorophyll concentrations were highest near the adaxial surface of the upper palisade, dropped with depth and then increased slightly in the bottom of the spongy mesophyll nearest the abaxial surface. In contrast, in UV‐B‐treated leaves chlorophyll concentrations were lowest at the adaxial surface of the upper palisade and increased with depth through the leaf. The most notable treatment difference in chlorophyll concentrations was in the upper palisade near the adaxial surface of leaves, where we estimate that chlorophyll concentrations in each 1‐μm‐thick paradermal layer were about 50% lower in UV‐B‐treated leaves than in controls. We found reduced electron transport capacity in UV‐B‐treated leaves, based on lower maximum fluorescence (Fm), variable to maximum fluorescence ratios (F,/Fm) and quantum yield of PSII electron transport (Y). However, the above were assessed from fluorometer measurements on the adaxial leaf surface and may reflect the markedly lower chlorophyll concentrations in the upper palisade of UV‐B‐treated leaves.

Chlorophyll fluorescence and photoacoustic characteristics in relationship to changes in chlorophyll and Ca2+ content of a Cu‐tolerant Silene compacta ecotype under Cu treatment

The effect of Cu toxicity on photosynthetic function, chlorophyll and Ca2+ content of Cu‐tolerant Silene compacta plants grown in nutrient solution was studied. Since, in plants grown under 8 μM Cu, the chlorophyll and Ca2+ concentration as well as the photosystem II (PSII) photochemistry were increased, compared to the control, the development of an adaptive mechanism of the Cu‐tolerant ecotype of S. compacta to 8 μM Cu is suggested. Increased Cu tolerance of the S. compacta ecotype reflects modulation of the photosynthetic apparatus to optimize photosynthesis. However, exposure of plants to 160 μM Cu resulted in a marked increase of the fraction of closed PSII centres and decreased quantum yield of PSII electron transport (ΦPSU) which was accompanied by a significant decline of relative quantum yield for O2 evolution (Aox/Apt). The concentration of chlorophyll and Ca2+ in leaves also decreased significantly under 160 μM Cu treatment. Photochemical quenching (qp) displayed a reduction as a result of perturbation of the photosynthetic electron transfer chain, while non‐photochemical quenching (qN) increased. High Cu treatment reduced photosynthetic productivity of S. compacta plants which can be attributed, in part, to pertubation of photosynthetic process and photosynthetic pigments as well as to Ca2+ loss.

Diagnosis of the Earliest Strain-Specific Interactions between Tobacco Mosaic Virus and Chloroplasts of Tobacco Leaves in Vivo by Means of Chlorophyll Fluorescence Imaging.

Fluorescence imaging was used to diagnose early stages of the strain-specific interactions between tobacco mosaic virus (strain PV230) and chloroplasts following infection of tobacco leaves (Nicotiana tabacum cv Xanthi). The earliest indication of interaction in tissues that ultimately become chlorotic was a reduction in chlorophyll fluorescence, and there was little fluorescence quenching compared with adjacent healthy tissues. Subsequently, fluorescence increased but remained unquenched. In the late stages fluorescence declined again in chlorotic regions as the chloroticmosaic symptoms developed. These in vivo data showing altered fluorescence yields confirm strain-specific interaction of virus coat protein with photosystem II (PSII) components in vitro, leading to photoinhibition and photooxidation of chlorophyll in infected cells and the development of visible chlorotic-mosaic symptoms. Although mechanisms leading to the low, unquenched fluorescence condition are not known, the intermediate high, unquenched fluorescence condition is consistent with impaired PSII electron transport as measured in vitro. Fluorescence lesions appear more rapidly and develop more extensively in high light, consistent with the faster and larger extent of symptom formation in high-light-grown leaves than in low-light-grown leaves.

Drought and chlorophyll fluorescence in field‐grown potato (Solanum tuberosum)

Light interception, stomatal conductance and chlorophyll fluorescence were measured in potato (Solanum tuberosum L.) grown either irrigated, or droughted from the time of plant emergence. Compared with the irrigated treatment, drought reduced both light interception and stomatal conductance. In both treatments, the yields of variable fluorescence in the dark‐ and light‐adapted states (Fy/Fm and F′v/F′m, respectively) were negatively correlated with photosynthetic photon flux density (PPFD) and mirrored daytime changes in PPFD. Photochemical quenching was positively correlated with PPFD, but the dominant effect of F′v/F′m resulted in a decrease in the quantum yield of photosystem II (PSII) electron transport with increasing PPFD.Drought had no significant effect on the functioning of PSII and the balance between photochemical and non‐photochemical quenching was unaffected. Non‐photochemical quenching was not increased by drought and the quantum yield of PSII electron transport was unaffected. It is concluded that, in leaves of droughted plants, excess energy, resultant of stomatal limitation of photosynthesis, was dissipated by photochemical quenching such as increased photorespiration.

Degradation of proteins from thylakoid membranes in senescing wheat leaves at high temperature

ABSTRACTThis investigation determined whether thylakoid proteins would be degraded more rapidly or not in senescing wheat (Triticum aestivum L. em. Thell.) leaves concurrently exposed to high temperatures. Excised leaves were pulse‐labelled with [35S]‐methionine for a 12 h period, and then incubated at 22,32 or 42°C for 0, 1, 2, or 3 d, before extracting a thylakoid enriched membrane sample. After electrophoretic separation, two prominent [35S]‐labelled protein bands were chosen for further analyses. Band A contained the D‐1 thylakoid protein and band B contained thylakoid proteins of the light harvesting complex (LHCII) associated with photosystem II (PSII). Total protein, [35S]‐labelled protein, band A protein, and band B protein within the thylakoid enriched membrane samples were measured. Unlabelled thylakoid enriched membrane samples, extracted from leaves given similar treatments, were used to measure uncoupled whole‐chain and photosystem II (PSII) electron transport and chlorophyll fluorescence. Accentuated decline in whole‐chain and PSII electron transport, increasing Fo values, and decreasing Fmax values were a result of high temperature injury in leaves treated at 42°C. None of the thylakoid enriched membrane protein fractions were degraded more rapidly in high‐temperature treated leaves. Degradation of the total [35S]‐labelled membrane proteins and band B was inhibited by the 42°C treatment. The results indicate that high temperature stress may disrupt some aspects of normal senescence.

Slow Degradation of the D1 Protein Is Related to the Susceptibility of Low-Light-Grown Pumpkin Plants to Photoinhibition

Photoinhibition of photosystem II (PSII) electron transport and subsequent degradation of the D1 protein were studied in pumpkin (Cucurbita pepo L.) leaves developed under high (1000 mumol m(-2) s(-1)) and low (80 mumol m(-2) s(-1)) photon flux densities. The low-light leaves were more susceptible to high light. This difference was greatly diminished when illumination was performed in the presence of chloramphenicol, indicating that a poor capacity to repair photodamaged PSII centers is decisive in the susceptibility of low-light leaves to photoinhibition. In fact, the first phases of the repair cycle, degradation and removal of photodamaged D1 protein from the reaction center complex, occurred slowly in low-light leaves, whereas in high-light leaves the degradation of the D1 protein more readily followed photoinhibition of PSII electron transport. A modified form of the D1 protein, with slightly slower electrophoretic mobility than the original D1, accumulated in the appressed thylakoid membranes of low-light leaves during illumination and was subsequently degraded only slowly.

Molecular mechanisms behind light-induced inhibition of photosystem II electron transport and degradation of reaction centre polypeptides

Excessively high light intensities are damaging to the photosynthetic apparatus. Photosynthetic organisms are therefore faced with the problem of maintaining sufficient excitation power under limiting light conditions whilst avoiding photodamage under high light conditions. The primary target for this photodamage is Photosystem II (PSII), which undergoes inhibition of its electron transport followed by irreversible damage to and degradation of the D~ and D 2 reaction centre polypeptides. This degradation is the initial event in a repair cycle of photodamaged complexes and is followed by protein synthesis and reassembly of functional PSII. This report will deal with recent information relevant to the molecular mechanisms of PSII electron transport impairment and of protein degradation, particularly the D I protein, and the chemical link between these two events. Data will be presented in support of the following sequence of reactions (Fig. 1), which take place at PSII under high-light stress: (i) overreduction of the acceptor side, leading to the formation of stably reduced QA species; (ii) these events facilitate the formation of chlorophyll triplets which react with molecular oxygen to form singlet oxygen; (iii) this highly reactive and damaging species will oxidize pigments and/or amino-acid residues leading to irreversible damage to the reaction centre, mainly to the D l protein; (iv) the oxidative damage induces a conformational change in the D~ protein which triggers it for

Protein phosphorylation and Mg2+ influence light harvesting and electron transport in chloroplast thylakoid membrane material containing only the chlorophyll-a/b-binding light-harvesting complex of photosystem II and photosystem I.

A material containing only photosystem I (PSI) and the chlorophyll-a/b-binding light-harvesting complex of PSII (LHC-II) has been isolated from the chloroplast thylakoid membrane by solubilization with Triton X-100. Fluorescence spectroscopy shows that, within the material, LHC-II is coupled to PSI for excitation-energy transfer and that this coupling is decreased by the presence of Mg2+, which also decreased PSI electron transport specifically at limiting light intensity. Inclusion of phosphorylated LHC-II within the material did not alter its structure, but gave decreased energy transfer to PSI and inhibition of electron transport which was independent of light intensity, implying effects of phosphorylation on both light harvesting and directly on electron transport. Inclusion of Mg2+ within the phosphorylated material gave decreased energy transfer, but slightly increased PSI electron transport. A cation-induced direct promotion of PSI electron transport was also observed in isolated PSI particles. The PSI/LHC-II material represents a model system for examining protein interactions during light-state adaptations and the possibility that LHC-II can contribute to the antenna of PSI in light state 2 in vivo is discussed.

Photoinhibition of photosynthesis represents a mechanism for the long-term regulation of photosystem II

The obligate shade plant, Tradescantia albiflora Kunth grown at 50 μmol photons · m(-2) s(-1) and Pisum sativum L. acclimated to two photon fluence rates, 50 and 300 μmol · m(-2) · s(-1), were exposed to photoinhibitory light conditions of 1700 μmol · m(-2) · s(-1) for 4 h at 22° C. Photosynthesis was assayed by measurement of CO2-saturated O2 evolution, and photosystem II (PSII) was assayed using modulated chlorophyll fluorescence and flash-yield determinations of functional reaction centres. Tradescantia was most sensitive to photoinhibition, while pea grown at 300 μmol · m(-2) · s(-1) was most resistant, with pea grown at 50 μmol · m(-2) · s(-1) showing an intermediate sensitivity. A very good correlation was found between the decrease of functional PSII reaction centres and both the inhibition of photosynthesis and PSII photochemistry. Photoinhibition caused a decline in the maximum quantum yield for PSII electron transport as determined by the product of photochemical quenching (qp) and the yield of open PSII reaction centres as given by the steady-state fluorescence ratio, F'vF'm, according to Genty et al. (1989, Biochim. Biophys. Acta 990, 81-92). The decrease in the quantum yield for PSII electron transport was fully accounted for by a decrease in F'vF'm, since qp at a given photon fluence rate was similar for photoinhibited and noninhibited plants. Under lightsaturating conditions, the quantum yield of PSII electron transport was similar in photoinhibited and noninhibited plants. The data give support for the view that photoinhibition of the reaction centres of PSII represents a stable, long-term, down-regulation of photochemistry, which occurs in plants under sustained high-light conditions, and replaces part of the regulation usually exerted by the transthylakoid ΔpH gradient. Furthermore, by investigating the susceptibility of differently lightacclimated sun and shade species to photoinhibition in relation to qp, i.e. the fraction of open-to-closed PSII reaction centres, we also show that irrespective of light acclimation, plants become susceptible to photoinhibition when the majority of their PSII reaction centres are still open (i.e. primary quinone acceptor oxidized). Photoinhibition appears to be an unavoidable consequence of PSII function when light causes sustained closure of more than 40% of PSII reaction centres.

The effects of photoinhibition on the photosynthetic light-response curve of green plant cells (Chlamydomonas reinhardtii)

The photosynthetic response to light can be accurately defined in terms of (1) the initial slope (quantum yield); (2) the asymptote (light-saturated rate); (3) the convexity (rate of bending); and (4) the intercept (dark respiration). The effects of photoinhibition [which damages the reaction centre of photosystem II (PSII)] on these four parameters were measured in optically thin cultures of green plant cells (Chlamydomonas reinhardtii). The convexity of the light-response curve decreased steadily from a value of 0.98 (indicating a sharply bending response) to zero (indicating Michaelis-Menten kinetics) in response to increasing photoinhibition. Photoinhibition was quantified from the quantum yield of inhibited cells relative to that of control cells. The quantum yield was estimated by applying linear regression to low-light data or by fitting a non-rectangular hyperbola. Assuming the initial slope is linear allowed comparison with earlier work. However, as the convexity was lowered this assumption resulted in a significant underestimate of the true quantum yield. Thus, the apparent level of photoinhibition required for a zero convexity and the initial decrease in light-saturated photosynthesis depended upon how the quantum yield was estimated. If the initial slope of the light response was assumed to be linear the critical level of inhibition was 60%. If the linear assumption was not made, the critical level was 40%. At the level of inhibition where the convexity reached zero, the light-saturated rate of photosynthesis also began to decrease, indicating that this level of inhibition caused photosynthesis to be limited at all light intensities by the rate of PSII electron transport. At this level of inhibition the Fm-Fi signal (where Fm is maximal chlorophyll fluorescence and Fi is intermediate chlorophyll fluorescence of dark adapted cells; Briantais et al. 1988) from the fluorescence induction curve was zero and the Fi-Fo signal (where Fo is initial chlorophyll fluorescence of dark adapted cells) was 30% of the control, indicating dramatic reduction or complete elimination of one type of PSII. These data do not contradict published mathematical models showing that the ratio of the maximum speed of electron transport in PSII relative to the maximum speed of plastoquinone electron transport can determine the convexity of the photosynthetic response to light.