Photoinhibition Of Photosystem II Research Articles

RNA interference (RNAi) of 2-Cys Prx gene enhances PSII photoinhibition but does not affect PSI activity in tobacco leaves under high-temperature stress

ABSTRACT This study aimed to evaluate the effects of 2-Cys Prx gene inhibition on photochemical reaction and reactive oxygen species (ROS) metabolism under high temperature (35°C) with low light (HT + LL) or high temperature with high light (HT + HL) in tobacco. The results showed that HT significantly increased the production of and H2O2 compared with CK (25°C). Particularly, the oxidative damage of RNAi plants was significantly greater than that of wild type (WT) under HT + HL treatment, possibly due to the inhibition of superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities. HT treatment inhibited the photosystem II (PSII) activity, and the oxygen evolution complex (OEC) was the main injury site. Notably, the photosystem I (PSI) activity of WT and RNAi plants did not change significantly under HT + LL treatment compared with CK. Although the PSI activity of WT and RNAi plants decreased significantly under HT + HL treatment, there was no significant difference between WT and RNAi plants. Collectively, these findings indicate that high light increases the photoinhibition of PSII and PSI and oxidative damage under high-temperature stress. The results also revealed that 2-Cys Prx plays a crucial role in alleviating oxidative damage and PSII photoinhibition under high-temperature stress in tobacco.

Dissecting photosynthetic electron transport and photosystems performance in Jerusalem artichoke (Helianthus tuberosus L.) under salt stress.

Jerusalem artichoke (Helianthus tuberosus L.), a vegetable with medical applications, has a strong adaptability to marginal barren land, but the suitability as planting material in saline land remains to be evaluated. This study was envisaged to examine salt tolerance in Jerusalem artichoke from the angle of photosynthetic apparatus stability by dissecting the photosynthetic electron transport process. Potted plants were exposed to salt stress by watering with a nutrient solution supplemented with NaCl. Photosystem I (PSI) and photosystem II (PSII) photoinhibition appeared under salt stress, according to the significant decrease in the maximal photochemical efficiency of PSI (△MR/MR0) and PSII. Consistently, leaf hydrogen peroxide (H2O2) concentration and lipid peroxidation were remarkably elevated after 8 days of salt stress, confirming salt-induced oxidative stress. Besides photoinhibition of the PSII reaction center, the PSII donor side was also impaired under salt stress, as a K step emerged in the prompt chlorophyll transient, but the PSII acceptor side was more vulnerable, considering the decreased probability of an electron movement beyond the primary quinone (ETo/TRo) upon depressed upstream electron donation. The declined performance of entire PSII components inhibited electron inflow to PSI, but severe PSI photoinhibition was not averted. Notably, PSI photoinhibition elevated the excitation pressure of PSII (1-qP) by inhibiting the PSII acceptor side due to the negative and positive correlation of △MR/MR0 with 1-qP and ETo/TRo, respectively. Furthermore, excessive reduction of PSII acceptors side due to PSI photoinhibition was simulated by applying a specific inhibitor blocking electron transport beyond primary quinone, demonstrating that PSII photoinhibition was actually accelerated by PSI photoinhibition under salt stress. In conclusion, PSII and PSI vulnerabilities were proven in Jerusalem artichoke under salt stress, and PSII inactivation, which was a passive consequence of PSI photoinhibition, hardly helped protect PSI. As a salt-sensitive species, Jerusalem artichoke was recommended to be planted in non-saline marginal land or mild saline land with soil desalination measures.

Zeaxanthin Epoxidase Activity Is Downregulated by Hydrogen Peroxide.

The xanthophyll zeaxanthin (Zx) serves important photoprotective functions in chloroplasts and is particularly involved in the dissipation of excess light energy as heat in the antenna of photosystem II (PSII). Zx accumulates under high-light (HL) conditions in thylakoid membranes and is reconverted to violaxanthin by Zx epoxidase (ZEP) in low light or darkness. ZEP activity is completely inhibited under long-lasting HL stress, and the ZEP protein becomes degraded along with the PSII subunit D1 during photoinhibition of PSII. This ZEP inactivation ensures that high levels of Zx are maintained under harsh HL stress. The mechanism of ZEP inactivation is unknown. Here, we investigated ZEP inactivation by reactive oxygen species (ROS) under in vitro conditions. Our results show that ZEP activity is completely inhibited by hydrogen peroxide (H2O2), whereas inhibition by singlet oxygen or superoxide seems rather unlikely. Due to the limited information about the amount of singlet oxygen and superoxide accumulating under the applied experimental conditions, however, a possible inhibition of ZEP activity by these two ROS cannot be generally excluded. Despite this limitation, our data support the hypothesis that the accumulation of ROS, in particular H2O2, might be responsible for HL-induced inactivation of ZEP under in vivo conditions.

Responses of Linear and Cyclic Electron Flow to Nitrogen Stress in an N-Sensitive Species Panax notoginseng.

Nitrogen (N) is a primary factor limiting leaf photosynthesis. However, the mechanism of N-stress-driven photoinhibition of the photosystem I (PSI) and photosystem II (PSII) is still unclear in the N-sensitive species such as Panax notoginseng, and thus the role of electron transport in PSII and PSI photoinhibition needs to be further understood. We comparatively analyzed photosystem activity, photosynthetic rate, excitation energy distribution, electron transport, OJIP kinetic curve, P700 dark reduction, and antioxidant enzyme activities in low N (LN), moderate N (MN), and high N (HN) leaves treated with linear electron flow (LEF) inhibitor [3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU)] and cyclic electron flow (CEF) inhibitor (methyl viologen, MV). The results showed that the increased application of N fertilizer significantly enhance leaf N contents and specific leaf N (SLN). Net photosynthetic rate (Pn) was lower in HN and LN plants than in MN ones. Maximum photochemistry efficiency of PSII (Fv/Fm), maximum photo-oxidation P700+ (Pm), electron transport rate of PSI (ETRI), electron transport rate of PSII (ETRII), and plastoquinone (PQ) pool size were lower in the LN plants. More importantly, K phase and CEF were higher in the LN plants. Additionally, there was not a significant difference in the activity of antioxidant enzyme between the MV- and H2O-treated plants. The results obtained suggest that the lower LEF leads to the hindrance of the formation of ΔpH and ATP in LN plants, thereby damaging the donor side of the PSII oxygen-evolving complex (OEC). The over-reduction of PSI acceptor side is the main cause of PSI photoinhibition under LN condition. Higher CEF and antioxidant enzyme activity not only protected PSI from photodamage but also slowed down the damage rate of PSII in P. notoginseng grown under LN.

Glycinebetaine mitigates tomato chilling stress by maintaining high-cyclic electron flow rate of photosystem I and stability of photosystem II.

Glycinebetaine alleviates chilling stress by protecting photosystems I and II in BADH-transgenic and GB-treated tomato plants, which can be an effective strategy for improving crop chilling tolerance. Tomato (Solanum lycopersicum) is one of the most cultivated vegetables in the world, but is highly susceptible to chilling stress and does not naturally accumulate glycinebetaine (GB), one of the most effective stress protectants. The protective mechanisms of GB on photosystem I (PSI) and photosystem II (PSII) against chilling stress, however, remain poorly understood. Here, we address this problem through exogenous GB application and generation of transgenic tomatoes (Moneymaker) with a gene encoding betaine aldehyde dehydrogenase (BADH), which is the key enzyme in the synthesis of GB, from spinach. Our results demonstrated that GB can protect chloroplast ultramicrostructure, alleviate PSII photoinhibition and maintain PSII stability under chilling stress. More importantly, GB increased the electron transfer between QA and QB and the redox potential of QB and maintained a high rate of cyclic electron flow around PSI, contributing to reduced production of reactive oxygen species, thereby mitigating PSI photodamage under chilling stress. Our results highlight the novel roles of GB in enhancing chilling tolerance via the protection of PSI and PSII in BADH transgenic and GB-treated tomato plants under chilling stress. Thus, introducing GB-biosynthetic pathway into tomato and exogenous GB application are effective strategies for improving chilling tolerance.

Regulation of Leaf Angle Protects Photosystem I under Fluctuating Light in Tobacco Young Leaves.

Fluctuating light is a typical light condition in nature and can cause selective photodamage to photosystem I (PSI). The sensitivity of PSI to fluctuating light is influenced by the amplitude of low/high light intensity. Tobacco mature leaves are tended to be horizontal to maximize the light absorption and photosynthesis, but young leaves are usually vertical to diminish the light absorption. Therefore, we tested the hypothesis that such regulation of the leaf angle in young leaves might protect PSI against photoinhibition under fluctuating light. We found that, upon a sudden increase in illumination, PSI was over-reduced in extreme young leaves but was oxidized in mature leaves. After fluctuating light treatment, such PSI over-reduction aggravated PSI photoinhibition in young leaves. Furthermore, the leaf angle was tightly correlated to the extent of PSI photoinhibition induced by fluctuating light. Therefore, vertical young leaves are more susceptible to PSI photoinhibition than horizontal mature leaves when exposed to the same fluctuating light. In young leaves, the vertical leaf angle decreased the light absorption and thus lowered the amplitude of low/high light intensity. Therefore, the regulation of the leaf angle was found for the first time as an important strategy used by young leaves to protect PSI against photoinhibition under fluctuating light. To our knowledge, we show here new insight into the photoprotection for PSI under fluctuating light in nature.

Photorespiration Alleviates Photoinhibition of Photosystem I under Fluctuating Light in Tomato.

Fluctuating light (FL) is a typical natural light stress that can cause photodamage to photosystem I (PSI). However, the effect of growth light on FL-induced PSI photoinhibition remains controversial. Plants grown under high light enhance photorespiration to sustain photosynthesis, but the contribution of photorespiration to PSI photoprotection under FL is largely unknown. In this study, we examined the photosynthetic performance under FL in tomato (Lycopersicon esculentum) plants grown under high light (HL-plants) and moderate light (ML-plants). After an abrupt increase in illumination, the over-reduction of PSI was lowered in HL-plants, resulting in a lower FL-induced PSI photoinhibition. HL-plants displayed higher capacities for CO2 fixation and photorespiration than ML-plants. Within the first 60 s after transition from low to high light, PSII electron transport was much higher in HL-plants, but the gross CO2 assimilation rate showed no significant difference between them. Therefore, upon a sudden increase in illumination, the difference in PSII electron transport between HL- and ML-plants was not attributed to the Calvin–Benson cycle but was caused by the change in photorespiration. These results indicated that the higher photorespiration in HL-plants enhanced the PSI electron sink downstream under FL, which mitigated the over-reduction of PSI and thus alleviated PSI photoinhibition under FL. Taking together, we here for the first time propose that photorespiration acts as a safety valve for PSI photoprotection under FL.

Photosystem I Inhibition, Protection and Signalling: Knowns and Unknowns.

Photosynthesis is the process that harnesses, converts and stores light energy in the form of chemical energy in bonds of organic compounds. Oxygenic photosynthetic organisms (i.e., plants, algae and cyanobacteria) employ an efficient apparatus to split water and transport electrons to high-energy electron acceptors. The photosynthetic system must be finely balanced between energy harvesting and energy utilisation, in order to limit generation of dangerous compounds that can damage the integrity of cells. Insight into how the photosynthetic components are protected, regulated, damaged, and repaired during changing environmental conditions is crucial for improving photosynthetic efficiency in crop species. Photosystem I (PSI) is an integral component of the photosynthetic system located at the juncture between energy-harnessing and energy consumption through metabolism. Although the main site of photoinhibition is the photosystem II (PSII), PSI is also known to be inactivated by photosynthetic energy imbalance, with slower reactivation compared to PSII; however, several outstanding questions remain about the mechanisms of damage and repair, and about the impact of PSI photoinhibition on signalling and metabolism. In this review, we address the knowns and unknowns about PSI activity, inhibition, protection, and repair in plants. We also discuss the role of PSI in retrograde signalling pathways and highlight putative signals triggered by the functional status of the PSI pool.

Oxidation of the reaction center chlorophyll of photosystem I is induced via close cooperation of photosystems II and I with progress of drought stress in soybean seedlings

ABSTRACT Drought is a serious abiotic stress that impairs plant productivity. We previously suggested that in rice (Oryza sativa L.) plants, the reaction center chlorophyll of photosystem I (PSI) (P700) is oxidized in response to drought stress via close cooperation of photosystem II (PSII) and PSI to protect PSI, whose fitness is crucial for photosynthesis and plant growth. It has rarely been reported whether such a defense system functions in other plant species. To collect another example, this subject was examined using soybean [Glycine max (L.)]. Irrigation of soil-cultured soybean seedlings was restricted to gradually impose drought stress. A decrease in the quantum efficiency of PSII [Y(II)], induction of non-photochemical quenching (NPQ) of excess light energy at PSII, reduction of the primary quinone electron acceptor of PSII (QA), and P700 oxidation proceeded before severe drought stress led to severe leaf wilting and photoinhibition of PSII and PSI. P700 oxidation, Y(II), and reduction of QA were strongly correlated in this period. These results are in accordance with those observed in rice. In contrast, such correlations disappeared when severely drought-stressed seedlings were included, as P700 oxidation was hampered while Y(II) further decreased in such seedlings. These results suggest that close cooperation of PSII and PSI leading to P700 oxidation functions to protect PSI against drought stress in soybean, and that similar defense system functions in soybean and rice unless drought stress are severe. The drought-stress responses of PSII and PSI are suggested to be regulated by acidification of thylakoid lumen in soybean, according to the changing patterns in the PSII and PSI parameters. Based on the similarity of the photochemistry of soybean and rice in response to drought stress, the effectiveness of P700 oxidation as an index for drought stress diagnosis is discussed. P700 oxidation is affected by factors other than drought stress such as N nutrition levels. However, effects of N nutrition levels are expected to be canceled by using another parameter obtained in P700 measurement as reference, suggesting the suitability of P700 oxidation for drought stress diagnosis.

The response of photosystem I to fluctuating light is influenced by leaf nitrogen content in tomato

Fluctuating light and nitrogen (N) deficiency can occur synchronously under natural growth conditions. However, little is known about the photosynthetic regulation under fluctuating light in plants grown with N deficiency. In the present study, we examined the effect of N supply on the response of photosystem I (PSI) to fluctuating light in tomato. Plants grown under high N concentration (HN-plants) had higher leaf N content, chlorophyll content and saturating CO2 assimilation rate than plants grown under middle and low N concentrations (MN- and LN-plants). After an abrupt increase in illumination for 10 s, all plants could not generate a sufficient proton gradient (ΔpH). Meanwhile, PSI was over-reduced in HN- and MN-plants but was highly oxidized in LN-plants. In LN-plants, the smallest PSII electron flow avoided an over-reduction of PSI under fluctuating light. After transition from low to high light, CEF gradually increased to the peak in 30 s in HN-plants but rapidly increased to the peak in 10 s in MN-plants. Such delayed activation of CEF in HN-plants accelerated the over-reduction of PSI. After fluctuating light treatment, HN-plants displayed the greatest PSI photoinhibition, followed by MN- and LN-plants. These results indicated that leaf N content significantly affected the response of PSI to fluctuating light in tomato.

Specific Incorporation of Polyunsaturated Fatty Acids into the sn-2 Position of Phosphatidylglycerol Accelerates Photodamage to Photosystem II under Strong Light.

Free fatty acids (FFAs) are generated by the reaction of lipases with membrane lipids. Generated polyunsaturated fatty acids (PUFAs) containing more than two double bonds have toxic effects in photosynthetic organisms. In the present study, we examined the effect of exogenous FFAs in the growth medium on the activity of photosystem II (PSII) under strong light in the cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis). PUFAs but not monounsaturated fatty acids accelerated the rate of photodamage to PSII by inactivating electron transfer at the oxygen-evolving complex. Moreover, supplemented PUFAs were specifically incorporated into the sn-2 position of phosphatidylglycerol (PG), which usually contains C16 fatty acids at the sn-2 position in Synechocystis cells. The disruption of the gene for an acyl-ACP synthetase reduced the effect of PUFAs on the photoinhibition of PSII. Thus, the specific incorporation of PUFAs into PG molecules requires acyl-ACP synthetase and leads to an unstable PSII, thereby accelerating photodamage to PSII. Our results are a breakthrough into elucidating the molecular mechanism of the toxicity of PUFAs to photosynthetic organisms.

Elevated CO2 Concentration Alters Photosynthetic Performances under Fluctuating Light in Arabidopsis thaliana.

In view of the current and expected future rise in atmospheric CO2 concentrations, we examined the effect of elevated CO2 on photoinhibition of photosystem I (PSI) under fluctuating light in Arabidopsis thaliana. At 400 ppm CO2, PSI showed a transient over-reduction within the first 30 s after transition from dark to actinic light. Under the same CO2 conditions, PSI was highly reduced after a transition from low to high light for 20 s. However, such PSI over-reduction greatly decreased when measured in 800 ppm CO2, indicating that elevated atmospheric CO2 facilitates the rapid oxidation of PSI under fluctuating light. Furthermore, after fluctuating light treatment, residual PSI activity was significantly higher in 800 ppm CO2 than in 400 ppm CO2, suggesting that elevated atmospheric CO2 mitigates PSI photoinhibition under fluctuating light. We further demonstrate that elevated CO2 does not affect PSI activity under fluctuating light via changes in non-photochemical quenching or cyclic electron transport, but rather from a rapid electron sink driven by CO2 fixation. Therefore, elevated CO2 mitigates PSI photoinhibition under fluctuating light at the acceptor rather than the donor side. Taken together, these observations indicate that elevated atmospheric CO2 can have large effects on thylakoid reactions under fluctuating light.

Ketocarotenoids accumulation in the leaves of engineered Brassica napus restricts photosynthetic efficiency and plant growth

Astaxanthin is a high-valued ketocarotenoid that is rarely synthesized in higher plants due to their lack of a carotenoid ketolase. Overexpression of a heterologous β-carotene ketolase gene coupled with other pathway genes in higher plants has triggered the production of various ketocarotenoids in the transformants. Here we reported that constitutive and simultaneous overexpression of four genes involved in astaxanthin biosynthesis in Brassica napus resulted in the accumulation of the nonnative ketocarotenoids such as astaxanthin and ketolutein, and the decreased production of chlorophylls, lutein and violaxanthin in leaves. As a result, transgenic plants showed lower non-photochemical quenching (NPQ) under high light, leading to moderate photoinhibition of photosystem II (PSII). Such PSII photoinhibition depressed linear electron transport and thus restricted the rate of CO2 assimilation. Consequently, transgenic plants grew slower and showed less biomass than wild type. These results indicated that endogenous synthesis of ketocarotenoids negatively affected photosynthesis and impaired plant growth in B. napus. Therefore, to prevent the side effect on normal growth, seed specific expression of the transgenes may be necessary for enhanced production of astaxanthin in seeds.

Glycinebetaine mitigated the photoinhibition of photosystem II at high temperature in transgenic tomato plants.

Photosystem II (PSII), especially the D1 protein, is highly sensitive to the detrimental impact of heat stress. Photoinhibition always occurs when the rate of photodamage exceeds the rate of D1 protein repair. Here, genetically engineered codA-tomato with the capability to accumulate glycinebetaine (GB) was established. After photoinhibition treatment at high temperature, the transgenic lines displayed more thermotolerance to heat-induced photoinhibition than the control line. GB maintained high expression of LeFtsHs and LeDegs and degraded the damaged D1 protein in time. Meanwhile, the increased transcription of synthesis-related genes accelerated the de novo synthesis of D1 protein. Low ROS accumulation reduced the inhibition of D1 protein translation in the transgenic plants, thereby reducing protein damage. The increased D1 protein content and decreased phosphorylated D1 protein (pD1) in the transgenic plants compared with control plants imply that GB may minimize photodamage and maximize D1 protein stability. As D1 protein exhibits a high turnover, PSII maybe repaired rapidly and efficiently in transgenic plants under photoinhibition treatment at high temperature, with the resultant mitigation of photoinhibition of PSII.

Influence of salinity stress on PSII in barley (Hordeum vulgare L.) genotypes, probed by chlorophyll-a fluorescence

ObjectivesChlorophyll-a fluorescence is an efficient tool to determine the photosynthetic capacity of plants and the health status of plants under normal or stress conditions including salinity stress. This study was aimed to elucidate changes in the efficiency of photosystem II (PSII) in barley genotypes differing in degree of salt tolerance, which can be used for phenotyping in the breeding program for developing salt-tolerant cultivars. MethodologyTwelve barley (Hordeum vulgare L.) genotypes were subjected to salt stress and salt stress reduced the growth of all barley genotypes, which is associated with a decline in chlorophyll and K+ contents (roots and leaves) and increase in Na+. Of the 12 barley genotypes, one salt-tolerant (B-10008) and one salt-sensitive barley genotype (B-14011) was selected to further investigate the structural stability of PSII using fast chlorophyll a kinetic analysis under salinity stress. ResultsThe shape of OJIP transients changed due to salt stress in both salt-sensitive and salt-tolerant barley genotypes indicating a disturbance in structural stability at various points of PSII. The detailed analysis of JIP-test parameters suggested that salt stress caused photoinhibition of PSII due to enhanced inactive reaction centers, reduced absorption flux (ABS/RC), low transfer of electrons per reaction center (ETO/RC) and enhanced accumulation of QA¯ (VJ) thus reducing primary photochemistry (TRO/RC, ɸPO). These changes in PSII resulted in the reduction of the maximum quantum yield of PSII (Fv/Fm) and performance index (PIABS). Moreover, salinity stress enhanced dissipation energy flux per reaction center (DIO/RC) as a protective measure to save PSII from photooxidative damage in thylakoid membrane. ConclusionThe appearance of positive K and L-bands supported the idea that salt stress dissociated the light-harvesting complex from core proteins of PSII, damaged oxygen-evolving complex and reduced the structural stability of PSII by disturbing the electron transfer between acceptor and donor sides of PSII especially in salt sensitive genotype (B-14011). Moreover, such an adverse effect of salt stress on PSII was less in salt-tolerant barley genotype (B-10008). Thus, some JIP-test parameters can be used as potential phenotype marker for screening salt-tolerant genotypes.

Contrasting photosynthesis, photoinhibition and oxidative damage in honeysuckle (Lonicera japonica Thunb.) under iso-osmotic salt and drought stresses

Honeysuckle (Lonicera japonica Thunb.) is a traditional Chinese medicinal crop and belongs to the glycophyte with certain salt tolerance. This study aimed to deeply dissect its salt adaptability by contrasting photosynthesis, photoinhibition and oxidative damage under moderate and severe iso-osmotic salt (150 and 300 mM NaCl) and drought (19.3 % and 28 % PEG-6000) stresses with hydroponic protocol. Photosynthesis was more susceptible to drought stress than iso-osmotic salt stress in honeysuckle according to drought-induced greater decrease in photosynthetic rate. In contrast to salt-induced mild PSⅡ and PSI photoinhibition, severe photosystem II (PSⅡ) and photosystem I (PSI) photoinhibition arose upon iso-osmotic drought stress, indicated by greater decreased the maximal photochemical efficiency of PSⅡ and PSI and remarkable loss of their reaction center proteins. However, PSⅡ and PSI interaction hardly contributed to salt stability of photosynthetic apparatus because of salt-induced finite restriction on electron flow from PSⅡ to PSI. Consistent with photosystems photoinhibition, leaf lipid peroxidation, H2O2 production and electrolyte leakage were elevated much greater by drought stress than iso-osmotic salt stress, confirming drought-induced severe oxidative stress in honeysuckle. Furthermore, the principal components analysis comprehensively showed higher salt adaptability in honeysuckle due to larger cluster separation upon drought stress than iso-osmotic salt stress. As an apparent reason, honeysuckle could prevent drought-induced tremendous leaf water loss upon iso-osmotic salt stress, and had a capacity to dispose accumulated Na+. Therefore, honeysuckle resembles halophytes in this respect and seems appropriate for planting in coastal saline land.

Photosystem I photoinhibition induced by fluctuating light depends on background low light irradiance

Fluctuating light (FL) can induce the preferential photoinhibition of photosystem I (PSI) in angiosperms. However, the underlying mechanisms have not been fully clarified. In this study, we examined the effects of low-light phases on FL-induced photoinhibition of PSI in Bletilla striata. We found that PSI was highly over-reduced within the first 20 s after transition from 59 to 1455 μmol photons m−2 s−1. However, such severe over-reduction of PSI was not observed in FL transition from 132 or 272–1455 μmol photons m−2 s−1. As a result, FL (59–1455) induced stronger PSI photoinhibition than FL (132–1455) and FL (272–1455). During low-light phase, a higher light intensity induced a higher proton gradient (ΔpH) across the thylakoid membranes, favoring the rapid formation of ΔpH after light increased and preventing an over-reduction of PSI. These results indicate that the low-light phase significantly affects the extent of PSI photoinhibition in FL. After transition from 59 or 132–1455 μmol photons m−2 s−1, CEF activity first increased and then decreased to the steady-state. However, such transient CEF stimulation was not observed during the transition from 272–1455 μmol photons m−2 s−1. Furthermore, the CEF activation in the high-light phase was highly dependent on the PSI reduction state. We conclude that CEF activation is finely regulated to optimize the trade-off between photoprotection and light use efficiency in FL.

Long-Chain Saturated Fatty Acids, Palmitic and Stearic Acids, Enhance the Repair of Photosystem II.

Free fatty acids (FFA) generated in cyanobacterial cells can be utilized for the biodiesel that is required for our sustainable future. The combination of FFA and strong light induces severe photoinhibition of photosystem II (PSII), which suppresses the production of FFA in cyanobacterial cells. In the present study, we examined the effects of exogenously added FFA on the photoinhibition of PSII in Synechocystis sp. PCC 6803. The addition of lauric acid (12:0) to cells accelerated the photoinhibition of PSII by inhibiting the repair of PSII and the de novo synthesis of D1. α-Linolenic acid (18:3) affected both the repair of and photodamage to PSII. Surprisingly, palmitic (16:0) and stearic acids (18:0) enhanced the repair of PSII by accelerating the de novo synthesis of D1 with the mitigation of the photoinhibition of PSII. Our results show chemical potential of FFA in the regulation of PSII without genetic manipulation.

Photorespiration Coupled With CO2 Assimilation Protects Photosystem I From Photoinhibition Under Moderate Poly(Ethylene Glycol)-Induced Osmotic Stress in Rice.

Photorespiration coupled with CO2 assimilation is thought to act as a defense system against photoinhibition caused by osmotic stress. In the present study, we examined whether such a mechanism is operative for the protection of photosystem I (PSI) in rice (Oryza sativa L.) including transgenic plants with decreased and increased Rubisco content (RBCS-antisense and RBCS-sense plants, respectively). All plants were hydroponically grown and moderate osmotic stress was imposed using hydroponic culture solutions containing poly(ethylene glycol) (PEG) at 16% or 20% (w/v) for 2 d. In wild-type plants, the rates of CO2 assimilation (A) were significantly decreased by the PEG treatment, whereas the photorespiration activity estimated from the rates of electron transport in photosystem II (PSII) and A were not affected. The maximal quantum efficiency of PSII (F v/F m) and the maximal activity of PSI (P m) were also not affected. In RBCS-antisense plants, A and the estimated photorespiration activity were considerably lower than those in wild-type plants in the presence or absence of the PEG treatment. P m and both F v/F m and P m decreased in the 16% PEG-treated and 20% PEG-treated RBCS-antisense plants, respectively. Thus, the decrease in Rubisco content led to the photoinhibition of PSI and PSII, indicating the importance of photorespiration coupled with CO2 assimilation for the protection of PSI from moderate PEG-induced osmotic stress. It was also shown that PSI was more sensitive to osmotic stress than PSII. In the PEG-treated wild-type and RBCS-antisense plants, osmotic-stress responses of the photosynthetic electron transport reactions upstream of PSI led to the oxidation of P700, which is thought to prevent PSI from over-reduction. Although such a defense system operated, it was not sufficient for the protection of PSI in RBCS-antisense plants. In addition, there were no large differences in the parameters measured between wild-type and RBCS-sense plants, as overproduction of Rubisco did not increase photorespiration activity.

Elevated Levels of Specific Carotenoids During Acclimation to Strong Light Protect the Repair of Photosystem II in Synechocystis sp. PCC 6803.

The tolerance of photosynthesis to strong light increases in photosynthetic organisms during acclimation to strong light. We investigated the role of carotenoids in the protection of photosystem II (PSII) from photoinhibition after acclimation to strong light in the cyanobacterium Synechocystis sp. PCC 6803. In cells that had been grown under strong light at 1,000 μmol photons m−2 s−1 (SL), specific carotenoids, namely, zeaxanthin, echinenone, and myxoxanthophyll, accumulated at high levels, and the photoinhibition of PSII was less marked than in cells that had been grown under standard growth light at 70 μmol photons m−2 s−1 (GL). The rate of photodamage to PSII, as monitored in the presence of lincomycin, did not differ between cells grown under SL and GL, suggesting that the mitigation of photoinhibition after acclimation to SL might be attributable to the enhanced ability to repair PSII. When cells grown under GL were transferred to SL, the mitigation of photoinhibition of PSII occurred in two distinct stages: a first stage that lasted 4 h and the second stage that occurred after 8 h. During the second stage, the accumulation of specific carotenoids was detected, together with enhanced synthesis de novo of proteins that are required for the repair of PSII, such as the D1 protein, and suppression of the production of singlet oxygen (1O2). In the ΔcrtRΔcrtO mutant of Synechocystis, which lacks zeaxanthin, echinenone, and myxoxanthophyll, the mitigation of photoinhibition of PSII, the enhancement of protein synthesis, and the suppression of production of 1O2 were significantly impaired during the second stage of acclimation. Thus, elevated levels of the specific carotenoids during acclimation to strong light appeared to protect protein synthesis from 1O2, with the resultant mitigation of photoinhibition of PSII.