Cyclic electron flow plays an important role in photoprotection for the resurrection plant <Emphasis Type="Italic">Paraboea</Emphasis><Emphasis Type="Italic">rufescens</Emphasis> under drought stress

Resurrection plants could survive severe drought stress, but the underlying mechanism for protecting their photosynthetic apparatus against drought stress is unclear. Cyclic electron flow (CEF) has been documented as a crucial mechanism for photoprotection in Arabidopsis and tobacco. We hypothesized that CEF plays an important role in protecting photosystem I (PSI) and photosystem II (PSII) against drought stress for resurrection plants. To address this hypothesis, the effects of mild drought stress on light energy distribution in PSII and P700 redox state were examined in a resurrection plant Paraboea rufescens. Cyclic electron flow was not activated below the photosynthetic photon flux density (PPFD) of 400 μmol m⁻² s⁻¹ in leaves without drought stress. However, CEF was activated under low light in leaves with mild drought stress, and the effective quantum yield of PSII significantly decreased. Meanwhile, non-photochemical quenching (NPQ) was significantly stimulated not only under high light but also under low light. Compared with the control, the fraction of overall P700 that cannot be oxidized in a given state (PSI acceptor side limitation) under high light was maintained at low level of 0.1 in leaves with water deficit, indicating that the over-reduction of the PSI acceptor side was prevented by the significant stimulation of CEF. Furthermore, methyl viologen could significantly increase the PSII photo-inhibition induced by high light compared with chloramphenicol. These results suggested that CEF is an important mechanism for protecting PSI and PSII from drought stress in resurrection plants.     

The different effects of chilling stress under moderate light intensity on photosystem II compared with photosystem I and subsequent recovery in tropical tree species

Tropical plants are sensitive to chilling temperatures above zero but it is still unclear whether photosystem I (PSI) or photosystem II (PSII) of tropical plants is mainly affected by chilling temperatures. In this study, the effect of 4°C associated with various light densities on PSII and PSI was studied in the potted seedlings of four tropical evergreen tree species grown in an open field, Khaya ivorensis, Pometia tomentosa, Dalbergia odorifera, and Erythrophleum guineense. After 8 h chilling exposure at the different photosynthetic flux densities of 20, 50, 100, 150 μmol m⁻² s⁻¹, the maximum quantum yield of PSII (F _v /F _m) in all of the four species decreased little, while the quantity of efficient PSI complex (P _m) remained stable in all species except E. guineense. However, after chilling exposure under 250 μmol m⁻² s⁻¹ for 24 h, F _v /F _m was severely photoinhibited in all species whereas P _m was relative stable in all plants except E. guineense. At the chilling temperature of 4°C, electron transport from PSII to PSI was blocked because of excessive reduction of primary electron acceptor of PSII. F _v /F _m in these species except E. guineense recovered to ~90% after 8 h recovery in low light, suggesting the dependence of the recovery of PSII on moderate PSI and/or PSII activity. These results suggest that PSII is more sensitive to chilling temperature under the moderate light than PSI in tropical trees, and the photoinhibition of PSII and closure of PSII reaction centers can serve to protect PSI.     

Competition between linear and cyclic electron flow in plants deficient in Photosystem I

Photosynthetic electron transport can involve either a linear flow from water to NADP, via Photosystems (PS) II and I or a cyclic flow just involving PSI. Little is known about factors regulating the relative flow through each of these pathways. We have examined photosynthetic electron transport through each system in plants of Arabidopsis thaliana in which either the PSI-D1 or PSI-E1 subunits of PSI have been knocked out. In both cases, this results in an imbalance in the turnover of PSI and PSII, such that PSII electron transport is limited by PSI turnover. Phosphorylation of light-harvesting complex II (LHCII) and its migration to PSI is enhanced but only partially reversible and not sufficient to balance photosystem turnover. In spite of this, cyclic electron flow is able to compete efficiently with PSI across a range of conditions. In dark-adapted leaves, the efficiency of cyclic relative to linear flow induced by far-red light is increased, implying that the limiting step of cyclic flow lies in the re-injection of electrons into the electron transport chain. Illumination of leaves with white light resulted in transient induction of a significant non-photochemical quenching in knockout plants which is probably high energy state quenching induced by cyclic electron flow. At high light and at low CO₂, non-photochemical quenching was greater in the knockout plants than in the wildtype. Comparison of PSI and PSII turnover under such conditions suggested that this is generated by cyclic electron flow around PSI. We conclude that, when the concentration of PSI is limiting, cyclic electron flow is still able to compete effectively with linear flow to maintain a high ΔpH to regulate photosynthesis.     

PGR5 and NDH-1 systems do not function as protective electron acceptors but mitigate the consequences of PSI inhibition

Avoidance of photoinhibition at photosystem (PS)I is based on synchronized function of PSII, PSI, Cytochrome b₆f and stromal electron acceptors. Here, we used a special light regime, PSI photoinhibition treatment (PIT), in order to specifically inhibit PSI by accumulating excess electrons at the photosystem (Tikkanen and Grebe, 2018). In the analysis, Arabidopsis thaliana WT was compared to the pgr5 and ndho mutants, deficient in one of the two main cyclic electron transfer pathways described to function as protective alternative electron acceptors of PSI. The aim was to investigate whether the PGR5 (pgr5) and the type I NADH dehydrogenase (NDH-1) (ndho) systems protect PSI from excess electron stress and whether they help plants to cope with the consequences of PSI photoinhibition. First, our data reveals that neither PGR5 nor NDH-1 system protects PSI from a sudden burst of electrons. This strongly suggests that these systems in Arabidopsis thaliana do not function as direct acceptors of electrons delivered from PSII to PSI – contrasting with the flavodiiron proteins that were found to make Physcomitrella patens PSI resistant to the PIT. Second, it is demonstrated that under light-limiting conditions, the electron transfer rate at PSII is linearly dependent on the amount of functional PSI in all genotypes, while under excess light, the PGR5-dependent control of electron flow at the Cytochrome b₆f complex overrides the effect of PSI inhibition. Finally, the PIT is shown to increase the amount of PGR5 and NDH-1 as well as of PTOX, suggesting that they mitigate further damage to PSI after photoinhibition rather than protect against it.     

Protection of photosystem I during sudden light stress depends on ferredoxin:NADP(H) reductase abundance and interactions

Plant tolerance to high light and oxidative stress is increased by overexpression of the photosynthetic enzyme Ferredoxin:NADP(H) reductase (FNR), but the specific mechanism of FNR-mediated protection remains enigmatic. It has also been reported that the localization of this enzyme within the chloroplast is related to its role in stress tolerance. Here, we dissected the impact of FNR content and location on photoinactivation of photosystem I (PSI) and photosystem II (PSII) during high light stress of Arabidopsis (Arabidopsis thaliana). The reaction center of PSII is efficiently turned over during light stress, while damage to PSI takes much longer to repair. Our results indicate a PSI sepcific effect, where efficient oxidation of the PSI primary donor (P700) upon transition from darkness to light, depends on FNR recruitment to the thylakoid membrane tether proteins: thylakoid rhodanase-like protein (TROL) and translocon at the inner envelope of chloroplasts 62 (Tic62). When these interactions were disrupted, PSI photoinactivation occurred. In contrast, there was a moderate delay in the onset of PSII damage. Based on measurements of ΔpH formation and cyclic electron flow, we propose that FNR location influences the speed at which photosynthetic control is induced, resulting in specific impact on PSI damage. Membrane tethering of FNR therefore plays a role in alleviating high light stress, by regulating electron distribution during short-term responses to light.

Altered location of a key enzyme involved in the post-photosystem I electron transport chain ameliorates damage to photosystem I during increasing light intensity.     

The role of light-harvesting complex I in excitation energy transfer from LHCII to photosystem I in Arabidopsis

Photosynthesis powers nearly all life on Earth. Light absorbed by photosystems drives the conversion of water and carbon dioxide into sugars. In plants, photosystem I (PSI) and photosystem II (PSII) work in series to drive the electron transport from water to NADP⁺. As both photosystems largely work in series, a balanced excitation pressure is required for optimal photosynthetic performance. Both photosystems are composed of a core and light-harvesting complexes (LHCI) for PSI and LHCII for PSII. When the light conditions favor the excitation of one photosystem over the other, a mobile pool of trimeric LHCII moves between both photosystems thus tuning their antenna cross-section in a process called state transitions. When PSII is overexcited multiple LHCIIs can associate with PSI. A trimeric LHCII binds to PSI at the PsaH/L/O site to form a well-characterized PSI–LHCI–LHCII supercomplex. The binding site(s) of the “additional” LHCII is still unclear, although a mediating role for LHCI has been proposed. In this work, we measured the PSI antenna size and trapping kinetics of photosynthetic membranes from Arabidopsis (Arabidopsis thaliana) plants. Membranes from wild-type (WT) plants were compared to those of the ΔLhca mutant that completely lacks the LHCI antenna. The results showed that “additional” LHCII complexes can transfer energy directly to the PSI core in the absence of LHCI. However, the transfer is about two times faster and therefore more efficient, when LHCI is present. This suggests LHCI mediates excitation energy transfer from loosely bound LHCII to PSI in WT plants.

The light-harvesting antennae of photosystem I facilitate energy transfer from trimeric light-harvesting complex II to photosystem I in the stroma lamellae membrane.     

The long-term response to fluctuating light quality is an important and distinct light acclimation mechanism that supports survival of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> under low light conditions

The long-term response (LTR) of higher plants to varying light qualities increases the photosynthetic yield; however, the benefit of this improvement for physiology and survival of plants is largely unknown, and its functional relation to other light acclimation responses has never been investigated. To unravel positive effects of the LTR we acclimated Arabidopsis thaliana for several days to light sources, which preferentially excite photosystem I (PSI) or photosystem II (PSII). After acclimation, plants revealed characteristic differences in chlorophyll fluorescence, thylakoid membrane stacking, phosphorylation state of PSII subunits and photosynthetic yield of PSII and PSI. These LTR-induced changes in the structure, function and efficiency of the photosynthetic machinery are true effects by light quality acclimation, which could not be induced by light intensity variations in the low light range. In addition, high light stress experiments indicated that the LTR is not involved in photoinhibition; however, it lowers non-photochemical quenching (NPQ) by directing more absorbed light energy into photochemical work. NPQ in turn is not essential for the LTR, since npq mutants performed a normal acclimation. We quantified the beneficial potential of the LTR by comparing wild-type plants with the LTR-deficient mutant stn7. The mutant exhibited a decreased effective quantum yield and produced only half of seeds when grown under fluctuating light quality conditions. Thus, the LTR represents a distinct acclimation response in addition to other already known responses that clearly improves plant physiology under low light conditions resulting in a pronounced positive effect on plant fitness.     

Response of photosynthesis to high light and drought for Arabidopsis thaliana grown under a UV-B enhanced light regime

Arabidopsis thaliana grown in a light regime that included ultraviolet-B (UV-B) radiation (6 kJ m⁻² d⁻¹) had similar light-saturated photosynthetic rates but up to 50% lower stomatal conductance rates, as compared to plants grown without UV-B radiation. Growth responses of Arabidopsis to UV-B radiation included lower leaf area (25%) and biomass (10%) and higher UV-B absorbing compounds (30%) and chlorophyll content (52%). Lower stomatal conductance rates for plants grown with UV-B radiation were, in part, due to lower stomatal density on the adaxial surface. Plants grown with UV-B radiation had more capacity to down regulate photochemical efficiency of photosystem II (PSII) as shown by up to 25% lower φ_PSII and 30% higher non-photochemical quenching of chlorophyll fluorescence under saturating light. These contributed to a smaller reduction in the maximum photochemical efficiency of PSII (F _v/F _m), greater dark-recovery of F _v/F _m, and higher light-saturated carbon assimilation and stomatal conductance and transpiration rates after a four-hour high light treatment for plants grown with UV-B radiation. Plants grown with UV-B were more tolerant to a 12 day drought treatment than plants grown without UV-B as indicated by two times higher photosynthetic rates and 12% higher relative water content. UV-B-grown plants also had three times higher proline content. Higher tolerance to drought stress for Arabidopsis plants grown under UV-B radiation may be attributed to both increased proline content and decreased stomatal conductance. Growth of Arabidopsis in a UV-B-enhanced light regime increased tolerance to high light exposure and drought stress.     

Photochemical activity and the structure of chloroplasts in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> L. mutants deficient in phytochrome A and B

The reduced content of photoreceptors, such as phytochromes, can decrease the efficiency of photosynthesis and activity of the photosystem II (PSII). For the confirmation of this hypothesis, the effect of deficiency in both phytochromes (Phy) A and B (double mutant, DM) in 7–27-day-old Arabidopsis thaliana plants on the photosynthetic activity was studied in absence and presence of UV-A radiation as a stress factor. The DM with reduced content of apoproteins of PhyA and PhyB and wild type (WT) plants with were grown in white and red light (WL and RL, respectively) of high (130 μmol quanta m^?2 s^?1) and low (40 μmol quanta m^?2 s^?1) intensity. For DM and WT grown in WL, no notable difference in the photochemical activity of PSII was observed. However, the resistance of the photosynthetic apparatus (PA) to UV-A and the rate of photosynthesis under light saturation were lower in the DM compared to those in the WT. Growth in RL, when the photoreceptors of blue light—cryptochromes—are inactive, resulted in the significant decrease of the photochemical activity of PSII in DM compared to that in WT including amounts of Q_B-non-reducing complexes of PSII and noticeable enhancement of thermal dissipation of absorbed light energy. In addition, marked distortion of the thylakoid membrane structure was observed for DM grown in RL. It is suggested that not only PhyA and PhyB but also cryptochromes are necessary for normal functioning of the PA and formation of the mechanisms of its resistance to UV-radiation.     

A novel method produces native light-harvesting complex II aggregates from the photosynthetic membrane revealing their role in nonphotochemical quenching

Nonphotochemical quenching (NPQ) is a mechanism of regulating light harvesting that protects the photosynthetic apparatus from photodamage by dissipating excess absorbed excitation energy as heat. In higher plants, the major light-harvesting antenna complex (LHCII) of photosystem (PS) II is directly involved in NPQ. The aggregation of LHCII is proposed to be involved in quenching. However, the lack of success in isolating native LHCII aggregates has limited the direct interrogation of this process. The isolation of LHCII in its native state from thylakoid membranes has been problematic because of the use of detergent, which tends to dissociate loosely bound proteins, and the abundance of pigment–protein complexes (e.g. PSI and PSII) embedded in the photosynthetic membrane, which hinders the preparation of aggregated LHCII. Here, we used a novel purification method employing detergent and amphipols to entrap LHCII in its natural states. To enrich the photosynthetic membrane with the major LHCII, we used Arabidopsis thaliana plants lacking the PSII minor antenna complexes (NoM), treated with lincomycin to inhibit the synthesis of PSI and PSII core proteins. Using sucrose density gradients, we succeeded in isolating the trimeric and aggregated forms of LHCII antenna. Violaxanthin- and zeaxanthin-enriched complexes were investigated in dark-adapted, NPQ, and dark recovery states. Zeaxanthin-enriched antenna complexes showed the greatest amount of aggregated LHCII. Notably, the amount of aggregated LHCII decreased upon relaxation of NPQ. Employing this novel preparative method, we obtained a direct evidence for the role of in vivo LHCII aggregation in NPQ.     

Acclimation of Arabidopsis thaliana to the light environment: changes in photosynthetic function

Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta was grown under light regimes of differing spectral qualities, which results in differences in the stoichiometries of the two photosynthetic reaction centres. The acclimative value of these changes was investigated by assessing photosynthetic function in these plants when exposed to two spectrally distinct actinic lights. Plants grown in an environment enriched in far-red light were better able to make efficient use of non-saturating levels of actinic light enriched in long-wavelength red light. Simultaneous measurements of chlorophyll fluorescence and absorption changes at 820 nm indicated that differences between plants grown under alternative light regimes can be ascribed to imbalances in excitation of photosystems I and II (PSI, PSII). Measurements of chlorophyll fluorescence emission and excitation spectra at 77 K provided strong evidence that there was little or no difference in the composition or function of PSI or PSII between the two sets of plants, implying that changes in photosynthetic stoichiometry are primarily responsible for the observed differences in photosynthetic function.Abbreviations Chl chlorophyll - FR far-red light - HF highirradiance FR-enriched light (400 mol·m^–2·s^–1, RFR = 0.72) - HW high-irradiance white light (400 mol·m^–2 1·1 s^–1RFR = 1.40) - LHCI, LHCII light-harvesting complex of PSI, PSII - qO quenching of dark-level chlorophyll fluorescence - qN non-photochemical quenching of variable chlorophyll fluorescence - qP photochemical quenching of variable chlorophyll fluorescence - R red light - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase We thank Dr. Sasha Ruban for assistance with the 77 K fluorescence measurements and for helpful discussions. This work was supported by Natural Environment Research Council Grant GR3/7571A.     

The occurrence of a thylakoid-localized small zinc finger protein in land plants

Previous studies showed that LOW QUANTUM YIELD OF PHOTOSYSTEM II 1 (LQY1), a small thylakoid zinc finger protein was involved in maintenance and repair of Photosystem II (PSII). Here the author provide additional evidence for the role of LQY1 in PSII maintenance and repair and further commentary on the occurrence of LQY1 protein among land plants. After exposure to high light, Arabidopsis thaliana mutants lacking functional LQY1 gene (At1g75690) are more photoinhibited than wild-type control plants; display higher total non-photochemical quenching and photoinhibitory quenching. These results are consistent with the initial observation that lqy1 mutants have lower PSII efficiency than wild-type plants after high-light treatment. The low-PSII-efficiency phenotype can be suppressed upon complementation of lqy1 mutants with the LQY1 gene from wild-type plants. This further demonstrates that LQY1 is important in maintaining the activity of photosystem II in Arabidopsis. LQY1 homologs are present in land plants but are absent from sequenced genomes of green algae and cyanobacteria, which may reflect plant adaptation to excess light stress during the transition to land.     

Acclimation of the photosynthetic apparatus and alterations in sugar metabolism in response to inoculation with endophytic fungi

The role of an endophytic Zygomycete Mucor sp. in growth promotion and adaptation of the photosynthetic apparatus to increased energy demands of its hosts Arabidopsis arenosa and Arabidopsis thaliana was evaluated. Inoculation with the fungus improved the water use efficiency of the plants and allowed for them to utilize incident light for photochemistry more effectively by upregulating the expression of several photosystem I‐ and II‐related genes and their respective proteins, proteins involved in light harvesting in PSII and PSI and carbon assimilation. This effect was independent of the ability of the plants to acquire nutrients from the soil. We hypothesize that the accelerated growth of the symbiotic plants resulted from an increase in their demand for carbohydrates and carbohydrate turnover (sink strength) that triggered a simultaneous upregulation of carbon assimilation. Arabidopsis plants inoculated with Mucor sp. exhibited upregulated expression in several genes encoding proteins involved in carbohydrate catabolism, sugar transport, and smaller starch grains that indicate a significant upregulation of carbohydrate metabolism.     

Acclimation of Arabidopsis thaliana to the light environment: regulation of chloroplast composition

The regulation by light of the composition of the photosynthetic apparatus was investigated in Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta. When grown in high- and low-irradiance white light, wild-type plants and photomorphogenic mutants showed large differences in their maximum photosynthetic rate and chlorophyll a/b ratios; such changes were abolished by growth in red light. Photosystem I (PSI) and PSII levels were measured in wild-type plants grown under a range of light environments; the results indicate that regulation of photosystem stoichiometry involves the specific detection of blue light. Supplementing red growth lights with low levels of blue light led to large increases in PSII content, while further increases in blue irradiance had the opposite effect; this latter response was abolished by the hy4 mutation, which affects certain events controlled by a blue-light receptor. Mutants defective in the phytochrome photoreceptors retained regulation of photosystem stoichiometry. We discuss the results in terms of two separate responses controlled by blue-light receptors: a blue-high-fluence response which controls photosystem stoichiometry; and a blue-low-fluence response necessary for activation of such control. Variation in the irradiance of the red growth light revealed that the blue-high-fluence response is attenuated by red light; this may be evidence that photosystem stoichiometry is controlled not only by photoreceptors, but also by photosynthetic metabolism.Abbreviations BHF blue-high-fluence - BLF blue-low-fluence - Chl chlorophyll - FR far-red light - LHCII light-harvesting complex of PSII - P_max maximum photosynthetic rate - R red light - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase This work was supported by Natural Environment Research Council Grant No. GR3/7571A. We would like to thank H. Smith (Botany Department, University of Leicester) and E. Murchie (INRA, Versailles) for helpful discussions.     

Desiccation of the resurrection plant <Emphasis Type="Italic">Haberlea rhodopensis</Emphasis> at high temperature

Haberlea rhodopensis plants, growing under low irradiance in their natural habitat, were desiccated to air-dry state at a similar light intensity (about 30 μmol m⁻² s⁻¹) under optimal (23/20°C, day/night) or high (38/30°C) temperature. Dehydration of plants at high temperature increased the rate of water loss threefold and had a more detrimental effect than either drought or high temperature alone. Water deficit decreased the photochemical activity of PSII and PSI and the rate of photosynthetic oxygen evolution, and these effects were stronger when desiccation was carried out at 38°C. Some reduction in the amount of the main PSI and PSII proteins was observed especially in severely desiccated Haberlea leaves. The results clearly showed that desiccation of the homoiochlorophyllous poikilohydric plant Haberlea rhodopensis at high temperature had more damaging effects than desiccation at optimal temperature and in addition recovery was slower. Increased thermal energy dissipation together with higher proline and carotenoid content in the course of desiccation at 38°C compared to desiccation at 23°C probably helped in overcoming the stress.     

Reversible changes in structure and function of photosynthetic apparatus of pea (Pisum sativum) leaves under drought stress

The effects of drought on photosynthesis have been extensively studied, whereas those on thylakoid organization are limited. We observed a significant decline in gas exchange parameters of pea (Pisum sativum) leaves under progressive drought stress. Chl a fluorescence kinetics revealed the reduction of photochemical efficiency of photosystem (PS)II and PSI. The non-photochemical quenching (NPQ) and the levels of PSII subunit PSBS increased. Furthermore, the light-harvesting complexes (LHCs) and some of the PSI and PSII core proteins were disassembled in drought conditions, whereas these complexes were reassociated during recovery. By contrast, the abundance of supercomplexes of PSII-LHCII and PSII dimer were reduced, whereas LHCII monomers increased following the change in the macro-organization of thylakoids. The stacks of thylakoids were loosely arranged in drought-affected plants, which could be attributed to changes in the supercomplexes of thylakoids. Severe drought stress caused a reduction of both LHCI and LHCII and a few reaction center proteins of PSI and PSII, indicating significant disorganization of the photosynthetic machinery. After 7 days of rewatering, plants recovered well, with restored chloroplast thylakoid structure and photosynthetic efficiency. The correlation of structural changes with leaf reactive oxygen species levels indicated that these changes were associated with the production of reactive oxygen species.     

Consequences of photosystem‐I damage and repair on photosynthesis and carbon use in Arabidopsis thaliana

Natural growth environments commonly include fluctuating conditions that can disrupt the photosynthetic energy balance and induce photoinhibition through inactivation of the photosynthetic apparatus. Photosystem II (PSII) photoinhibition is efficiently reversed by the PSII repair cycle, whereas photoinhibited photosystem I (PSI) recovers much more slowly. In the current study, treatment of the Arabidopsis thaliana mutant proton gradient regulation 5 (pgr5) with excess light was used to compromise PSI functionality in order to investigate the impact of photoinhibition and subsequent recovery on photosynthesis and carbon metabolism. The negative impact of PSI photoinhibition on CO₂ fixation was especially deleterious under low irradiance. Impaired starch accumulation after PSI photoinhibition was reflected in reduced respiration in the dark, but this was not attributed to impaired sugar synthesis. Normal chloroplast and mitochondrial metabolisms were shown to recover despite the persistence of substantial PSI photoinhibition for several days. The results of this study indicate that the recovery of PSI function involves the reorganization of the light‐harvesting antennae, and suggest a pool of surplus PSI that can be recruited to support photosynthesis under demanding conditions.     

Physico-chemical Property of Rare Earths—Effects on the Energy Regulation of Photosystem II in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Photosystem II (PSII) from Arabidopsis thaliana treated by lanthanum (La³⁺), cerium (Ce³⁺), and neodymium (Nd³⁺) were isolated to investigate the effects of 4f electron characteristics and alternation valence of rare earth elements (REEs) on PSII function regulation comparatively. Results showed that REE treatment could induce the generous expression of LhcII b in A. thaliana and increase the content of light-harvesting complex II and its trimer on the thylakoid membrane significantly. Meanwhile, the light absorption in the red and blue region and fluorescence quantum yield near 683 nm were obviously increased; oxygen evolution rate was greatly improved too, suggesting that REEs could enhance the efficiency of light absorption, regulate excitation energy distribution from photosystem I (PSI) to PSII, and thus increase the activity of photochemical reaction and oxygen evolution accordingly. The efficiency order of the four treatments was Ce³⁺ > Nd³⁺ > La³⁺ > control.     

Photosynthetic Properties of Photosystem Ⅱ in Arabidopsis thaliana Ipa1 Mutant

In a previous study, we characterized a high chlorophyll fluorescence Ipal mutant of Arabidopsis thallana, in which approximately 20% photosystem （PS） Ⅱ protein is accumulated. In the present study, analysis of fluorescence decay kinetics and thermoluminescence profiles demonstrated that the electron transfer reaction on either the donor or acceptor side of PSII remained largely unaffected in the Ipa1 mutant. In the mutant, maximal photochemical efficiency （Fv/Fm, where Fm is the maximum fluorescence yield and Fv is variable fluorescence） decreased with increasing light intensity and remained almost unchanged in wildtype plants under different light conditions. The Fv/Fm values also increased when mutant plants were transferred from standard growth light to low light conditions. Analysis of PSll protein accumulation further confirmed that the amount of PSll reaction center protein is correlated with changes in Fv/Fm in Ipal plants. Thus, the assembled PSll in the mutant was functional and also showed increased photosensitivity compared with wild-type plants.     

Digitonin-sensitive LHCII enlarges the antenna of Photosystem I in stroma lamellae of Arabidopsis thaliana after far-red and blue-light treatment

Light drives photosynthesis. In plants it is absorbed by light-harvesting antenna complexes associated with Photosystem I (PSI) and photosystem II (PSII). As PSI and PSII work in series, it is important that the excitation pressure on the two photosystems is balanced. When plants are exposed to illumination that overexcites PSII, a special pool of the major light-harvesting complex LHCII is phosphorylated and moves from PSII to PSI (state 2). If instead PSI is over-excited the LHCII complex is dephosphorylated and moves back to PSII (state 1). Recent findings have suggested that LHCII might also transfer energy to PSI in state 1. In this work we used a combination of biochemistry and (time-resolved) fluorescence spectroscopy to investigate the PSI antenna size in state 1 and state 2 for Arabidopsis thaliana. Our data shows that 0.7 ± 0.1 unphosphorylated LHCII trimers per PSI are present in the stroma lamellae of state-1 plants. Upon transition to state 2 the antenna size of PSI in the stroma membrane increases with phosphorylated LHCIIs to a total of 1.2 ± 0.1 LHCII trimers per PSI. Both phosphorylated and unphosphorylated LHCII function as highly efficient PSI antenna.