Energy Transfer from Phycobilisomes to Photosystems of Nostoc flagelliforme Born. et Flah. During the Rewetting Course and Its Physiological Significance

During the non-frost season, the condensation of dew makes Nostocflagelliforme Born. et Flah., a highly drought-tolerant terrestrial cyanobacterium, frequently undergo rehydration-dehydration.Rehydration begins in the dark at night. After rewetting in the dark, photochemical activity and the structure of photosystem (PS)II were not recovered at all; the structure of PSI, energy transfer in phycobilisomes, and energy transfer from phycobilisomes to PSI were recovered within 5 min, as in the light. The recovery of energy transfer from phycobilisomes to PSII was light dependent and energy transfer from phycobilisomes to PSII was only partially recovered in the dark. These results suggest that the two-trigger control (water and light) of photosynthetic recovery may make IV, flagelliforme avoid unnecessary energy consumption and, at the same time, the partial recovery of energy transfer from phycobilisomes to PSII in the dark could help N. flagelliforme accumulate more photosynthetic products during the transient period of rehydration-dehydration.     

EFFECTS OF POTASSIUM ON THE PHOTOSYNTHETIC RECOVERY OF THE TERRESTRIAL CYANOBACTERIUM,NOSTOC FLAGELLIFORME (CYANOPHYCEAE) DURING REHYDRATION1

Effects of potassium on the photosynthetic recovery of Nostoc flagelliforme (Berk. & Curtis) Bornet & Flahault were investigated to determine its exact role during rehydration. Potassium enhanced recovery of the ability to reduce the primary quinone‐type acceptor (Q_A) and plastoquinone (PQ) pool and the area over the fluorescence rise curve was increased by 127%. The proportions of closed PSII reaction centers at phases J and I and the net rate of closure of PSII reaction centers were decreased by, respectively, 19%, 8%, and 23% with the addition of potassium, due to changes in the ability of PSII for multiple turnovers needed to reduce the PQ pool. Potassium significantly enhanced the probability of electron transfer beyond Q_A and the recovery of electron transport flux per PSII reaction center. Electron transport from water to methyl viologen for samples rehydrated in K⁺‐free BG₁₁ medium was 54% of those with the addition of potassium. However, electron flow from water to p‐benzoquinone and from reduced 2,6‐dichlorophenol‐indophenol to methyl viologen showed little change with the addition of potassium. The fast phase and slow phase of millisecond delayed light emission and the ATP content for samples rehydrated in K⁺‐free BG₁₁ medium were, respectively, 71.6%, 50.7%, and 77.1% of those with the addition of potassium. These suggested that potassium affected electron transfer from PQ to plastocyanin through the cytochrome b₆f complex and the proton motive force across the thylakoid membranes, probably reflecting its role in charge balance during H⁺ transport by the cytochrome b₆f complex.     

Low temperature and high light dependent dynamic photoprotective strategies in Arabidopsis thaliana

Arabidopsis thaliana has been recognized as a chilling tolerant species based on analysis of resistance to low temperature stress, however, the mechanisms involved in this tolerance are not yet clarified. The low temperature-induced effects are exacerbated when plants are exposed to low temperatures in the presence of high light irradiance but the experimental data on the impact of light intensity during cold stress and its influence during recovery from stress are rather limited. The main objective of this study was to re-examine the photosynthetic responses of A. thaliana plants to short term (6 days) low temperature stress (12/10°C) under optimal (150 μmol m⁻² s⁻¹) and high light (500 μmol m⁻² s⁻¹) intensity and the subsequent recovery from the stress. Simultaneous measurements of the in vivo and in vitro functional performance of both photosystem II (PSII) and photosystem I (PSI), as well as, net photosynthesis, low temperature (77 K) chlorophyll fluorescence and immunoblot analysis of the relative abundance of PSII and PSI reaction center proteins were used to evaluate the role of light in the development of possible protective mechanisms during low temperature stress and the consequent recovery from exposure to low temperature and different light intensities. The results presented clearly suggest that Arabidopsis plants can employ a number of highly dynamic photoprotective strategies depending on the light intensity. These strategies include one based on LHCII quenching and two other quenching mechanisms localized within the PSII and PSI reaction centers, which are all expressed to different extent depending on the severity of the photoinhibitory treatments under low temperature stress 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.     

POTASSIUM ALLEVIATES THE INHIBITORY ACTION OF AMMONIUM UPON THE PHOTOSYNTHETIC RECOVERY OF NOSTOC FLAGELLIFORME (CYANOPHYCEAE) DURING REHYDRATION1

Effects of ammonium on the photosynthetic recovery of Nostoc flagelliforme Berk. et M. A. Curtis were assayed when being rehydrated in low‐K⁺ or high‐K⁺ medium. Its photosynthetic recovery was K⁺ limited after 3 years of dry storage. The potassium absorption of N. flagelliforme reached the maximum after 3 h rehydration in low‐K⁺ medium but at 5 min in high‐K⁺ medium. The K⁺ content of N. flagelliforme rehydrated in high‐K⁺ medium was much higher than that in low‐K⁺ medium. The maximal PSII quantum yield (F_v/F_m) value of N. flagelliforme decreased significantly when samples were rehydrated in low‐K⁺ medium treated with 5 mM NH₄Cl. However, the treatment of 20 mM NH₄Cl had little effect on its F_v/F_m value in high‐K⁺ medium. The relative F_v/F_m 24 h EC₅₀ (concentration at which 50% inhibition occurred) value of NH₄⁺ in high‐K⁺ medium (64.35 mM) was much higher than that in low‐K⁺ medium (22.17 mM). This finding indicated that high K⁺ could alleviate the inhibitory action of NH₄⁺ upon the photosynthetic recovery of N. flagelliforme during rehydration. In the presence of 10 mM tetraethylammonium chloride (TEACl), the relative F_v/F_m 24 h EC₅₀ value of NH₄⁺ was increased to 46.34 and 70.78 mM, respectively, in low‐K⁺ and high‐K⁺ media. This observation suggested that NH₄⁺ entered into N. flagelliforme cells via the K⁺ channel. Furthermore, NH₄⁺ could decrease K⁺ absorption in high‐K⁺ medium.     

Reduced photoinhibition with stem curling in the resurrection plant Selaginella lepidophylla

Summary Selaginella lepidophylla, the resurrection plant, curls dramatically during desiccation and the hypothesis that curling may help limit bright light-induced damage during desiccation and rehydration was tested under laboratory conditions. Restraint of curling during desiccation at 25° C and a constant irradiance of 2000 mol m^–2 s^]t-1 significantly decreased PSII and whole-chain electron transport and the F_v/F_m fluorescence yield ratio following rehydration relative to unrestrained plants. Normal curling during desiccation at 37.5°C and 200 mol m^–2 s^–1 irradiance did not fully protect against photoinhibition or chlorophyll photooxidation indicating that some light-induced damage occurred early in the desiccation process before substantial curling. Photosystem I electron transport was less inhibited by high-temperature, high-irradiance desiccation than either PSII or whole-chain electron transport and PSI was not significantly affected by restraint of curling during desiccation at 25°C and high irradiance. Previous curling also helped prevent photoinhibition of PSII electron transport and loss of whole-plant photosynthetic capacity as the plants uncurled during rehydration at high light. These results demonstrate that high-temperature desiccation exacerbated photoinhibition, PSI was less photoinhibited than PSII or whole-chain electron transport, and stem curling ameliorated bright light-induced damage helping to make rapid recovery of photosynthetic competence possible when the plants are next wetted.     

Effect of high temperature on dehydration-induced alterations in photosynthetic characteristics of the resurrection plant Haberlea rhodopensis

The effect of high temperature (HT) and dehydration on the activity of photosynthetic apparatus and its ability to restore membrane properties, oxygen evolution, and energy distribution upon rehydration were investigated in a resurrection plant, Haberlea rhodopensis. Plants growing under low irradiance in their natural habitat were desiccated to air-dry state at a similar light intensity [about 30 μol(photon) m^?2 s^?1] under optimal day/night (23/20°C) or high (38/30°C) temperature. Our results showed that HT alone reduced the photosynthetic activity and desiccation of plants at 38°C and it had more detrimental effect compared with desiccation at 23°C. The study on isolated thylakoids demonstrated increased distribution of excitation energy to PSI as a result of the HT treatment, which was enhanced upon the desiccation. It could be related to partial destacking of thylakoid membranes, which was confirmed by electron microscopy data. In addition, the surface charge density of thylakoid membranes isolated from plants desiccated at 38°C was higher in comparison with those at 23°C, which was in agreement with the decreased membrane stacking. Dehydration led to a decrease of amplitudes of oxygen yields and to a loss of the oscillation pattern. Following rehydration, the recovery of CO₂ assimilation and fluorescence properties were better when desiccation was performed at optimal temperature compared to high temperature. Rehydration resulted in partial recovery of the amplitudes of flash oxygen yields as well as of population of S₀ state in plants desiccated at 23°C. However, it was not observed in plants dehydrated at 38°C.     

Lead induced changes in phosphorylation of PSII proteins in low light grown pea plants

Light-intensity and redox-state induced thylakoid proteins phosphorylation involved in structural changes and in regulation of protein turnover. The presence of heavy metal ions triggers a wide range of cellular responses including changes in plant growth and photosynthesis. Plants have evolved a number of mechanisms to protect photosynthetic apparatus. We have characterized the effect of lead on PSII protein phosphorylation in pea (Pisum sativum L.) plants grown in low light conditions. Pb ions affected only slightly photochemical efficiency of PSII and had no effect on organization of thylakoid complexes. Lead activated strongly phosphorylation of PSII core D1 protein and dephosphorylation of this protein did not proceed in far red light. D1 protein was also not degraded in this conditions. However, phosphorylation of LHCII proteins was not affected by lead. These results indicate that Pb²⁺ stimulate the phosphorylation of PSII core proteins and by disturbing the disassembly of supercomplexes play a role in PSII repair mechanism. LHCII phosphorylation could control the distribution of energy between the photosystems in low light conditions. This demonstrates that plants may respond to heavy metals by induction different pathways responsible for protein protection under stress conditions.     

Non-photochemical quenching-dependent acclimation and thylakoid organization of Chlamydomonas reinhardtii to high light stress

Light is essential for all photosynthetic organisms while an excess of it can lead to damage mainly the photosystems of the thylakoid membrane. In this study, we have grown Chlamydomonas reinhardtii cells in different intensities of high light to understand the photosynthetic process with reference to thylakoid membrane organization during its acclimation process. We observed, the cells acclimatized to long-term response to high light intensities of 500 and 1000 µmol m^?2 s^?1 with faster growth and more biomass production when compared to cells at 50 µmol m^?2 s^?1 light intensity. The ratio of Chl a/b was marginally decreased from the mid-log phase of growth at the high light intensity. Increased level of zeaxanthin and LHCSR3 expression was also found which is known to play a key role in non-photochemical quenching (NPQ) mechanism for photoprotection. Changes in photosynthetic parameters were observed such as increased levels of NPQ, marginal change in electron transport rate, and many other changes which demonstrate that cells were acclimatized to high light which is an adaptive mechanism. Surprisingly, PSII core protein contents have marginally reduced when compared to peripherally arranged LHCII in high light-grown cells. Further, we also observed alterations in stromal subunits of PSI and low levels of PsaG, probably due to disruption of PSI assembly and also its association with LHCI. During the process of acclimation, changes in thylakoid organization occurred in high light intensities with reduction of PSII supercomplex formation. This change may be attributed to alteration of protein–pigment complexes which are in agreement with circular dichoism spectra of high light-acclimatized cells, where decrease in the magnitude of psi-type bands indicates changes in ordered arrays of PSII–LHCII supercomplexes. These results specify that acclimation to high light stress through NPQ mechanism by expression of LHCSR3 and also observed changes in thylakoid protein profile/supercomplex formation lead to low photochemical yield and more biomass production in high light condition.

     

Effect of protein modification by malondialdehyde on the interaction between the oxygen-evolving complex 33 kDa protein and photosystem II core proteins

Previously we observed that the oxygen-evolving complex 33 kDa protein (OEC33) which stabilizes the Mn cluster in photosystem II (PSII), was modified with malondialdehyde (MDA), an end-product of peroxidized polyunsaturated fatty acids, and the modification increased in heat-stressed plants (Yamauchi et al. 2008). In this study, we examined whether the modification of OEC33 with MDA affects its binding to the PSII complex and causes inactivation of the oxygen-evolving complex. Purified OEC33 and PSII membranes that had been removed of extrinsic proteins of the oxygen-evolving complex (PSII∆OEE) of spinach (Spinacia oleracea) were separately treated with MDA. The binding was diminished when both OEC33 and PSII∆OEE were modified, but when only OEC33 or PSII∆OEE was treated, the binding was not impaired. In the experiment using thylakoid membranes, release of OEC33 from PSII and corresponding loss of oxygen-evolving activity were observed when thylakoid membranes were treated with MDA at 40°C but not at 25°C. In spinach leaves treated at 40°C under light, maximal efficiency of PSII photochemistry (F _v/F _m ratio of chlorophyll fluorescence) and oxygen-evolving activity decreased. Simultaneously, MDA contents in heat-stressed leaves increased, and OEC33 and PSII core proteins including 47 and 43 kDa chlorophyll-binding proteins were modified with MDA. In contrast, these changes were to a lesser extent at 40°C in the dark. These results suggest that MDA modification of PSII proteins causes release of OEC33 from PSII and it is promoted in heat and oxidative conditions.     

Photoinhibition of Photosynthetic Oxygen Production and its Recovery in the Subtidal Red Alga Polyneura hilliae

In the marine red alga Polyneura hilliae photoinhibition of photosynthesis was investigated by measuring the photosynthetic oxygen production. The extent of photoinhibition in this alga depends on the fluence rate as well as on the time of exposure. Strongly photoinhibited algae recover slowly. Full recovery is reached only in weak light after 3 days. Two phases of recovery can be distinguished: a first relatively fast phase of recovery that is independent of light, and a second slow phase which begins after about 6 hours and requires weak light. Consequently, even in complete darkness partial recovery is observed. An action spectrum of photoinhibition and its comparison with the in vivo absorption spectrum of Polyneura demonstrates that photoinhibition is mainly caused by light which is absorbed by phycobiliproteins, chlorophyll a and carotenoids. This fact and the ineffectiveness of light above 686 nm clearly indicate that the main photoinhibition site in this alga is PS II. No photosynthetic oxygen production is detectable after 3 h irradiation at 402 nm, 150 μmol quanta m^?2s^?1. As photosynthetic activity recovers slowly and to a limited extent, the suppression of oxygen production is apparently only in part the result of photoinhibition sensu strictu and in part due to permanent photodamage.     

Effect of partial or complete elimination of light‐harvesting complexes on the surface electric properties and the functions of cyanobacterial photosynthetic membranes

Influence of the modification of the cyanobacterial light‐harvesting complex [i.e. phycobilisomes (PBS)] on the surface electric properties and the functions of photosynthetic membranes was investigated. We used four PBS mutant strains of Synechocystis sp. PCC6803 as follows: PAL (PBS‐less), CK (phycocyanin‐less), BE (PSII‐PBS‐less) and PSI‐less/apcE^? (PSI‐less with detached PBS). Modifications of the PBS content lead to changes in the cell morphology and surface electric properties of the thylakoid membranes as well as in their functions, such as photosynthetic oxygen‐evolving activity, P700 kinetics and energy transfer between the pigment–protein complexes. Data reveal that the complete elimination of PBS in the PAL mutant causes a slight decrease in the electric dipole moments of the thylakoid membranes, whereas significant perturbations of the surface charges were registered in the membranes without assembled PBS–PSII macrocomplex (BE mutant) or PSI complex (PSI‐less mutant). These observations correlate with the detected alterations in the membrane structural organization. Using a polarographic oxygen rate electrode, we showed that the ratio of the fast to the slow oxygen‐evolving PSII centers depends on the partial or complete elimination of light‐harvesting complexes, as the slow operating PSII centers dominate in the PBS‐less mutant and in the mutant with detached PBS.     

Acclimation of Arabidopsis thaliana to the light environment: Changes in composition of the photosynthetic apparatus

Acclimation to changes in the light environment was investigated in Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta. Plants grown under four light regimes showed differences in their development, morphology, photosynthetic performance and in the composition of the photosynthetic apparatus. Plants grown under high light showed higher maximum rates of oxygen evolution and lower levels of light-harvesting complexes than their low light-grown counterparts; plants transferred to low light showed rapid changes in maximum photosynthetic rate and chlorophyll-a/b ratio as they became acclimated to the new environment. In contrast, plants grown under lights of differing spectral quality showed significant differences in the ratio of photosystem II to photosystem I. These changes are consistent with a model in which photosynthetic metabolism provides signals which regulate the composition of the thylakoid membrane.Abbreviations Aac1 gene encoding actin - Chl chlorophyll - F far-red-enriched light (R:FR = 0.72) - FR far-red light - H high light (400 mol · m^–2 · s^–1) - L low light (100 ml · m^–2 · s^–1) - LHCII light-harvesting complex of PSII - Lhcb genes encoding the proteins of LHCII - R red light - Rbcs genes encoding the small subunit of Rubisco - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - W white light (R:FR = 1.40) 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 (University of Sheffield) for helpful discussions.     

LIGHT DEPENDENCY OF PHOTOSYNTHETIC RECOVERY DURING WETTING AND THE ACCLIMATION OF PHOTOSYNTHETIC APPARATUS TO LIGHT FLUCTUATION IN A TERRESTRIAL CYANOBACTERIUM NOSTOC COMMUNE1

The PSII photochemical activity in a terrestrial cyanobacterium Nostoc commune Vaucher ex Bornet et Flahault during rewetting was undetectable in the dark but was immediately recognized in the light. The maximum quantum yield of PSII (F_v/F_m) during rewetting in the light rose to 85% of the maximum within ～30 min and slowly reached the maximum within 6 h, while with rewetting in the darkness for 6 h and then exposure to light the recovery of F_v/F_m required only ～3 min. These results suggested that recovery of photochemical activity might depend on two processes, light dependence and light independence, and the activation of photosynthetic recovery in the initial phase was severely light dependent. The inhibitor experiments showed that the recovery of F_v/F_m was not affected by chloramphenicol (CMP), but severely inhibited by 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) in the light, suggesting that the light‐dependent recovery of photochemical activity did not require de novo protein synthesis but required activation of PSII associated with electron flow to plastoquinone. Furthermore, the test indicated that the lower light intensity and the red light were of benefit to its activation of photochemical activity. In an outdoor experiment of diurnal changes of photochemical activity, our results showed that PSII photochemical activity was sensitive to light fluctuation, and the nonphotochemical quenching (NPQ) was rapidly enhanced at noon. Furthermore, the test suggested that the repair of PSII by de novo protein synthesis played an important role in the acclimation of photosynthetic apparatus to high light, and the heavily cloudy day was more beneficial for maintaining high photochemical activity.     

EFFECTS OF SALINITY AND LIGHT INTENSITY ON THE RESUMPTION OF PHOTOSYNTHESIS IN REHYDRATED CYANOBACTERIAL MATS FROM BAJA CALIFORNIA SUR,MEXICO1

Lyngbya mats in the intertidal of the Laguna Ojo de Liebre are metabolically active for only a few days every 2 weeks during spring tides, with environmental conditions varying greatly during these periods of hydration. Pulse amplitude modulated fluorometry (PAM) and oxygen measurements were used to measure photosynthetic activity during the first few hours after rehydration under various light intensities and salinities. Upon rehydration, a transitory maximum in respiratory activity (10–30 min) occurred before the resumption of photosynthesis, which could recover in about 2 h. Salinities outside the mats' natural range (35–50 psu) were detrimental to photosynthetic recovery. Both high (100 psu) and low (0–10 psu) salinities slowed recovery as well as lowered the overall photosynthetic yield. Photosynthesis was initiated earlier and recovered more rapidly with increasing light intensity. In addition, the positive effect of light on rates of recovery was disproportionately greater at lower salinities (10–25 psu) where high light (500 W·m^?2) counteracted the negative effects of low‐salt stress early in recovery. However, higher light intensities became photoinhibitory later in recovery (>2 h). Photosynthesis did not recover uniformly within the mat. Filaments deeper in the mat most likely recovered later than those near the surface due to high light attenuation. The ability of the mats to tolerate desiccation and take advantage of hydration periods even when conditions are suboptimal enables these mats to predominate in the intertidal environment.     

Changes in some thylakoid membrane proteins and pigments upon desiccation of the resurrection plant Haberlea rhodopensis

The changes in some proteins involved in the light reactions of photosynthesis of the resurrection plant Haberlea rhodopensis were examined in connection with desiccation. Fully hydrated (control) and completely desiccated plants (relative water content (RWC) 6.5%) were used for thylakoid preparations. The chlorophyll (Chl) a to Chl b ratios of thylakoids isolated from control and desiccated leaves were very similar, which was also confirmed by measuring their absorption spectra. HPLC analysis revealed that β-carotene content was only slightly enhanced in desiccated leaves compared with the control, but the zeaxanthin level was strongly increased. Desiccation of H. rhodopensis to an air-dried state at very low light irradiance led to a little decrease in the level of D1, D2, PsbS and PsaA/B proteins in thylakoids, but a relative increase in LHC polypeptides. To further elucidate whether the composition of the protein complexes of the thylakoid membranes had changed, we performed a separation of solubilized thylakoids on sucrose density gradients. In contrast to spinach, Haberlea thylakoids appeared to be much more resistant to the same solubilization procedure, i.e. complexes were not separated completely and complexes of higher density were found. However, the fractions analyzed provided clear evidence for a move of part of the antenna complexes from PSII to PSI when plants became desiccated. This move was also confirmed by low temperature emission spectra of thylakoids.Overall, the photosynthetic proteins remained comparatively stable in dried Haberlea leaves when plants were desiccated under conditions similar to their natural habitat. Low light during desiccation was enough to induce a rise in the xanthophyll zeaxanthin and β-carotene. Together with the extensive leaf shrinkage and some leaf folding, increased zeaxanthin content and the observed shift in antenna proteins from PSII to PSI during desiccation of Haberlea contributed to the integrity of the photosynthetic apparatus, which is important for rapid recovery after rehydration.     

Perturbations at the chloride site during the photosynthetic oxygen-evolving cycle

Photosystem II (PSII) catalyzes the oxidation of water to O₂ at the manganese-containing, oxygen-evolving complex (OEC). Photoexcitation of PSII results in the oxidation of the OEC; four sequential oxidation reactions are required for the generation and release of molecular oxygen. Therefore, with flash illumination, the OEC cycles among five S _n states. Chloride depletion inhibits O₂ evolution. However, the binding site of chloride in the OEC is not known, and the role of chloride in oxygen evolution has not as yet been elucidated. We have employed reaction-induced FT-IR spectroscopy and selective flash excitation, which cycles PSII samples through the S state transitions. On the time scale employed, these FT-IR difference spectra reflect long-lived structural changes in the OEC. Bromide substitution supports oxygen evolution and was used to identify vibrational bands arising from structural changes at the chloride-binding site. Contributions to the vibrational spectrum from bromide-sensitive bands were observed on each flash. Sulfate treatment led to an elimination of oxygen evolution activity and of the FT-IR spectra assigned to the S₃ to S₀ (third flash) and S₀ to S₁ transitions (fourth flash). However, sulfate treatment changed, but did not eliminate, the FT-IR spectra obtained with the first and second flashes. Solvent isotope exchange in chloride-exchanged samples suggests flash-dependent structural changes, which alter protein dynamics during the S state cycle. Supported by NSF MCB 03-55421.     

Protection of the photosynthetic apparatus against dehydration stress in the resurrection plant Craterostigma pumilum

The group of homoiochlorophyllous resurrection plants evolved the unique capability to survive severe drought stress without dismantling the photosynthetic machinery. This implies that they developed efficient strategies to protect the leaves from reactive oxygen species (ROS) generated by photosynthetic side reactions. These strategies, however, are poorly understood. Here, we performed a detailed study of the photosynthetic machinery in the homoiochlorophyllous resurrection plant Craterostigma pumilum during dehydration and upon recovery from desiccation. During dehydration and rehydration, C. pumilum deactivates and activates partial components of the photosynthetic machinery in a specific order, allowing for coordinated shutdown and subsequent reinstatement of photosynthesis. Early responses to dehydration are the closure of stomata and activation of electron transfer to oxygen accompanied by inactivation of the cytochrome b₆f complex leading to attenuation of the photosynthetic linear electron flux (LEF). The decline in LEF is paralleled by a gradual increase in cyclic electron transport to maintain ATP production. At low water contents, inactivation and supramolecular reorganization of photosystem II becomes apparent, accompanied by functional detachment of light‐harvesting complexes and interrupted access to plastoquinone. This well‐ordered sequence of alterations in the photosynthetic thylakoid membranes helps prepare the plant for the desiccated state and minimize ROS production.     

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.