Intersystem Electron Transport Chain Research Articles

Integrative Transcriptome and Chlorophyll Fluorescence Test Analysis Shed New Light on the Leaf Senescence Mechanism of Zoysia japonica

Zoysia japonica is an important warm-season turfgrass used worldwide. The decreased aesthetic quality and functionality during leaf senescence hamper its further utilization. However, information about the transcriptional mechanism and genes involved in leaf senescence in Z. japonica needs to be more extensive. Therefore, to better understand leaf senescence in Z. japonica, we investigated the integrated analysis of chlorophyll fluorescence test (JIP-test) and RNA sequencing (RNA-seq) of mature and senescent leaves. First, we identified 22,049 genes, of which 4038 were differentially expressed genes (DEGs). The results for gene expression profiles were evaluated using quantitative real-time PCR. A total of 2515 genes have homologous genes in other plants. The matched known-function SAGs are mainly involved in chlorophyll degradation and plant hormone response. A total of 539 differentially expressed transcription factor genes, including AP2/ERF-ERF, NAC, WRKY, bHLH, and MYB, were identified to be associated with leaf senescence. Next, senescence represses chlorophyll biosynthesis while upregulating chlorophyll degradation. Senescence harms the integrity and functionality of PSII, PSI, and the intersystem electron transport chain. In addition, IAA biosynthesis was inhibited, whereas ABA and ET biosynthesis were activated in leaf senescence, and senescence activates signal transduction of IAA, ABA, and ET. These findings add to our understanding of the regulatory mechanism of leaf senescence. The senescence-associated genes are candidate targets for providing new insight into leaf senescence modeling in Z. japonica. They provided a theoretical foundation to reveal the functions of senescence-associated genes and chlorophyll catabolic genes involved in leaf senescence.

The noninvasive monitoring of the redox status of photosynthetic electron transport chains in Hibiscus rosa-sinensis and Tradescantia leaves

We present an approach to the noninvasive determination of the electron capacity of the intersystem pool of electron carriers in chloroplasts in situ. As apt experimental models, we used the leaves of Hibiscus rosa-sinensis and Tradescantia species. Electron paramagnetic resonance and optical response of P700 (the primary electron donor in Photosystem I) were applied to measuring electron transport in chloroplasts. Electron capacities of the intersystem electron transport chain (ETC) were determined from redox transients of P700 upon chromatic transitions (white light → far-red light). During the induction period, we observed the nonmonotonic changes in the number of electron equivalents in the intersystem ETC per P700 (parameter Q). In Hibiscus rosa-sinensis, the light-induced rise of Q from ≈2.5 (in the dark) to Q ≈ 12 was followed by its decrease to Q ≈ 6. The data obtained are discussed in the context of pH-dependent regulation of electron transport in chloroplasts, which provides the well-balanced operation of the intersystem ETC. The decay of Q is explained by the attenuation of Photosystem II activity due to the lumen acidification and the acceleration of plastoquinol re-oxidation as a result of the Calvin-Benson cycle activation. Our computer model of electron and proton transport coupled to ATP synthesis in chloroplasts was used to analyze the up and down feedbacks responsible for pH-dependent regulation of electron transport in chloroplasts. The procedures introduced here may be important for subsequent works aimed at defining the plastoquinone participation in regulation of photosynthetic processes in chloroplasts in situ.

Functional characterization of the chlorophyll b reductase gene NYC1 associated with chlorophyll degradation and photosynthesis in Zoysia japonica

Chlorophyll plays essential roles in photosynthesis and its degradation should be regulated precisely. Chlorophyll b reductase encoded by the gene NYC1 is responsible for catalyzing chlorophyll b degradation and maintaining the stability of photosystems. Although NYC1s have been characterized in many plants, their function in the photosynthetic machinery and the underlying regulatory mechanisms at the whole transcriptome level remain elusive. In this study, the ZjNYC1 gene was cloned from the important warm-season turfgrass Zoysia japonica. Analysis of the subcellular localization revealed that ZjNYC1 was localized in the chloroplasts, and results of the bimolecular fluorescence complementation (BiFC) assay indicated that ZjNYC1 could interact in vivo with the NYC1-like protein, ZjNOL. Overexpression of ZjNYC1 accelerated the degradation of chlorophyll, and promoted senescence with accumulation of ABA and reactive oxygen species (ROS) in Arabidopsis. Expression of ZjNYC1 rescued the stay-green phenotype of the Arabidopsis nyc1 mutant. An integrated analysis of chlorophyll fluorescence test (JIP-test) and RNA-sequencing revealed that ZjNYC1 negatively affected the efficiency of the PSII, PSI, and the intersystem electron transport chain. Furthermore, the Calvin cycle and photorespiration were suppressed, but ABA and ethylene synthesis and signal transduction pathways were activated in the transgenic lines. Collectively, these results demonstrated that ZjNYC1 plays important roles in accelerating chlorophyll degradation and senescence, and caused negative affect on the integrity and functionality of the photosystems.

Photomorphogenesis in the Picocyanobacterium Cyanobium gracile Includes Increased Phycobilisome Abundance Under Blue Light, Phycobilisome Decoupling Under Near Far-Red Light, and Wavelength-Specific Photoprotective Strategies.

Photomorphogenesis is a process by which photosynthetic organisms perceive external light parameters, including light quality (color), and adjust cellular metabolism, growth rates and other parameters, in order to survive in a changing light environment. In this study we comprehensively explored the light color acclimation of Cyanobium gracile, a common cyanobacterium in turbid freshwater shallow lakes, using nine different monochromatic growth lights covering the whole visible spectrum from 435 to 687 nm. According to incident light wavelength, C. gracile cells performed great plasticity in terms of pigment composition, antenna size, and photosystem stoichiometry, to optimize their photosynthetic performance and to redox poise their intersystem electron transport chain. In spite of such compensatory strategies, C. gracile, like other cyanobacteria, uses blue and near far-red light less efficiently than orange or red light, which involves moderate growth rates, reduced cell volumes and lower electron transport rates. Unfavorable light conditions, where neither chlorophyll nor phycobilisomes absorb light sufficiently, are compensated by an enhanced antenna size. Increasing the wavelength of the growth light is accompanied by increasing photosystem II to photosystem I ratios, which involve better light utilization in the red spectral region. This is surprisingly accompanied by a partial excitonic antenna decoupling, which was the highest in the cells grown under 687 nm light. So far, a similar phenomenon is known to be induced only by strong light; here we demonstrate that under certain physiological conditions such decoupling is also possible to be induced by weak light. This suggests that suboptimal photosynthetic performance of the near far-red light grown C. gracile cells is due to a solid redox- and/or signal-imbalance, which leads to the activation of this short-term light acclimation process. Using a variety of photo-biophysical methods, we also demonstrate that under blue wavelengths, excessive light is quenched through orange carotenoid protein mediated non-photochemical quenching, whereas under orange/red wavelengths state transitions are involved in photoprotection.

Revisiting cyanobacterial state transitions

Photosynthetic organisms are exposed to a fluctuating environment in which light intensity and quality change continuously. Specific illumination of either photosystem (PSI or PSII) creates an energy imbalance, leading to the reduction or oxidation of the intersystem electron transport chain. This redox imbalance could trigger the formation of dangerous reactive oxygen species. Cyanobacteria, like plants and algae, have developed a mechanism to re-balance this preferential excitation of either reaction center, called state transitions. State transitions are triggered by changes in the redox state of the membrane-soluble plastoquinone (PQ) pool. In plants and green algae, these changes in redox potential are sensed by Cytochrome b6f, which interacts with a specific kinase that triggers the movement of the main PSII antenna (the light-harvesting complex II). By contrast, although cyanobacterial state transitions have been studied extensively, there is still no agreement about the molecular mechanism, the PQ redox state sensor and the signaling pathways involved. In this review, we aimed to critically evaluate the results published on cyanobacterial state transitions, and discuss the “new” and “old” models in the subject. The phycobilisome and membrane contributions to this physiological process were addressed and the current hypotheses regarding its signaling transduction pathway were discussed.

Early Brassica Crops Responses to Salinity Stress: A Comparative Analysis Between Chinese Cabbage, White Cabbage, and Kale.

Soil salinity is severely affecting crop productivity in many countries, particularly in the Mediterranean area. To evaluate early plant responses to increased salinity and characterize tolerance markers, three important Brassica crops – Chinese cabbage (Brassica rapa ssp. pekinensis), white cabbage (B. oleracea var. capitata) and kale (B. oleracea var. acephala) were subjected to short-term (24 h) salt stress by exposing them to NaCl at concentrations of 50, 100, or 200 mM. Physiological (root growth, photosynthetic performance parameters, and Na+/K+ ratio) and biochemical parameters (proline content and lipid peroxidation as indicated by malondialdehyde, MDA, levels) in the plants’ roots and leaves were then measured. Photosynthetic parameters such as the total performance index PItotal (describing the overall efficiency of PSI, PSII and the intersystem electron transport chain) appeared to be the most salinity-sensitive parameter and informative stress marker. This parameter was decreased more strongly in Chinese cabbage than in white cabbage and kale. It indicated that salinity reduced the capacity of the photosynthetic system for efficient energy conversion, particularly in Chinese cabbage. In parallel with the photosynthetic impairments, the Na+/K+ ratio was highest in Chinese cabbage leaves and lowest in kale leaves while kale root is able to keep high Na+/K+ ratio without a significant increase in MDA. Thus Na+/K+ ratio, high in root and low in leaves accompanying with low MDA level is an informative marker of salinity tolerance. The crops’ tolerance was positively correlated with levels of the stress hormone abscisic acid (ABA) and negatively correlated with levels of jasmonic acid (JA), and jasmonoyl-L-isoleucine (JA-Ile). Furthermore, salinity induced contrasting changes in levels of the growth-promoting hormones brassinosteroids (BRs). The crop’s tolerance was positively correlated with levels of BR precursor typhasterol while negatively with the active BR brassinolide. Principal Component Analysis revealed correlations in observed changes in phytohormones, biochemical, and physiological parameters. Overall, the results show that kale is the most tolerant of the three species and Chinese cabbage the most sensitive to salt stress, and provide holistic indications of the spectrum of tolerance mechanisms involved.

Electron transport kinetics in the diazotrophic cyanobacterium Trichodesmium spp. grown across a range of light levels.

The diazotrophic cyanobacterium Trichodesmium is a major contributor to marine nitrogen fixation. We analyzed how light acclimation influences the photophysiological performance of Trichodesmium IMS101 during exponential growth in semi-continuous nitrogen fixing cultures under light levels of 70, 150, 250, and 400 μmol photons m(-2) s(-1), across diel cycles. There were close correlations between growth rate, trichome length, particulate organic carbon and nitrogen assimilation, and cellular absorbance, which all peaked at 150 μmol photons m(-2) s(-1). Growth rate was light saturated by about 100 μmol photons m(-2) s(-1) and was photoinhibited above 150 μmol photons m(-2) s(-1). In contrast, the light level (I k) to saturate PSII electron transport (e (-) PSII(-1) s(-1)) was much higher, in the range of 450-550 μmol photons m(-2) s(-1), and increased with growth light. Growth rate correlates with the absorption cross section as well as with absorbed photons per cell, but not to electron transport per PSII; this disparity suggests that numbers of PSII in a cell, along with the energy allocation between two photosystems and the state transition mechanism underlie the changes in growth rates. The rate of state transitions after a transfer to darkness increased with growth light, indicating faster respiratory input into the intersystem electron transport chain.

Alternative photosynthetic electron flow to oxygen in marine Synechococcus

Cyanobacteria dominate the world's oceans where iron is often barely detectable. One manifestation of low iron adaptation in the oligotrophic marine environment is a decrease in levels of iron-rich photosynthetic components, including the reaction center of photosystem I and the cytochrome b 6f complex [R.F. Strzepek and P.J. Harrison, Photosynthetic architecture differs in coastal and oceanic diatoms, Nature 431 (2004) 689–692.]. These thylakoid membrane components have well characterised roles in linear and cyclic photosynthetic electron transport and their low abundance creates potential impediments to photosynthetic function. Here we show that the marine cyanobacterium Synechococcus WH8102 exhibits significant alternative electron flow to O 2, a potential adaptation to the low iron environment in oligotrophic oceans. This alternative electron flow appears to extract electrons from the intersystem electron transport chain, prior to photosystem I. Inhibitor studies demonstrate that a propyl gallate-sensitive oxidase mediates this flow of electrons to oxygen, which in turn alleviates excessive photosystem II excitation pressure that can often occur even at relatively low irradiance. These findings are also discussed in the context of satisfying the energetic requirements of the cell when photosystem I abundance is low.

Nonphotosynthetic Reduction of the Intersystem Electron Transport Chain of Chloroplasts Following Heat Stress. The Pool Size of Stromal Reductants¶

The properties of a negative transient signal (negative peak) observed during the first seconds of the induction of the photoacoustic (PA) signal in dark-adapted barley leaves treated with methyl viologen (MV) and diuron and then exposed to high temperatures have been examined. Under those conditions no electron donation from photosystem II (PSII) occurred, and electron flow through PSI could be supported only by soluble reductants located in the chloroplast stroma. The negative peak was observed only if the PA signal had been monitored at low, and not high, frequencies. The peak obviously originated from the oxygen consumption by PSI. The size of the peak increased as the temperature of preheating was raised from 39 to 45°C. The size of the peak decreased exponentially with a half-time of 3.7 s during illumination under low light. This decrease was found to be much faster under strong light. The recovery of the peak during dark acclimation required several minutes. It is concluded that the negative peak reflects the oxygen consumption supported by stromal reductants, their pool being rapidly exhausted under light in the presence of MV. The maximal size of the pool was calculated as 140 eq:P700 in dark-adapted leaves.

Effects of ambient versus reduced UV‐B radiation on high arctic Salix arctica assessed by measurements and calculations of chlorophyll a fluorescence parameters from fluorescence transients

A UV‐B exclusion‐experiment was conducted in the high Arctic Zackenberg, NE Greenland, in which Salix arctica leaves during most of the growing season were fixed perpendicular to the solar zenith angle, thereby receiving maximal solar radiation. Covered with Teflon and Mylar foil, the leaves received approximately 90 and 40% of the ambient UV‐B irradiance, respectively. The effects were examined through recordings of chlorophyll a fluorescence transients, determination of biomass and analysis of total carbon and nitrogen content and amount of soluble flavonoids in the leaves. The processing of light was analysed by means of the chlorophyll a fluorescence transient, using the so‐called JIP test, as evolved by Reto J. Strasser and his coworkers. Reduction of the UV‐B irradiance caused a rise in many of the fluorescence parameters during July, but not in August (late season). Thus increases in the efficiency that an absorbed photon will be trapped by the PSII reaction centre with the resultant reduction of QA to QA– (ET0/ABS = FV/FM) and the efficiency that an electron residing on QA– will enter the intersystem electron transport chain (ET0/TR0) were observed in reduced UV‐B. Moreover, estimated per cross‐section of leaf sample, the number of active PSII reaction centres (RC/CSM) and electron transport rate (ETM/CSM) and all performance indexes (PIABS, PICSo and PICSm) were increased in reduced UV‐B. The total soluble flavonoid content was highest in ambient UV‐B. The treatment effects on fluorescence parameters that were directly measured (e.g. F0 and FM) and those that were derived (e.g. quantum efficiencies, parameters per PSII reaction centres and per cross‐section of leaf sample) are discussed in relation to one another, in relation to daily and seasonal variation, and from the perspective of evaluating the relative importance of UV‐B of donor and acceptor side capacity in Photosystem II. In conclusion, the experimental set‐up and non‐invasive measurements proved to be a sensitive method to screen for effects of UV‐B stress.

EPR study of electron transport in the cyanobacterium Synechocystis sp. PCC 6803: Oxygen-dependent interrelations between photosynthetic and respiratory electron transport chains

In this work, we investigated electron transport processes in the cyanobacterium Synechocystis sp. PCC 6803, with a special emphasis focused on oxygen-dependent interrelations between photosynthetic and respiratory electron transport chains. Redox transients of the photosystem I primary donor P700 and oxygen exchange processes were measured by the EPR method under the same experimental conditions. To discriminate between the factors controlling electron flow through photosynthetic and respiratory electron transport chains, we compared the P700 redox transients and oxygen exchange processes in wild type cells and mutants with impaired photosystem II and terminal oxidases ( CtaI, CydAB, CtaDEII). It was shown that the rates of electron flow through both photosynthetic and respiratory electron transport chains strongly depended on the transmembrane proton gradient and oxygen concentration in cell suspension. Electron transport through photosystem I was controlled by two main mechanisms: (i) oxygen-dependent acceleration of electron transfer from photosystem I to NADP +, and (ii) slowing down of electron flow between photosystem II and photosystem I governed by the intrathylakoid pH. Inhibitor analysis of P700 redox transients led us to the conclusion that electron fluxes from dehydrogenases and from cyclic electron transport pathway comprise 20–30% of the total electron flux from the intersystem electron transport chain to P700 +.

Stoichiometry of the photosynthetic apparatus and phycobilisome structure of the cyanobacterium Plectonema boryanum UTEX 485 are regulated by both light and temperature.

The role of growth temperature and growth irradiance on the regulation of the stoichiometry and function of the photosynthetic apparatus was examined in the cyanobacterium Plectonema boryanum UTEX 485 by comparing mid-log phase cultures grown at either 29 degrees C/150 micromol m(-2) s(-1), 29 degrees C/750 micromol m(-2) s(-1), 15 degrees C/150 micromol m(-2) s(-1), or 15 degrees C/10 micromol m(-2) s(-1). Cultures grown at 29 degrees C/750 micromol m(-2) s(-1) were structurally and functionally similar to those grown at 15 degrees C/150 micromol m(-2) s(-1), whereas cultures grown at 29 degrees C/150 micromol m(-2) s(-1) were structurally and functionally similar to those grown at 15 degrees C/10 micromol m(-2) s(-1). The stoichiometry of specific components of the photosynthetic apparatus, such as the ratio of photosystem (PS) I to PSII, phycobilisome size and the relative abundance of the cytochrome b(6)/f complex, the plastoquinone pool size, and the NAD(P)H dehydrogenase complex were regulated by both growth temperature and growth irradiance in a similar manner. This indicates that temperature and irradiance may share a common sensing/signaling pathway to regulate the stoichiometry and function of the photosynthetic apparatus in P. boryanum. In contrast, the accumulation of neither the D1 polypeptide of PSII, the large subunit of Rubisco, nor the CF(1) alpha-subunit appeared to be regulated by the same mechanism. Measurements of P700 photooxidation in vivo in the presence and absence of inhibitors of photosynthetic electron transport coupled with immunoblots of the NAD(P)H dehydrogenase complex in cells grown at either 29 degrees C/750 micromol m(-2) s(-1) or 15 degrees C/150 micromol m(-2) s(-1) are consistent with an increased flow of respiratory electrons into the photosynthetic intersystem electron transport chain maintaining P700 in a reduced state relative to cells grown at either 29 degrees C/150 micromol m(-2) s(-1) or 15 degrees C/10 micromol m(-2) s(-1). These results are discussed in terms of acclimation to excitation pressure imposed by either low growth temperature or high growth irradiance.

Kinetics of NADPH-Induced Non-Photochemical Reduction of the Plastoquinone Pool in Spinach Chloroplasts

Kinetics of non-photochemical reduction of the photosynthetic intersystem electron transport chain by exogenous NADPH was examined in osmotically lysed spinach chloroplasts by chlorophyll (Chl) fluorescence measurements under anaerobic condition. Upon the addition of NADPH, the apparent F0 increased sigmoidally, and the value of the maximal slope was calculated to give the reduction rate of plastoquinone (PQ) pool. Application of 5 µM antimycin A lowered significantly both the ceiling and the rate of the NADPH-induced Chl fluorescence increase, while the suppressive effect of 10 µM rotenone was slighter. This indicated that dark reduction of the PQ pool by NADPH in spinach chloroplasts under O2-limitation condition could be attributed mainly to the pathway catalysed sequentially by ferredoxin-NADP+ oxidoreductase (FNR) and ferredoxin-plastoquinone reductase (FQR), rather than that mediated by NAD(P)H dehydro- genase (NDH).

Nonphotosynthetic reduction of the intersystem electron transport chain of chloroplasts following heat stress. The pool size of stromal reductants.

The properties of a negative transient signal (negative peak) observed during the first seconds of the induction of the photoacoustic (PA) signal in dark-adapted barley leaves treated with methyl viologen (MV) and diuron and then exposed to high temperatures have been examined. Under those conditions no electron donation from photosystem II (PSII) occurred, and electron flow through PSI could be supported only by soluble reductants located in the chloroplast stroma. The negative peak was observed only if the PA signal had been monitored at low, and not high, frequencies. The peak obviously originated from the oxygen consumption by PSI. The size of the peak increased as the temperature of preheating was raised from 39 to 45 degrees C. The size of the peak decreased exponentially with a half-time of 3.7 s during illumination under low light. This decrease was found to be much faster under strong light. The recovery of the peak during dark acclimation required several minutes. It is concluded that the negative peak reflects the oxygen consumption supported by stromal reductants, their pool being rapidly exhausted under light in the presence of MV. The maximal size of the pool was calculated as 140 eq: P700 in dark-adapated leaves.

Nonphotosynthetic Reduction of the Intersystem Electron Transport Chain of Chloroplasts Following Heat Stress. Steady-state Rate

The consequence of elevated temperatures in the range of 39-51 degrees C on the steady-state rate of light-induced electron transport through photosystem I (PSI) supported by stromal reductants was studied in intact barley leaves using photoacoustic and chlorophyll fluorescence techniques. Measurable electron flow through PSI in diuron-treated leaves occurred only after exposure to temperatures above 37 degrees C. The steady-state rate of the above diuron-insensitive electron flow with methyl viologen as electron acceptor was estimated to be 3.7 mu eq m-2 s-1 or 0.018 mu eq mumol chlorophyll-1 s-1 in leaves exposed for 5 min to 45 degrees C.

Redox control of psbA gene expression in the cyanobacterium Synechocystis PCC 6803. Involvement of the cytochrome b(6)/f complex.

We investigated the role of the redox state of the photosynthetic and respiratory electron transport chains on the regulation of psbA expression in Synechocystis PCC 6803. Different means to modify the redox state of the electron carriers were used: (a) dark to oxidize the whole electron transport chain; (b) a shift from dark to light to induce its reduction; (c) the chemical interruption of the electron flow at different points to change the redox state of specific electron carriers; and (d) the presence of glucose to maintain a high reducing power in darkness. We show that changes in the redox state of the intersystem electron transport chain induce modifications of psbA transcript production and psbA mRNA stability. Reduction of the intersystem electron carriers activates psbA transcription and destabilizes the mRNA, while their oxidation induces a decrease in transcription and a stabilization of the transcript. Furthermore, our data suggest that the redox state of one of the electron carriers between the plastoquinone pool and photosystem I influences not only the expression of the psbA gene, but also that of other two photosynthetic genes, psaE and cpcBA. As a working hypothesis, we propose that the occupancy of the Q(0) site in the cytochrome b(6)/f complex may be involved in this regulation.

Nonphotosynthetic Reduction of the Intersystem Electron Transport Chain of Chloroplasts Following Heat Stress. Steady-state Rate

Nonphotosynthetic Reduction of the Intersystem Electron Transport Chain of Chloroplasts Following Heat Stress. Steady-state Rate

Photosynthesis of Transgenic P ssu-ipt Tobacco

The elevated content of endogenous cytokinin (CK), as the result of the introduction of the chimeric Pssu-ipt gene in tobacco, induced changes in growth, water regime and photosynthesis of transgenic grafts and rooted plants grown in a greenhouse. Despite closed stomata remarkable water stress was detected in Pssu-ipt plants. The ABA content was lower than in wild type tobacco, indicative of disturbances in ABA transport or metabolism. The rates of net photosynthesis (measured as CO2 uptake or O2 evolution) decreased by more than 50% in Pssu-ipt grafts and by 20% in Pssu-ipt rooted plants, respectively. The partial reactions of the electron transport chain in transgenic Pssu-ipt tobacco were differently influenced: the activity of the reaction centre of PSII was hardly affected; the PS I activity and the intersystem electron transport chain was inhibited up to 70%, particularly in the Pssu-ipt grafts. Although the slight decrease in the potential photochemical efficiency of PSII, expressed as Fv/Fm, was found in transgenic plants, the actual quantum yields, photochemical efficiencies, and qP under steady-state conditions indicated that transgenic plants were not seriously limited by the redox state of QA. The reaction centre of PSII was well preserved. In transgenic grafts, photophosphorylation capacity was strongly reduced, which could correspond with lower non-photochemical quenching. The above mentioned changes are the results of elevated CK content per se rather than the effect of altered water relations in the plants, caused by the disproportion of shoots and root system in both Pssu-ipt grafts and plants.

The chloroplast Ndh complex mediates the dark reduction of the plastoquinone pool in response to heat stress in tobacco leaves

We have examined the effects of heat stress on electron transfer in the thylakoid membrane of an engineered plastid ndh deletion mutant, Δ1, incapable of performing the Ndh-mediated reduction of the plastoquinone pool in the chloroplast. Upon heat stress in the dark, the rate of PSII-independent reduction of PSI after subsequent illumination by far-red light is dramatically enhanced in both Δ1 and a wild-type control plant (WT). In contrast, in the dark, only the WT shows an increase in the reduction state of the plastoquinone pool. We conclude that the heat stress-induced reduction of the intersystem electron transport chain can be mediated by Ndh-independent pathways in the light but that in the dark the dominant pathway for reduction of the plastoquinone pool is catalysed by the Ndh complex. Our results therefore demonstrate a functional role for the Ndh complex in the dark.

Adjustment of thylakoid plastoquinone content and Photosystem I electron donor pool size in response to growth temperature and growth irradiance in winter rye (Secale cereale L.)

Winter rye (Secale cereale L. cv Musketeer) grown at 5 °C/250 µmol photons m−2 s−1 exhibited a relative reduction state of PS II comparable to that of rye grown at 20 °C but high light (800 µmol photons m−2 s−1) (1-qP = 0.32) whereas winter rye grown at 20 °C/250 µmol photons m−2 s−1 exhibited values of 1-qP (≈ 0.15) comparable to plants grown at 5 °C but low light (50 µmol photons m−2 s−1). The apparent size of the electron donor pool to PS I, estimated either in vivo or in vitro in the presence of methylviologen by ΔA820 was positively correlated with the relative reduction state of PS II under the steady-state growth conditions. Immunoblotting of rye thylakoid polypeptides indicated that the relative contents of Lhcb1, Lhcb2, D1, Cyt f, PC, PsaA/PsaB heterodimer and the β-subunit of ATPase complex exhibited minimal changes on a Chl basis. In contrast, a 2-fold increase in plastoquinone A content was associated with increasing growth irradiance at growth temperatures of either 5 or 20 °C. We suggest that the increases in the apparent size of the electron donor pool to PS I associated with rye grown at either 5 °C/250 µmol photons m−2 s−1or 20 °C/800 µmol photons m−2 s−1 may be explained by an increased thylakoid plastoquinone A content, coupled with possible enhanced PS I cyclic electron transport and/or increased capacity for electron donation from the stroma to the intersystem electron transport chain. The results are discussed with respect to photosynthetic adjustment to changes in PS II ‘excitation pressure’ in winter rye.