Control of the photosynthetic electron transport by PQ diffusion microdomains in thylakoids of higher plants.
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We investigate the role of plastoquinone (PQ) diffusion in the control of the photosynthetic electron transport. A control analysis reveals an unexpected flux control of the whole chain electron transport by photosystem (PS) II. The contribution of PSII to the flux control of whole chain electron transport was high in stacked thylakoids (control coefficient, CJ(PSII) =0.85), but decreased after destacking (CJ(PSII)=0.25). From an 'electron storage' experiment, we conclude that in stacked thylakoids only about 50 to 60% of photoreducable PQ is involved in the light-saturated linear electron transport. No redox equilibration throughout the membrane between fixed redox groups at PSII and cytochrome (cyt) bf complexes, and the diffusable carrier PQ is achieved. The data support the PQ diffusion microdomain concept by Lavergne et al. [J. Lavergne, J.-P. Bouchaud, P. Joliot, Biochim. Biophys. Acta 1101 (1992) 13-22], but we come to different conclusions about size, structure and size distribution of domains. From an analysis of cyt b6 reduction, as a function of PSII inhibition, we conclude that in stacked thylakoids about 70% of PSII is located in small domains, where only 1 to 2 PSII share a local pool of a few PQ molecules. Thirty percent of PSII is located in larger domains. No small domains were found in destacked thylakoids. We present a structural model assuming a hierarchy of specific, strong and weak interactions between PSII core, light harvesting complexes (LHC) II and cyt bf. Peripheral LHCII's may serve to connect PSII-LHCII supercomplexes to a flexible protein network, by which small closed lipid diffusion compartments are formed. Within each domain, PQ moves rapidly and shuttles electrons between PSII and cyt bf complexes in the close vicinity. At the same time, long range diffusion is slow. We conclude, that in high light, cyt bfcomplexes located in distant stromal lamellae (20 to 30%) are not involved in the linear electron transport. (+info)
Dephosphorylation of photosystem II reaction center proteins in plant photosynthetic membranes as an immediate response to abrupt elevation of temperature.
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Kinetic studies of protein dephosphorylation in photosynthetic thylakoid membranes revealed specifically accelerated dephosphorylation of photosystem II (PSII) core proteins at elevated temperatures. Raising the temperature from 22 degrees C to 42 degrees C resulted in a more than 10-fold increase in the dephosphorylation rates of the PSII reaction center proteins D1 and D2 and of the chlorophyll a binding protein CP43 in isolated spinach (Spinacia oleracea) thylakoids. In contrast the dephosphorylation rates of the light harvesting protein complex and the 9-kD protein of the PSII (PsbH) were accelerated only 2- to 3-fold. The use of a phospho-threonine antibody to measure in vivo phosphorylation levels in spinach leaves revealed a more than 20-fold acceleration in D1, D2, and CP43 dephosphorylation induced by abrupt elevation of temperature, but no increase in light harvesting protein complex dephosphorylation. This rapid dephosphorylation is catalyzed by a PSII-specific, intrinsic membrane protein phosphatase. Phosphatase assays, using intact thylakoids, solubilized membranes, and the isolated enzyme, revealed that the temperature-induced lateral migration of PSII to the stroma-exposed thylakoids only partially contributed to the rapid increase in the dephosphorylation rate. Significant activation of the phosphatase coincided with the temperature-induced release of TLP40 from the membrane into thylakoid lumen. TLP40 is a peptidyl-prolyl cis-trans isomerase, which acts as a regulatory subunit of the membrane phosphatase. Thus dissociation of TLP40 caused by an abrupt elevation in temperature and activation of the membrane protein phosphatase are suggested to trigger accelerated repair of photodamaged PSII and to operate as possible early signals initiating other heat shock responses in chloroplasts. (+info)
Isolation and analysis of thermotolerant mutants of wheat.
(75/2508)
Thermotolerant mutants of wheat cv. Guardian were isolated by selecting survivors from 5-d-old seedlings of M2 populations exposed to 47 degrees C for 90 min. Progeny testing, using triphenyl tetrazolium chloride reduction as a measure of tissue viability, following heat stress treatment for 120 min at each of three temperatures (32, 38 and 50 degrees C), confirmed the thermotolerant nature of seedlings of 13 mutants. Mutants were isolated at frequencies of 0.1% and 0.2% following the use of sodium azide and ethyl methanesulphonate, respectively. The relative thermotolerance of ten of the mutants and 'Guardian' was then tested by exposing plants to heat stresses of 38 degrees C for 6 h in every 24 h for five successive days at one of four growth stages between seedling and anthesis. Pmax (light-saturated net photosynthetic rate) and chlorophyll content were compared in stressed and unstressed plants. The Pmax of 'Guardian' was depressed by at least 23% by heat stress at each stage; this inhibition was least at ear emergence and greatest at anthesis, the latter being associated with reduced sink size as a result of lowered seed set. The stress-induced inhibition of Pmax in 'Guardian' plants at anthesis had not recovered 3 d after removal of the stress. Mutant lines exhibited different developmental profiles of Pmax thermostability. Mutant tht (thermotolerant) 10, for example, exhibited partial thermostability at each growth stage tested while the Pmax of mutant tht 2 was completely unaffected by heat stress at second node and ear emergence, but was as inhibited as that of 'Guardian' at anthesis; heat stress applied at anthesis in tht 2, but not tht 10, was associated with reduced seed set. Generally, the inhibitory effect of heat stress on Pmax in the mutants was reflected in declines in chlorophyll content. The ten mutants were grouped into nine categories, on the basis of thermotolerance characteristics. (+info)
The effects of ethylene, depressed oxygen and elevated carbon dioxide on antioxidant profiles of senescing spinach leaves.
(76/2508)
It has been suggested that antioxidants play a role in regulating or modulating senescence dynamics of plant tissues. Ethylene has been shown to promote early plant senescence while controlled atmospheres (CA; reduced O2 levels and elevated CO2 levels) can delay its onset and/or severity. In order to examine the possible importance of various antioxidants in the regulation of senescence, detached spinach (Spinacia oleracea L.) leaves were stored for 35 d at 10 degrees C in one of three different atmospheres: (1) ambient air (0.3% CO2, 21.5% O2, 78.5% N2), (2) ambient air + 10 ppm ethylene to promote senescence, or (3) CA (10% CO2, 0.8% O2 and 89.2% N2) to delay senescence. At weekly intervals, material was assessed for activities of the antioxidant enzymes ascorbate peroxidase (ASPX; EC 1.11.1.11), catalase (CAT; EC 1.11.1.6), dehydroascorbate reductase (DHAR; EC 1.8.5.4), glutathione reductase (GR; EC 1.6.4.2), monodehydroascorbate reductase (MDHAR; EC 1.6.5.4), and superoxide dismutase (SOD; EC 1.15.1.1), and concentrations of the water-soluble antioxidant compounds ascorbate and glutathione. Indicators of the rate and severity of senescence (lipid peroxidation, chlorophyll, and soluble protein levels) were also determined. Results indicated that the rate and severity of senescence was similar between the leaves stored in ambient air or CA until day 35, at which point the ambient air-stored leaves exhibited a sharp increase in lipid peroxidation. Tissues under both storage regimes demonstrated significant declines only in levels of ASPX, CAT, and ascorbate. Glutathione content in the CA-stored tissue also significantly dropped, but only on day 35. In contrast, spinach leaves stored in ambient air + ethylene experienced a rapid decrease in levels of all the antioxidants assessed except SOD. Declines in levels of ASPX, CAT, and ascorbate over the 35 d storage period regardless of the composition of the storage atmosphere suggests that regulation of H2O2 levels plays an important role in both the dynamics and severity of post-harvest senescence of spinach. (+info)
Five ways to stay green.
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The relationship between carbon income and expenditure over the life of a leaf is described and related to the productivity benefits of altering the timing of senescence initiation. In genetic variants with delayed leaf senescence ('stay-greens') deconstruction of the photosynthetic apparatus during leaf senescence is partially or completely prevented. Although the staygreen phenotype is superficially similar in all species and genotypes, the genetic and physiological routes to the trait are diverse. In one type of stay-green, chlorophyll catabolism is disabled. Legumes and monocots with pigment breakdown lesions are discussed. Sorghum is presented as an example of another kind of stay-green in which perennial tendencies have been bred into a monocarpic annual crop species. Transgenic approaches are briefly discussed (enhanced endogenous cytokinins, reduced ethylene production or perception). An alternative route towards making a stay-green phenotype is through quantitative trait mapping and marker-assisted selection. Loci for greenness in pearl millet have been identified, some of which are associated with drought responses or flowering time. Finally the question of the limits on stay-green as a productivity-enhancing character is addressed. (+info)
Photosynthetic electron sinks in transgenic tobacco with reduced amounts of Rubisco: little evidence for significant Mehler reaction.
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Transgenic tobacco (Nicotiana tabacum L. cv. W38) plants with an antisense gene directed against the mRNA of the small subunit of Rubisco were used to investigate the role of O2 as an electron acceptor during photosynthesis. The reduction in Rubisco has reduced the capacity for CO2-fixation in these plants without a similar reduction in electron transport capacity. Concurrent measurements of chlorophyll fluorescence and CO2 assimilation at different CO2 and O2 partial pressures showed close linear relationships between chloroplast electron transport rates calculated from chlorophyll fluorescence and those calculated from CO2-fixation. These relationships were similar for wild-type and transgenic plants, indicating that the reduced capacity for CO2 fixation in the transgenic plants did not result in extra electron transport not associated with the photosynthetic carbon reduction (PCR) or photorespiratory carbon oxidation (PCO) cycle. This was further investigated with mass spectrometric measurements of 16O2 and 18O2 exchange made concurrently with measurements of chlorophyll fluorescence. In all tobacco lines the rates of 18O2 uptake in the dark were similar to the 18O2 uptake rates at very high CO2 partial pressures in the light. Rates of oxygenase activity calculated from 18O2 uptake at the compensation point were linearly related to the Rubisco content of leaves. The ratios of oxygenase to carboxylase rates were calculated from measurements of 16O2 evolution and 18O2 uptake at the compensation point. These ratios were lower in the transgenic plants, consistent with their higher CO2 compensation points. It is concluded that although there may be some electron transport to O2 to balance conflicting demands of NADPH to ATP requirements, this flux must decrease in proportion with the reduced demand for ATP and NADPH consumption in the transgenic lines. The altered balance between electron transport and Rubisco capacity, however, does not result in rampant electron transport to O2 or other electron transport acceptors in the absence of PCR and PCO cycle activity. (+info)
Chlorophyll fluorescence analysis has become one of the most powerful and widely used techniques available to plant physiologists and ecophysiologists. This review aims to provide an introduction for the novice into the methodology and applications of chlorophyll fluorescence. After a brief introduction into the theoretical background of the technique, the methodology and some of the technical pitfalls that can be encountered are explained. A selection of examples is then used to illustrate the types of information that fluorescence can provide. (+info)
High CO2-mediated down-regulation of photosynthetic gene transcripts is caused by accelerated leaf senescence rather than sugar accumulation.
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The influence of elevated atmospheric CO2 on transcript levels of photosynthetic genes was investigated in leaves of Nicotiana tabacum cv. SamsunNN and cv. Wisconsin38 plants. Plants were grown under ambient (400 ppm) and elevated (800/1,000 ppm) atmospheric CO2, and transcript levels were determined in leaves of different age. Down-regulation of photosynthetic gene transcripts was apparent in senesing leaves only. A correlation between transcript levels and leaf contents of soluble sugars could not be found. To investigate whether a shift in leaf ontogeny would be involved in the regulation of photosynthetic genes transgenic tobacco plants expressing either the gus or ipt gene under control of the senescence-specific SAG-12 promoter [Gan, S. and Amasino, R.M. (1995) Science 270, 1986-1988] were included in our studies. As expected SAG-12-driven GUS activity increased with leaf age. This increase of GUS activity was stimulated by elevated atmospheric CO2, accompanied by a loss of chlorophyll and the down-regulation of photosynthetic genes, verifying that high CO2 accelerates leaf ontogeny. Senescence as well as down-regulation of photosynthetic genes could be delayed by ipt expression. Levels of soluble sugars were indistinguishable from wild type or even slightly elevated in ipt transgenic plants. Therefore, sugar accumulation as a cause for down-regulation of photosynthetic genes under high CO2 can be excluded. It appears more likely that the high CO2-mediated decline in photosynthetic gene transcripts is due to a temporal shift in leaf ontogeny. (+info)