Role of membrane H+ transport and plasmalemma excitability in pattern formation, long-distance transport and photosynthesis of Characean algae

Computer modeling of electron and proton transport in chloroplasts.

This review is focused on pH-dependent mechanisms of regulation of photosynthetic electron transport and ATP synthesis in chloroplasts. The light-induced acidification of the thylakoid lumen is known to decelerate the plastoquinol oxidation by the cytochrome b 6 f complex, thus impeding the electron flow between photosystem II and photosystem I. Acidification of the lumen also triggers the dissipation of excess energy in the light-harvesting antenna of photosystem II, thereby protecting the photosynthetic apparatus against a solar stress. After brief description of structural and functional organization of the chloroplast electron transport chain, our attention is focused on the nature of the rate-limiting step of electron transfer between photosystem II and photosystem I. In the context of pH-dependent mechanism of photosynthetic control in chloroplasts, the mechanisms of plastoquinol oxidation by the cytochrome b 6 f complex have been considered. The light-induced alkalization of stroma is another factor of pH-dependent regulation of electron transport in chloroplasts. Alkalization of stroma induces activation of the Bassham-Benson-Calvin cycle reactions, thereby promoting efflux of electrons from photosystem I to NADP(+). The mechanisms of the light-induced activation of ATP synthase are briefly considered.

Electron transport in chloroplasts isolated from desiccated sunflower (Helianthus annuus L. cv. Russian Mammoth) leaves was compared with electron transport in sunflower chloroplasts in sorbitol-containing media having various osmotic potentials. In media having low osmotic potentials and dichloroindophenol as electron acceptor, the activity for electron transport was inhibited, but the inhibition was much less than that due to comparable desiccation in vivo. The inhibition at low osmotic potentials was rapidly reversed by returning the chloroplasts to media having high osmotic potentials, but the activity of chloroplasts from desiccated tissue showed no reversal when the chloroplasts were placed in media having high osmotic potentials. Nevertheless, the inhibition of chloroplast activity due to desiccation in vivo was basically reversible, because chloroplasts recovered quickly when they were rehydrated in vivo. The large differences between desiccation in vivo and exposure to low osmotic potential in vivo indicate that osmotic solutions did not reproduce the effects of tissue desiccation. It is concluded that decreases in the Gibbs free energy of water due to decreased osmotic potentials probably have only a small effect on electron transport in chloroplasts from desiccated tissue and do not account for the major effects of leaf desiccation on electron transport.

A mathematical model is presented that describes the key steps of photosynthetic electron transport and transmembrane proton transfer in chloroplasts. Numerical modeling has been performed with due regard for regulatory processes at the donor and acceptor parts of photosystem (PS) I. The influence of pH-dependent activation of the Calvin cycle enzymes and energy dissipation in PS II (nonphotochemical quenching of chlorophyll fluorescence) on the light-induced redox transients of P700, plastoquinone, and NADP as well as on the changes in intrathylakoid pH and ATP level is examined. It is demonstrated that pH-dependent regulatory processes alter the distribution of electron fluxes on the acceptor side of PS I and the total rate of electron flow between PS II and PS I. The light-induced activation of the Calvin cycle leads to significant enhancement of the electron flow from PS I to NADP+ and attenuation of the electron flow to molecular oxygen.

Ca 2+ and calmodulin antagonists inhibit the proton gradients associated with non-cyclic and cylic photophosphorylation in spinach chloroplasts

Photosynthetic carbon metabolism is initiated by ribulose-bisphosphate carboxylase/oxygenase (Rubisco), which uses both CO2 and O2 as substrates. One 2-phosphoglycolate (P-glycolate) molecule is produced for each O2 molecule fixed. P-glycolate has been considered to be metabolized exclusively via the oxidative photosynthetic carbon cycle. This paper reports an additional pathway for P-glycolate and glycolate metabolism in the chloroplasts. Light-dependent glycolate or P-glycolate oxidation by osmotically shocked chloroplasts from the algae Dunaliella or spinach leaves was measured by three electron acceptors, methyl viologen (MV), potassium ferricyanide, or dichloroindophenol. Glycolate oxidation was assayed with 3-(3,4)-dichlorophenyl)-1,1-dimethylurea (DCMU) as oxygen uptake in the presence of MV at a rate of 9 mol per mg of chlorophyll per h. Washed thylakoids from spinach leaves oxidized glycolate at a rate of 22 mol per mg of chlorophyll per h. This light-dependent oxidation was inhibited completely by SHAM, an inhibitor of quinone oxidoreductase, and 75% by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), which inhibits electron transfer from plastoquinone to the cytochrome b6f complex. SHAM stimulated severalfold glycolate excretion by algal cells, Dunaliella or Chlamydomonas, and by isolated Dunaliella chloroplasts. Glycolate and P-glycolate were oxidized about equally well to glyoxylate and phosphate. On the basis of results of inhibitor action, the possible site which accepts electrons from glycolate or P-glycolate is a quinone after the DCMU site but before the DBMIB site. This glycolate oxidation is a light-dependent, SHAM-sensitive, glycolate-quinone oxidoreductase system that is associated with photosynthetic electron transport in the chloroplasts.

Results of simulation of electron and proton transport in higher plant chloroplasts, taking into account the lateral heterogeneity of their lamellar system, were summarized. The existence of heterogeneous lateral profiles of pH inside thylakoids and in gaps between the thylakoids of grana was predicted. The basic kinetic relationships were simulated for photoinduced redox transformations of P700, the primary electron donor for PS1. It was shown that, along with changes in pH inside thylakoids, an essential role in controlling the electron transport in chloroplasts can belong to alkalinization of the gap between thylakoids of grana, caused by deceleration of diffusion of hydrogen ions from the stroma to the PS2 complexes in thylakoids of grana.

A low molecular weight phytotoxin produced by Phoma tracheiphila, the cause of mal secco disease in citrus

The analysis of electron and proton transport in chloroplasts of higher plants has been carried out on the basis of a mathematical model that takes into account the pH-dependent regulation of electron transport and the operation of the ATP synthase. Numerical experiments aimed at simulation of these processes under pseudocyclic electron transport (water–water cycle) have shown good agreement with experimental data on the kinetics of electron transfer to photosystem 1 (PS1) in class B chloroplasts in metabolic states corresponding to high (state 3) and low (state 4) ATP synthase activity. The simulation of electron transport processes that took into account the Calvin–Benson cycle (CBC), cyclic electron transport around PS1, pH-dependent heat dissipation of energy in photosystem 2 (PS2), and nonphotochemical quenching (NPQ) made it possible to estimate the contribution of these factors to the kinetics of induction phenomena in chloroplasts in situ. It has been shown that the multiphase kinetics of the photooxidation of P700 (a primary electron donor in PS1) reflects the redistribution of electron flows between cyclic and non-cyclic electron transfer pathways, caused by the activation of CBC due to the alkalization of the stroma, as well as the change of the limiting stage in the electron transport chain, induced by a decrease in the intrathylakoid pH (pHin). The electron flux between PS2 and PS1 decelerates with pHin decrease, which may be caused by the reduced rate of plastoquinol oxidation and attenuated activity of PS2 due to NPQ.

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

Effects of cyclosis on chloroplast–cytoplasm interactions revealed with localized lighting in Characean cells at rest and after electrical excitation

The effects of growth stimulants 6-benzylaminopurine (6-BAP), gibberellic acid (GA3) and 1-naphthaleneacetic acid (1-NAA) as well as gibberellin inhibitors tebuconazole (EW-250), ethephon (2-chloroethylphosphonic acid, 2-CEPA) and chloromequate chloride (ССС-750), which differ in their action mechanism, on growth, development, leaf apparatus formation, CO2 and H2O gas exchange, photochemical activity of Photosystem II (PSII) and productivity traits of sweet pepper plants were studied. It was shown that treatment with growth stimulants increased, and gibberellin inhibitors decreased the linear sizes of sweet pepper plants of the Antei variety. It was established that all growth regulators, except for 2-CEPA, increased the number and the mass of leaves on the plant. Under the action of all preparations, except for 2-CEPA, the area of the leaves increased. Gibberellin inhibitors and 6-BAP significantly increased the amount of chlorophyll in pepper leaves. However, it decreased under the action of GA3 and did not practically change in treatment with 1-NAA. All growth substances, except GA3, increased the total chlorophyll content in the plant. The impact of growth regulators on the activity of photosynthetic processes was more pronounced at the stage of fruit formation than at the flowering stage. The CO2 assimilation rate at the flowering stage increased under the treatment of 1-NAA, 6-BAP, 2-CEPA and EW-250, but decreased under the action of GA3 and CCC-750. At the same time, all studied growth regulators increased the CO2 assimilation rate at the stage of fruit formation. Changes in the CO2 assimilation rate were closely correlated with changes in stomatal conductance (r = 0.79—0.85). Growth regulators increased transpiration in the light at fruit formation stage while the transpiration in the dark was reduced at the flowering stage. Growth regulators increased the operating quantum efficiency of PSII in the light, photochemical quenching of chlorophyll fluorescence, and intensity of linear electron transport in chloroplasts, and reduced non-photochemical quenching (NPQ) fluorescence. The specified morphological, physiological and biochemical changes in plants of sweet pepper of the Antei variety contributed to improvement of crop productivity traits. The use of growth stimulants 6-BAP and GA3, and retardants EW-250 and ССС-750 was most effective.

In this work, we have studied photosynthetic electron transport in chloroplasts of two “contrasting” species of Cucumis genus, the shade-tolerant species Cucumis sativus (cucumber) and the light-loving species Cucumis melo (melon). Plants were acclimated to moderate (50–125 μmole photons m−2 s−1) or high light (850–1000 μmole photons m−2 s−1). Parameters of a fast induction of chlorophyll a fluorescence, emitted from photosystem 2 (PS2), were determined using a conventional OJIP test. For monitoring the turnover of photosystem 1 (PS1) reaction centers \({\text{{Р}}}_{{700}}^{ + }\), we used electron paramagnetic resonance. The shade-tolerant (C. sativus) and light-loving (C. melo) species, acclimation to high or low light irradiation, revealed substantial difference in their response to variations of light intensity. Photosynthetic activity of shade-tolerant species C. sativus revealed higher sensitivity to light intensity during acclimation as compared to C. melo. In the course of the long-term acclimation (more than 2 months) of С. sativum to high light (≥ 500 μmole photons m−2 m−1), a photochemical activity of PS2 decreased. This was not the case, however, for leaves of C. melo. In С. sativus leaves, a decrease in photochemical activity of PS2 caused by acclimation to high light was reversible, demonstrating the recovery after the attenuation of irradiation intensity. Plants of both species acclimated to high and low light also revealed significant differences in the two-phase kinetics of \({\text{{Р}}}_{{700}}^{ + }\) redox transients. In the leaves of plants acclimated to strong light, we observed a lag-phase in the kinetics of \({\text{{Р}}}_{{700}}^{ + }\) photooxidation that could be attributed to cyclic electron transport (CET) around PS1. The ratio of the signals induced by white light and far-red light (707 nm) was higher in plants acclimated to strong light. This effect can be explained by the enhancement of CET and optimization of the energy balance at excess of light, protecting plants from oxidative stress. The data obtained are discussed in the context of the problem of photosynthesis optimization upon fluctuations of light intensity.

Cadmium (Cd) is a widely spread pollutant and can be easily taken up by crop from soil, resulting in a serious health issue for humans. The objective of this study was to comparatively investigate the photosynthetic activity, chlorophyll a fluorescence, chlorophyll contents, and spectral reflectance in mature and young leaves of soybean plants after being treated with different concentrations of Cd for 10 days. The photosynthetic rate, chlorophyll contents, actual photochemical efficiency of PSII, and photochemical quenching in the young leaves decreased more significantly with increasing concentrations of Cd in the nutrient solution, compared with those in the mature leaves, though the young leaves had less Cd concentrations. Thus, there was more excessive excited energy produced in the young leaves than that in the mature leaves. In the young leaves, due to more excessive excited energy, more reactive oxygen species may be generated, which further damaged the photosynthetic apparatus. It was supported by the fact that the decrease of reflectance in near-infrared wavelengths of the young leaves was more noticeable than that of the mature leaves. In addition, the chlorophyll a fluorescence transients of the young leaves was significantly different from that in the mature leaves, indicating that the electron transport of young leaves were inhibited much more severely than that of the mature leaves. These observations imply that the responses of photosynthetic activity of soybean leaves to Cd stress depend on their growth stage, and the Cd-induced inhibition of photosynthetic activity might be attributed to the decrease in chlorophyll contents and the decrease in mesophyll CO2 assimilation ability cause by the Cd, which further decreased the consumption of ATP and NADPH, leading to accumulation of NADPH on the acceptor sides of the PSI, and then feedback inhibited electron transport in chloroplasts.

This work aimed to evaluate if gas exchange and PSII photochemical activity in maize are affected by different irradiance levels during short-term exposure to elevated CO2. For this purpose gas exchange and chlorophyll a fluorescence were measured on maize plants grown at ambient CO2 concentration (control CO2) and exposed for 4 h to short-term treatments at 800 μmol(CO2) mol-1 (high CO2) at a photosynthetic photon flux density (PPFD) of either 1,000 μmol m-2 s-1 (control light) or 1,900 μmol m-2 s-1 (high light). At control light, high-CO2 leaves showed a significant decrease of net photosynthetic rate (P N) and a rise in the ratio of intercellular to ambient CO2 concentration (C i/C a) and water-use efficiency (WUE) compared to control CO2 leaves. No difference between CO2 concentrations for PSII effective photochemistry (ΦPSII), photochemical quenching (qp) and nonphotochemical quenching (NPQ) was detected. Under high light, high-CO2 leaves did not differ in P N, C i/C a, ΦPSII and NPQ, but showed an increase of WUE. These results suggest that at control light photosynthetic apparatus is negatively affected by high CO2 concentration in terms of carbon gain by limitations in photosynthetic dark reaction rather than in photochemistry. At high light, the elevated CO2 concentration did not promote an increase of photosynthesis and photochemistry but only an improvement of water balance due to increased WUE.