The Effect of Light Intensity and Spectral Composition on Electron Transport in Chloroplasts in situ and in silico

Computer modeling of electron and proton transport in chloroplasts.

Light-induced redox transformations of P700, the reaction center of photosystem I, were studied by EPR method depending on the illumination conditions of plant leaves (intensity and spectral composition of the active light). Within the framework of a mathematical model, the key stages of electron transfer along the noncyclic electron transport chain containing photosystems I and II and mobile transporters (plastoquinone, plastocyanin, ferredoxin) and the processes of trans-thylakoid proton transfer associated with ATP synthesis were considered. The mechanisms of pH-dependent regulation of electron transport in chloroplasts at the acceptor and donor sites of photosystem I have been analyzed. The modeling results are in agreement with experimental data on the kinetics of light-induced transformations of P700 in chloroplasts of higher plants. The results obtained are discussed in the context of "short-term" pH-dependent mechanisms of electron transport regulation in chloroplasts in situ.

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.

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.

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

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

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

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.

Black net shade treatment attenuates flavonoid biosynthesis in tea plants, while the effect of light quality is still unclear. We investigated the flavonoid and transcriptome profiles of tea leaves under different light conditions, using black nets with different shade percentages, blue, yellow and red nets to alter the light intensity and light spectral composition in the fields. Flavonol glycosides are more sensitive to light intensity than catechins, with a reduction percentage of total flavonol glycosides up to 79.6% compared with 38.7% of total catechins under shade treatment. A total of 29,292 unigenes were identified, and the KEGG result indicated that flavonoid biosynthesis was regulated by both light intensity and light spectral composition while phytohormone signal transduction was modulated under blue net shade treatment. PAL, CHS, and F3H were transcriptionally downregulated with light intensity. Co-expression analysis showed the expressions of key transcription factors MYB12, MYB86, C1, MYB4, KTN80.4, and light signal perception and signaling genes (UVR8, HY5) had correlations with the contents of certain flavonoids (p < 0.05). The level of abscisic acid in tea leaves was elevated under shade treatment, with a negative correlation with TFG content (p < 0.05). This work provides a potential route of changing light intensity and spectral composition in the field to alter the compositions of flavor substances in tea leaves and regulate plant growth, which is instructive to the production of summer/autumn tea and matcha.

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.

BAL (2,3-dithiopropan-1-ol) treatment of chloroplasts has previously been reported to induce a block in electron transport from water to NADP+ at a site preceding plastocyanin [Belkin et al. (1980) Biochim. Biophys. Acta 766, 563-569]. In the present work the block was further characterized. The following properties of BAL treatment are described. Inhibition of electron transport from water to lipophilic acceptors but not to silicomolybdate. Inhibition of the slow, sigmoidal phase of chlorophyll a fluorescence induction. Inability of N,N,N',N',-tetramethyl-p-phenylenediamine to bypass the inhibition of NADP+ photoreduction with water as the electron donor. Inhibition of electron transport from externally added quinols to NADP+. Inhibition of cytochrome f reduction by photosystem II, but not its oxidation by photosystem I. Inhibition of cytochrome b6 turnover and cytochrome f rereduction after single-turnover flash illumination under cyclic electron-flow conditions. The BAL-induced block is therefore located between the secondary quinone acceptor (QB) and the cytochrome b6f complex. It was further found that (a) the isolated cytochrome complex is not inhibited after BAL treatment; (b) BAL-reacted plastoquinone-1 inhibits electron transport in chloroplasts; (c) BAL does not inhibit electron transport in chromatophores of Rhodospirilum rubrum or Rhodopseudomonas capsulata. It is suggested that the inhibition of electron transport in chloroplasts results from specific reaction of BAL with the endogenous plastoquinone.

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.

The effect of dimethyl sulphoxide on electron transport in chloroplasts