Cytochrome B6f Research Articles

The interplay between LHCSR and PSBS proteins provides photoprotection in Chlamydomonas reinhardtii pgr5 mutant under high light

Cyclic electron transport (CET) is a vital alternative route that protects against photodamage and aids in energy production. This process depends on proton gradient regulation 5 (PGR5) and PGRL1-dependent pathways associated with CET. The exact roles of these proteins in photosystem I photochemistry under prolonged high light conditions are not fully understood. Continuous light adaptation hinges on two critical mechanisms: alterations in the proton motive force (pmf) and adjustments in the ratio of proteins activated by high light that dissipate excess light through non-photochemical quenching (NPQ). To explore this, we studied pgrl1 and pgr5 mutants to gauge their roles in balancing photochemistry and photoacclimation. These mutants showed inhibited growth, reduced photosynthetic efficiency, and a lowered pmf, leading to diminished non-photochemical energy quenching (qE) under high light. Prolonged high light exposure slowed down unregulated energy losses Y(NO), and relaxation helped regulate photosynthetic activity by increasing photoinhibitory quenching (qI), thus preventing further damage to the photosystem. The precise balance between the two pmf components, ΔpH and Δψ, is critical for controlling photochemistry and photoacclimation, yet remains elusive. In pgr5 reduced pmf led to an accumulation of cytochrome b6f under high light, and a decrease in the ΔpH component increased the Δψ component's role in photosynthetic acclimation. Notably, light-harvesting complex stress response protein 3 (LHCSR3) showed decreased expression in pgrl1, whereas pgr5 exhibited no expression of both LHCSR3 and LHCSR1 under high light conditions. Moreover, continuous increase in PSBS protein accumulation in pgr5 suggests enhanced photoprotection in the absence of LHCSR3 under high light. The study provides significant insights into how cyclic electron transport (CET) regulates photoprotective proteins LHCSR and PSBS, influencing Chlamydomonas' survival strategies.

Plastocyanin and Cytochrome f Complex Structures Obtained by NMR, Molecular Dynamics, and AlphaFold 3 Methods Compared to Cryo-EM Data.

Plastocyanin is a small mobile protein that facilitates electron transfer through the formation of short-lived protein-protein complexes with cytochrome bf and photosystem 1. Due to the transient nature of plastocyanin-cytochrome f complex, the lack of a long-lived tight complex makes it impossible to determine its structure by X-ray diffraction analysis. Up to today, a number of slightly different structures of such complexes have been obtained by experimental and computer methods. Now, artificial intelligence gives us the possibility to predict the structures of intermolecular complexes. In this study, we compare encounter and final complexes obtained by Brownian and molecular dynamics methods, as well as the structures predicted by AlphaFold 3, with NMR and cryo-EM data. Surprisingly, the best match for the plastocyanin electron density obtained by cryo-EM was demonstrated by an AlphaFold 3 structure. The orientation of plastocyanin in this structure almost completely coincides with its orientation obtained by molecular dynamics calculation, and, at the same time, it is different from the orientation of plastocyanin predicted on the basis of NMR data. This is even more unexpected given that only NMR structures for the plastocyanin-cytochrome f complex are available in the PDB database, which was used to train AlphaFold 3.

Molecular basis of plastoquinone reduction in plant cytochrome b6f.

A multi-subunit enzyme, cytochrome b6f (cytb6f), provides the crucial link between photosystems I and II in the photosynthetic membranes of higher plants, transferring electrons between plastoquinone (PQ) and plastocyanin. The atomic structure of cytb6f is known, but its detailed catalytic mechanism remains elusive. Here we present cryogenic electron microscopy structures of spinach cytb6f at 1.9 Å and 2.2 Å resolution, revealing an unexpected orientation of the substrate PQ in the haem ligand niche that forms the PQ reduction site (Qn). PQ, unlike Qn inhibitors, is not in direct contact with the haem. Instead, a water molecule is coordinated by one of the carbonyl groups of PQ and can act as the immediate proton donor for PQ. In addition, we identify water channels that connect Qn with the aqueous exterior of the enzyme, suggesting that the binding of PQ in Qn displaces water through these channels. The structures confirm large movements of the head domain of the iron-sulfur protein (ISP-HD) towards and away from the plastoquinol oxidation site (Qp) and define the unique position of ISP-HD when a Qp inhibitor (2,5-dibromo-3-methyl-6-isopropylbenzoquinone) is bound. This work identifies key conformational states of cytb6f, highlights fundamental differences between substrates and inhibitors and proposes a quinone-water exchange mechanism.

Inhibition of sulfur assimilation by S-benzyl-L-cysteine: Impacts on growth, photosynthesis, and leaf proteome of maize plants

Sulfur is an essential nutrient for various physiological processes, including protein synthesis and enzyme activation. We aimed to evaluate how S-benzyl-L-cysteine (SBC), an inhibitor of the sulfur assimilation pathway, affects maize plants' growth, photosynthesis, and leaf proteomic profile. Thus, maize plants were grown for 14 days in vermiculite supplemented with SBC. Photosynthesis was assessed using light and CO2 response curves and chlorophyll a fluorescence. Leaf proteome analysis was conducted to evaluate photosynthetic protein biosynthesis, and ROS content was quantified to assess oxidative stress. Applying SBC resulted in a significant decrease in the growth of maize plants. The gas exchange analysis revealed that maize plants exhibited a diminished rate of CO2 assimilation attributable to both stomatal and non-stomatal limitations. Furthermore, SBC suppressed the activity of important elements involved in the photosynthetic electron transport chain (including photosystems I and II, cytochrome b6f, and ATP synthase) and enzymes responsible for the Calvin cycle, some of which have sulfur-containing prosthetic groups. Consequently, the diminished electron flow rate resulted in a substantial increase in the levels of ROS within the leaves. Our research highlights the crucial role of SBC in disrupting maize photosynthesis by limiting L-cysteine and assimilated sulfur availability, which are essential for the synthesis of protein and prosthetic groups and photosynthetic processes, emphasizing the potential of OAS-TL as a new herbicide site of action.

Uracil phosphoribosyltransferase is required to establish a functional cytochrome b6f complex.

Arabidopsis uracil phosphoribosyltransferase (UPP) is an essential enzyme and plants lacking this enzyme are strongly compromised in chloroplast function. Our analysis of UPP amiRNA mutants has confirmed that this vital function is crucial to establish a fully functional photosynthesis as the RIESKE iron sulfur protein (PetC) is almost absent, leading to a block in photosynthetic electron transport. Interestingly, this function appears to be unrelated to nucleotide homeostasis since nucleotide levels were not altered in the studied mutants. Transcriptomics and proteomic analysis showed that protein homeostasis but not gene expression is most likely responsible for this observation and high light provoked an upregulation of protease levels, including thylakoid filamentation temperature-sensitive 1, 5 (FtsH), caseinolytic protease proteolytic subunit 1 (ClpP1), and processing peptidases, as well as components of the chloroplast protein import machinery in UPP amiRNA lines. Strongly reduced PetC amounts were not only detected by immunoblot from mature plants but in addition in a de-etiolation experiment with young seedlings and are causing reduced high light-induced non-photochemical quenching Φ(NPQ) but increased unregulated energy dissipation Φ(NO). This impaired photosynthesis results in an inability to induce flavonoid biosynthesis. In addition, the levels of the osmoprotectants raffinose, proline, and fumarate were found to be reduced. In sum, our work suggests that UPP assists in stabilization PetC during import, processing or targeting to the thylakoid membrane, or protects it against proteolytic degradation.

Manganese deficiency alters photosynthetic electron transport in Marchantia polymorpha

Manganese (Mn) is considered as an essential element for plant growth. Mn starvation has been shown to affect photosystem II, the site of the Mn4CaO5 cluster responsible for water oxidation. Less is known on the effect of Mn starvation on photosystem I. Here we studied the effects of Mn deficiency in vivo on redox changes of P700 and plastocyanin (Pc) in the liverwort Marchantia polymorpha using the KLAS-NIR spectrophotometer. Far-red illumination is used to excite preferentially photosystem I, thus facilitating cyclic electron transport. Under Mn starvation, we observed slower oxidation of P700 and a decrease in the Pc signal relative to P700. The lower Pc content under Mn deficiency was confirmed by western blots. Re-reduction kinetics of P700+ and Pc+ were faster in Mn deficient thalli than in the control. The above findings show that the kinetics studied under Mn deficiency not only depend on the number of available reductants but also on how quickly electrons are transferred from stromal donors via the intersystem chain to Pc+ and P700+. We suggest that under Mn deficiency a structural reorganization of the thylakoid membrane takes place favoring the formation of supercomplexes between ferredoxin, cytochrome b6f complex, Pc and photosystem I, and thus an enhanced cyclic electron transport.

Enhancing Photosynthesis and Plant Productivity through Genetic Modification.

Enhancing crop photosynthesis through genetic engineering technologies offers numerous opportunities to increase plant productivity. Key approaches include optimizing light utilization, increasing cytochrome b6f complex levels, and improving carbon fixation. Modifications to Rubisco and the photosynthetic electron transport chain are central to these strategies. Introducing alternative photorespiratory pathways and enhancing carbonic anhydrase activity can further increase the internal CO2 concentration, thereby improving photosynthetic efficiency. The efficient translocation of photosynthetically produced sugars, which are managed by sucrose transporters, is also critical for plant growth. Additionally, incorporating genes from C4 plants, such as phosphoenolpyruvate carboxylase and NADP-malic enzymes, enhances the CO2 concentration around Rubisco, reducing photorespiration. Targeting microRNAs and transcription factors is vital for increasing photosynthesis and plant productivity, especially under stress conditions. This review highlights potential biological targets, the genetic modifications of which are aimed at improving photosynthesis and increasing plant productivity, thereby determining key areas for future research and development.

Light-Driven H2 Production in Chlamydomonas reinhardtii: Lessons from Engineering of Photosynthesis.

In the green alga Chlamydomonas reinhardtii, hydrogen production is catalyzed via the [FeFe]-hydrogenases HydA1 and HydA2. The electrons required for the catalysis are transferred from ferredoxin (FDX) towards the hydrogenases. In the light, ferredoxin receives its electrons from photosystem I (PSI) so that H2 production becomes a fully light-driven process. HydA1 and HydA2 are highly O2 sensitive; consequently, the formation of H2 occurs mainly under anoxic conditions. Yet, photo-H2 production is tightly coupled to the efficiency of photosynthetic electron transport and linked to the photosynthetic control via the Cyt b6f complex, the control of electron transfer at the level of photosystem II (PSII) and the structural remodeling of photosystem I (PSI). These processes also determine the efficiency of linear (LEF) and cyclic electron flow (CEF). The latter is competitive with H2 photoproduction. Additionally, the CBB cycle competes with H2 photoproduction. Consequently, an in-depth understanding of light-driven H2 production via photosynthetic electron transfer and its competition with CO2 fixation is essential for improving photo-H2 production. At the same time, the smart design of photo-H2 production schemes and photo-H2 bioreactors are challenges for efficient up-scaling of light-driven photo-H2 production.

S-Benzyl-L-cysteine Inhibits Growth and Photosynthesis, and Triggers Oxidative Stress in Ipomoea grandifolia

L-cysteine, a precursor of essential components for plant growth, is synthesized by the cysteine synthase complex, which includes O-acetylserine(thiol) lyase (OAS-TL) and serine acetyltransferase. In this work, we investigated how S-benzyl-L-cysteine (SBC), an OAS-TL inhibitor, affects the growth, photosynthesis, and oxidative stress of Ipomoea grandifolia plants. SBC impaired gas exchange and chlorophyll a fluorescence, indicating damage that compromised photosynthesis and reduced plant growth. Critical parameters such as the electron transport rate (J), triose phosphate utilization (TPU), light-saturation point (LSP), maximum carboxylation rate of Rubisco (Vcmax), and light-saturated net photosynthetic rate (PNmax) decreased by 19%, 20%, 22%, 23%, and 24%, respectively. The photochemical quenching coefficient (qP), quantum yield of photosystem II photochemistry (ϕPSII), electron transport rate through PSII (ETR), and stomatal conductance (gs) decreased by 12%, 19%, 19%, and 34%, respectively. Additionally, SBC decreased the maximum fluorescence yield (Fm), variable fluorescence (Fv), and chlorophyll (SPAD index) by 14%, 15%, and 15%, respectively, indicating possible damage to the photosynthetic apparatus. SBC triggered root oxidative stress by increasing malondialdehyde, reactive oxygen species, and conjugated dienes by 30%, 55%, and 61%, respectively. We hypothesize that dysfunctions in sulfur-containing components of the photosynthetic electron transport chain, such as the cytochrome b6f complex, ferredoxin, and the iron–sulfur (Fe-S) centers are the cause of these effects, which ultimately reduce the efficiency of electron transport and hinder photosynthesis in I. grandifolia plants. In short, our findings suggest that targeting OAS-TL with inhibitors like SBC could be a promising strategy for the development of novel herbicides.

The stromal side of the cytochrome b6f complex regulates state transitions.

In oxygenic photosynthesis, state transitions distribute light energy between PSI and PSII. This regulation involves reduction of the plastoquinone pool, activation of the state transitions 7 (STT7) protein kinase by the cytochrome (cyt) b6f complex, and phosphorylation and migration of light harvesting complexes II (LHCII). In this study, we show that in Chlamydomonas reinhardtii, the C-terminus of the cyt b6 subunit PetB acts on phosphorylation of STT7 and state transitions. We used site-directed mutagenesis of the chloroplast petB gene to truncate (remove L215b6) or elongate (add G216b6) the cyt b6 subunit. Modified complexes are devoid of heme ci and degraded by FTSH protease, revealing that salt bridge formation between cyt b6 (PetB) and Subunit IV (PetD) is essential to the assembly of the complex. In double mutants where FTSH is inactivated, modified cyt b6f accumulated but the phosphorylation cascade was blocked. We also replaced the arginine interacting with heme ci propionate (R207Kb6). In this modified complex, heme ci is present but the kinetics of phosphorylation are slower. We show that highly phosphorylated forms of STT7 accumulated transiently after reduction of the PQ pool and represent the active forms of the protein kinase. The phosphorylation of the LHCII targets is favored at the expense of the protein kinase, and the migration of LHCII toward PSI is the limiting step for state transitions.

Analytical fingerprint of the interactions between quinones and bioenergetic membranes in Chlamydomonas reinhardtii

The use of intact photosynthetic organisms (e.g. microalgae or cyanobacteria) for biotechnological approaches is a promising avenue to extract sustainable energy from oxygenic photosynthesis. More particularly, exogenous quinones can act as electron shuttles to reroute part of the photosynthetic electron flow of living cells to an outer collecting electrode. This encouraging approach is however hampered by reported poisoning or side-effects of exogenous quinones on the cell bioenergetics. In order to contribute to understand those effects, we investigated the modes and sites of interaction of two model quinones (2,6-DCBQ and 2,6-DMBQ) with the respiratory and photosynthetic electron transfer chains of the green alga Chlamydomonas reinhardtii. By considering different analytical tools (chlorophyll a fluorescence, transient absorption spectrometry, O2 consumption rate), the two exogenous quinones are shown to hamper the photosynthetic electron transfer from photosystem II (PSII) to the cytochrome b6f and, at longer term, PSII damage. In addition, the investigated quinones initiate the suppression of mitochondrial respiration, illustrated by the decrease of O2 consumption. This results in the diminution of the ATP exchanges between mitochondrion and chloroplast responsible for the generation of the proton motive force across the thylakoid in darkness, and in turn affects the performances of the CF1FO ATPase. For all those effects, 2,6-DCBQ was more effective than 2,6-DMBQ in agreement with its higher redox potential and partition coefficient values. This work provides a new framework for the study of biophotovoltaic devices using photosynthetic organisms and quinones as mediators and could be extended to find the best candidates combining efficient bioelectricity production and limited toxicity.

Improved estimation of gross primary productivity (GPP) using solar-induced chlorophyll fluorescence (SIF) from photosystem II

Solar-induced chlorophyll fluorescence (SIF) contains contributions from both photosystem I (PSI) and photosystem II (PSII). In theory, SIF emitted from PSII (SIFPSII) should be extracted from at-sensor SIF to quantify photosynthetic CO2 assimilation, as PSI fluorescence yield is nearly insensitive to changes in photochemical yield. In many SIF-related studies, the fraction of chlorophyll-absorbed energy allocated to PSII (β2), a key factor controlling the flux of excitation energy for SIFPSII, is simply assigned a fixed value. However, β2 is regulated in response to variations in environmental conditions to avoid an energy imbalance between PSI and PSII. By quantifying the regulating effect of the cytochrome b6f complex (Cyt b6f) on the electron transport from PSII to PSI, and its interaction with energy dissipation in both photosystems, we develop a framework to estimate β2 and PSII fluorescence yield (ΦF2), two key determinants for SIFPSII, from SIF emission, PAR, air temperature, the maximum carboxylase activity of Rubisco (Vcmax), and the maximum activity of Cyt b6f (JCB6F_max). The framework is equipped with a two-leaf scheme, enabling us to estimate SIFPSII for sunlit and shaded leaves from top-of-canopy SIF observations (SIFTOC). Our simulation results showed that β2 and ΦF2 tend to change in opposite directions with varying PAR, making the contribution of PSII to SIFTOC relatively constant. We compare gross primary productivity (GPP) mechanistically estimated from SIFPSII obtained with fixed (GPPSIF_Fβ) and dynamic β2 (GPPSIF_Dβ) against the eddy-tower-derived GPP (GPPEC) at a winter-wheat experiment site. At a half-hourly time scale, GPPSIF_Dβ is better than GPPSIF_Fβ, showing higher correlations with GPPEC (R2 = 0.74 versus R2 = 0.64 and RMSE = 6.61 μmol m−2 s−1 versus RMSE = 7.67 μmol m−2 s−1). The study provides a practical way to estimate the contribution of SIFPSII to SIFTOC, giving a better theoretical basis to SIF-based GPP estimation models.

Photosynthetic control at the cytochrome b6f complex.

Photosynthetic control (PCON) is a protective mechanism that prevents light-induced damage to PSI by ensuring the rate of NADPH and ATP production via linear electron transfer (LET) is balanced by their consumption in the CO2 fixation reactions. Protection of PSI is a priority for plants since they lack a dedicated rapid-repair cycle for this complex, meaning that any damage leads to prolonged photoinhibition and decreased growth. The imbalance between LET and the CO2 fixation reactions is sensed at the level of the transthylakoid ΔpH, which increases when light is in excess. The canonical mechanism of PCON involves feedback control by ΔpH on the plastoquinol oxidation step of LET at cytochrome b6f. PCON thereby maintains the PSI special pair chlorophylls (P700) in an oxidized state, which allows excess electrons unused in the CO2 fixation reactions to be safely quenched via charge recombination. In this review we focus on angiosperms, consider how photo-oxidative damage to PSI comes about, explore the consequences of PSI photoinhibition on photosynthesis and growth, discuss recent progress in understanding PCON regulation, and finally consider the prospects for its future manipulation in crop plants to improve photosynthetic efficiency.

A pgr5 suppressor screen uncovers two distinct suppression mechanisms and links cytochrome b6f complex stability to PGR5.

PROTON GRADIENT REGULATION5 (PGR5) is thought to promote cyclic electron flow, and its deficiency impairs photosynthetic control and increases photosensitivity of photosystem (PS) I, leading to seedling lethality under fluctuating light (FL). By screening for Arabidopsis (Arabidopsis thaliana) suppressor mutations that rescue the seedling lethality of pgr5 plants under FL, we identified a portfolio of mutations in 12 different genes. These mutations affect either PSII function, cytochrome b6f (cyt b6f) assembly, plastocyanin (PC) accumulation, the CHLOROPLAST FRUCTOSE-1,6-BISPHOSPHATASE1 (cFBP1), or its negative regulator ATYPICAL CYS HIS-RICH THIOREDOXIN2 (ACHT2). The characterization of the mutants indicates that the recovery of viability can in most cases be explained by the restoration of PSI donor side limitation, which is caused by reduced electron flow to PSI due to defects in PSII, cyt b6f, or PC. Inactivation of cFBP1 or its negative regulator ACHT2 results in increased levels of the NADH dehydrogenase-like complex. This increased activity may be responsible for suppressing the pgr5 phenotype under FL conditions. Plants that lack both PGR5 and DE-ETIOLATION-INDUCED PROTEIN1 (DEIP1)/NEW TINY ALBINO1 (NTA1), previously thought to be essential for cyt b6f assembly, are viable and accumulate cyt b6f. We suggest that PGR5 can have a negative effect on the cyt b6f complex and that DEIP1/NTA1 can ameliorate this negative effect.

Identification of micronutrient deficiency related miRNA and their targets in Triticum aestivum using bioinformatics approach

Identification of miRNAs and their target proteins infer their functions to understand the biological processes of miRNAs and their involvement in plant growth and development. The homology-based approach (BLAST suite) was used for the identification of miRNA related to Zn and Cu deficiency in the bread wheat genome. Calculated the coding potential for the precursor miRNA and then predicted their secondary structure through RNAfold. PmiREN online server identified the miRNA target wheat protein. Further, STRING database predicted the biological relevance of the target protein. This in-silico study has identified the 3 miRNAs of the respective family of miR528, miR397, and miR168 of Triticum aestivum related to Cu and Zn deficiency. Out of the 42 targets for tae-miR397c; one of the targets is MFS domain-containing protein that contributes to “electron transfer” between photosystem700 and the “cytochrome b6-f complex” in photosystem and rest of the targets are laccase protein; involved in cell wall ligning deposition. Tae-miR528c has 6 targets; four are uncategorized proteins and the remaining two targets viz. GRF-type domain-containing protein and phytocyanin domain-containing protein are responsible for Zn ion binding and participate in electron transfer activity. The protein-protein interactions (PPIs) have found the various proteins that are associated with these identified miRNAs (tae-miR397c and tae-miR528c) target the protein that could be annotated further for their role in plant growth and development. The current computational hypothesis has developed a fast and robust pipeline to identify plant miRNAs and their targets compared to other used approaches.

Proton Gradient Regulation 5 determines reserve partitioning between starch and lipids in C. reinhardtii.

Nutrient deprivation induces reserve accumulation in unicellular algae. An absence of nitrogen in the growth media results in the reorganization of the photosynthetic apparatus and triggers an increase in starch and triacylglyceride (TAG) accumulation in different algal species. Here we study the integration of photosynthetic regulatory mechanisms with carbon partitioning under N stress in C. reinhardtii. The mutant, proton gradient regulation 5 (pgr5) is impaired in photosynthetic cyclic electron flow resulting in low chloroplastic ATP/NADPH ratios. Over a time course, under both mixotrophic and phototrophic conditions, the pgr5 mutant did not accumulate starch in the first three days, but rather degraded its meagre reserves. In contrast, there was a high TAG content in the pgr5 mutant which we show, is not linked to a selective increase in autophagy in pgr5. In all strains, proteins involved in alternative electron pathways are upregulated while Photosystem II and chlorophyll are strongly degraded; pgr5 only preferentially preserved some cyt b6f complex. Our results show that low ATP/NADPH ratios due to an absence of cyclic electron flow in pgr5 result in the mobilization of starch and strong TAG accumulation from the onset of N stress in Chlamydomonas.

Dynamic modulation of transthylakoid electric potential by chloroplast ATP synthases

The light-induced transthylakoid membrane potential (ΔΨm) can function as a driving force to help catalyzing the formation of ATP molecules, proving a tight connection between ΔΨm and the ATP synthase. Naturally, a question can be raised on the effects of altered functioning of ATP synthases on regulating ΔΨm, which is attractive in the area of photosynthetic research. Lots of findings, when making efforts of solving this difficulty, can offer an in-depth understanding into the mechanism behind. However, the functional network on modulating ΔΨm is highly interdependent. It is difficult to comprehend the consequences of altered activity of ATP synthases on adjusting ΔΨm because parameters that have influences on ΔΨm would themselves be affected by ΔΨm. In this work, a computer model was applied to check the kinetic changes in polarization/depolarization across the thylakoid membrane (TM) regulated by the modified action of ATP synthases. The computing data revealed that under the extreme condition by numerically “switching off” the action of the ATP synthase, the complete inactivation of ATP synthase would markedly impede proton translocation at the cytb6f complex. Concurrently, the KEA3 (CLCe) porter, actively pumping protons into the stroma, further contributes to achieving a sustained low level of ΔΨm. Besides, the quantitative consequences on every particular component of ΔΨm adjusted by the modified functioning of ATP synthases were also explored. By employing the model, we bring evidence from the theoretical perspective that the ATP synthase is a key factor in forming a transmembrane proton loop thereby maintaining a propriate steady-state ΔΨm to meet variable environmental conditions.

Integrative Physiological, Transcriptome, and Proteome Analyses Provide Insights into the Photosynthetic Changes in Maize in a Maize-Peanut Intercropping System.

Intercropping is a traditional and sustainable planting method that can make rational use of natural resources such as light, temperature, fertilizer, water, and CO2. Due to its efficient resource utilization, intercropping, in particular, maize and legume intercropping, is widespread around the world. However, the molecular details of these pathways remain largely unknown. In this study, physiological, transcriptome, and proteome analyses were compared between maize monocropping and maize-peanut intercropping. The results show that an intercropping system enhanced the ability of carbon fixation and carboxylation of maize leaves. Apparent quantum yield (AQY), the light-saturated net photosynthetic rate (LSPn), the light saturation point (LSP), and the light compensation point (LCP) were increased by 11.6%, 9.4%, 8.9%, and 32.1% in the intercropping system, respectively; carboxylation efficiency (CE), the CO2 saturation point (Cisat), the Rubisco maximum carboxylation rate (Vcmax), the maximum electron transfer rate (Jmax), and the triose phosphate utilization rate (TPU) were increased by 28.5%, 7.3%, 18.7%, 29.2%, and 17.0%, respectively; meanwhile, the CO2 compensation point (Γ) decreased by 22.6%. Moreover, the transcriptome analysis confirmed the presence of 588 differentially expressed genes (DEGs), and the numbers of up-regulated and down-regulated genes were 383 and 205, respectively. The DEGs were primarily concerned with ribosomes, plant hormone signal transduction, and photosynthesis. Furthermore, 549 differentially expressed proteins (DEPs) were identified in the maize leaves in both the maize monocropping and maize-peanut intercropping systems. Bioinformatics analysis revealed that 186 DEPs were related to 37 specific KEGG pathways in each of the two treatment groups. Based on the physiological, transcriptome, and proteome analyses, it was demonstrated that the photosynthetic characteristics in maize leaves can be improved by maize-peanut intercropping. This may be related to PS I, PS II, cytochrome b6f complex, ATP synthase, and photosynthetic CO2 fixation, which is caused by the improved CO2 carboxylation efficiency. Our results provide a more in-depth understanding of the high yield and high-efficiency mechanism in maize and peanut intercropping.

Peculiarities of DNP-INT and DBMIB as inhibitors of the photosynthetic electron transport.

Inhibitory analysis is a useful tool for studying cytochrome b6f complex in the photosynthetic electron transport chain. Here, we examine the inhibitory efficiency of two widely used inhibitors of the plastoquinol oxidation in the cytochrome b6f complex, namely 2,4-dinitrophenyl ether of 2-iodo-4-nitrothymol (DNP-INT) and 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DBMIB). Using isolated thylakoids from pea and arabidopsis, we demonstrate that inhibitory activity of DNP-INT and DBMIB is enhanced by increasing irradiance, and this effect is due to the increase in the rate of electron transport. However, the accumulation of protons in the thylakoid lumen at low light intensity has opposite effects on the inhibitory activity of DNP-INT and DBMIB, namely increasing the activity of DNP-INT and restricting the activity of DBMIB. These results allow for the refinement of the conditions under which the use of these inhibitors leads to the complete inhibition of plastoquinol oxidation in the cytochrome b6f complex, thereby broadening our understanding of the operation of the cytochrome b6f complex under conditions of steady-state electron transport.

Electron transport in chloroplasts: regulation and alternative pathways of electron transfer

This work represents an overview of electron transport regulation in chloroplasts as considered in the context of structure-function organization of photosynthetic apparatus in plants. A basic focus of the article is concentrated on a bifurcated oxidation of plastoquinol by the cytochrome b6f complex, which represents the rate-limiting step of electron transfer between photosystems 2 and 1. Electron transport along the chains of the noncyclic, cyclic and pseudocyclic electron flow, their relationships to generation of the trans-thylakoid difference in electrochemical potentials of protons in chloroplasts, and the pH-dependent mechanisms of regulation of the cytochrome b6f complex, are considered. Redox reactions with the participation of molecular oxygen and ascorbate, the alternative mediators of electron transport in chloroplasts, have also been discussed.