Carbon Fixation Reactions Research Articles

A new strategy of carbon-energy coupling transfer enhancement by formate on phototrophic green microalga Chlamydomonas reinhardtii

Low efficient CO2 and solar energy transfer in aquatic microalgae cultivation are always challenges and have consumed huge efforts during the last two decades. With the development of solar fuels technologies, formate can come from the reduction of CO2 with green H2. As a water-soluble solar fuel, formate is a good carbon-energy coupling enhancement carrier for biosynthesis. However, formate is also an inhibitor to the electron transfer chain (ETC) in the cell. Based on the chloroplast engineering, a series of Chlamydomonas reinhardtii mutants with chloroplast transit peptide chimeric NADPH-dependent formate dehydrogenase (FDH) were constructed, and these mutants were named as PsFDHs. Results showed that exogenous FDH alleviated the inhibition of formate on the photosynthetic system and promoted the growth, and a win–win result obtained between the natural and artificial photosynthesis: 40 % more carbon from formate was fixed than heterotrophic only, and 62 % increase in dry weight (DW), while no obvious formate consumption was detected in the wide type (WT). The maximum overall biomass productivity was 15.4 mg·L−1·h−1, which was 36 % higher than that of WT. It’s postulated that formate enhanced the carbon and energy transportation close to carbon fixation reaction center, and photosynthesis made up for energy shortages in the conversion of formate to biomass. The results showed the potential of formate in achieving a higher-than-nature artificial-natural hybrid photosynthesis by microalgae, to meet requirements of both the carbon neutralization and protein supply for human nutrients.

The thylakoid proton antiporter KEA3 regulates photosynthesis in response to the chloroplast energy status

Plant photosynthesis contains two functional modules, the light-driven reactions in the thylakoid membrane and the carbon-fixing reactions in the chloroplast stroma. In nature, light availability for photosynthesis often undergoes massive and rapid fluctuations. Efficient and productive use of such variable light supply requires an instant crosstalk and rapid synchronization of both functional modules. Here, we show that this communication involves the stromal exposed C-terminus of the thylakoid K+-exchange antiporter KEA3, which regulates the ΔpH across the thylakoid membrane and therefore pH-dependent photoprotection. By combining in silico, in vitro, and in vivo approaches, we demonstrate that the KEA3 C-terminus senses the energy state of the chloroplast in a pH-dependent manner and regulates transport activity in response. Together our data pinpoint a regulatory feedback loop by which the stromal energy state orchestrates light capture and photoprotection via multi-level regulation of KEA3.

Diffusional and Biochemical Limitations to Photosynthesis Under Water Deficit for Field-Grown Cotton.

Water deficit stress limits net photosynthetic rate (AN), but the relative sensitivities of underlying processes such as thylakoid reactions, ATP production, carbon fixation reactions, and carbon loss processes to water deficit stress in field-grown upland cotton require further exploration. Therefore, the objective of the present study was to assess (1) the diffusional and biochemical mechanisms associated with water deficit-induced declines in AN and (2) associations between water deficit-induced variation in oxidative stress and energy dissipation for field-grown cotton. Water deficit stress was imposed for three weeks during the peak bloom stage of cotton development, causing significant reductions in leaf water potential and AN. Among diffusional limitations, mesophyll conductance was the major contributor to the AN decline. Several biochemical processes were adversely impacted by water deficit. Among these, electron transport rate and RuBP regeneration were most sensitive to AN-limiting water deficit. Carbon loss processes (photorespiration and dark respiration) were less sensitive than carbon assimilation, contributing to the water deficit-induced declines in AN. Increased energy dissipation via non-photochemical quenching or maintenance of electron flux to photorespiration prevented oxidative stress. Declines in AN were not associated with water deficit-induced variation in ATP production. It was concluded that diffusional limitations followed by biochemical limitations (ETR and RuBP regeneration) contributed to declines in AN, carbon loss processes partially contributed to the decline in AN, and increased energy dissipation prevented oxidative stress under water deficit in field-grown cotton.

Mechanism of interaction between hydration–carbonation of C2S

The physical and microscopic properties of γ-C2S and β-C2S under hydration, carbonation, and hydration–carbonation cycle reactions were investigated. The hydration–carbonation cycle reaction facilitated the carbon sequestration of C2S, and the generation of CaCO3 filled the pores of the matrix and enhanced the matrix density in the carbon fixation reaction process. In the subsequent hydration and reaction process, the formation of hydration products increased the number of pores in the matrix and reduced the matrix density. Post-hydration, γ-C2S slurry became powder and had no mechanical strength. However, after carbonation reaction and two carbonation–hydration cycles, γ-C2S exhibited better mechanical properties than β-C2S.

Analysis of state 1—state 2 transitions by genome editing and complementation reveals a quenching component independent from the formation of PSI-LHCI-LHCII supercomplex in Arabidopsis thaliana

BackgroundThe light-harvesting antennae of photosystem (PS) I and PSII are pigment-protein complexes responsible of the initial steps of sunlight conversion into chemical energy. In natural environments plants are constantly confronted with the variability of the photosynthetically active light spectrum. PSII and PSI operate in series but have different optimal excitation wavelengths. The prompt adjustment of light absorption by photosystems is thus crucial to ensure efficient electron flow needed to sustain downstream carbon fixing reactions. Fast structural rearrangements equilibrate the partition of excitation pressure between PSII and PSI following the enrichment in the red (PSII-favoring) or far-red (PSI-favoring) spectra. Redox imbalances trigger state transitions (ST), a photoacclimation mechanism which involves the reversible phosphorylation/dephosphorylation of light harvesting complex II (LHCII) proteins by the antagonistic activities of the State Transition 7 (STN7) kinase/TAP38 phosphatase enzyme pair. During ST, a mobile PSII antenna pool associates with PSI increasing its absorption cross section. LHCII consists of assorted trimeric assemblies of Lhcb1, Lhcb2 and Lhcb3 protein isoforms (LHCII), several being substrates of STN7. However, the precise roles of Lhcb phosphorylation during ST remain largely elusive.ResultsWe inactivated the complete Lhcb1 and Lhcb2 gene clades in Arabidopsis thaliana and reintroduced either wild type Lhcb1.3 and Lhcb2.1 isoforms, respectively, or versions lacking N-terminal phosphorylatable residues proposed to mediate state transitions. While the substitution of Lhcb2.1 Thr-40 prevented the formation of the PSI-LHCI-LHCII complex, replacement of Lhcb1.3 Thr-38 did not affect the formation of this supercomplex, nor did influence the amplitude or kinetics of PSII fluorescence quenching upon state 1—state 2 transition.ConclusionsPhosphorylation of Lhcb2 Thr-40 by STN7 alone accounts for ≈ 60% of PSII fluorescence quenching during state transitions. Instead, the presence of Thr-38 phosphosite in Lhcb1.3 was not required for the formation of the PSI-LHCI-LHCII supercomplex nor for re-equilibration of the plastoquinone redox state. The Lhcb2 phosphomutant was still capable of ≈ 40% residual fluorescence quenching, implying that a yet uncharacterized, STN7-dependent, component of state transitions, which is unrelated to Lhcb2 Thr-40 phosphorylation and to the formation of the PSI-LHCI-LHCII supercomplex, contributes to the equilibration of the PSI/PSII excitation pressure upon plastoquinone over-reduction.

Engineering the Biosynthesis of prFMN Promotes the Conversion between Styrene/CO2 and Cinnamic Acid Catalyzed by the Ferulic Acid Decarboxylase Fdc1

Enzymatic decarboxylation and carboxylation are emerging as prospective processes to produce high-value compounds under mild conditions. Ferulic acid decarboxylase Fdc1 catalyzes broad substrate tolerance against α, β-unsaturated carboxylic acids, and provides green routes for carbon dioxide fixation with the reversible carboxylation, while the activity of the enzyme is limited by the indispensable cofactor prenylated flavin (prFMN), which is unstable and is rarely detected in nature. In this study, a prFMN efficient synthesis route was built using six exogenous enzymes introduced into E. coli cells, leading to the construction of a powerful cell catalyst named SC-6. Based on the metabolic analysis, the results indicated that the reduction of FMN to FMNH2 was the bottleneck in prFMN synthesis pathway, and introducing FMN reductase increased the production of prFMN 3.8-fold compared with the common flavin prenyltransferase UbiX overexpression strain. Using SC-6 cell catalyst, the decarboxylation activity of Fdc1 increased more than 20 times with cinnamic acid and 4-acetoxycinnamic acid as substrates. Furthermore, the reversible carboxylation reaction was carried out, and the cell catalyst presented 20 times carbon dioxide fixation activity using styrene to produce cinnamic acid. Finally, the maximum yield of cinnamic acid catalyzed by SC-6 achieved 833.68 ± 34.51 mM·mg−1 in two hours. The constructed prFMN pathway in vivo provides fundamentals for efficient decarboxylation and carbon fixation reactions catalyzed by prFMN-dependent enzymes.

Effects of chilling on the photosynthetic performance of the CAM orchid Phalaenopsis.

Crassulacean acid metabolism (CAM) is one of the three main metabolic adaptations for CO2 fixation found in plants. A striking feature for these plants is nocturnal carbon fixation and diurnal decarboxylation of malic acid to feed Rubisco with CO2 behind closed stomata, thereby saving considerable amounts of water. Compared to the effects of high temperatures, drought, and light, much less information is available about the effects of chilling temperatures on CAM plants. In addition a lot of CAM ornamentals are grown in heated greenhouses, urging for a deeper understanding about the physiological responses to chilling in order to increase sustainability in the horticultural sector. The present study focuses on the impact of chilling temperatures (10°C) for 3 weeks on the photosynthetic performance of the obligate CAM orchid Phalaenopsis 'Edessa'. Detailed assessments of the light reactions were performed by analyzing chlorophyll a fluorescence induction (OJIP) parameters and the carbon fixation reactions by measuring diel leaf gas exchange and diel metabolite patterns. Results showed that chilling already affected the light reactions after 24h. Whilst the potential efficiency of photosystem II (PSII) (Fv/Fm) was not yet influenced, a massive decrease in the performance index (PIabs) was noticed. This decrease did not depict an overall downregulation of PSII related energy fluxes since energy absorption and dissipation remained uninfluenced whilst the trapped energy and reduction flux were upregulated. This might point to the presence of short-term adaptation mechanisms to chilling stress. However, in the longer term the electron transport chain from PSII to PSI was affected, impacting both ATP and NADPH provision. To avoid over-excitation and photodamage plants showed a massive increase in thermal dissipation. These considerations are also in line with carbon fixation data showing initial signs of cold adaptation by achieving comparable Rubisco activity compared to unstressed plants but increasing daytime stomatal opening in order to capture a higher proportion of CO2 during daytime. However, in accordance with the light reactions data, Rubisco activity declined and stomatal conductance and CO2 uptake diminished to near zero levels after 3 weeks, indicating that plants were not successful in cold acclimation on the longer term.

State transition is quiet around pyrenoid and LHCII phosphorylation is not essential for thylakoid deformation in Chlamydomonas 137c

Photosynthetic organisms have developed a regulation mechanism called state transition (ST) to rapidly adjust the excitation balance between the two photosystems by light-harvesting complex II (LHCII) movement. Though many researchers have assumed coupling of the dynamic transformations of the thylakoid membrane with ST, evidence of that remains elusive. To clarify the above-mentioned coupling in a model organism Chlamydomonas, here we used two advanced microscope techniques, the excitation-spectral microscope (ESM) developed recently by us and the superresolution imaging based on structured-illumination microscopy (SIM). The ESM observation revealed ST-dependent spectral changes upon repeated ST inductions. Surprisingly, it clarified a less significant ST occurrence in the region surrounding the pyrenoid, which is a subcellular compartment specialized for the carbon-fixation reaction, than that in the other domains. Further, we found a species dependence of this phenomenon: 137c strain showed the significant intracellular inhomogeneity of ST occurrence, whereas 4A+ strain hardly did. On the other hand, the SIM observation resolved partially irreversible fine thylakoid transformations caused by the ST-inducing illumination. This fine, irreversible thylakoid transformation was also observed in the STT7 kinase-lacking mutant. This result revealed that the fine thylakoid transformation is not induced solely by the LHCII phosphorylation, suggesting the highly susceptible nature of the thylakoid ultrastructure to the photosynthetic light reactions.

Fine-tuning of polarized hydroxyapatite for the catalytic conversion of dinitrogen to ammonium under mild conditions

Polarized hydroxyapatite (p-HAp), a calcium phosphate catalyst obtained at high temperature under intense electric field, has been used for the synthesis of ammonium starting from N2 and liquid water at low pressure (<6 bar) and temperatures below 120 °C. The success of the nitrogen fixation process has been demonstrated by isotope labelling experiments using 15N2. Considering the optimal reaction conditions for the production of ammonium, the yield is as high as 154.6 ± 25.8 μmol/g of catalyst. The proposed synthesis exhibits three important advantages for its utilization in green chemistry environmental processes related to the recycling of polluted air. These are: i) the catalysts converts CO2 into valuable chemical products in addition of transforming N2 in ammonium; ii) the final energy balance is very favorable since no external electrical field is necessary to promote nitrogen and carbon fixation reactions; and iii) products are easily transferred to water favoring their extraction and avoiding the saturation of the catalyst.

Hydroxyapatite-based biphasic catalysts with plasticity properties and its potential in carbon dioxide fixation

• Polarized hydroxyapatite (HAp) fixates carbon into C1, C2 and C3 molecules. • Polarized brushite (Bru) is not catalytically active. • The synergy between Hap and Bru gives plasticity to Hap-Bru catalysts. • The HAp/Bru ratio in the catalyst regulates the C1/C3 ratio in the products. • The HAp-Bru system is a promising green alternative to metal-based electrocatalysts. The design of catalysts with controlled selectivity at will, also known as catalytic plasticity, is a very attractive approach for the recycling of carbon dioxide (CO 2 ). In this work, we study how catalytically active hydroxyapatite (HAp) and brushite (Bru) interact synergistically, allowing the production of formic acid or acetic acid depending on the HAp/Bru ratio in the catalyst. Raman, wide angle X-ray scattering, X-ray photoelectron spectroscopy, scanning electron microscopy and electrochemical impedance spectroscopy studies, combined with an exhaustive revision of the crystalline structure of the catalyst at the atomic level, allowed to discern how the Bru phase can be generated and stabilized at high temperatures. Results clearly indicate that the presence of OH – groups to maintain the crystalline structural integrity in conjunction with Ca 2+ ions less bonded to the lattice fixate carbon into C1, C2 and C3 molecules from CO 2 and allow the evolution from formic to acetic acid and acetone. In this way, the plasticity of the HAp-Bru system is demonstrated, representing a promising green alternative to the conventional metal-based electrocatalysts used for CO 2 fixation. Thus, the fact that no electric voltage is necessary for the CO 2 reduction has a very favorable impact in the final energetic net balance of the carbon fixation reaction.

Functional division of f-type and m-type thioredoxins to regulate the Calvin cycle and cyclic electron transport around photosystem I.

Redox regulation of chloroplast proteins is necessary to adjust photosynthetic performance with changes in light. The thioredoxin (Trx) system plays a central role in this process. Chloroplast-localized classical Trx is a small redox-active protein that regulates many target proteins by reducing their disulfide bonds in a light-dependent manner. Arabidopsis thaliana mutants lacking f-type Trx (trx f1f2) or m-type Trx (trx m124-2) have been reported to show delayed reduction of Calvin cycle enzymes. As a result, the trx m124-2 mutant exhibits growth defects. Here, we characterized a quintuple mutant lacking both Trx f and Trx m to investigate the functional complementarity of Trx f and Trx m. The trx f1f2 m124-2 quintuple mutant was newly obtained by crossing, and is analyzed here for the first time. The growth defects of the trx m124-2 mutant were not enhanced by the lack of Trx f. In contrast, deficiencies of both Trxs additively suppressed the reduction of Calvin cycle enzymes, resulting in a further delay in the initiation of photosynthesis. Trx f appeared to be necessary for the rapid activation of the Calvin cycle during the early induction of photosynthesis. To perform effective photosynthesis, plants seem to use both Trxs in a coordinated manner to activate carbon fixation reactions. In contrast, the PROTON GRADIENT REGULATION 5 (PGR5)-dependent cyclic electron transport around photosystem I was regulated by Trx m, but not by Trx f. Lack of Trx f did not affect the activity and regulation of the PGR5-dependent pathway. Trx f may have a higher specificity for target proteins, whereas Trx m has a variety of target proteins to regulate overall photosynthesis and other metabolic reactions in the chloroplasts.

The Effects of Different Solid Content Carbon Nanotubes and Silver Quantum Dots on Potential Toxicity to Plants through Direct Effects on Carbon and Light Reactions of Photosynthesis

We examined the effects of two types of carbon nanotubes (CNTs) and one type of silver quantum dot (Ag-QD) on potential plant toxicity through effects on plant gas exchange across four different experiments. First, Arabidopsis thaliana seeds were directly grown in growth medium containing 75% solid content CNTs at concentrations of 24.93µg/ml and 53.55 µg/ml in petri dishes. Second, A. thaliana seeds were directly grown in growth medium containing 95% solid content CNTs at concentrations of 4μg/ml; or third, 18 nm Ag-QDs at a concentration of 4μg/ ml. Fourth, we grew A. thaliana in soil for 6 weeks and added the 95% solid content CNT suspension at increasing concentrations of 10, 30, 90, 150, 190, 250 μg/ ml each week. The 75% solid content CNT, and the CNTRENE® C100LM material production waste produced for disposal, had no negative effects on growth or gas exchange. We found that gas exchange in petri dish grown A. thaliana was greatly negatively affected by the Ag-QD, and relatively marginally negatively affected by the 95% solid content CNT. There were significant reductions in photosynthesis rates and related light and carbon fixation reactions in both the Ag-QD and 95% solid content CNT A. thaliana grown in petri dishes. We found that gas exchange in soil grown A. thaliana was unaffected by 95% solid content CNTs, even at very high concentrations. These findings have implications for understanding toxicity of engineered nanoparticles on plant and animal health, public awareness, and environmental remediation.

Ammonium Bicarbonate Significantly Accelerates the Microdroplet Reactions of Amines with Carbon Dioxide.

The reactions between amines and carbon dioxide (CO2) are among the most commonly used and important carbon fixation reactions at present. Microdroplets generated by electrospray ionization (ESI) have been proved to increase the conversion ratio (RC) of amines. In this work, we confirmed that the presence of ammonium bicarbonate (NH4HCO3) in ESI microdroplets significantly increased the RC of amines. The RC went up remarkably with the increase in the concentration of NH4HCO3 from 0.5 to 20 mM. The RC of N,N-dibutyl-1,3-propanediamine (DBPA) reached 93.7% under 20 mM NH4HCO3, which was significantly higher than previous reports. The rise in RC became insignificant when the concentration of NH4HCO3 was increased beyond 20 mM. Further investigations were made on the mechanism of the phenomenon. According to the results, it was suggested that NH4HCO3 decomposed into CO2 and formed microbubbles within the microdroplets of ESI. The microbubbles acted as direct internal CO2 sources. The conversion reactions occurred at the liquid-gas interface. The formation of CO2 microbubbles remarkably increased the total area of the interface, thus promoting the conversion reactions. 13C-labeled experiments confirmed that NH4HCO3 acted as an internal CO2 source. Factors that influenced the RC of the reaction were optimized. Pure water was proved to be the optimal solvent. Lower temperature of the mass spectrometer's entrance capillary was beneficial to the stabilization of the product carbamic acids. The sample flow rate of ESI was crucial to the RC. It determined the initial sizes of the microdroplet. Lower flow rates ensured higher RC of amines. The present work implied that NH4HCO3 could be a superior medium for CO2 capture and utilization. It might offer an alternative choice for future CO2 conversion research studies. In addition, our study also provided evidence that NH4HCO3 decomposed and generated microbubbles in the droplets during ESI. Attention should be paid to this when using NH4HCO3 as an additive in mass spectrometry-based analysis.

LPA2 protein is involved in photosystemII assembly in Chlamydomonas reinhardtii.

SummaryPhotosynthetic eukaryotes require the proper assembly of photosystem II (PSII) in order to strip electrons from water and fuel carbon fixation reactions. In Arabidopsis thaliana, one of the PSII subunits (CP43/PsbC) was suggested to be assembled into the PSII complex via its interaction with an auxiliary protein called Low PSII Accumulation 2 (LPA2). However, the original articles describing the role of LPA2 in PSII assembly have been retracted. To investigate the function of LPA2 in the model organism for green algae, Chlamydomonas reinhardtii, we generated knockout lpa2 mutants by using the CRISPR‐Cas9 target‐specific genome editing system. Biochemical analyses revealed the thylakoidal localization of LPA2 protein in the wild type (WT), whereas lpa2 mutants were characterized by a drastic reduction in the levels of D1, D2, CP47 and CP43 proteins. Consequently, reduced PSII supercomplex accumulation, chlorophyll content per cell, PSII quantum yield and photosynthetic oxygen evolution were measured in the lpa2 mutants, leading to the almost complete impairment of photoautotrophic growth. Pulse‐chase experiments demonstrated that the absence of LPA2 protein caused reduced PSII assembly and reduced PSII turnover. Taken together, our data indicate that, in C. reinhardtii, LPA2 is required for PSII assembly and proper function.

Shotgun metagenomics reveals distinct functional diversity and metabolic capabilities between 12 000-year-old permafrost and active layers on Muot da Barba Peider (Swiss Alps).

The warming-induced thawing of permafrost promotes microbial activity, often resulting in enhanced greenhouse gas emissions. The ability of permafrost microorganisms to survive the in situ sub-zero temperatures, their energetic strategies and their metabolic versatility in using soil organic materials determine their growth and functionality upon thawing. Hence, functional characterization of the permafrost microbiome, particularly in the underexplored mid-latitudinal alpine regions, is a crucial first step in predicting its responses to the changing climate, and the consequences for soil–climate feedbacks. In this study, for the first time, the functional potential and metabolic capabilities of a temperate mountain permafrost microbiome from central Europe has been analysed using shotgun metagenomics. Permafrost and active layers from the summit of Muot da Barba Peider (MBP) [Swiss Alps, 2979 m above sea level (a.s.l.)] revealed a strikingly high functional diversity in the permafrost (north-facing soils at a depth of 160 cm). Permafrost metagenomes were enriched in stress-response genes (e.g. cold-shock genes, chaperones), as well as in genes involved in cell defence and competition (e.g. antiviral proteins, antibiotics, motility, nutrient-uptake ABC transporters), compared with active-layer metagenomes. Permafrost also showed a higher potential for the synthesis of carbohydrate-active enzymes, and an overrepresentation of genes involved in fermentation, carbon fixation, denitrification and nitrogen reduction reactions. Collectively, these findings demonstrate the potential capabilities of permafrost microorganisms to thrive in cold and oligotrophic conditions, and highlight their metabolic versatility in carbon and nitrogen cycling. Our study provides a first insight into the high functional gene diversity of the central European mountain permafrost microbiome. Our findings extend our understanding of the microbial ecology of permafrost and represent a baseline for future investigations comparing the functional profiles of permafrost microbial communities at different latitudes.

Effect of sugar feedback regulation on major genes and proteins of photosynthesis in sugarcane leaves

Productivity of sugarcane (Saccharum spp.) relies upon sucrose production in leaves and movement to sinks. The feedback regulatory effect of sugar upon photosynthesis balances this process involving Phosphoenolpyruvate carboxylase (PEPCase) and Rubisco where greater understanding in this area may allow manipulation to achieve higher yields. Accumulation of sucrose in leaves and decreased photosynthesis are early symptoms of the condition called yellow canopy syndrome (YCS) in sugarcane, which presents as a system in which to study sucrose feedback regulation. This work investigates changes in gene expression and protein abundance which coincide with the sugar accumulation in the leaves of YCS symptomatic sugarcane. During the early-stage of sugar accumulation, the levels of the Photosystem II core protein D1, and PsbQ of the oxygen-evolving complex decreased significantly. Transcript levels of these proteins also decreased, suggesting both nuclear and chloroplast gene expression were affected early in sugar build-up of YCS development. Transcript level of primary carbon fixation reactions enzyme NADP malate dehydrogenase was especially downregulated. However, PEPCase, decarboxylation and re-fixation (Rubisco) enzymes were not negatively regulated at the transcript or protein abundance level. Phosphoenolpyruvate carboxykinase was upregulated in both gene expression and protein abundance. The Calvin cycle in the bundle sheath was sensitive through the CP12 protein. Two isoforms of CP12 were found, one of which showed downregulation which coincided with a decrease in CP12 protein. This suggests transcript and protein decrease of PEPCase and Rubisco may be secondary regulation points of the sugar feedback regulation process upon photosynthesis in sugarcane leaves.

Two chloroplast thioredoxin systems differentially modulate photosynthesis in Arabidopsis depending on light intensity and leaf age.

SUMMARYVarious regulatory mechanisms have evolved in plants to optimize photosynthetic activity under fluctuating light. Thioredoxins (TRX) are members of the regulatory network balancing activities of light and carbon fixation reactions in chloroplasts. We have studied the impact of two chloroplast TRX systems, the ferredoxin‐dependent TRX reductase (FTR) and the NADPH‐dependent TRX reductase C (NTRC) on regulation of photosynthesis by mutants lacking or overexpressing a component of either system. Plants were subjected to image‐based phenotyping and chlorophyll fluorescence measurements that allow long‐term monitoring of the development and photosynthetic activity of the rosettes, respectively. Our experiments demonstrate that NTRC and FTR systems respond differently to variation of light intensity. NTRC was an indispensable regulator of photosynthesis in young leaves, at light‐intensity transitions and under low light intensities limiting photosynthesis, whereas steady‐state exposure of plants to growth or higher light intensities diminished the need of NTRC in regulation of photosynthesis. In fluctuating light, overexpression of NTRC increased the quantum yield of Photosystem II (YII) at low light and stimulated the relaxation of non‐photochemical quenching (NPQ) after high light exposure, indicating that overexpression of NTRC improves leaf capacity to convert light energy to chemical energy under these conditions. Overexpression of chimeric protein (NTR‐TRXf) containing both the thioredoxin reductase and TRXf activity on an ntrc mutant background, did not completely recover either growth or steady‐state photosynthetic activity, whereas OE‐NTR‐TRXf plants exposed to fluctuating light regained the wild‐type level of Y(II) and NPQ.

Maize GOLDEN2-LIKE genes enhance biomass and grain yields in rice by improving photosynthesis and reducing photoinhibition

Photosynthetic efficiency is a major target for improvement of crop yield potential under agricultural field conditions. Inefficiencies can occur in many steps of the photosynthetic process, from chloroplast biogenesis to functioning of the light harvesting and carbon fixation reactions. Nuclear-encoded GOLDEN2-LIKE (GLK) transcription factors regulate some of the earliest steps by activating target genes encoding chloroplast-localized and photosynthesis-related proteins. Here we show that constitutive expression of maize GLK genes in rice leads to enhanced levels of chlorophylls and pigment-protein antenna complexes, and that these increases lead to improved light harvesting efficiency via photosystem II in field-grown plants. Increased levels of xanthophylls further buffer the negative effects of photoinhibition under high or fluctuating light conditions by facilitating greater dissipation of excess absorbed energy as heat. Significantly, the enhanced photosynthetic capacity of field-grown transgenic plants resulted in increased carbohydrate levels and a 30–40% increase in both vegetative biomass and grain yield.

Does the stromal concentration of Pi control chloroplast ATP synthase protein amount in contrasting growth environments?

ABSTRACTChanges in the growth environment can generate imbalances in chloroplast photosynthetic metabolism. Under water deficit, stomatal closure limits CO2 availability such that the production of ATP and NADPH by the thylakoid membrane-localized electron transport chain may not match the consumption of these energy intermediates by the stroma-localized Calvin-Benson cycle, thus challenging energy balance. Alternatively, in an elevated CO2 atmosphere, carbon fixation by the Calvin-Benson cycle may outpace the activity of downstream carbohydrate-utilizing processes, thus challenging carbon balance. Our previous studies have shown that, in both of the above scenarios, a mitochondrial alternative oxidase contributes to maintaining energy or carbon balance, highlighting the importance of photosynthesis-respiration interactions in optimizing photosynthesis in different growth environments. In these previous studies, we observed aberrant amounts of chloroplast ATP synthase protein across the different transgenic plant lines and growth conditions, compared to wild-type. Based on these observations, we develop here the hypothesis that an important determinant of chloroplast ATP synthase protein amount is the stromal concentration of inorganic phosphate. ATP synthase is a master regulator of photosynthesis. Coarse control of ATP synthase protein amount by the stromal inorganic phosphate status could provide a means to coordinate the electron transport and carbon fixation reactions of photosynthesis.

Chloroplast thioredoxin systems dynamically regulate photosynthesis in plants.

Photosynthesis is a highly regulated process in photoautotrophic cells. The main goal of the regulation is to keep the basic photosynthetic reactions, i.e. capturing light energy, conversion into chemical energy and production of carbohydrates, in balance. The rationale behind the evolution of strong regulation mechanisms is to keep photosynthesis functional under all conditions encountered by sessile plants during their lifetimes. The regulatory mechanisms may, however, also impair photosynthetic efficiency by overriding the photosynthetic reactions in controlled environments like crop fields or bioreactors, where light energy could be used for production of sugars instead of dissipation as heat and down-regulation of carbon fixation. The plant chloroplast has a high number of regulatory proteins called thioredoxins (TRX), which control the function of chloroplasts from biogenesis and assembly of chloroplast machinery to light and carbon fixation reactions as well as photoprotective mechanisms. Here, we review the current knowledge of regulation of photosynthesis by chloroplast TRXs and assess the prospect of improving plant photosynthetic efficiency by modification of chloroplast thioredoxin systems.