Over-expressing the C(3) photosynthesis cycle enzyme Sedoheptulose-1-7 Bisphosphatase improves photosynthetic carbon gain and yield under fully open air CO(2) fumigation (FACE)

Structure of the Calvin-Benson-Bassham sedoheptulose-1,7-bisphosphatase from the model microalga Chlamydomonas reinhardtii

The Calvin-Benson-Bassham cycle (CBBC) performs carbon fixation in photosynthetic organisms. Among the eleven enzymes that participate in the pathway, sedoheptulose-1,7-bisphosphatase (SBPase) is expressed in photo-autotrophs and catalyzes the hydrolysis of sedoheptulose-1,7-bisphosphate (SBP) to sedoheptulose-7-phosphate (S7P). SBPase, along with nine other enzymes in the CBBC, contributes to the regeneration of ribulose-1,5-bisphosphate, the carbon-fixing co-substrate used by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). The metabolic role of SBPase is restricted to the CBBC, and a recent study revealed that the three-dimensional structure of SBPase from the moss Physcomitrium patens was found to be similar to that of fructose-1,6-bisphosphatase (FBPase), an enzyme involved in both CBBC and neoglucogenesis. In this study we report the first structure of an SBPase from a chlorophyte, the model unicellular green microalga Chlamydomonas reinhardtii. By combining experimental and computational structural analyses, we describe the topology, conformations, and quaternary structure of Chlamydomonas reinhardtii SBPase (CrSBPase). We identify active site residues and locate sites of redox- and phospho-post-translational modifications that contribute to enzymatic functions. Finally, we observe that CrSBPase adopts distinct oligomeric states that may dynamically contribute to the control of its activity.

Ectopic Expression of a Transmembrane Protein KaCyt b6 from a Red Seaweed Kappaphycus alvarezii in Transgenic Tobacco Augmented the Photosynthesis and Growth

Cytochrome b6f complex is a thylakoid membrane-localized protein and catalyses the transfer of electrons from plastoquinol to plastocyanin in photosynthetic electron transport chain. In the present study, Cytochrome b6 (KaCyt b6) gene from Kappaphycus alvarezii (a red seaweed) was overexpressed in tobacco. A 935 base pair (bp) long KaCyt b6 cDNA contained an open reading frame of 648 bp encoding a protein of 215 amino acids with an expected isoelectric point of 8.67 and a molecular mass of 24.37 kDa. The KaCyt b6 gene was overexpressed in tobacco under control of CaMV35S promoter. The transgenic tobacco had higher electron transfer rate and photosynthetic yield over wild-type and vector control tobacco. The KaCyt b6 tobacco also exhibited significantly higher photosynthetic gas exchange (PN) and improved water use efficiency. The transgenic plants had higher ratio of PN and intercellular CO2. The KaCyt b6 transgenic tobacco showed higher estimates of photosystem II quantum yield, higher activity of the water-splitting complex, PSII photochemistry, and photochemical quenching. The basal quantum yield of nonphotochemical processes in PSII was recorded lower in KaCyt b6 tobacco. Transgenic tobacco contained higher contents of carotenoids and total chlorophyll and also had better ratios of chlorophyll a and b, and carotenoids and total chlorophyll contents hence improved photosynthetic efficiency and production of sugar and starch. The KaCyt b6 transgenic plants performed superior under control and greenhouse conditions. To the best of our knowledge through literature survey, this is the first report on characterization of KaCyt b6 gene from K. alvarezii for enhanced photosynthetic efficiency and growth in tobacco.

Perspectives on improving photosynthesis to increase crop yield

Improving photosynthesis, the fundamental process by which plants convert light energy into chemical energy, is a key area of research with great potential for enhancing sustainable agricultural productivity and addressing global food security challenges. This perspective delves into the latest advancements and approaches aimed at optimizing photosynthetic efficiency. Our discussion encompasses the entire process, beginning with light harvesting and its regulation and progressing through the bottleneck of electron transfer. We then delve into the carbon reactions of photosynthesis, focusing on strategies targeting the enzymes of the Calvin-Benson-Bassham (CBB) cycle. Additionally, we explore methods to increase carbon dioxide (CO2) concentration near the Rubisco, the enzyme responsible for the first step of CBB cycle, drawing inspiration from various photosynthetic organisms, and conclude this section by examining ways to enhance CO2 delivery into leaves. Moving beyond individual processes, we discuss two approaches to identifying key targets for photosynthesis improvement: systems modeling and the study of natural variation. Finally, we revisit some of the strategies mentioned above to provide a holistic view of the improvements, analyzing their impact on nitrogen use efficiency and on canopy photosynthesis.

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.

The end game(s) of photosynthetic carbon metabolism

The year 2024 marks 70 years since the general outline of the carbon pathway in photosynthesis was published. Although several alternative pathways are now known, it is remarkable how many organisms use the reaction sequence described 70 yrs ago, which is now known as the Calvin-Benson cycle or variants such as the Calvin-Benson-Bassham cycle or Benson-Calvin cycle. However, once the carbon has entered the Calvin-Benson cycle and is converted to a 3-carbon sugar, it has many potential fates. This review will examine the last stages of photosynthetic metabolism in leaves. In land plants, this process mostly involves the production of sucrose provided by an endosymbiont (the chloroplast) to its host for use and transport to the rest of the plant. Photosynthetic metabolism also usually involves the synthesis of starch, which helps maintain respiration in the dark and enables the symbiont to supply sugars during both the day and night. Other end products made in the chloroplast are closely tied to photosynthetic CO2 assimilation. These include serine from photorespiration and various amino acids, fatty acids, isoprenoids, and shikimate pathway products. I also describe 2 pathways that can short circuit parts of the Calvin-Benson cycle. These final processes of photosynthetic metabolism play many important roles in plants.

Engineering Rubisco to enhance CO2 utilization

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a pivotal enzyme that mediates the fixation of CO2. As the most abundant protein on earth, Rubisco has a significant impact on global carbon, water, and nitrogen cycles. However, the significantly low carboxylation activity and competing oxygenase activity of Rubisco greatly impede high carbon fixation efficiency. This review first summarizes the current efforts in directly or indirectly modifying plant Rubisco, which has been challenging due to its high conservation and limitations in chloroplast transformation techniques. However, recent advancements in understanding Rubisco biogenesis with the assistance of chaperones have enabled successful heterologous expression of all Rubisco forms, including plant Rubisco, in microorganisms. This breakthrough facilitates the acquisition and evaluation of modified proteins, streamlining the measurement of their activity. Moreover, the establishment of a screening system in E. coli opens up possibilities for obtaining high-performance mutant enzymes through directed evolution. Finally, this review emphasizes the utilization of Rubisco in microorganisms, not only expanding their carbon-fixing capabilities but also holding significant potential for enhancing biotransformation processes.

Kinetic modeling identifies targets for engineering improved photosynthetic efficiency in potato (Solanum tuberosum cv. Solara)

Potato (Solanum tuberosum) is a significant non-grain food crop in terms of global production. However, its yield potential might be raised by identifying means to release bottlenecks within photosynthetic metabolism, from the capture of solar energy to the synthesis of carbohydrates. Recently, engineered increases in photosynthetic rates in other crops have been directly related to increased yield - how might such increases be achieved in potato? To answer this question, we derived the photosynthetic parameters Vcmax and Jmax to calibrate a kinetic model of leaf metabolism (e-Photosynthesis) for potato. This model was then used to simulate the impact of manipulating the expression of genes and their protein products on carbon assimilation rates in silico through optimizing resource investment among 23 photosynthetic enzymes, predicting increases in photosynthetic CO2 uptake of up to 67%. However, this number of manipulations would not be practical with current technologies. Given a limited practical number of manipulations, the optimization indicated that an increase in amounts of three enzymes - Rubisco, FBP aldolase, and SBPase - would increase net assimilation. Increasing these alone to the levels predicted necessary for optimization increased photosynthetic rate by 28% in potato.

Is chloroplast size optimal for photosynthetic efficiency?

Improving photosynthetic efficiency has recently emerged as a promising way to increase crop production in a sustainable manner. While chloroplast size may affect photosynthetic efficiency in several ways, we aimed to explore whether chloroplast size manipulation can be a viable approach to improving photosynthetic performance. Several tobacco (Nicotiana tabacum) lines with contrasting chloroplast sizes were generated via manipulation of chloroplast division genes to assess photosynthetic performance under steady-state and fluctuating light. A selection of lines was included in a field trial to explore productivity. Lines with enlarged chloroplasts underperformed in most of the measured traits. Lines with smaller and more numerous chloroplasts showed a similar efficiency compared with wild-type (WT) tobacco. Chloroplast size only weakly affected light absorptance and light profiles within the leaf. Increasing chloroplast size decreased mesophyll conductance (gm ) but decreased chloroplast size did not increase gm . Increasing chloroplast size reduced chloroplast movements and enhanced non-photochemical quenching. The chloroplast smaller than WT appeared to be no better than WT for photosynthetic efficiency and productivity under field conditions. The results indicate that chloroplast size manipulations are therefore unlikely to lead to higher photosynthetic efficiency or growth.

Phosphoribulokinase abundance is not limiting the Calvin-Benson-Bassham cycle in Chlamydomonas reinhardtii

Improving photosynthetic efficiency in plants and microalgae is of utmost importance to support the growing world population and to enable the bioproduction of energy and chemicals. Limitations in photosynthetic light conversion efficiency can be directly attributed to kinetic bottlenecks within the Calvin-Benson-Bassham cycle (CBBC) responsible for carbon fixation. A better understanding of these bottlenecks in vivo is crucial to overcome these limiting factors through bio-engineering. The present study is focused on the analysis of phosphoribulokinase (PRK) in the unicellular green alga Chlamydomonas reinhardtii. We have characterized a PRK knock-out mutant strain and showed that in the absence of PRK, Chlamydomonas cannot grow photoautotrophically while functional complementation with a synthetic construct allowed restoration of photoautotrophy. Nevertheless, using standard genetic elements, the expression of PRK was limited to 40% of the reference level in complemented strains and could not restore normal growth in photoautotrophic conditions suggesting that the CBBC is limited. We were subsequently able to overcome this initial limitation by improving the design of the transcriptional unit expressing PRK using diverse combinations of DNA parts including PRK endogenous promoter and introns. This enabled us to obtain strains with PRK levels comparable to the reference strain and even overexpressing strains. A collection of strains with PRK levels between 16% and 250% of WT PRK levels was generated and characterized. Immunoblot and growth assays revealed that a PRK content of ≈86% is sufficient to fully restore photoautotrophic growth. This result suggests that PRK is present in moderate excess in Chlamydomonas. Consistently, the overexpression of PRK did not increase photosynthetic growth indicating that that the endogenous level of PRK in Chlamydomonas is not limiting the Calvin-Benson-Bassham cycle under optimal conditions.

Research Progress in Improving Photosynthetic Efficiency

Photosynthesis is the largest mass- and energy-conversion process on Earth, and it is the material basis for almost all biological activities. The efficiency of converting absorbed light energy into energy substances during photosynthesis is very low compared to theoretical values. Based on the importance of photosynthesis, this article summarizes the latest progress in improving photosynthesis efficiency from various perspectives. The main way to improve photosynthetic efficiency is to optimize the light reactions, including increasing light absorption and conversion, accelerating the recovery of non-photochemical quenching, modifying enzymes in the Calvin cycle, introducing carbon concentration mechanisms into C₃ plants, rebuilding the photorespiration pathway, de novo synthesis, and changing stomatal conductance. These developments indicate that there is significant room for improvement in photosynthesis, providing support for improving crop yields and mitigating changes in climate conditions.

Seasonal climate conditions impact the effectiveness of improving photosynthesis to increase soybean yield

Context

Photosynthetic stimulations have shown promising outcomes in improving crop photosynthesis, including soybean. However, it is still unclear to what extent these changes can impact photosynthetic assimilation and yield under long-term field climate conditions.

Objective

In this paper, we present a systematic evaluation of the response of canopy photosynthesis and yield to two critical parameters in leaf photosynthesis: the maximum carboxylation rate of ribulose-1,5-bisphosphate carboxylase/oxygenase (Vcmax) and the maximum electron transport of the ribulose-1,5-bisphosphate regeneration rate (Jmax).

Methods

Using the field-scale crop model Soybean-BioCro and ten years of observed climate data in Urbana, Illinois, U.S., we conducted sensitivity experiments to estimate the changes in canopy photosynthesis, leaf area index, and biomass due to the changes in Vcmax and Jmax.

Results

The results show that 1) Both the canopy photosynthetic assimilation (An) and pod biomass yields were more sensitive to the changes in Jmax, particularly at high atmospheric carbon-dioxide concentrations ([CO2]); 2) Higher [CO2] undermined the effectiveness of increasing the two parameters to improve An and yield; 3) Under the same [CO2], canopy light interception and canopy respiration were key factors that undermined improvements in An and yield; 4) A canopy with smaller leaf area index tended to have a higher yield improvement, and 5) Increases in assimilations and yields were highly dependent on growing-season climatic conditions. The solar radiation, temperature, and relative humidity were the main climate drivers that impacted the yield improvement, and they had opposite correlations with improved yield during the vegetative phase compared to the reproductive phase.

Conclusions

In a world with elevated [CO2], genetic engineering crop photosynthesis should focus more on improving Jmax. Further, long-term climate conditions and seasonal variations must be considered to determine the improvements in soybean canopy photosynthesis and yield at the field scale.

Implications

Quantifying the effectiveness of changing Vcmax and Jmax helps understand their individual and combined contributions to potential improvements in assimilation and yield. This work provides a framework for evaluating how altering the photosynthetic rate parameters impacts soybean yield and assimilation under different seasonal climate scenarios at the field scale.

Two decades of fumigation data from the Soybean Free Air Concentration Enrichment facility

The Soybean Free Air Concentration Enrichment (SoyFACE) facility is the longest running open-air carbon dioxide and ozone enrichment facility in the world. For over two decades, soybean, maize, and other crops have been exposed to the elevated carbon dioxide and ozone concentrations anticipated for late this century. The facility, located in East Central Illinois, USA, exposes crops to different atmospheric concentrations in replicated octagonal ~280 m² Free Air Concentration Enrichment (FACE) treatment plots. Each FACE plot is paired with an untreated control (ambient) plot. The experiment provides important ground truth data for predicting future crop productivity. Fumigation data from SoyFACE were collected every four seconds throughout each growing season for over two decades. Here, we organize, quality control, and collate 20 years of data to facilitate trend analysis and crop modeling efforts. This paper provides the rationale for and a description of the SoyFACE experiments, along with a summary of the fumigation data and collation process, weather and ambient data collection procedures, and explanations of air pollution metrics and calculations.

Increased sedoheptulose-1,7-bisphosphatase content in Setaria viridis does not affect C4 photosynthesis

Sedoheptulose-1,7-bisphosphatase (SBPase) is one of the rate-limiting enzymes of the Calvin cycle, and increasing the abundance of SBPase in C3 plants provides higher photosynthetic rates and stimulates biomass and yield. C4 plants usually have higher photosynthetic rates because they operate a biochemical CO2-concentrating mechanism between mesophyll and bundle sheath cells. In the C4 system, SBPase and other enzymes of the Calvin cycle are localized to the bundle sheath cells. Here we tested what effect increasing abundance of SBPase would have on C4 photosynthesis. Using green foxtail millet (Setaria viridis), a model C4 plant of NADP-ME subtype, we created transgenic plants with 1.5 to 3.2 times higher SBPase content compared to wild-type plants. Transcripts of the transgene were found predominantly in the bundle sheaths suggesting the correct cellular localization of the protein. The abundance of ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit was not affected in transgenic plants overexpressing SBPase, and neither was leaf chlorophyll content or photosynthetic electron transport parameters. We found no association between SBPase content in S. viridis and saturating rates of CO2 assimilation. Moreover, a detailed analysis of CO2 assimilation rates at different CO2 partial pressures, irradiances, and leaf temperatures showed no improvement of photosynthesis in plants overexpressing SBPase. We discuss the potential implications of these results for understanding the role of SBPase in regulation of C4 photosynthesis.

Bacterial Form II Rubisco can support wild-type growth and productivity in Solanum tuberosum cv. Desiree (potato) under elevated CO2

The last decade has seen significant advances in the development of approaches for improving both the light harvesting and carbon fixation pathways of photosynthesis by nuclear transformation, many involving multigene synthetic biology approaches. As efforts to replicate these accomplishments from tobacco into crops gather momentum, similar diversification is needed in the range of transgenic options available, including capabilities to modify crop photosynthesis by chloroplast transformation. To address this need, here we describe the first transplastomic modification of photosynthesis in a crop by replacing the native Rubisco in potato with the faster, but lower CO2-affinity and poorer CO2/O2 specificity Rubisco from the bacterium Rhodospirillum rubrum. High level production of R. rubrum Rubisco in the potRr genotype (8 to 10 µmol catalytic sites m2) allowed it to attain wild-type levels of productivity, including tuber yield, in air containing 0.5% (v/v) CO2. Under controlled environment growth at 25°C and 350 µmol photons m2 PAR, the productivity and leaf biochemistry of wild-type potato at 0.06%, 0.5%, or 1.5% (v/v) CO2 and potRr at 0.5% or 1.5% (v/v) CO2 were largely indistinguishable. These findings suggest that increasing the scope for enhancing productivity gains in potato by improving photosynthate production will necessitate improvement to its sink-potential, consistent with current evidence productivity gains by eCO2 fertilization for this crop hit a ceiling around 560 to 600 ppm CO2.

Climate change challenges, plant science solutions

Climate change is a defining challenge of the 21st century, and this decade is a critical time for action to mitigate the worst effects on human populations and ecosystems. Plant science can play an important role in developing crops with enhanced resilience to harsh conditions (e.g. heat, drought, salt stress, flooding, disease outbreaks) and engineering efficient carbon-capturing and carbon-sequestering plants. Here, we present examples of research being conducted in these areas and discuss challenges and open questions as a call to action for the plant science community.

Understanding source-sink interactions: Progress in model plants and translational research to crops

Agriculture is facing a massive increase in demand per hectare as a result of an ever-expanding population and environmental deterioration. While we have learned much about how environmental conditions and diseases impact crop yield, until recently considerably less was known concerning endogenous factors, including within-plant nutrient allocation. In this review, we discuss studies of source-sink interactions covering both fundamental research in model systems under controlled growth conditions and how the findings are being translated to crop plants in the field. In this respect we detail efforts aimed at improving and/or combining C₃, C₄, and CAM modes of photosynthesis, altering the chloroplastic electron transport chain, modulating photorespiration, adopting bacterial/algal carbon-concentrating mechanisms, and enhancing nitrogen- and water-use efficiencies. Moreover, we discuss how modulating TCA cycle activities and primary metabolism can result in increased rates of photosynthesis and outline the opportunities that evaluating natural variation in photosynthesis may afford. Although source, transport, and sink functions are all covered in this review, we focus on discussing source functions because the majority of research has been conducted in this field. Nevertheless, considerable recent evidence, alongside the evidence from classical studies, demonstrates that both transport and sink functions are also incredibly important determinants of yield. We thus describe recent evidence supporting this notion and suggest that future strategies for yield improvement should focus on combining improvements in each of these steps to approach yield optimization.

Genetic manipulation of photosynthesis to enhance crop productivity under changing environmental conditions

Current global agricultural production needs to be increased to feed the unconstrained growing population. The changing climatic condition due to anthropogenic activities also makes the conditions more challenging to meet the required crop productivity in the future. The increase in crop productivity in the post green revolution era most likely became stagnant, or no major enhancement in crop productivity observed. In this review article, we discuss the emerging approaches for the enhancement of crop production along with dealing to the future climate changes like rise in temperature, increase in precipitation and decrease in snow and ice level, etc. At first, we discuss the efforts made for the genetic manipulation of chlorophyll metabolism, antenna engineering, electron transport chain, carbon fixation, and photorespiratory processes to enhance the photosynthesis of plants and to develop tolerance in plants to cope with changing environmental conditions. The application of CRISPR to enhance the crop productivity and develop abiotic stress-tolerant plants to face the current changing climatic conditions is also discussed.

Elevated inorganic carbon and salinity enhances photosynthesis and ATP synthesis in picoalga Picocystis salinarum as revealed by label free quantitative proteomics

Saline soda lakes are of immense ecological value as they niche some of the most exclusive haloalkaliphilic communities dominated by bacterial and archaeal domains, with few eukaryotic algal representatives. A handful reports describe Picocystis as a key primary producer with great production rates in extremely saline alkaline habitats. An extremely haloalkaliphilic picoalgal strain, Picocystis salinarum SLJS6 isolated from hypersaline soda lake Sambhar, Rajasthan, India, grew robustly in an enriched soda lake medium containing mainly Na₂CO₃, 50 g/l; NaHCO_3, 50 g/l, NaCl, 50 g/l (salinity ≈150‰) at pH 10. To elucidate the molecular basis of such adaptation to high inorganic carbon and NaCl concentrations, a high-throughput label-free quantitation based quantitative proteomics approach was applied. Out of the total 383 proteins identified in treated samples, 225 were differentially abundant proteins (DAPs), of which 150 were statistically significant (p < 0.05) including 70 upregulated and 64 downregulated proteins after 3 days of growth in highly saline-alkaline medium. Most DAPs were involved in photosynthesis, oxidative phosphorylation, glucose metabolism and ribosomal structural components envisaging that photosynthesis and ATP synthesis were central to the salinity-alkalinity response. Key components of photosynthetic machinery like photosystem reaction centres, adenosine triphosphate (ATP) synthase ATP, Rubisco, Fructose-1,6-bisphosphatase, Fructose-bisphosphate aldolase were highly upregulated. Enzymes peptidylprolyl isomerases (PPIase), important for correct protein folding showed remarkable marked-up regulation along with other chaperon proteins indicating their role in osmotic adaptation. Enhanced photosynthetic activity exhibited by P. salinarum in highly saline-alkaline condition is noteworthy as photosynthesis is suppressed under hyperosmotic conditions in most photosynthetic organisms. The study provided the first insights into the proteome of extremophilic alga P. salinarum exhibiting extraordinary osmotic adaptation and proliferation in polyextreme conditions prevailing in saline sodic ecosystems, potentially unraveling the basis of resilience in this not so known organism and paves the way for a promising future candidate for biotechnological applications and model organism for deciphering the molecular mechanisms of osmotic adaptation. The mass spectrometry proteomics data is available at the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD037170.

Multigene manipulation of photosynthetic carbon metabolism enhances the photosynthetic capacity and biomass yield of cucumber under low-CO2 environment

Solar greenhouses are important in the vegetable production and widely used for the counter-season production in the world. However, the CO₂ consumed by crops for photosynthesis after sunrise is not supplemented and becomes chronically deficient due to the airtight structure of solar greenhouses. Vegetable crops cannot effectively utilize light resources under low-CO₂ environment, and this incapability results in reduced photosynthetic efficiency and crop yield. We used cucumber as a model plant and generated several sets of transgenic cucumber plants overexpressing individual genes, including β-carbonic anhydrase 1 (CsβCA1), β-carbonic anhydrase 4 (CsβCA4), and sedoheptulose-1,7-bisphosphatase (CsSBP); fructose-1,6-bisphosphate aldolase (CsFBA), and CsβCA1 co-expressing plants; CsβCA4, CsSBP, and CsFBA co-expressing plants (14SF). The results showed that the overexpression of CsβCA1, CsβCA4, and 14SF exhibited higher photosynthetic and biomass yield in transgenic cucumber plants under low-CO₂ environment. Further enhancements in photosynthesis and biomass yield were observed in 14SF transgenic plants under low-CO₂ environment. The net photosynthesis biomass yield and photosynthetic rate increased by 49% and 79% compared with those of the WT. However, the transgenic cucumbers of overexpressing CsFBA and CsSBP showed insignificant differences in photosynthesis and biomass yield compared with the WT under low-CO₂.environment. Photosynthesis, fluorescence parameters, and enzymatic measurements indicated that CsβCA1, CsβCA4, CsSBP, and CsFBA had cumulative effects in photosynthetic carbon assimilation under low-CO₂ environment. Co-expression of this four genes (CsβCA1, CsβCA4, CsSBP, and CsFBA) can increase the carboxylation activity of RuBisCO and promote the regeneration of RuBP. As a result, the 14SF transgenic plants showed a higher net photosynthetic rate and biomass yield even under low-CO₂environment.These findings demonstrate the possibility of cultivating crops with high photosynthetic efficiency by manipulating genes involved in the photosynthetic carbon assimilation metabolic pathway.

Reduction in chloroplastic ribulose-5-phosphate-3-epimerase decreases photosynthetic capacity in Arabidopsis

Chloroplast ribulose-5-phosphate-3-epimerase (RPE) is a critical enzyme involved in the Calvin-Benson cycle and oxidative pentose phosphate pathways in higher plants. Three Arabidopsis rpe mutants with reduced level of RPE were identified through their high NPQ (nonphotochemical quenching) phenotype upon illumination, and no significant difference of plant size was found between these rpe mutants and WT (wild type) plants under growth chamber conditions. A decrease in RPE expression to a certain extent leads to a decrease in CO₂ fixation, V _cmax and J _max. Photosynthetic linear electron transport was partially inhibited and activity of ATP synthase was also decreased in the rpe mutants, but the levels of thylakoid protein complexes and other Calvin-Benson cycle enzymes in rpe mutants were not affected. These results demonstrate that some degree of reduction in RPE expression decreases carbon fixation in chloroplasts, which in turn feedback inhibits photosynthetic electron transport and ATP synthase activity due to the photosynthetic control. Taken together, this work provides evidence that RPE plays an important role in the Calvin-Benson cycle and influences the photosynthetic capacity of chloroplasts.

Climate change-associated multifactorial stress combination: A present challenge for our ecosystems

Humans negatively influence Earth ecosystems and biodiversity causing global warming, climate change as well as man-made pollution. Recently, the number of different stress factors have increased, and when impacting simultaneously, the multiple stress conditions cause dramatic declines in plant and ecosystem health. Although much is known about how plants and ecosystems are affected by each individual stress, recent research efforts have diverted into how these biological systems respond to several of these stress conditions applied together. Studies of such "multifactorial stress combination" concept have reported a severe decrease in plant survival and microbiome biodiversity along the increasing number of factors in a consistent directional trend. In addition, these results are in concert with studies about how ecosystems and microbiota are affected by natural conditions imposed by climate change. Therefore, all this evidence should serve as an important warning in order to decrease pollutants, create strategies to deal with global warming, and increase the tolerance of plants to multiple stressful factors in combination. Here we review recent studies focused on the impact of abiotic stresses on plants, agrosystems and different ecosystems including forests and microecosystems. In addition, different strategies to mitigate the impact of climate change in ecosystems are discussed.

Overexpression of DfRaf from Fragrant Woodfern (Dryopteris fragrans) Enhances High-Temperature Tolerance in Tobacco (Nicotiana tabacum)

Heat stress seriously affects medicinal herbs’ growth and yield. Rubisco accumulation factor (Raf) is a key mediator regulating the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which plays important roles in carbon assimilation and the Calvin cycle in plants. Raf has been studied in many plants, but has rarely been studied in the important medicinal plant fragrant woodfern (Dryopteris fragrans). The aim of this study was to analyze the effects of Raf on carbohydrate metabolism and the response to heat stress in medicinal plants. In this study, high temperature treatment upregulated the expression of DfRaf, which was significantly higher than that of phosphoribokinase (DfPRK), Rubisco small subunits (DfRbcS), Rubisco large subunits (DfRbcL) and Rubisco activase (DfRCA). The subcellular localization showed that the DfRaf proteins were primarily located in the nucleus; DfPRK, DfRbcS, DfRbcL and DfRCA proteins were primarily located in the chloroplast. We found that overexpression of DfRaf led to increased activity of Rubisco, RCA and PRK under high-temperature stress. The H₂O₂, O₂⁻ and MDA content of the DfRaf-OV-L2 and DfRaf-OV-L6 transgenic lines were significantly lower than those of WT and VC plants under high-temperature stress. The photosynthetic pigments, proline, soluble sugar content and ROS-scavenging ability of the DfRaf-OV-L2 and DfRaf-OV-L6 transgenic lines were higher than those of WT and VC plants under high-temperature stress. The results showed that overexpression of the DfRaf gene increased the Rubisco activity, which enhanced the high-temperature tolerance of plants.

A novel variant of the Calvin-Benson cycle bypassing fructose bisphosphate

The Calvin–Benson cycle (CB cycle) is quantitatively the most important metabolic pathway for CO₂ fixation. In the canonical CB cycle, fructose 6-phosphate (F6P), fructose 1,6-bisphosphate (FBP), sedoheptulose 7-phosphate (S7P), and sedoheptulose 1,7-bisphosphate (SBP) appear as essential intermediates, where F6P is formed from FBP by the fructose 1,6-bisphosphatase (FBPase) reaction, and S7P is formed from SBP by the sedoheptulose 1,7-bisphosphatase (SBPase) reaction. Although the involvement of SBP and SBPase in the canonical CB cycle is consistent with the reported dependency of photosynthetic carbon metabolism on SBPase, the involvement of FBP and FBPase is not completely consistent with the reported FBP- or FBPase-related findings such as, although with a diminished growth rate, an Arabidopsis mutant lacking FBPase grew photoautotrophically in soil. Here, we show a novel variant of the CB cycle involving SBP, SBPase, and transaldolase, but neither FBP nor FBPase. This novel variant, named the S7P-removing transaldolase variant, bypasses FBP. This variant explains the FBP- or FBPase-related findings more easily than the canonical CB cycle as well as the dependency of photosynthetic carbon metabolism on SBPase and further suggests that co-overexpression of SBPase and transaldolase can be a strategy for enhancing photosynthetic carbon metabolism, which is important for the global environment.

Regulation of Calvin-Benson cycle enzymes under high temperature stress

The Calvin-Benson cycle (CBC) consists of three critical processes, including fixation of CO₂ by Rubisco, reduction of 3-phosphoglycerate (3PGA) to triose phosphate (triose-P) with NADPH and ATP generated by the light reactions, and regeneration of ribulose 1,5-bisphosphate (RuBP) from triose-P. The activities of photosynthesis-related proteins, mainly from the CBC, were found more significantly affected and regulated in plants challenged with high temperature stress, including Rubisco, Rubisco activase (RCA) and the enzymes involved in RuBP regeneration, such as sedoheptulose-1,7-bisphosphatase (SBPase). Over the past years, the regulatory mechanism of CBC, especially for redox-regulation, has attracted major interest, because balancing flux at the various enzymatic reactions and maintaining metabolite levels in a range are of critical importance for the optimal operation of CBC under high temperature stress, providing insights into the genetic manipulation of photosynthesis. Here, we summarize recent progress regarding the identification of various layers of regulation point to the key enzymes of CBC for acclimation to environmental temperature changes along with open questions are also discussed.

Here comes the sun: How optimization of photosynthetic light reactions can boost crop yields

Photosynthesis started to evolve some 3.5 billion years ago CO₂ is the substrate for photosynthesis and in the past 200-250 years, atmospheric levels have approximately doubled due to human industrial activities. However, this time span is not sufficient for adaptation mechanisms of photosynthesis to be evolutionarily manifested. Steep increases in human population, shortage of arable land and food, and climate change call for actions, now. Thanks to substantial research efforts and advances in the last century, basic knowledge of photosynthetic and primary metabolic processes can now be translated into strategies to optimize photosynthesis to its full potential in order to improve crop yields and food supply for the future. Many different approaches have been proposed in recent years, some of which have already proven successful in different crop species. Here, we summarize recent advances on modifications of the complex network of photosynthetic light reactions. These are the starting point of all biomass production and supply the energy equivalents necessary for downstream processes as well as the oxygen we breathe.

Metabolomic analysis elucidates how shade conditions ameliorate the deleterious effects of greening (Huanglongbing) disease in citrus

Under tropical and subtropical environments, citrus leaves are exposed to excess sunlight, inducing photoinhibition. Huanglongbing (HLB, citrus greening), a devastating phloem-limited disease putatively caused by Candidatus Liberibacter asiaticus, exacerbates this challenge with additional photosynthetic loss and excessive starch accumulation. A combined metabolomics and physiological approach was used to elucidate whether shade alleviates the deleterious effects of HLB in field-grown citrus trees, and to understand the underlying metabolic mechanisms related to shade-induced morpho-physiological changes in citrus. Using metabolite profiling and multinomial logistic regression, we identified pivotal metabolites altered in response to shade. A core metabolic network associated with shade conditions was identified through pathway enrichment analysis and metabolite mapping. We measured physio-biochemical responses and growth and yield characteristics. With these, the relationships between metabolic network and the variables measured above were investigated. We found that moderate-shade alleviates sink limitation by preventing excessive starch accumulation and increasing foliar sucrose levels. Increased growth and fruit yield in shaded compared with non-shaded trees were associated with increased photosystem II efficiency and leaf carbon fixation pathway metabolites. Our study also shows that, in HLB-affected trees under shade, the signaling of plant hormones (auxins and cytokinins) and nitrogen supply were downregulated with reducing new shoot production likely due to diminished needs of cell damage repair and tissue regeneration under shade. Overall, our findings provide the first glimpse of the complex dynamics between cellular metabolites and leaf physiological functions in citrus HLB pathosystem under shade, and reveal the mechanistic basis of how shade ameliorates HLB disease.

Amelioration of plant responses to drought under elevated CO2 by rejuvenating photosynthesis and nitrogen use efficiency: implications for future climate-resilient crops

The contemporary global agriculture is beset with serious threats from diverse eco-environmental conditions causing decreases in crop yields by ~ 15%. These yield losses might increase further due to climate change scenarios leading to increased food prices triggering social unrest and famines. Urbanization and industrialization are often associated with rapid increases in greenhouse gases (GHGs) especially atmospheric CO₂ concentration [(CO₂)]. Increase in atmospheric [CO₂] significantly improved crop photosynthesis and productivity initially which vary with plant species, genotype, [CO₂] exposure time and biotic as well as abiotic stress factors. Numerous attempts have been made using different plant species to unravel the physiological, cellular and molecular effects of elevated [CO₂] as well as drought. This review focuses on plant responses to elevated [CO₂] and drought individually as well as in combination with special reference to physiology of photosynthesis including its acclimation. Furthermore, the functional role of nitrogen use efficiency (NUE) and its relation to photosynthetic acclimation and crop productivity under elevated [CO₂] and drought are reviewed. In addition, we also discussed different strategies to ameliorate the limitations of ribulose-1,5-bisphosphate (RuBP) carboxylation and RuBP regeneration. Further, improved stomatal and mesophyll conductance and NUE for enhanced crop productivity under fast changing global climate conditions through biotechnological approaches are also discussed here. We conclude that multiple gene editing approaches for key events in photosynthetic processes would serve as the best strategy to generate resilient crop plants with improved productivity under fast changing climate.

Activation of CDC48 and acetyltransferase encoding genes contributes to enhanced abiotic stress tolerance and improved productivity traits in rice

World-wide crop productivity is highly impacted by various extreme environmental conditions. In the present investigation, activation tagged (AT) line A10-Ds-RFP6 of rice endowed with improved agronomic attributes was tested for its tolerance ability against drought and salinity stress conditions as well as identification of genes associated with these traits. Under both drought and salinity stress conditions, A10-Ds-RFP6 line exhibited increased seed germination rates and improved plant growth characteristics at seedling, vegetative and reproductive stages as compared to wild-type (WT) plants. Moreover, A10-Ds-RFP6 revealed effective antioxidant systems resulting in decreased accumulation of reactive oxygen species and delayed stress symptoms compared to WT plants. Reduced accumulation of malondialdehyde with concomitant increase in proline and soluble sugars in A10-Ds-RFP6 line further endorse its improved stress tolerance levels. Furthermore, A10-Ds-RFP6 disclosed enhanced plant water content, photosynthetic efficiency, stomatal conductance, water use efficiency and maximum quantum yield compared to WT plants. TAIL and qRT-PCR analyses of AT rice line revealed the integration site of Ds element in the genome and increased expression levels of CDC48 and acetyltransferase genes involved in various aspects of plant development and stress tolerance. As such, the promising AT line plausibly serve as a rare genetic resource for fortifying stress tolerance and productivity traits of elite rice cultivars.

Designing the Crops for the Future; The CropBooster Program

Simple Summary

Our climate is changing and the world population is growing to an estimated 10 billion people by 2050. This may cause serious problems in global food supply, protection of the environment and safeguarding Earth’s biodiversity. To face these challenges, agriculture will have to adapt and a key element in this will be the development of “future-proof” crops. These crops will not only have to be high-yielding, but also should be able to withstand future climate conditions and will have to make very efficient use of scarce resources such as water, phosphorus and minerals. Future crops should not only sustainably give access to sufficient, nutritious, and diverse food to a worldwide growing population, but also support the circular bio-based economy and contribute to a lower atmospheric CO₂ concentration to counteract global warming. Future-proofing our crops is an urgent issue and a challenging goal that only can be realized by large-scale, international research cooperation. We call for international action and propose a pan-European research and innovation initiative, the CropBooster Program, to mobilize the European plant research community and all interested actors in agri-food research and innovation to face the challenge.

Abstract

The realization of the full objectives of international policies targeting global food security and climate change mitigation, including the United Nation’s Sustainable Development Goals, the Paris Climate Agreement COP21 and the European Green Deal, requires that we (i) sustainably increase the yield, nutritional quality and biodiversity of major crop species, (ii) select climate-ready crops that are adapted to future weather dynamic and (iii) increase the resource use efficiency of crops for sustainably preserving natural resources. Ultimately, the grand challenge to be met by agriculture is to sustainably provide access to sufficient, nutritious and diverse food to a worldwide growing population, and to support the circular bio-based economy. Future-proofing our crops is an urgent issue and a challenging goal, involving a diversity of crop species in differing agricultural regimes and under multiple environmental drivers, providing versatile crop-breeding solutions within wider socio-economic-ecological systems. This goal can only be realized by a large-scale, international research cooperation. We call for international action and propose a pan-European research initiative, the CropBooster Program, to mobilize the European plant research community and interconnect it with the interdisciplinary expertise necessary to face the challenge.

Improving crop yield and resilience through optimization of photosynthesis: panacea or pipe dream?

Increasing the speed of breeding to enhance crop productivity and adaptation to abiotic stresses is urgently needed. The perception that a second Green Revolution should be implemented is widely established within the scientific community and among stakeholders. In recent decades, different alternatives have been proposed for increasing crop yield through manipulation of leaf photosynthetic efficiency. However, none of these has delivered practical or relevant outputs. Indeed, the actual increases in photosynthetic rates are not expected to translate into yield increases beyond 10-15%. Furthermore, instantaneous rates of leaf photosynthesis are not necessarily the reference target for research. Yield is the result of canopy photosynthesis, understood as the contribution of laminar and non-laminar organs over time, within which concepts such as canopy architecture, stay-green, or non-laminar photosynthesis need to be taken into account. Moreover, retrospective studies show that photosynthetic improvements have been more common at the canopy level. Nevertheless, it is crucial to place canopy photosynthesis in the context of whole-plant functioning, which includes sink-source balance and transport of photoassimilates, and the availability and uptake of nutrients, such as nitrogen in particular. Overcoming this challenge will only be feasible if a multiscale crop focus combined with a multidisciplinary scientific approach is adopted.

Overproduction of Chloroplast Glyceraldehyde-3-Phosphate Dehydrogenase Improves Photosynthesis Slightly under Elevated [CO2] Conditions in Rice

Chloroplast glyceraldehyde-3-phosphate dehydrogenase (GAPDH) limits the regeneration of ribulose 1,5-bisphosphate (RuBP) in the Calvin-Benson cycle. However, it does not always limit the rate of CO2 assimilation. In the present study, the effects of overproduction of GAPDH on the rate of CO2 assimilation under elevated [CO2] conditions, where the capacity for RuBP regeneration limits photosynthesis, were examined in transgenic rice (Oryza sativa). GAPDH activity was increased to 3.2- and 4.5-fold of the wild-type levels by co-overexpression of the GAPDH genes, GAPA and GAPB, respectively. In the transgenic rice plants, the rate of CO2 assimilation under elevated [CO2] conditions increased by approximately 10%, whereas that under normal and low [CO2] conditions was not affected. These results indicate that overproduction of GAPDH is effective in improving photosynthesis under elevated [CO2] conditions, although its magnitude is relatively small. By contrast, biomass production of the transgenic rice plants was not greater than that of wild-type plants under elevated [CO2] conditions, although starch content tended to increase marginally.

Knocking out NEGATIVE REGULATOR OF PHOTOSYNTHESIS 1 increases rice leaf photosynthesis and biomass production in the field

Improving photosynthesis is a major approach to increasing crop yield potential. Here we identify a transcription factor as a negative regulator of photosynthesis, which can be manipulated to increase rice photosynthesis and plant biomass in the field. This transcription factor, named negative regulator of photosynthesis 1 (NRP1; Os07g0471900), was identified through a co-expression analysis using rice leaf RNA sequencing data. NRP1 expression showed significantly negative correlation with the expression of many genes involved in photosynthesis. Knocking out NRP1 led to greater photosynthesis and increased biomass in the field, while overexpression of NRP1 decreased photosynthesis and biomass. Transcriptomic data analysis shows that NRP1 can negatively regulate the expression of photosynthetic genes. Protein transactivation experiments show that NRP1 is a transcription activator, implying that NRP1 may indirectly regulate photosynthetic gene expression through an unknown regulator. This study shows that combination of bioinformatics analysis with transgenic testing can be used to identify new regulators to improve photosynthetic efficiency in crops.

Perspectives on improving light distribution and light use efficiency in crop canopies

Plant stands in nature differ markedly from most seen in modern agriculture. In a dense mixed stand, plants must vie for resources, including light, for greater survival and fitness. Competitive advantages over surrounding plants improve fitness of the individual, thus maintaining the competitive traits in the gene pool. In contrast, monoculture crop production strives to increase output at the stand level and thus benefits from cooperation to increase yield of the community. In choosing plants with higher yields to propagate and grow for food, humans may have inadvertently selected the best competitors rather than the best cooperators. Here, we discuss how this selection for competitiveness has led to overinvestment in characteristics that increase light interception and, consequently, sub-optimal light use efficiency in crop fields that constrains yield improvement. Decades of crop canopy modeling research have provided potential strategies for improving light distribution in crop canopies, and we review the current progress of these strategies, including balancing light distribution through reducing pigment concentration. Based on recent research revealing red-shifted photosynthetic pigments in algae and photosynthetic bacteria, we also discuss potential strategies for optimizing light interception and use through introducing alternative pigment types in crops. These strategies for improving light distribution and expanding the wavelengths of light beyond those traditionally defined for photosynthesis in plant canopies may have large implications for improving crop yield and closing the yield gap.

Phosphoglucoisomerase Is an Important Regulatory Enzyme in Partitioning Carbon out of the Calvin-Benson Cycle

Phosphoglucoisomerase (PGI) isomerizes fructose 6-phosphate (F6P) and glucose 6-phosphate (G6P) in starch and sucrose biosynthesis. Both plastidic and cytosolic isoforms are found in plant leaves. Using recombinant enzymes and isolated chloroplasts, we have characterized the plastidic and cytosolic isoforms of PGI. We have found that the Arabidopsis plastidic PGI K m for G6P is three-fold greater compared to that for F6P and that erythrose 4-phosphate is a key regulator of PGI activity. Additionally, the K m of spinach plastidic PGI can be dynamically regulated in the dark compared to the light and increases by 200% in the dark. We also found that targeting Arabidopsis cytosolic PGI into plastids of Nicotiana tabacum disrupts starch accumulation and degradation. Our results, in combination with the observation that plastidic PGI is not in equilibrium, indicates that PGI is an important regulatory enzyme that restricts flow and acts as a one-way valve preventing backflow of G6P into the Calvin-Benson cycle. We propose the PGI may be manipulated to improve flow of carbon to desired targets of biotechnology.

Role of sedoheptulose-1,7 bisphosphatase in low light tolerance of rice (Oryza sativa L.)

Rice grain yield is drastically reduced under low light especially in kharif (wet) season due to cloudy weather during most part of crop growth. Therefore, 50-60% of yield penalty was observed. To overcome this problem, identification of low light tolerant rice genotypes with a high buffering capacity trait such as photosynthetic rate has to be developed. Sedoheptulose-1,7 bisphosphatase, a light-regulated enzyme, plays pivotal role in the Calvin cycle by regenerating the substrate (RuBP) for RuBisCo and therefore, indirectly regulates the influx of CO₂ for this crucial process. We found a potential role of SBPase expression and activity in low light tolerant and susceptible rice genotypes by analyzing its influence on net photosynthetic rate and biomass. We observed a significant relationship of yield with photosynthesis, SBPase expression and activity especially under low light conditions. Two tolerant and two susceptible rice genotypes were used for the present study. Tolerant genotypes exhibited significant but least reduction compared to susceptible genotypes in the expression and activity of SBPase, which was also manifested in its photosynthetic rate and finally in the grain yield under low light. However, susceptible genotypes showed significant reduction in SBPase activity along with photosynthesis and grain yield suggesting that tracking the expression and activity of SBPase could form a simple and reliable method to identify the low light tolerant rice cultivars. The data were analyzed using the Indostat 7.5, Tukey-Kramer method through Microsoft Excel 2019 and PAST4.0 software. The significant association of SBPase activity with the grain yield, net assimilation rate, electron transfer rate, biomass and grain weight were observed under low light stress. These traits should be considered while selecting and breeding for low light tolerant cultivars. Thus, SBPase plays a major role in the low light tolerance mechanism in rice.

Retrospective analysis of biochemical limitations to photosynthesis in 49 species: C4 crops appear still adapted to pre-industrial atmospheric [CO2 ]

Leaf CO₂ uptake (A) in C₄ photosynthesis is limited by the maximum apparent rate of PEPc carboxylation (V_pmax ) at low intercellular [CO₂ ] (c_i ) with a sharp transition to a c_i -saturated rate (V_max ) due to co-limitation by ribulose-1:5-bisphosphate carboxylase/oxygenase (Rubisco) and regeneration of PEP. The response of A to c_i has been widely used to determine these two parameters. V_max and V_pmax depend on different enzymes but draw on a shared pool of leaf resources, such that resource distribution is optimized, and A maximized, when V_max and V_pmax are co-limiting. We collected published A/c_i curves in 49 C₄ species and assessed variation in photosynthetic traits between phylogenetic groups, and as a function of atmospheric [CO₂ ]. The balance of V_max -V_pmax varied among evolutionary lineages and C₄ subtypes. Operating A was strongly V_max -limited, such that re-allocation of resources from V_pmax towards V_max was predicted to improve A by 12% in C₄ crops. This would not require additional inputs but rather altered partitioning of existing leaf nutrients, resulting in increased water and nutrient-use efficiency. Optimal partitioning was achieved only in plants grown at pre-industrial atmospheric [CO₂ ], suggesting C₄ crops have not adjusted to the rapid increase in atmospheric [CO₂ ] of the past few decades.

Multi-gene metabolic engineering of tomato plants results in increased fruit yield up to 23%

The capacity to assimilate carbon and nitrogen, to transport the resultant sugars and amino acids to sink tissues, and to convert the incoming sugars and amino acids into storage compounds in the sink tissues, are key determinants of crop yield. Given that all of these processes have the potential to co-limit growth, multiple genetic interventions in source and sink tissues, plus transport processes may be necessary to reach the full yield potential of a crop. We used biolistic combinatorial co-transformation (up to 20 transgenes) for increasing C and N flows with the purpose of increasing tomato fruit yield. We observed an increased fruit yield of up to 23%. To better explore the reconfiguration of metabolic networks in these transformants, we generated a dataset encompassing physiological parameters, gene expression and metabolite profiling on plants grown under glasshouse or polytunnel conditions. A Sparse Partial Least Squares regression model was able to explain the combination of genes that contributed to increased fruit yield. This combinatorial study of multiple transgenes targeting primary metabolism thus offers opportunities to probe the genetic basis of metabolic and phenotypic variation, providing insight into the difficulties in choosing the correct combination of targets for engineering increased fruit yield.

Photosynthesis: basics, history and modelling

BACKGROUND

With limited agricultural land and increasing human population, it is essential to enhance overall photosynthesis and thus productivity. Oxygenic photosynthesis begins with light absorption, followed by excitation energy transfer to the reaction centres, primary photochemistry, electron and proton transport, NADPH and ATP synthesis, and then CO2 fixation (Calvin-Benson cycle, as well as Hatch-Slack cycle). Here we cover some of the discoveries related to this process, such as the existence of two light reactions and two photosystems connected by an electron transport 'chain' (the Z-scheme), chemiosmotic hypothesis for ATP synthesis, water oxidation clock for oxygen evolution, steps for carbon fixation, and finally the diverse mechanisms of regulatory processes, such as 'state transitions' and 'non-photochemical quenching' of the excited state of chlorophyll a.

SCOPE

In this review, we emphasize that mathematical modelling is a highly valuable tool in understanding and making predictions regarding photosynthesis. Different mathematical models have been used to examine current theories on diverse photosynthetic processes; these have been validated through simulation(s) of available experimental data, such as chlorophyll a fluorescence induction, measured with fluorometers using continuous (or modulated) exciting light, and absorbance changes at 820 nm (ΔA820) related to redox changes in P700, the reaction centre of photosystem I.

CONCLUSIONS

We highlight here the important role of modelling in deciphering and untangling complex photosynthesis processes taking place simultaneously, as well as in predicting possible ways to obtain higher biomass and productivity in plants, algae and cyanobacteria.

Stimulating photosynthetic processes increases productivity and water-use efficiency in the field

Previous studies have demonstrated that the independent stimulation of either electron transport or RuBP regeneration can increase the rate of photosynthetic carbon assimilation and plant biomass. In this paper, we present evidence that a multigene approach to simultaneously manipulate these two processes provides a further stimulation of photosynthesis. We report on the introduction of the cyanobacterial bifunctional enzyme fructose-1,6-bisphosphatase/sedoheptulose-1,7-bisphosphatase or the overexpression of the plant enzyme sedoheptulose-1,7-bisphosphatase, together with the expression of the red algal protein cytochrome c6, and show that a further increase in biomass accumulation under both glasshouse and field conditions can be achieved. Furthermore, we provide evidence that the stimulation of both electron transport and RuBP regeneration can lead to enhanced intrinsic water-use efficiency under field conditions.

A single nucleotide substitution at the 3'-end of SBPase gene involved in Calvin cycle severely affects plant growth and grain yield in rice

BACKGROUND

Calvin cycle plays a crucial role in carbon fixation which provides the precursors of organic macromolecules for plant growth and development. Currently, no gene involved in Calvin cycle has been identified in monocotyledonous plants through mutant or/and map-based cloning approach.

RESULTS

Here, we isolated a low-tillering mutant, c6635, in rice (Oryza sativa). The mutant displayed light green leaves and intensely declined pigment contents and photosynthetic capacity at early growth stage. Moreover, its individual plant showed a much smaller size, and most individuals produced only two tillers. At mature stage, its productive panicles, grain number and seed setting rate were significantly decreased, which lead to a sharp reduction of the grain yield. We confirmed that a single nucleotide mutation in LOC_Os04g16680 gene encoding sedoheptulose 1,7-bisphosphatase (SBPase) involved in Calvin cycle was responsible for the mutant phenotype of c6635 through map-based cloning, MutMap analysis and complementation experiments. Sequence analysis suggested that the point mutation caused an amino acid change from Gly-364 to Asp at the C-terminal of SBPase. In addition, OsSBPase gene was mainly expressed in leaf, and the encoded protein was located in chloroplast. The mutation of OsSBPase could significantly affect expression levels of some key genes involved in Calvin cycle.

CONCLUSIONS

We successfully identified a SBPase gene in monocotyledonous plants. Meanwhile, we demonstrated that a single nucleotide substitution at the 3'-end of this gene severely affects plant growth and grain yield, implying that the Gly-364 at the C-terminal of SBPase could play an important role in SBPase function in rice.

Photosynthesis in a Changing Global Climate: Scaling Up and Scaling Down in Crops

Photosynthesis is the major process leading to primary production in the Biosphere. There is a total of 7000bn tons of CO2 in the atmosphere and photosynthesis fixes more than 100bn tons annually. The CO2 assimilated by the photosynthetic apparatus is the basis of crop production and, therefore, of animal and human food. This has led to a renewed interest in photosynthesis as a target to increase plant production and there is now increasing evidence showing that the strategy of improving photosynthetic traits can increase plant yield. However, photosynthesis and the photosynthetic apparatus are both conditioned by environmental variables such as water availability, temperature, [CO2], salinity, and ozone. The "omics" revolution has allowed a better understanding of the genetic mechanisms regulating stress responses including the identification of genes and proteins involved in the regulation, acclimation, and adaptation of processes that impact photosynthesis. The development of novel non-destructive high-throughput phenotyping techniques has been important to monitor crop photosynthetic responses to changing environmental conditions. This wealth of data is being incorporated into new modeling algorithms to predict plant growth and development under specific environmental constraints. This review gives a multi-perspective description of the impact of changing environmental conditions on photosynthetic performance and consequently plant growth by briefly highlighting how major technological advances including omics, high-throughput photosynthetic measurements, metabolic engineering, and whole plant photosynthetic modeling have helped to improve our understanding of how the photosynthetic machinery can be modified by different abiotic stresses and thus impact crop production.

Proteomic changes may lead to yield alteration in maize under carbon dioxide enriched condition

In the present study, the effect of elevated CO2 on growth, physiology, yield and proteome was studied on two maize (Zea mays L.) varieties grown under Free-air CO2 enrichment. Growth in high CO2 (530 ppm) did not affect either photosynthesis or pigment contents in both varieties. Reduced MDA content, antioxidant and antioxidative enzymes levels were observed in both varieties in response to high CO2. PEHM-5 accumulated more biomass than SMH-3031 under eCO2. PEHM-5 also had more seed starch and total soluble sugar than SMH-3031. However, SMH-3031 had increased number of seed per cob than PEHM-5. Interestingly, thousand seed weight was significantly increased in PEHM-5 only, while it was decreased in SMH-3031 under eCO2. We observed increased seed size in PEHM-5, while the size of the SMH-3031 seeds remained unaltered. Leaf proteomics revealed more abundance of proteins related to Calvin cycle, protein synthesis assembly and degradation, defense and redox homeostasis in PEHM-5 that contributed to better growth and yield in elevated CO2. While in SMH-3031 leaf, proteins related to Calvin cycle, defense and redox homeostasis were less abundant in elevated CO2 resulting in average growth and yield. The results showed a differential response of two maize varieties to eCO2.

Linking fundamental science to crop improvement through understanding source and sink traits and their integration for yield enhancement

Understanding processes in sources and sinks that contribute to crop yields has taken years of painstaking research. For crop yield improvement, processes need to be understood as standalone mechanisms in addition to how these mechanisms perform at the crop level; currently there is often a chasm between the two. Fundamental mechanisms need to be considered in the context of crop ideotypes and the agricultural environment which is often more water limited than carbon limited. Different approaches for improvement should be considered, namely is there genetic variation? Or if not, could genetic modification, genome editing, or alternative approaches be utilized? Currently, there are few examples where genetic modification has improved intrinsic yield in the field for commercial application in a major crop. Genome editing, particularly of negative yield regulators as a first step, is providing new opportunities. Here we highlight key mechanisms in source and sink, arguing that for large yield increases integration of key processes is likely to produce the biggest successes within the framework of crop ideotypes with optimized phenology. We highlight a plethora of recent papers that show breakthroughs in fundamental science and the promise of the trehalose 6-phosphate signalling pathway, which regulates carbohydrate allocation which is key for many crop traits.

Mutation of SlSBPASE Aggravates Chilling-Induced Oxidative Stress by Impairing Glutathione Biosynthesis and Suppressing Ascorbate-Glutathione Recycling in Tomato Plants

Sedoheptulose-1,7-bisphosphatase (SBPase) is a crucial enzyme for photosynthetic carbon assimilation in the Calvin-Benson cycle. Previous studies have shown that overexpression of SBPase is advantageous to chilling tolerance in plants; however, the mechanisms of SBPase acting in the improvement of chilling tolerance remain largely unknown. In the present study, we aimed to uncover the essential role of SBPase in the response of tomato plants to oxidative stress induced by low temperature. To fulfill that, we performed an array of comparative studies between slsbpase mutant plants that we previously generated using CRISPR/Cas9 genome editing system and their wild-type counterparts under chilling stress. It was observed that following a 24 h chilling treatment, slsbpase mutant plants accumulated higher levels of reactive oxygen species (ROS) than wild-type plants and consequently, more severe lipid peroxidation occurred in slsbpase plants. Activity assay of antioxidant enzymes showed that mutation in SlSBPASE significantly decreased activities of peroxidase (POD) and ascorbate peroxidase (APX), but surprisingly did not significantly alter activities of superoxide dismutase (SOD) and catalase (CAT) under the chilling condition. Notably, mutation in SlSBPASE reduced the contents of total ascorbate (AsA) and total glutathione (GSH) and suppressed the recycling of AsA and GSH in chilling-stressed tomato plants. In addition, activities of two GSH biosynthetic enzymes (gamma-glutamylcysteine synthetase and glutathione synthetase) and transcript abundance of their coding genes (GSH1 and GSH2) were markedly reduced in slsbpase mutant plants in comparison with those in wild-type plants under chilling stress. Furthermore, exogenous GSH remarkably mitigated chilling damage in slsbpase plants. Collectively, these results support that mutation in SlSBPASE aggravates chilling-induced oxidative stress by suppressing GSH biosynthesis and AsA-GSH recycling and suggest that SBPase is required for optimal response to chilling stress in tomato plants. The findings also shed light on the idea to mitigate chilling-induced damages by genetically manipulating a photosynthetic enzyme in plants.

Photosynthetic efficiency and mesophyll conductance are unaffected in Arabidopsis thaliana aquaporin knock-out lines

Improving photosynthetic efficiency is widely regarded as a major route to achieving much-needed yield gains in crop plants. In plants with C3 photosynthesis, increasing the diffusion conductance for CO2 transfer from substomatal cavity to chloroplast stroma (gm) could help to improve the efficiencies of CO2 assimilation and photosynthetic water use in parallel. The diffusion pathway from substomatal cavity to chloroplast traverses cell wall, plasma membrane, cytosol, chloroplast envelope membranes, and chloroplast stroma. Specific membrane intrinsic proteins of the aquaporin family can facilitate CO2 diffusion across membranes. Some of these aquaporins, such as PIP1;2 in Arabidopsis thaliana, have been suggested to exert control over gm and the magnitude of the CO2 assimilation flux, but the evidence for a direct physiological role of aquaporins in determining gm is limited. Here, we estimated gm with four different methods under a range of light intensities and CO2 concentrations in two previously characterized pip1;2 knock-out lines as well as pip1;3 and pip2;6 knock-out lines, which have not been previously evaluated for a role in gm. This study presents the most in-depth analysis of gm in Arabidopsis aquaporin knock-out mutants to date. Surprisingly, all methods failed to show any significant differences between the pip1;2, pip1;3, or pip2;6 mutants and the Col-0 control.

Genetic Engineering for Global Food Security: Photosynthesis and Biofortification

Increasing demands for food and resources are challenging existing markets, driving a need to continually investigate and establish crop varieties with improved yields and health benefits. By the later part of the century, current estimates indicate that a >50% increase in the yield of most of the important food crops including wheat, rice and barley will be needed to maintain food supplies and improve nutritional quality to tackle what has become known as 'hidden hunger'. Improving the nutritional quality of crops has become a target for providing the micronutrients required in remote communities where dietary variation is often limited. A number of methods to achieve this have been investigated over recent years, from improving photosynthesis through genetic engineering, to breeding new higher yielding varieties. Recent research has shown that growing plants under elevated [CO2] can lead to an increase in Vitamin C due to changes in gene expression, demonstrating one potential route for plant biofortification. In this review, we discuss the current research being undertaken to improve photosynthesis and biofortify key crops to secure future food supplies and the potential links between improved photosynthesis and nutritional quality.

Biotechnological strategies for improved photosynthesis in a future of elevated atmospheric CO2

The improvement of photosynthesis using biotechnological approaches has been the focus of much research. It is now vital that these strategies be assessed under future atmospheric conditions. The demand for crop products is expanding at an alarming rate due to population growth, enhanced affluence, increased per capita calorie consumption, and an escalating need for plant-based bioproducts. While solving this issue will undoubtedly involve a multifaceted approach, improving crop productivity will almost certainly provide one piece of the puzzle. The improvement of photosynthetic efficiency has been a long-standing goal of plant biotechnologists as possibly one of the last remaining means of achieving higher yielding crops. However, the vast majority of these studies have not taken into consideration possible outcomes when these plants are grown long-term under the elevated CO₂ concentrations (e[CO₂]) that will be evident in the not too distant future. Due to the considerable effect that CO₂ levels have on the photosynthetic process, these assessments should become commonplace as a means of ensuring that research in this field focuses on the most effective approaches for our future climate scenarios. In this review, we discuss the main biotechnological research strategies that are currently underway with the aim of improving photosynthetic efficiency and biomass production/yields in the context of a future of e[CO₂], as well as alternative approaches that may provide further photosynthetic benefits under these conditions.

Carbon assimilation in crops at high temperatures

Global temperatures are rising, and higher rates of temperature increase are projected over land areas that encompass the globe's major agricultural regions. In addition to increased growing season temperatures, heat waves are predicted to become more common and severe. High temperatures can inhibit photosynthetic carbon gain of crop plants and thus threaten productivity, the effects of which may interact with other aspects of climate change. Here, we review the current literature assessing temperature effects on photosynthesis in key crops with special attention to field studies using crop canopy heating technology and in combination with other climate variables. We also discuss the biochemical reactions related to carbon fixation that may limit crop photosynthesis under warming temperatures and the current strategies for adaptation. Important progress has been made on several adaptation strategies demonstrating proof-of-concept for translating improved photosynthesis into higher yields. These are now poised to test in important food crops.