Ensuring an adequate food supply amidst a growing global population and climate change challenges necessitates innovative strategies to enhance crop productivity. Previous studies have demonstrated that the simultaneous stimulation of different photosynthesis-related processes can increase the rate of photosynthetic carbon assimilation and plant biomass. This study evaluates an approach based on modelling aimed at simultaneously increasing photosynthetic and sink capacities in Nicotiana tabacum by overexpressing three key enzymes: sedoheptulose-1,7-bisphosphatase (SBPase), fructose-1,6-bisphosphate aldolase (FBP aldolase), and ADP-glucose pyrophosphorylase (AGPase). Our results showed that this strategy does not significantly improve growth or carbon assimilation in Nicotiana tabacum under the tested conditions. This suggests that while the model informing our work offers a valuable framework, its application may require adjustments based on species and environmental conditions. Future research should explore these genetic modifications in species with larger sink capacities and under a range of growth conditions to fully realize the potential of photosynthetic optimization.

Photosynthesis underpins life on Earth, serving as the primary energy source while regulating global carbon and water cycles, thereby shaping climate and vegetation. Advancing photosynthesis research is essential for improving crop productivity and refining photosynthesis models across scales, ultimately addressing critical global challenges such as food security and environmental sustainability. This minireview synthesizes a selection of recent advancements presented at the 2nd European Congress of Photosynthesis Research, focusing on improving photosynthesis efficiency and modelling across the scales. We explore strategies to optimize light harvesting and carbon fixation, leading to canopy level improvements. Alongside synthetic biology, we examine recent advances in harnessing natural variability in key photosynthetic traits, considering both methodological innovations and the vast reservoir of opportunities they present. Additionally, we highlight unique insights gained from plants adapted to extreme environments, offering pathways to improve photosynthetic efficiency and resilience simultaneously. We emphasize the importance of a holistic approach, integrating dynamic modeling of metabolic processes to bridge these advancements. Beyond photosynthesis improvements, we discuss the progress of improving photosynthesis simulations, particularly through improved parametrization of mesophyll conductance, crucial for enhancing leaf-to-global scale simulations. Recognizing the need for greater interdisciplinary collaboration to tackle the grand challenges put on photosynthesis research, we highlight two initiatives launched at the congress-an open science platform and a dedicated journal for plant ecophysiology. We conclude this minireview with a forward-looking outline, highlighting key next steps toward achieving meaningful improvements in photosynthesis, yield, resilience and modeling.

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.

Several long-term studies have provided strong support demonstrating that growing crops under elevated [CO2] can increase photosynthesis and result in an increase in yield, flavour and nutritional content (including but not limited to Vitamins C, E and pro-vitamin A). In the case of tomato, increases in yield by as much as 80% are observed when plants are cultivated at 1000 ppm [CO2], which is consistent with current commercial greenhouse production methods in the tomato fruit industry. These results provide a clear demonstration of the potential for elevating [CO2] for improving yield and quality in greenhouse crops. The major focus of this review is to bring together 50 years of observations evaluating the impact of elevated [CO2] on fruit yield and fruit nutritional quality. In the final section, we consider the need to engineer improvements to photosynthesis and nitrogen assimilation to allow plants to take greater advantage of elevated CO2 growth conditions.

In plants, glyceraldehyde-3-phosphate dehydrogenase (GAPDH; EC 1.2.1.12) reversibly converts 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate coupled with the reduction of NADPH to NADP(+). The GAPDH enzyme that functions in the Calvin-Benson cycle is assembled either from 4 glyceraldehyde-3-phosphate dehydrogenase A (GAPA) subunit proteins forming a homotetramer (A(4)) or from 2 GAPA and 2 glyceraldehyde-3-phosphate dehydrogenase B (GAPB) subunit proteins forming a heterotetramer (A(2)B(2)). The relative importance of these 2 forms of GAPDH in determining the rate of photosynthesis is unknown. To address this question, we measured the photosynthetic rates of Arabidopsis (Arabidopsis thaliana) plants containing reduced amounts of the GAPDH A and B subunits individually and jointly, using T-DNA insertion lines of GAPA and GAPB and transgenic GAPA and GAPB plants with reduced levels of these proteins. Here, we show that decreasing the levels of either the A or B subunits decreased the maximum efficiency of CO2 fixation, plant growth, and final biomass. Finally, these data showed that the reduction in GAPA protein to 9% wild-type levels resulted in a 73% decrease in carbon assimilation rates. In contrast, eliminating GAPB protein resulted in a 40% reduction in assimilation rates. This work demonstrates that the GAPA homotetramer can compensate for the loss of GAPB, whereas GAPB alone cannot compensate fully for the loss of the GAPA subunit.

The Calvin-Benson-Bassham (CBB) cycle is the ancestral CO2 assimilation pathway and is found in all photosynthetic organisms. Biochemical extensions to the CBB cycle have evolved that allow the resulting pathways to act as CO2 concentrating mechanisms, either spatially in the case of C4 photosynthesis or temporally in the case of Crassulacean acid metabolism (CAM). While the biochemical steps in the C4 and CAM pathways are known, questions remain on their integration and regulation with CBB cycle activity. The application of omic and transgenic technologies is providing a more complete understanding of the biochemistry of C4 and CAM species and will also provide insight into the CBB cycle in these plants. As the global population increases, new solutions are required to increase crop yields and meet demands for food and other bioproducts. Previous work in C3 species has shown that increasing carbon assimilation through genetic manipulation of the CBB cycle can increase biomass and yield. There may also be options to improve photosynthesis in species using C4 photosynthesis and CAM through manipulation of the CBB cycle in these plants. This is an underexplored strategy and requires more basic knowledge of CBB cycle operation in these species to enable approaches for increased productivity.

The C3 cycle is the primary biochemical pathway for fixation of atmospheric CO₂ in over 85% of plants, including the major crop species. This pathway is autocatalytic involving eleven enzymes catalysing 13 reactions and provides carbon compounds directly for the synthesis of isoprenoids, sucrose, starch, phenylpropanoids, thiamine and nucleotides. One focus of research on the C3 cycle has been to identify bottlenecks to provide strategies for improving crop yield. There have been one or two notable successes from this work, mainly in model species and in controlled environment conditions. Given that the C₃ cycle is highly conserved it is reasonable to predict that lessons learned from model plants will be transferable to crops. We provide an update on the current understanding of the biochemistry, regulation and limitations of the C3 cycle, and future questions to be addressed to enable improvement of the cycle to boost crop yields.

Sedoheptulose-1,7-bisphosphatase (SBPase) is one of the rate-limiting enzymes of the Calvin cycle, and, in C3 plants, increasing the abundance of SBPase is known to provide higher photosynthetic rates and stimulate 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 Calvin cycle are localised to the bundle sheath cells. Here we tested what effect increasing abundance of SBPase would have on C4 photosynthesis. Using 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 localisation of the protein. Abundance of RBCL, the large subunit of Rubisco, was not affected in transgenic plants overexpressing SBPase, and neither was relative chlorophyll content or photosynthetic electron transport parameters. We found no correlation between SBPase content in S. viridis and saturating rates of CO2 assimilation. Moreover, detailed analysis of CO2 assimilation rates at different CO2 partial pressure, irradiance and leaf temperature, showed no improvement of photosynthesis in plants overexpressing SBPase. We discuss potential implications of these results for understanding the regulation of C4 photosynthesis.

Photosynthesis is fundamental for plant growth and yield. The Cytochrome b 6 f complex catalyses a rate-limiting step in thylakoid electron transport and therefore represents an important point of regulation of photosynthesis. Here we show that overexpression of a single core subunit of Cytochrome b 6 f , the Rieske FeS protein, led to up to a 40% increase in the abundance of the complex in Nicotiana tabacum (tobacco) and was accompanied by an enhanced in vitro Cytochrome f activity, indicating a full functionality of the complex. Analysis of transgenic plants overexpressing Rieske FeS by the light-induced fluorescence transients technique revealed a more oxidised primary quinone acceptor of Photosystem II (QA) and plastoquinone pool and a faster electron transport from the plastoquinone pool to Photosystem I upon changes in irradiance, compared to control plants. A faster establishing of qE, the energy-dependent component of non-photochemical quenching, in transgenic plants suggested a more rapid build-up of the transmembrane proton gradient, also supporting the increased in vivo Cytochrome b 6 f activity. However, there was no consistent increase in steady-state rates of electron transport or CO2 assimilation in plants overexpressing Rieske FeS grown in either laboratory conditions or in field trials, suggesting that the in vivo activity of the complex was only transiently increased upon changes in irradiance. Our results show that overexpression of Rieske FeS in tobacco enhances abundance of functional Cytochrome b 6 f and electron transport capacity and may have a potential to increase plant productivity if combined with other traits. One-sentence summary Increased abundance of Cytochrome b 6 f complex leads to transient increases in photosynthetic electron transport rate in tobacco.

In this study, four tobacco transformants overexpressing the inorganic carbon transporter B gene (ictB) were screened for photosynthetic performance relative to the wild type (WT) in field-based conditions. The WT and transgenic tobacco plants were evaluated for photosynthetic performance to determine the maximum rate of carboxylation (Vc, max), maximum rate of electron transport (Jmax), the photosynthetic compensation point (Γ*), quantum yield of PSII (ΦPSII), and mesophyll conductance (gm). Additionally, all plants were harvested to compare differences in above-ground biomass. Overall, transformants did not perform better than the WT on photosynthesis-, biomass-, and leaf composition-related traits. This is in contrast to previous studies that have suggested significant increases in photosynthesis and yield with the overexpression of ictB, although not widely evaluated under field conditions. These findings suggest that the benefit of ictB is not universal and may only be seen under certain growth conditions. While there is certainly still potential benefit to utilizing ictB in the future, further effort must be concentrated on understanding the underlying function of the gene and in which environmental conditions it offers the greatest benefit to crop performance. As it stands at present, it is possible that ictB overexpression may be largely favorable in controlled environments, such as greenhouses.

The Calvin–Benson (CB) cycle fixes carbon dioxide from the atmosphere into carbon compounds that are then used to make an array of molecules essential for plant growth and development. The efficiency of this cycle is known to be lower than its potential maximum and the goal of improving it has been an objective of plant scientist over the last 20 years. A major limitation to the CB cycle is the first enzyme in the cycle, ribulose 1,5-bisphosphatase, but evidence has also accumulated identifying additional enzymes sedoheptulose 1,7-bisphosphatase and fructose 1,5 bisphosphate aldolase, which share some of this limitation. In this chapter, we review the progress of the work to achieve improvements in CO2 assimilation through improving the function of CB cycle by the (1) engineering of rubisco, (2) introduction of novel pathways or mechanisms to improve the function of rubisco, and (3) direct manipulation of the levels of activity of CB cycle enzymes. Examples are given of successes obtained using these approaches and the limitations of the current approaches and the future prospects are discussed in the context of rising CO2 levels.

The Calvin-Benson-Bassham (CBB) cycle is arguably the most important pathway on earth, capturing CO2 from the atmosphere and converting it into organic molecules, providing the basis for life on our planet. This cycle has been intensively studied over the 50 yr since it was elucidated, and it is highly conserved across nature, from cyanobacteria to the largest of our land plants. Eight out of the 11 enzymes in this cycle catalyse the regeneration of ribulose-1-5 bisphosphate (RuBP), the CO2 acceptor molecule. The potential to manipulate RuBP regeneration to improve photosynthesis has been demonstrated in a number of plant species, and the development of new technologies, such as omics and synthetic biology provides exciting future opportunities to improve photosynthesis and increase crop yields.

The communication between chloroplasts and mitochondria, which depends on the inter-organellar exchange of carbon skeletons, energy, and reducing equivalents, is essential for maintaining efficient respiratory metabolism and photosynthesis.We devised a multi-transgene approach to manipulate the leaf energy and redox balance in tobacco (Nicotiana tabacum) while monitoring the in vivo cytosolic redox status of NAD(H) using the biosensor c-Peredox-mCherry.Our strategy involved altering the shuttling capacity of the chloroplast by (1) increasing the chloroplast malate valve capacity by overexpression of the chloroplast malate valve transporter pOMT from Arabidopsis (AtpOMT1) while (2) reducing the activity of the chloroplast triose-phosphate/3-phosphoglycerate shuttle by knocking down the cytosolic NAD-dependent glyceraldehyde 3-phosphate dehydrogenase (NtGAPC).This was accompanied by (3) alterations to the export of reducing equivalents in the mitochondria by knocking down the mitochondrial malate dehydrogenase (NtmMDH) and ( 4) an increased expression of the mitochondrial fission regulator FIS1A from Arabidopsis (AtFIS1A).The multi-transgene tobacco plants were analysed in glasshouse conditions and showed significant increases in the cytosolic NADH:NAD + in the dark when transcript levels for NtGAPC or NtmMDH were knocked down.In addition, principal component analysis and Spearman correlation analyses showed negative correlations between average transcript levels for the gene targets and parameters related to chlorophyll fluorescence and plant growth.Our results highlight the importance of the shuttling of energy and reducing equivalents from chloroplasts and mitochondria to support photosynthesis and growth and suggest an important role for the dual 2-oxoglutarate/malate and oxaloacetate/malate transporter (pOMT).

C4 photosynthesis is characterized by the compartmentalization of the processes of atmospheric uptake of CO2 and its conversion into carbohydrate between mesophyll and bundle-sheath cells. As a result, most of the enzymes participating in the Calvin-Benson-Bassham (CBB) cycle, including RubisCO, are highly expressed in bundle-sheath cells. There is evidence that changes in the regulatory sequences of RubisCO contribute to its bundle-sheath-specific expression, however, little is known about how the spatial-expression pattern of other CBB cycle enzymes is regulated. In this study, we use a computational approach to scan for transcription factor binding sites in the regulatory regions of the genes encoding CBB cycle enzymes, SBPase, FBPase, PRK, and GAPDH-B, of C3 and C4 grasses. We identified potential cis-regulatory elements present in each of the genes studied here, regardless of the photosynthetic path used by the plant. The trans-acting factors that bind these elements have been validated in A. thaliana and might regulate the expression of the genes encoding CBB cycle enzymes. In addition, we also found C4-specific transcription factor binding sites in the genes encoding CBB cycle enzymes that could potentially contribute to the pathway-specific regulation of gene expression. These results provide a foundation for the functional analysis of the differences in regulation of genes encoding CBB cycle enzymes between C3 and C4 grasses.

Previous studies have demonstrated that 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 multi-gene 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 overexpression of the plant enzyme sedoheptulose-1,7-bisphosphatase, together with expression of the red algal protein cytochrome c 6 , and show that a further increase in biomass accumulation under both glasshouse and field conditions can be achieved. Furthermore, we provide evidence that the simultaneous stimulation of electron transport and RuBP regeneration can lead to enhanced intrinsic water use efficiency under field conditions. One sentence summary Simultaneous stimulation of RuBP regeneration and electron transport results in improvements in biomass yield in glasshouse and field grown tobacco.

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.