Overexpression of the RieskeFeS Protein Increases Electron Transport Rates and Biomass Yield

Morphophysiological and proteomic profiling unveiling mechanisms underlying nitrogen use efficiency in popcorn (Zea mays var. everta)

In this study we hypothesize that the contrasting nitrogen use efficiency (NUE) between popcorn (Zea mays var. everta) inbred lines P2 (high NUE) and L80 (low NUE) is driven by distinct morphophysiological responses and proteomic profiles found in leaves and roots. To elucidate the mechanisms involved, plants were cultivated in a greenhouse under high (100% N) and low (10% N) nitrogen conditions, in a randomized complete block design with two factorial treatment arrangements and seven blocks. Morphological and physiological traits such as photochemical and non-photochemical quenching, quantum yield of photosystem II, and potential photosynthesis were evaluated. Compared to L80, under low N, P2 exhibited 25.9% greater leaf area, 22.4% taller plants, 21.7% thicker stems and 113% higher shoot dry mass, as well as higher values of photochemical and non-photochemical quenching and quantum yield of photosystem II that drove to a maximum photosynthesis 16.5% higher than L80. Comparative proteomic analysis of the leaves identified 215 differentially accumulated proteins (DAPs) in P2 and 168 DAPs in L80, while in roots, 127 DAPs were observed in P2 and 172 in L80. Notably, in leaves, the response to oxidative stress, energy metabolism, and photosynthesis represented the main differences between P2 and L80. In roots, the nitrate transport, ammonium assimilation, and amino acid metabolism appear to have contributed to the improved NUE in P2. Consequently, this study provides valuable insights into the molecular mechanisms underlying NUE and opens avenues for molecular breeding aimed at selecting superior genotypes for the development of a more sustainable agriculture.

Adaptive responses of plants to light stress: mechanisms of photoprotection and acclimation. A review

Plants depend on solar energy for growth via oxygenic photosynthesis. However, when light levels exceed the optimal range for photosynthesis, it causes abiotic stress and harms plant physiology. In response to excessive light, plants activate a series of signaling pathways starting from the chloroplast and affecting the entire plant, leading to stress-specific physiological changes. These signals prompt various physiological and biochemical adjustments aimed at counteracting the negative impacts of high light intensity, including photodamage and photoinhibition. Mechanisms to protect against light stress involve scavenging of chloroplastic reactive oxygen species (ROS), adjustments in chloroplast and stomatal positioning, and increased anthocyanin production to safeguard the photosynthetic machinery. Given that this machinery is a primary target for stress-induced damage, plants have evolved acclimation strategies like dissipating thermal energy via non-photochemical quenching (NPQ), repairing Photosystem II (PSII), and regulating the transcription of photosynthetic proteins. Fluctuating light presents a less severe but consistent stress, which has not been extensively studied. Nevertheless, current research indicates that state transitions and cyclic electron flow play crucial roles in helping plants adapt to varying light conditions. This review encapsulates the latest understanding of plant physiological and biochemical responses to both high light and low light stress.

Exploring the Potential of Selenium-Containing Amine (Se-AMA) to Enhance Photosynthesis and Leaf Water Content: New Avenues for Carbonic Anhydrase Modulation in Arabidopsis thaliana

Global changes and growing demands have led to the development of new molecular approaches to improve crop physiological performances. Carbonic anhydrase (CA) enzymes, ubiquitous across various life kingdoms, stand out for their critical roles in plant photosynthesis and water relations. We hypothesize that the modulators of human CAs could affect plant physiology. Our research demonstrated that foliar treatments with a synthetic selenium-containing CA activator (Se-AMA) influenced the physiological performances of Arabidopsis thaliana. Se-AMA increased net photosynthesis (A + 31.7%) and stomatal conductance (gs + 48.2%) at 100 µM, with the most notable effects after 10 days of treatment. Se-AMA at 300 µM proved to be even more effective, boosting A and gs by 19.9% and 55.3%, respectively, already after 3 days of application. Morning treatment with Se-AMA at 300 µM enhanced photosynthetic performances throughout the day, suggesting that the positive effect of Se-AMA lasted for several hours. Additionally, Se-AMA increased water content in plants by 17.1%, suggesting that Se-AMA treatment may have improved plant water absorption and resource management. This effect might be linked to Se-AMA's role in modulating specific CA isoforms working with aquaporins. Although preliminary, these findings suggest that Se-AMA could enhance plant physiological performances under the conditions of non-limiting water availability.

Enhanced salt stress tolerance in plants without growth penalty through increased photosynthesis activity by plastocyanin from Antarctic moss

Salinity poses a significant challenge to plant growth and crop productivity by adversely affecting crucial processes, including photosynthesis. Efforts to enhance abiotic stress tolerance in crops have been hindered by the trade-off effect, where increased stress resistance is accompanied by growth reduction. In this study, we identified and characterized a plastocyanin gene (PaPC) from the Antarctic moss Polytrichastrum alpinum, which enhanced photosynthesis and salt stress tolerance in Arabidopsis thaliana without compromising growth. While there were no differences in growth and salt tolerance between the wild type and Arabidopsis plastocyanin genes (AtPC1 and AtPC2)-overexpressing plants, PaPC-overexpressing plants demonstrated superior photosynthetic efficiency, increased biomass, and enhanced salt tolerance. Similarly, PaPC-overexpressing rice plants exhibited improved yield potential and photosynthetic efficiency under both normal and salt stress conditions. Key amino acid residues in PaPC responsible for this enhanced functionality were identified, and their substitution into AtPC2 conferred improved photosynthetic performance and stress tolerance in Arabidopsis, tobacco, and tomato. These findings not only highlight the potential of extremophiles as valuable genetic resources but also suggest a photosynthesis-based strategy for developing stress-resilient crops without a growth penalty.

Multiplex Expression Cassette Assembly: A flexible and versatile method for building complex genetic circuits in conventional vectors

The manipulation of multiple transcription units for simultaneous and coordinated expression is not only key to building complex genetic circuits to accomplish diverse functions in synthetic biology, but is also important in crop breeding for significantly improved productivity and overall performance. However, building constructs with multiple independent transcription units for fine-tuned and coordinated regulation is complicated and time-consuming. Here, we introduce the Multiplex Expression Cassette Assembly (MECA) method, which modifies canonical vectors compatible with Golden Gate Assembly, and then uses them to produce multi-cassette constructs. By embedding the junction syntax in primers that are used to amplify functional elements, MECA is able to make complex constructs using only one intermediate vector and one destination vector via two rounds of one-pot Golden Gate assembly reactions, without the need for dedicated vectors and a coherent library of standardized modules. As a proof-of-concept, we modified eukaryotic and prokaryotic expression vectors to generate constructs for transient expression of green fluorescent protein and β-glucuronidase in Nicotiana benthamiana, genome editing to block monoterpene metabolism in tomato glandular trichomes, production of betanin in tobacco and synthesis of β-carotene in Escherichia coli. Additionally, we engineered the stable production of thymol and carvacrol, bioactive compounds from Lamiaceae family plants, in glandular trichomes of tobacco. These results demonstrate that MECA is a flexible, efficient and versatile method for building complex genetic circuits, which will not only play a critical role in plant synthetic biology, but also facilitate improving agronomic traits and pyramiding traits for the development of next-generation elite crops.

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.

Photosynthetic control at the cytochrome b6f complex

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

Improving photosynthetic efficiency of rice via over-expressing a ferredoxin-like protein gene from Methanothermobacter thermautotrophicus

Ferredoxins (Fds) are crucial in various essential plant metabolic processes, including photosynthesis, fermentation and aerobic nitrogen fixation, due to their role in electron transport rate (ETR). However, the full scope of ferredoxin's function across prokaryotes and eukaryotic plants remains less understood. This study investigated the effect of MtFd from Methanothermobacter thermoautotrophicus on rice photosynthetic efficiency. We found that MtFd was localized in the chloroplasts of rice protoplasts. Transgenic analysis showed that MtFd significantly enhanced the photosynthetic capacity compared to the wild-type plants. This enhancement was evident through increased ETR, NADPH content and net photosynthetic rates, as well as decreased non-photochemical quenching (NPQ). Despite similar biomass to wild type plants, MtFd transgenic plants exhibited a marked increase in grain size and the 1000-grian weight. The elevated ETR and surplus free electrons in transgenic plants result in a considerable rise in cellular ROS content, which in turn enhances the enzymatic activity of the antioxidant system. In summary, our findings suggest that introducing the Fd protein from M. thermoautotrophicus into transgenic rice improves photosynthetic efficiency by accelerating ETR, which triggers the cellular oxidative stress response.

The atypical Dof transcriptional factor OsDes1 contributes to stay-green, grain yield, and disease resistance in rice

The regulation of leaf senescence and disease resistance plays a crucial role in determining rice grain yield and quality, which are important to meet the ever-increasing food demands of the world. Here, we identified an atypical Dof transcriptional factor OsDes1 that contributes to the stay-green phenotype, grain yield, and disease resistance in rice. The expression level of OsDes1 is positively associated with stay-green in natural variations of japonica rice, suggesting that OsDes1 would be alternatively used in breeding programs. Mechanistically, OsDes1 targets the promoter of the Rieske FeS protein gene OsPetC to activate its expression and interacts with OsPetC to protect against its degradation, thus promoting stay-green and ultimately improving the grain yield. OsDes1 also binds to the promoter region of defense-related genes, such as OsPR1b, and activates their expression, leading to enhanced disease resistance. These findings offer a potential strategy for breeding rice to enhance grain yield and disease resistance.

Photosynthesis: Genetic Strategies Adopted to Gain Higher Efficiency

The global challenge of feeding an ever-increasing population to maintain food security requires novel approaches to increase crop yields. Photosynthesis, the fundamental energy and material basis for plant life on Earth, is highly responsive to environmental conditions. Evaluating the operational status of the photosynthetic mechanism provides insights into plants' capacity to adapt to their surroundings. Despite immense effort, photosynthesis still falls short of its theoretical maximum efficiency, indicating significant potential for improvement. In this review, we provide background information on the various genetic aspects of photosynthesis, explain its complexity, and survey relevant genetic engineering approaches employed to improve the efficiency of photosynthesis. We discuss the latest success stories of gene-editing tools like CRISPR-Cas9 and synthetic biology in achieving precise refinements in targeted photosynthesis pathways, such as the Calvin-Benson cycle, electron transport chain, and photorespiration. We also discuss the genetic markers crucial for mitigating the impact of rapidly changing environmental conditions, such as extreme temperatures or drought, on photosynthesis and growth. This review aims to pinpoint optimization opportunities for photosynthesis, discuss recent advancements, and address the challenges in improving this critical process, fostering a globally food-secure future through sustainable food crop production.

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.

Physiological and molecular mechanisms of the inhibitory effects of artemisinin on Microcystis aeruginosa and Chlorella pyrenoidosa

Artemisinin, a novel plant allelochemical, has attracted attention for its potential selective inhibitory effects on algae, yet to be fully explored. This study compares the sensitivity and action targets of Microcystis aeruginosa (M. aeruginosa) and Chlorella pyrenoidosa (C. pyrenoidosa) to artemisinin algaecide (AMA), highlighting their differences. Results indicate that at high concentrations, AMA displaces the natural PQ at the QB binding site within M. aeruginosa photosynthetic system, impairing the D1 protein repair function. Furthermore, AMA disrupts electron transfer from reduced ferredoxin (Fd) to NADP+ by interfering with the iron-sulfur clusters in the ferredoxin-NADP+ reductases (FNR) domain of Fd. Moreover, significant reactive oxygen species (ROS) accumulation triggers oxidative stress and interrupts the tricarboxylic acid cycle, hindering energy acquisition. Notably, AMA suppresses arginine synthesis in M. aeruginosa, leading to reduced microcystins (MCs) release. Conversely, C. pyrenoidosa counters ROS accumulation via photosynthesis protection, antioxidant defenses, and by regulating intracellular osmotic pressure, accelerating damaged protein degradation, and effectively repairing DNA for cellular detoxification. Additionally, AMA stimulates the expression of DNA replication-related genes, facilitating cell proliferation. Our finding offer a unique approach for selectively eradicating cyanobacteria while preserving beneficial algae, and shed new light on employing eco-friendly algicides with high specificity.

Modulation of physio-biochemical and photosynthesis parameters by overexpressing SbPIP2 gene improved abiotic stress tolerance of transgenic tobacco

The present study aims to explore the potential of a plasma-membrane localized PIP2-type aquaporin protein sourced from the halophyte Salicornia brachiata to alleviate salinity and water deficit stress tolerance in a model plant through transgenic intervention. Transgenic plants overexpressing SbPIP2 gene showed improved physio-biochemical parameters like increased osmolytes (proline, total sugar, and amino acids), antioxidants (polyphenols), pigments and membrane stability under salinity and drought stresses compared to control plants [wild type (WT) and vector control (VC) plants]. Multivariate statistical analysis showed that, under water and salinity stresses, osmolytes, antioxidants and pigments were correlated with SbPIP2-overexpressing (SbPIP2-OE) plants treated with salinity and water deficit stress, suggesting their involvement in stress tolerance. As aquaporins are also involved in CO2 transport, SbPIP2-OE plants showed enhanced photosynthesis performance than wild type upon salinity and drought stresses. Photosynthetic gas exchange (net CO2 assimilation rate, PSII efficiency, ETR, and non-photochemical quenching) were significantly higher in SbPIP2-OE plants compared to control plants (wild type and vector control plants) under both unstressed and stressed conditions. The higher quantum yield for reduction of end electron acceptors at the PSI acceptor side [Φ( R0 )] in SbPIP2-OE plants compared to control plants under abiotic stresses indicates a continued PSI functioning, leading to retained electron transport rate, higher carbon assimilation, and less ROS-mediated injuries. In conclusion, the SbPIP2 gene functionally validated in the present study could be a potential candidate for engineering abiotic stress resilience in important crops.

Characterization of PetM cytochrome b6f subunit 7 domain-containing protein in tomato

In recent years, multiple advances have been made in understanding the photosynthetic machinery in model organisms. Knowledge transfer to horticultural important fruit crops is challenging and time-consuming due to restrictions in gene editing tools and prolonged life cycles. Here, we characterize a gene encoding a PetM domain-containing protein in tomato. The CRISPR/Cas9 knockout lines of the PetM showed impairment in the chloroplastic electron transport rate (ETR), reduced CO2 assimilation, and reduction of carotenoids and chlorophylls (Chl) under several light conditions. Further, growth-condition-dependent elevation or repression of Chl a/b ratios and de-epoxidation states were identified, underlining possible impairment compensation mechanisms. However, under low light and glasshouse conditions, there were basal levels in CO2 assimilation and ETR, indicating a potential role of the PetM domain in stabilizing the cytochrome b6f complex (Cb6f) under higher light irradiance and increasing its quantum efficiency. This suggests a potential evolutionary role in which this domain might stabilize the site of the Cb6f regulating ratios of cyclic and linear electron transport and its potential importance during the conquest of terrestrial ecosystems during which plants were exposed to higher irradiance. Finally, the results are discussed with regard to metabolism and their implication to photosynthesis from an agronomic perspective.

A Ubiquitin-Based Module Directing Protein-Protein Interactions in Chloroplasts

A promising approach for the genetic engineering of multiprotein complexes in living cells involves designing and reconstructing the interaction between two proteins that lack native affinity. Thylakoid-embedded multiprotein complexes execute the light reaction of plant photosynthesis, but their engineering remains challenging, likely due to difficulties in accurately targeting heterologous membrane-bound proteins to various sub-compartments of thylakoids. In this study, we developed a ubiquitin-based module (Nub-Cub) capable of directing interactions in vivo between two chloroplast proteins lacking native affinities. We applied this module to genetically modify thylakoid multiprotein complexes. We demonstrated the functionality of the Nub-Cub module in the model organism Arabidopsis thaliana. Employing this system, we successfully modified the Photosystem II (PSII) complex by ectopically attaching an extrinsic subunit of PSII, PsbTn1, to CP26-a component of the antenna system of PSII. Surprisingly, this mandatory interaction between CP26 and PsbTn1 in plants impairs the efficiency of electron transport in PSII and unexpectedly results in noticeable defects in leaf development. Our study not only offers a general strategy to modify multiprotein complexes embedded in thylakoid membranes but it also sheds light on the possible interplay between two proteins without native interaction.

Optimizing the electron transport chain to sustainably improve photosynthesis

Genetically improving photosynthesis is a key strategy to boosting crop production to meet the rising demand for food and fuel by a rapidly growing global population in a warming climate. Many components of the photosynthetic apparatus have been targeted for genetic modification for improving photosynthesis. Successful translation of these modifications into increased plant productivity in fluctuating environments will depend on whether the electron transport chain (ETC) can support the increased electron transport rate without risking overreduction and photodamage. At present atmospheric conditions, the ETC appears suboptimal and will likely need to be modified to support proposed photosynthetic improvements and to maintain energy balance. Here, I derive photochemical equations to quantify the transport capacity and the corresponding reduction level based on the kinetics of redox reactions along the ETC. Using these theoretical equations and measurements from diverse C3/C4 species across environments, I identify several strategies that can simultaneously increase the transport capacity and decrease the reduction level of the ETC. These strategies include increasing the abundances of reaction centers, cytochrome b6f complexes, and mobile electron carriers, improving their redox kinetics, and decreasing the fraction of secondary quinone-nonreducing photosystem II reaction centers. I also shed light on several previously unexplained experimental findings regarding the physiological impacts of the abundances of the cytochrome b6f complex and plastoquinone. The model developed, and the insights generated from it facilitate the development of sustainable photosynthetic systems for greater crop yields.

Determinants of photochemical characteristics of the photosynthetic electron transport chain of maize

Introduction

The photosynthetic electron transport chain (ETC) is the bridge that links energy harvesting during the photophysical reactions at one end and energy consumption during the biochemical reactions at the other. Its functioning is thus fundamental for the proper balance between energy supply and demand in photosynthesis. Currently, there is a lack of understanding regarding how the structural properties of the ETC are affected by nutrient availability and plant developmental stages, which is a major roadblock to comprehensive modeling of photosynthesis.

Methods

Redox parameters reflect the structural controls of ETC on the photochemical reactions and electron transport. We conducted joint measurements of chlorophyll fluorescence (ChlF) and gas exchange under systematically varying environmental conditions and growth stages of maize and sampled foliar nutrient contents. We utilized the recently developed steady-state photochemical model to infer redox parameters of electron transport from these measurements.

Results and discussion

We found that the inferred values of these photochemical redox parameters varied with leaf macronutrient content. These variations may be caused either directly by these nutrients being components of protein complexes on the ETC or indirectly by their impacts on the structural integrity of the thylakoid and feedback from the biochemical reactions. Also, the redox parameters varied with plant morphology and developmental stage, reflecting seasonal changes in the structural properties of the ETC. Our findings will facilitate the parameterization and simulation of complete models of photosynthesis.

Expression of the plastocyanin gene PETE2 in Camelina sativa improves seed yield and salt tolerance

Plastocyanin functions as an electron carrier in the photosynthetic electron transport chain, located at the thylakoid membrane. In several species, endogenous plastocyanin levels are correlated with the photosynthetic electron transport rate. Overexpression of plastocyanin genes in Arabidopsis thaliana increases plant size, but this phenomenon has not been observed in crop species. Here, we investigated the effects of heterologous expression of a gene encoding a plastocyanin isoform from Arabidopsis, AtPETE2, in the oil seed crop Camelina sativa under standard growth conditions and under salt stress. AtPETE2 heterologous expression enhanced photosynthetic activity in Camelina, accelerating plant development and improving seed yield under standard growth conditions. Additionally, CsPETE2 from Camelina was induced by salt stress and AtPETE2 expression lines had larger primary roots and more lateral roots than the wild type. AtPETE2 expression lines also had larger seeds and higher total seed yield under long-term salt stress compared with non-transgenic Camelina. Our results demonstrate that increased plastocyanin levels in Camelina can enhance photosynthesis and productivity, as well as tolerance to osmotic and salt stresses. Heterologous expression of plastocyanin may be a useful strategy to mitigate crop stress in saline soils.

Securing maize reproductive success under drought stress by harnessing CO2 fertilization for greater productivity

Securing maize grain yield is crucial to meet food and energy needs for the future growing population, especially under frequent drought events and elevated CO2 (eCO2) due to climate change. To maximize the kernel setting rate under drought stress is a key strategy in battling against the negative impacts. Firstly, we summarize the major limitations to leaf source and kernel sink in maize under drought stress, and identified that loss in grain yield is mainly attributed to reduced kernel set. Reproductive drought tolerance can be realized by collective contribution with a greater assimilate import into ear, more available sugars for ovary and silk use, and higher capacity to remobilize assimilate reserve. As such, utilization of CO2 fertilization by improved photosynthesis and greater reserve remobilization is a key strategy for coping with drought stress under climate change condition. We propose that optimizing planting methods and mining natural genetic variation still need to be done continuously, meanwhile, by virtue of advanced genetic engineering and plant phenomics tools, the breeding program of higher photosynthetic efficiency maize varieties adapted to eCO2 can be accelerated. Consequently, stabilizing maize production under drought stress can be achieved by securing reproductive success by harnessing CO2 fertilization.

Machine learning in photosynthesis: Prospects on sustainable crop development

Improving photosynthesis is a promising avenue to increase food security. Studying photosynthetic traits with the aim to improve efficiency has been one of many strategies to increase crop yield but analyzing large data sets presents an ongoing challenge. Machine learning (ML) represents a ubiquitous tool that can provide a more elaborate data analysis. Here we review the application of ML in various domains of photosynthetic research, as well as in photosynthetic pigment studies. We highlight how correlating hyperspectral data with photosynthetic parameters to improve crop yield could be achieved through various ML algorithms. We also propose strategies to employ ML in promoting photosynthetic pigment research for furthering crop yield.

Glyceraldehyde-3-phosphate dehydrogenase subunits A and B are essential to maintain photosynthetic efficiency

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 (A4) or from 2 GAPA and 2 glyceraldehyde-3-phosphate dehydrogenase B (GAPB) subunit proteins forming a heterotetramer (A2B2). 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.

Modelling plants across scales of biological organisation for guiding crop improvement

Grain yield improvement in globally important staple crops is critical in the coming decades if production is to keep pace with growing demand; so there is increasing interest in understanding and manipulating plant growth and developmental traits for better crop productivity. However, this is confounded by complex cross-scale feedback regulations and a limited ability to evaluate the consequences of manipulation on crop production. Plant/crop modelling could hold the key to deepening our understanding of dynamic trait-crop-environment interactions and predictive capabilities for supporting genetic manipulation. Using photosynthesis and crop growth as an example, this review summarises past and present experimental and modelling work, bringing about a model-guided crop improvement thrust, encompassing research into: (1) advancing cross-scale plant/crop modelling that connects across biological scales of organisation using a trait dissection-integration modelling principle; (2) improving the reliability of predicted molecular-trait-crop-environment system dynamics with experimental validation; and (3) innovative model application in synergy with cross-scale experimentation to evaluate G×M×E and predict yield outcomes of genetic intervention (or lack of it) for strategising further molecular and breeding efforts. The possible future roles of cross-scale plant/crop modelling in maximising crop improvement are discussed.

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.

An intrinsically disordered region-containing protein mitigates the drought-growth trade-off to boost yields

Drought stress poses a serious threat to global agricultural productivity and food security. Plant resistance to drought is typically accompanied by a growth deficit and yield penalty. Herein, we report a previously uncharacterized, dicotyledon-specific gene, Stress and Growth Interconnector (SGI), that promotes growth during drought in the oil crop rapeseed (Brassica napus) and the model plant Arabidopsis (Arabidopsis thaliana). Overexpression of SGI conferred enhanced biomass and yield under water-deficient conditions, whereas corresponding CRISPR SGI mutants exhibited the opposite effects. These attributes were achieved by mediating reactive oxygen species (ROS) homeostasis while maintaining photosynthetic efficiency to increase plant fitness under water-limiting environments. Further spatial-temporal transcriptome profiling revealed dynamic reprogramming of pathways for photosynthesis and stress responses during drought and the subsequent recovery. Mechanistically, SGI represents an intrinsically disordered region-containing protein that interacts with itself, catalase isoforms, dehydrins, and other drought-responsive positive factors, restraining ROS generation. These multifaceted interactions stabilize catalases in response to drought and facilitate their ROS-scavenging activities. Taken altogether, these findings provide insights into currently underexplored mechanisms to circumvent trade-offs between plant growth and stress tolerance that will inform strategies to breed climate-resilient, higher yielding crops for sustainable agriculture.

Mass screening of rice mutant populations at low CO2 for identification of lowered photorespiration and respiration rates

INTRODUCTION

Identifying rice (Oryza sativa) germplasm with improved efficiency of primary metabolism is of utmost importance in order to increase yields. One such approach can be attained through screening genetically diverse populations under altered environmental conditions. Growth or treatment under low carbon dioxide (CO₂) concentrations can be used as a means of revealing altered leaf photorespiration, respiration and other metabolic variants.

METHODS

We developed a pipeline for very high throughput treatment of gamma- and ethyl methanesulfonate- (EMS) induced mutant populations of IR64 rice seedlings at very low CO₂ for 7 days. 1050 seedlings per batch at 5^th leaf stage were exposed to 60 ppm CO₂ for the first day and 30 ppm for the remaining three days. Following this, putative candidates were identified by measuring chlorophyll depletion using SPAD. Screening results showed a distinct difference between the mutants and the WTs.

RESULTS AND DISCUSSION

The mean chlorophyll loss in WTs ranged from 65% to 11% respectively, whereas in the mutant lines chlorophyll loss ranged from 0 to 100%, suggesting considerable phenotypic variation. Rice mutants with a reduced chlorophyll reduction (<10%) were identified as 'Chlorophyll retention mutants' (CRMs) under low CO₂ stress. In total, 1909 mutant lines (14,000 seedlings) were screened for chlorophyll content under 30 ppm CO_2, with 26 lines selected for detailed screening. These 26 putative candidates were self-seeded to produce an M₅ generation, used to determine the genetic control of the altered response to low CO_2. Gas exchange of light and CO₂ response revealed that there were significant variations among photosynthetic properties in two selected rice mutants. The CO₂ compensation points in the absence of photorespiration and leaf respiration rates were lower than the WTs and anatomical analyses showed that CRM 29 had improved mesophyll cell area. We propose that this approach is useful for generating new material for breeding rice with improved primary metabolism.

Identification of photosynthetic parameters for superior yield of two super hybrid rice varieties: A cross-scale study from leaf to canopy

Enhancing photosynthetic capacity is widely accepted as critical to advancing crop yield. Therefore, identifying photosynthetic parameters positively related to biomass accumulation in elite cultivars is the major focus of current rice research. In this work, we assessed leaf photosynthetic performance, canopy photosynthesis, and yield attributes of super hybrid rice cultivars Y-liangyou 3218 (YLY3218) and Y-liangyou 5867 (YLY5867) at tillering stage and flowering stage, using inbred super rice cultivars Zhendao11(ZD11) and Nanjing 9108 (NJ9108) as control. A diurnal canopy photosynthesis model was applied to estimate the influence of key environmental factors, canopy attributes, and canopy nitrogen status on daily aboveground biomass increment (AMDAY). Results showed that primarily the light-saturated photosynthetic rate at tillering stage contributed to the advancing yield and biomass of super hybrid rice in comparison to inbred super rice, and the light-saturated photosynthetic rate between them was similar at flowering stage. At tillering stage, the higher CO2 diffusion capacity, together with higher biochemical capacity (i.e., maximum carboxylation rate of Rubisco, maximum electron transport rate (J max), and triose phosphate utilization rate) favored leaf photosynthesis of super hybrid rice. Similarly, AMDAY in super hybrid rice was higher than inbred super rice at tillering stage, and comparable at flowering stage partially due to increased canopy nitrogen concentration (SLNave) of inbred super rice. At tillering stage, model simulation revealed that replacement of J max and g m in inbred super rice by super hybrid rice always had a positive effect on AMDAY, and the averaged AMDAY increment was 5.7% and 3.4%, respectively. Simultaneously, the 20% enhancement of total canopy nitrogen concentration through the improvement of SLNave (TNC-SLNave) resulted in the highest AMDAY across cultivars, with an average increase of 11.2%. In conclusion, the advancing yield performance of YLY3218 and YLY5867 was due to the higher J max and g m at tillering stage, and TCN-SLNave is a promising target for future super rice breeding programs.

Integrated transcriptomics and metabolomics analysis reveals key regulatory network that response to cold stress in common Bean (Phaseolus vulgaris L.)

Cold temperatures can be detrimental to crop survival and productivity. Breeding progress can be improved by understanding the molecular basis of low temperature tolerance. We investigated the key routes and critical metabolites related to low temperature resistance in cold-tolerant and -sensitive common bean cultivars 120 and 093, respectively. Many potential genes and metabolites implicated in major metabolic pathways during the chilling stress response were identified through transcriptomics and metabolomics research. Under chilling stress, the expression of many genes involved in lipid, amino acid, and flavonoid metabolism, as well as metabolite accumulation increased in the two bean types. Malondialdehyde (MDA) content was lower in 120 than in 093. Regarding amino acid metabolism, 120 had a higher concentration of acidic amino acids than 093, whereas 093 had a higher concentration of basic amino acids. Methionine accumulation was clearly higher in 120 than in 093. In addition, 120 had a higher concentration of many types of flavonoids than 093. Flavonoids, methionine and malondialdehyde could be used as biomarkers of plant chilling injury. Transcriptome analysis of hormone metabolism revealed considerably greater, expression of abscisic acid (ABA), gibberellin (GA), and jasmonic acid (JA) in 093 than in 120 during chilling stress, indicating that hormone regulation modes in 093 and 120 were different. Thus, chilling stress tolerance is different between 093 and 120 possibly due to transcriptional and metabolic regulation.

Photosynthetic Gains in Super-Nodulating Mutants of Medicago truncatula under Elevated Atmospheric CO2 Conditions

Legumes are generally considered to be more responsive to elevated CO2 (eCO2) conditions due to the benefits provided by symbiotic nitrogen fixation. In response to high carbohydrate demand from nodules, legumes display autoregulation of nodulation (AON) to restrict nodules to the minimum number necessary to sustain nitrogen supply under current photosynthetic levels. AON mutants super-nodulate and typically grow smaller than wild-type plants under ambient CO2. Here, we show that AON super-nodulating mutants have substantially higher biomass under eCO2 conditions, which is sustained through increased photosynthetic investment. We examined photosynthetic and physiological traits across super-nodulating rdn1-1 (Root Determined Nodulation) and sunn4 (Super Numeric Nodules) and non-nodulating nfp1 (Nod Factor Perception) Medicago truncatula mutants. Under eCO2 conditions, super-nodulating plants exhibited increased rates of carboxylation (Vcmax) and electron transport (J) relative to wild-type and non-nodulating counterparts. The substantially higher rate of CO2 assimilation in eCO2-grown sunn4 super-nodulating plants was sustained through increased production of key photosynthetic enzymes, including Rieske FeS. We hypothesize that AON mutants are carbon-limited and can perform better at eCO2 through improved photosynthesis. Nodulating legumes, especially those with higher nitrogen fixation capability, are likely to out-perform non-nodulating plants under future CO2 conditions and will be important tools for understanding carbon and nitrogen partitioning under eCO2 conditions and future crop improvements.

The thylakoid membrane protein NTA1 is an assembly factor of the cytochrome b6f complex essential for chloroplast development in Arabidopsis

The cytochrome b_6f (Cyt b_6f) complex is a multisubunit protein complex in chloroplast thylakoid membranes required for photosynthetic electron transport. Here we report the isolation and characterization of the new tiny albino 1 (nta1) mutant in Arabidopsis, which has severe defects in Cyt b_6f accumulation and chloroplast development. Gene cloning revealed that the nta1 phenotype was caused by disruption of a single nuclear gene, NTA1, which encodes an integral thylakoid membrane protein conserved across green algae and plants. Overexpression of NTA1 completely rescued the nta1 phenotype, and knockout of NTA1 in wild-type plants recapitulated the mutant phenotype. Loss of NTA1 function severely impaired the accumulation of multiprotein complexes related to photosynthesis in thylakoid membranes, particularly the components of Cyt b_6f. NTA1 was shown to directly interact with four subunits (Cyt b₆/PetB, PetD, PetG, and PetN) of Cyt b_6f through the DUF1279 domain and C-terminal sequence to mediate their assembly. Taken together, our results identify NTA1 as a new and key regulator of chloroplast development that plays essential roles in assembly of the Cyt b_6f complex by interacting with multiple Cyt b_6f subunits.

Enhancing the light reactions of photosynthesis: Strategies, controversies, and perspectives

Photosynthesis is central to life on Earth, employing sunlight, water, and carbon dioxide to produce chemical energy and oxygen. It is generally accepted that boosting its efficiency offers one promising way to increase crop yields under agronomically realistic conditions. Since the components, structure, and regulatory mechanisms of the light reactions of photosynthesis are well understood, concepts for enhancing the process have been suggested and partially tested. These approaches vary in complexity, from targeting single components to comprehensive redesign of the whole process on the scales from single cells or tissues to whole canopies. Attempts to enhance light utilization per leaf, by decreasing pigmentation, increasing levels of photosynthetic proteins, prolonging the lifespan of the photosynthetic machinery, or massive reconfiguration of the photosynthetic machinery and the incorporation of nanomaterials, are discussed in this review first. Secondly, strategies intended to optimize the acclimation of photosynthesis to changes in the environment are presented, including redesigning mechanisms to dissipate excess excitation energy (e.g., non-photochemical quenching) or reduction power (e.g., flavodiiron proteins). Moreover, schemes for improving acclimation, inspired by natural or laboratory-induced adaptation, are introduced. However, all these endeavors are still in an early exploratory phase and/or have not resulted in the desired outcome, largely because photosynthesis is embedded within large networks of closely interacting cellular and metabolic processes, which can vary among species and even cultivars. This explains why integrated, systems-wide approaches are required to achieve the breakthroughs required for effectively increasing crop yields.

Structural insights into photosynthetic cyclic electron transport

During photosynthesis, light energy is utilized to drive sophisticated biochemical chains of electron transfers, converting solar energy into chemical energy that feeds most life on earth. Cyclic electron transfer/flow (CET/CEF) plays an essential role in efficient photosynthesis, as it balances the ATP/NADPH ratio required in various regulatory and metabolic pathways. Photosystem I, cytochrome b₆f, and NADH dehydrogenase (NDH) are large multisubunit protein complexes embedded in the thylakoid membrane of the chloroplast and key players in NDH-dependent CEF pathway. Furthermore, small mobile electron carriers serve as shuttles for electrons between these membrane protein complexes. Efficient electron transfer requires transient interactions between these electron donors and acceptors. Structural biology has been a powerful tool to advance our knowledge of this important biological process. A number of structures of the membrane-embedded complexes, soluble electron carrier proteins, and transient complexes composed of both have now been determined. These structural data reveal detailed interacting patterns of these electron donor-acceptor pairs, thus allowing us to visualize the different parts of the electron transfer process. This review summarizes the current state of structural knowledge of three membrane complexes and their interaction patterns with mobile electron carrier proteins.

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.

A cross-scale analysis to understand and quantify the effects of photosynthetic enhancement on crop growth and yield across environments

Photosynthetic manipulation provides new opportunities for enhancing crop yield. However, understanding and quantifying the importance of individual and multiple manipulations on the seasonal biomass growth and yield performance of target crops across variable production environments is limited. Using a state-of-the-art cross-scale model in the APSIM platform we predicted the impact of altering photosynthesis on the enzyme-limited (A_c ) and electron transport-limited (A_j ) rates, seasonal dynamics in canopy photosynthesis, biomass growth, and yield formation via large multiyear-by-location crop growth simulations. A broad list of promising strategies to improve photosynthesis for C₃ wheat and C₄ sorghum were simulated. In the top decile of seasonal outcomes, yield gains were predicted to be modest, ranging between 0% and 8%, depending on the manipulation and crop type. We report how photosynthetic enhancement can affect the timing and severity of water and nitrogen stress on the growing crop, resulting in nonintuitive seasonal crop dynamics and yield outcomes. We predicted that strategies enhancing A_c alone generate more consistent but smaller yield gains across all water and nitrogen environments, A_j enhancement alone generates larger gains but is undesirable in more marginal environments. Large increases in both A_c and A_j generate the highest gains across all environments. Yield outcomes of the tested manipulation strategies were predicted and compared for realistic Australian wheat and sorghum production. This study uniquely unpacks complex cross-scale interactions between photosynthesis and seasonal crop dynamics and improves understanding and quantification of the potential impact of photosynthesis traits (or lack of it) for crop improvement research.

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.

Improving crop yield potential: Underlying biological processes and future prospects

The growing world population and global increases in the standard of living both result in an increasing demand for food, feed and other plant-derived products. In the coming years, plant-based research will be among the major drivers ensuring food security and the expansion of the bio-based economy. Crop productivity is determined by several factors, including the available physical and agricultural resources, crop management, and the resource use efficiency, quality and intrinsic yield potential of the chosen crop. This review focuses on intrinsic yield potential, since understanding its determinants and their biological basis will allow to maximize the plant's potential in food and energy production. Yield potential is determined by a variety of complex traits that integrate strictly regulated processes and their underlying gene regulatory networks. Due to this inherent complexity, numerous potential targets have been identified that could be exploited to increase crop yield. These encompass diverse metabolic and physical processes at the cellular, organ and canopy level. We present an overview of some of the distinct biological processes considered to be crucial for yield determination that could further be exploited to improve future crop productivity.

Enhanced abundance and activity of the chloroplast ATP synthase in rice through the overexpression of the AtpD subunit

ATP, produced by the light reactions of photosynthesis, acts as the universal cellular energy cofactor fuelling all life processes. Chloroplast ATP synthase produces ATP using the proton motive force created by solar energy-driven thylakoid electron transport reactions. Here we investigate how increasing abundance of ATP synthase affects leaf photosynthesis and growth of rice, Oryza sativa variety Kitaake. We show that overexpression of AtpD, the nuclear-encoded subunit of the chloroplast ATP synthase, stimulates both abundance of the complex, confirmed by immunodetection of thylakoid complexes separated by Blue Native-PAGE, and ATP synthase activity, detected as higher proton conductivity of the thylakoid membrane. Plants with increased AtpD content had higher CO2 assimilation rates when a stepwise increase in CO2 partial pressure was imposed on leaves at high irradiance. Fitting of the CO2 response curves of assimilation revealed that plants overexpressing AtpD had a higher electron transport rate (J) at high CO2, despite having wild-type-like abundance of the cytochrome b6f complex. A higher maximum carboxylation rate (Vcmax) and lower cyclic electron flow detected in transgenic plants both pointed to an increased ATP production compared with wild-type plants. Our results present evidence that the activity of ATP synthase modulates the rate of electron transport at high CO2 and high irradiance.

Rieske FeS overexpression in tobacco provides increased abundance and activity of cytochrome b6 f

Photosynthesis is fundamental for plant growth and yield. The cytochrome b₆ 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 b₆ 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 (Q_A ) and plastoquinone pool and faster electron transport from the plastoquinone pool to photosystem I upon changes in irradiance, compared to control plants. A faster establishment of q_E , the energy-dependent component of nonphotochemical quenching, in transgenic plants suggests a more rapid buildup of the transmembrane proton gradient, also supporting the increased in vivo cytochrome b₆ 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 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 the abundance of functional cytochrome b₆ f and may have the potential to increase plant productivity if combined with other traits.

New horizons for building pyrenoid-based CO2-concentrating mechanisms in plants to improve yields

Many photosynthetic species have evolved CO2-concentrating mechanisms (CCMs) to improve the efficiency of CO2 assimilation by Rubisco and reduce the negative impacts of photorespiration. However, the majority of plants (i.e. C3 plants) lack an active CCM. Thus, engineering a functional heterologous CCM into important C3 crops, such as rice (Oryza sativa) and wheat (Triticum aestivum), has become a key strategic ambition to enhance yield potential. Here, we review recent advances in our understanding of the pyrenoid-based CCM in the model green alga Chlamydomonas reinhardtii and engineering progress in C3 plants. We also discuss recent modeling work that has provided insights into the potential advantages of Rubisco condensation within the pyrenoid and the energetic costs of the Chlamydomonas CCM, which, together, will help to better guide future engineering approaches. Key findings include the potential benefits of Rubisco condensation for carboxylation efficiency and the need for a diffusional barrier around the pyrenoid matrix. We discuss a minimal set of components for the CCM to function and that active bicarbonate import into the chloroplast stroma may not be necessary for a functional pyrenoid-based CCM in planta. Thus, the roadmap for building a pyrenoid-based CCM into plant chloroplasts to enhance the efficiency of photosynthesis now appears clearer with new challenges and opportunities.

Single-molecule real-time sequencing of the full-length transcriptome of Halophila beccarii

Ecologically, Halophila beccarii Asch. is considered as a colonizing or a pioneer seagrass species and a “tiny but mighty” seagrass species, since it may recover quickly from disturbance generally. The use of transcriptome technology can provide a better understanding of the physiological processes of seagrasses. To date, little is known about the genome and transcriptome information of H. beccarii. In this study, we used single molecule real-time (SMRT) sequencing to obtain full-length transcriptome data and characterize the transcriptome structure. A total of 11,773 of the 15,348 transcripts were successfully annotated in seven databases. In addition, 1573 long non-coding RNAs, 8402 simple sequence repeats and 2567 transcription factors were predicted in all the transcripts. A GO analysis showed that 5843 transcripts were divided into three categories, including biological process (BP), cellular component (CC) and molecular function (MF). In these three categories, metabolic process (1603 transcripts), protein-containing complex (515 transcripts) and binding (3233 transcripts) were the primary terms in BP, CC, and MF, respectively. The major types of transcription factors were involved in MYB-related and NF-YB families. To the best of our knowledge, this is the first report of the transcriptome of H. beccarii using SMRT sequencing technology.

Non-rolling flag leaves use an effective mechanism to reduce water loss and light-induced damage under drought stress

BACKGROUND AND AIMS

The study reports on four different types of flag leaf rolling under soil drought in relation to the level of cell wall-bound phenolics. The flag leaf colonization by aphids, as a possible bioindicator of the accumulation of cell wall-bound phenolics, was also estimated.

METHODS

The proteins of the photosynthetic apparatus that form its core and are crucial for maintaining its stability (D1/PsbA protein), limit destructive effects of light (PsbS, a protein binding carotenoids in the antennas) and participate in efficient electron transport between photosystems II (PSII) and PSI (Rieske iron-sulfur protein of the cytochrome b6f complex) were evaluated in two types of flag leaf rolling. Additionally, biochemical and physiological reactions to drought stress in rolling and non-rolling flag leaves were compared.

KEY RESULTS

The study identified four types of genome-related types of flag leaf rolling. The biochemical basis for these differences was a different number of phenolic molecules incorporated into polycarbohydrate structures of the cell wall. In an extreme case of non-rolling dehydrated flag leaves, they were found to accumulate high amounts of cell wall-bound phenolics that limited cell water loss and protected the photosynthetic apparatus against excessive light. PSII was also additionally protected against excess light by the accumulation of photosynthetic apparatus proteins that ensured stable and efficient transport of excitation energy beyond PSII and its dissipation as far-red fluorescence and heat. Our analysis revealed a new type of flag leaf rolling brought about by an interaction between wheat and rye genomes, and resulting in biochemical specialization of flexible, rolling and rigid, non-rolling parts of the flag leaf. The study confirmed limited aphid colonization of the flag leaves with enhanced content of cell wall-bound phenolics.

CONCLUSIONS

Non-rolling leaves developed effective adaptation mechanisms to reduce both water loss and photoinhibitory damage to the photosynthetic apparatus under drought stress.

Using synthetic biology to improve photosynthesis for sustainable food production

Photosynthesis is responsible for the primary productivity and maintenance of life on Earth, boosting biological activity and contributing to the maintenance of the environment. In the past, traditional crop improvement was considered sufficient to meet food demands, but the growing demand for food coupled with climate change has modified this scenario over the past decades. However, advances in this area have not focused on photosynthesis per se but rather on fixed carbon partitioning. In short, other approaches must be used to meet an increasing agricultural demand. Thus, several paths may be followed, from modifications in leaf shape and canopy architecture, improving metabolic pathways related to CO₂ fixation, the inclusion of metabolic mechanisms from other species, and improvements in energy uptake by plants. Given the recognized importance of photosynthesis, as the basis of the primary productivity on Earth, we here present an overview of the latest advances in attempts to improve plant photosynthetic performance. We focused on points considered key to the enhancement of photosynthesis, including leaf shape development, RuBisCO reengineering, Calvin-Benson cycle optimization, light use efficiency, the introduction of the C₄ cycle in C₃ plants and the inclusion of other CO₂ concentrating mechanisms (CCMs). We further provide compelling evidence that there is still room for further improvements. Finally, we conclude this review by presenting future perspectives and possible new directions on this subject.

Overexpression of cytoplasmic C4 Flaveria bidentis carbonic anhydrase in C3 Arabidopsis thaliana increases amino acids, photosynthetic potential, and biomass

An important method to improve photosynthesis in C3 crops, such as rice and wheat, is to transfer efficient C4 characters to them. Here, cytosolic carbonic anhydrase (CA: βCA3) of the C4 Flaveria bidentis (Fb) was overexpressed under the control of 35 S promoter in Arabidopsis thaliana, a C3 plant, to enhance its photosynthetic efficiency. Overexpression of CA resulted in a better supply of the substrate HCO 3 - for the endogenous phosphoenolpyruvate carboxylase in the cytosol of the overexpressers, and increased its activity for generating malate that feeds into the tricarboxylic acid cycle. This provided additional carbon skeleton for increased synthesis of amino acids aspartate, asparagine, glutamate, and glutamine. Increased amino acids contributed to higher protein content in the transgenics. Furthermore, expression of FbβCA3 in Arabidopsis led to a better growth due to expression of several genes leading to higher chlorophyll content, electron transport, and photosynthetic carbon assimilation in the transformants. Enhanced CO2 assimilation resulted in increased sugar and starch content, and plant dry weight. In addition, transgenic plants had lower stomatal conductance, reduced transpiration rate, and higher water-use efficiency. These results, taken together, show that expression of C4 CA in the cytosol of a C3 plant can indeed improve its photosynthetic capacity with enhanced water-use efficiency.

3dCAP-Wheat: An Open-Source Comprehensive Computational Framework Precisely Quantifies Wheat Foliar, Nonfoliar, and Canopy Photosynthesis

Canopy photosynthesis is the sum of photosynthesis of all above-ground photosynthetic tissues. Quantitative roles of nonfoliar tissues in canopy photosynthesis remain elusive due to methodology limitations. Here, we develop the first complete canopy photosynthesis model incorporating all above-ground photosynthetic tissues and validate this model on wheat with state-of-the-art gas exchange measurement facilities. The new model precisely predicts wheat canopy gas exchange rates at different growth stages, weather conditions, and canopy architectural perturbations. Using the model, we systematically study (1) the contribution of both foliar and nonfoliar tissues to wheat canopy photosynthesis and (2) the responses of wheat canopy photosynthesis to plant physiological and architectural changes. We found that (1) at tillering, heading, and milking stages, nonfoliar tissues can contribute ~4, ~32, and ~50% of daily gross canopy photosynthesis (A_cgross; ~2, ~15, and ~-13% of daily net canopy photosynthesis, A_cnet) and absorb ~6, ~42, and ~60% of total light, respectively; (2) under favorable condition, increasing spike photosynthetic activity, rather than enlarging spike size or awn size, can enhance canopy photosynthesis; (3) covariation in tissue respiratory rate and photosynthetic rate may be a major factor responsible for less than expected increase in daily A_cnet; and (4) in general, erect leaves, lower spike position, shorter plant height, and proper plant densities can benefit daily A_cnet. Overall, the model, together with the facilities for quantifying plant architecture and tissue gas exchange, provides an integrated platform to study canopy photosynthesis and support rational design of photosynthetically efficient wheat crops.

Cryo-EM structures of the Synechocystis sp. PCC 6803 cytochrome b6f complex with and without the regulatory PetP subunit

In oxygenic photosynthesis, the cytochrome b₆f (cytb₆f) complex links the linear electron transfer (LET) reactions occurring at photosystems I and II and generates a transmembrane proton gradient via the Q-cycle. In addition to this central role in LET, cytb₆f also participates in a range of processes including cyclic electron transfer (CET), state transitions and photosynthetic control. Many of the regulatory roles of cytb₆f are facilitated by auxiliary proteins that differ depending upon the species, yet because of their weak and transient nature the structural details of these interactions remain unknown. An apparent key player in the regulatory balance between LET and CET in cyanobacteria is PetP, a ∼10 kDa protein that is also found in red algae but not in green algae and plants. Here, we used cryogenic electron microscopy to determine the structure of the Synechocystis sp. PCC 6803 cytb₆f complex in the presence and absence of PetP. Our structures show that PetP interacts with the cytoplasmic side of cytb₆f, displacing the C-terminus of the PetG subunit and shielding the C-terminus of cytochrome b₆, which binds the heme c_n cofactor that is suggested to mediate CET. The structures also highlight key differences in the mode of plastoquinone binding between cyanobacterial and plant cytb₆f complexes, which we suggest may reflect the unique combination of photosynthetic and respiratory electron transfer in cyanobacterial thylakoid membranes. The structure of cytb₆f from a model cyanobacterial species amenable to genetic engineering will enhance future site-directed mutagenesis studies of structure-function relationships in this crucial ET complex.

Mining for allelic gold: finding genetic variation in photosynthetic traits in crops and wild relatives

Improvement of photosynthetic traits in crops to increase yield potential and crop resilience has recently become a major breeding target. Synthetic biology and genetic technologies offer unparalleled opportunities to create new genetics for photosynthetic traits driven by existing fundamental knowledge. However, large 'gene bank' collections of germplasm comprising historical collections of crop species and their relatives offer a wealth of opportunities to find novel allelic variation in the key steps of photosynthesis, to identify new mechanisms and to accelerate genetic progress in crop breeding programmes. Here we explore the available genetic resources in food and fibre crops, strategies to selectively target allelic variation in genes underpinning key photosynthetic processes, and deployment of this variation via gene editing in modern elite material.

A model-guided holistic review of exploiting natural variation of photosynthesis traits in crop improvement

Breeding for improved leaf photosynthesis is considered as a viable approach to increase crop yield. Whether it should be improved in combination with other traits has not been assessed critically. Based on the quantitative crop model GECROS that interconnects various traits to crop productivity, we review natural variation in relevant traits, from biochemical aspects of leaf photosynthesis to morpho-physiological crop characteristics. While large phenotypic variations (sometimes >2-fold) for leaf photosynthesis and its underlying biochemical parameters were reported, few quantitative trait loci (QTL) were identified, accounting for a small percentage of phenotypic variation. More QTL were reported for sink size (that feeds back on photosynthesis) or morpho-physiological traits (that affect canopy productivity and duration), together explaining a much greater percentage of their phenotypic variation. Traits for both photosynthetic rate and sustaining it during grain filling were strongly related to nitrogen-related traits. Much of the molecular basis of known photosynthesis QTL thus resides in genes controlling photosynthesis indirectly. Simulation using GECROS demonstrated the overwhelming importance of electron transport parameters, compared with the maximum Rubisco activity that largely determines the commonly studied light-saturated photosynthetic rate. Exploiting photosynthetic natural variation might significantly improve crop yield if nitrogen uptake, sink capacity, and other morpho-physiological traits are co-selected synergistically.

The red seaweed Kappaphycus alvarezii antiporter gene (KaNa+/H+) confers abiotic stress tolerance in transgenic tobacco

BACKGROUND

Plant establishment, growth, development and productivity are adversely affected by abiotic stresses that are dominant characteristics of environmentally challenged/degraded habitats created in the Anthropocene. Crop breeding for climate resilience properties is need of the hour to sustain the crop productivity. We report on the characterization of Kappaphycus alvarezii (a red seaweed) Na+/H+ antiporter gene (KaNa+/H+) for enhanced salt and osmotic stress tolerance.

METHODS

The KaNa+/H+ antiporter gene was cloned and over-expressed in tobacco under the control of CaMV35S promoter. Transgenic analysis was carried out to assess the stress tolerance ability of tobacco over-expressing KaNa+/H+ antiporter gene.

RESULTS

Over-expression of KaNa+/H+ gene improved the seed germination and seed vigor index under stress. Transgenic plants grew better and exhibited delayed leaf senescence. Improved K+/Na+, carotenoid/total chlorophyll and relative water content; lower accumulation of reactive oxygen species (ROS), MDA and Na+; lower electrolyte leakage; better membrane stability index and accumulation of K+, photosynthetic pigment, starch, sugar, free amino acid, proline and polyphenol contents indicated better physiological health of the transgenic tobacco under stress. Transgenic tobacco exhibited higher photosynthesis, photosystem II efficiency, electron transfer rate, photochemical quenching and activity of water splitting complex. Compared with control tobacco, transgenic tobacco exhibited higher expression of stress-defence genes under stress and better recovery after long-term osmotic stress.

CONCLUSIONS

Lower Na+ cytotoxicity, lower accumulation of ROS and maintenance of the membrane integrity helped transgenic tobacco to maintain the physiological functioning under stress. Present results established K. alvarezii as a potential gene resource and the KaNa+/H+ antiporter gene as a potential candidate gene in molecular breeding of crops for development of the degraded land.

Transcription Profiles Reveal Age-Dependent Variations of Photosynthetic Properties and Sugar Metabolism in Grape Leaves (Vitis vinifera L.)

Leaves, considered as the ‘source’ organs, depend on the development stages because of the age-dependent photosynthesis and assimilation of leaves. However, the molecular mechanisms of age-dependent limitations on the function of leaves are seldom reported. In the present study, the photosynthesis-related characteristics and photoassimilates were investigated in grape leaves at six different age groups (Ll to L6) at micro-morphological, biochemical, and molecular levels. These results showed lower expression levels of genes associated with stomatal development, and chl biosynthesis resulted in fewer stomata and lowered chlorophyll a/b contents in L1 when compared to L3 and L5. The DEGs between L5 and L3/L1 were largely distributed at stomatal movement, carbon fixation, and sucrose and starch metabolism pathways, such as STOMATAL ANION CHANNEL PROTEIN 1 (SLAC1), FRUCTOSE-1,6-BISPHOSPHATE ALDOLASE (FBA1), SUCROSE-PHOSPHATE SYNTHASE (SPP1), and SUCROSE-PHOSPHATE PHOSPHATASE (SPS2, 4). These genes could be major candidate genes leading to increased photosynthesis capacity and sugar content in L5. The accumulation of starch grains in the chloroplast and palisade tissue of L5 and higher transcription levels of genes related to starch biosynthesis in L5 further supported the high ability of L5 to produce photoassimilates. Hence, our results provide insights for understanding different photosynthetic functions in age-dependent leaves in grape plants at the molecular level.