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Plant carbon metabolism is impacted by rising CO2 concentrations and temperatures, but also feeds back onto the climate system to help determine the trajectory of future climate change. Here we review how photosynthesis, photorespiration and respiration are affected by increasing atmospheric CO2 concentrations and climate warming, both separately and in combination. We also compile data from the literature on plants grown at multiple temperatures, focusing on net CO2 assimilation rates and leaf dark respiration rates measured at the growth temperature (A growth and R growth, respectively). Our analyses show that the ratio of A growth to R growth is generally homeostatic across a wide range of species and growth temperatures, and that species that have reduced A growth at higher growth temperatures also tend to have reduced R growth, while species that show stimulations in A growth under warming tend to have higher R growth in the hotter environment. These results highlight the need to study these physiological processes together to better predict how vegetation carbon metabolism will respond to climate change.

As global temperatures rise, improving crop yields will require enhancing the thermotolerance of crops. One approach for improving thermotolerance is using bioengineering to increase the thermostability of enzymes catalysing essential biological processes. Photorespiration is an essential recycling process in plants that is integral to photosynthesis and crop growth. The enzymes of photorespiration are targets for enhancing plant thermotolerance as this pathway limits carbon fixation at elevated temperatures. We explored the effects of temperature on the activity of the photorespiratory enzyme glycerate kinase (GLYK) from various organisms and the homologue from the thermophilic alga Cyanidioschyzon merolae was more thermotolerant than those from mesophilic plants, including Arabidopsis thaliana. To understand enzyme features underlying the thermotolerance of C. merolae GLYK (CmGLYK), we performed molecular dynamics simulations using AlphaFold-predicted structures, which revealed greater movement of loop regions of mesophilic plant GLYKs at higher temperatures compared to CmGLYK. Based on these simulations, hybrid proteins were produced and analysed. These hybrid enzymes contained loop regions from CmGLYK replacing the most mobile corresponding loops of AtGLYK. Two of these hybrid enzymes had enhanced thermostability, with melting temperatures increased by 6 °C. One hybrid with three grafted loops maintained higher activity at elevated temperatures. Whilst this hybrid enzyme exhibited enhanced thermostability and a similar Km for ATP compared to AtGLYK, its Km for glycerate increased threefold. This study demonstrates that molecular dynamics simulation-guided structure-based recombination offers a promising strategy for enhancing the thermostability of other plant enzymes with possible application to increasing the thermotolerance of plants under warming climates.

As plants are sessile organisms that have to attune their physiology and morphology continuously to varying environmental challenges in order to survive and reproduce, they have evolved complex and integrated environment–cell, cell–cell, and cell–organelle signalling circuits that regulate and trigger the required adjustments (such as alteration of gene expression). Although reactive oxygen species (ROS) are essential components of this network, their pathways are not yet completely unravelled. In addition to the intrinsic chemical properties that define the array of interaction partners, mobility, and stability, ROS signalling specificity is obtained via the spatiotemporal control of production and scavenging at different organellar and subcellular locations (e.g. chloroplasts, mitochondria, peroxisomes, and apoplast). Furthermore, these cellular compartments may crosstalk to relay and further fine-tune the ROS message. Hence, plant cells might locally and systemically react upon environmental or developmental challenges by generating spatiotemporally controlled dosages of certain ROS types, each with specific chemical properties and interaction targets, that are influenced by interorganellar communication and by the subcellular location and distribution of the involved organelles, to trigger the suitable acclimation responses in association with other well-established cellular signalling components (e.g. reactive nitrogen species, phytohormones, and calcium ions). Further characterization of this comprehensive ROS signalling matrix may result in the identification of new targets and key regulators of ROS signalling, which might be excellent candidates for engineering or breeding stress-tolerant plants.

The non-proteinogenic amino acid ornithine links several stress response pathways. From a previous study we know that ornithine accumulates in response to low CO.sub.2 . To investigate ornithine accumulation in plants, we shifted plants to either low CO.sub.2 or low light. Both conditions increased carbon limitation, but only low CO.sub.2 also increased the rate of photorespiration. Changes in metabolite profiles of light- and CO.sub.2 -limited plants were quite similar. Several amino acids that are known markers of senescence accumulated strongly under both conditions. However, urea cycle intermediates respond differently between the two treatments. While the levels of both ornithine and citrulline were much higher in plants shifted to 100 ppm CO.sub.2 compared to those kept in 400 ppm CO.sub.2, their metabolite abundance did not significantly change in response to a light limitation. Furthermore, both ornithine and citrulline accumulation is independent from sugar starvation. Exogenous supplied sugar did not significantly change the accumulation of the two metabolites in low CO.sub.2 -stressed plants, while the accumulation of other amino acids was reduced by about 50%. Gene expression measurements showed a reduction of the entire arginine biosynthetic pathway in response to low CO.sub.2 . Genes in both proline biosynthesis and degradation were induced. Hence, proline did not accumulate in response to low CO.sub.2 like observed for many other stresses. We propose that excess of nitrogen re-fixed during photorespiration can be alternatively stored in ornithine and citrulline under low CO.sub.2 conditions. Furthermore, ornithine is converted to pyrroline-5-carboxylate by the action of [delta]OAT.

Plant RuBisCo, a complex of eight large and eight small subunits, catalyzes the fixation of CO₂ in photosynthesis. The low catalytic efficiency of RuBisCo provides strong motivation to reengineer the enzyme with the goal of increasing crop yields. However, genetic manipulation has been hampered by the failure to express plant RuBisCo in a bacterial host. We achieved the functional expression of Arabidopsis thaliana RuBisCo in Escherichia coli by coexpressing multiple chloroplast chaperones. These include the chaperonins Cpn60/Cpn20, RuBisCo accumulation factors 1 and 2, RbcX, and bundle-sheath defective-2 (BSD2). Our structural and functional analysis revealed the role of BSD2 in stabilizing an end-state assembly intermediate of eight RuBisCo large subunits until the small subunits become available. The ability to produce plant RuBisCo recombinantly will facilitate efforts to improve the enzyme through mutagenesis.

In some of our most useful crops (such as rice and wheat), photosynthesis produces toxic by-products that reduce its efficiency. Photorespiration deals with these by-products, converting them into metabolically useful components, but at the cost of energy lost. South et al. constructed a metabolic pathway in transgenic tobacco plants that more efficiently recaptures the unproductive by-products of photosynthesis with less energy lost (see the Perspective by Eisenhut and Weber). In field trials, these transgenic tobacco plants were ∼40% more productive than wild-type tobacco plants. Science , this issue p. eaat9077 ; see also p. 32 Tobacco plants carrying engineered glycolate metabolic pathways showed as much as 40% greater productivity than wild-type plants in field trials. Photorespiration is required in C 3 plants to metabolize toxic glycolate formed when ribulose-1,5-bisphosphate carboxylase-oxygenase oxygenates rather than carboxylates ribulose-1,5-bisphosphate. Depending on growing temperatures, photorespiration can reduce yields by 20 to 50% in C 3 crops. Inspired by earlier work, we installed into tobacco chloroplasts synthetic glycolate metabolic pathways that are thought to be more efficient than the native pathway. Flux through the synthetic pathways was maximized by inhibiting glycolate export from the chloroplast. The synthetic pathways tested improved photosynthetic quantum yield by 20%. Numerous homozygous transgenic lines increased biomass productivity between 19 and 37% in replicated field trials. These results show that engineering alternative glycolate metabolic pathways into crop chloroplasts while inhibiting glycolate export into the native pathway can drive increases in C 3 crop yield under agricultural field conditions.

Net photosynthetic CO 2 assimilation rate ( A n ) decreases at leaf temperatures above a relatively mild optimum ( T opt ) in most higher plants. This decline is often attributed to reduced CO 2 conductance, increased CO 2 loss from photorespiration and respiration, reduced chloroplast electron transport rate ( J ), or deactivation of Ribulose-1,5-bisphosphate Carboxylase Oxygenase (Rubisco). However, it is unclear which of these factors can best predict species independent declines in A n at high temperature. We show that independent of species, and on a global scale, the observed decline in A n with rising temperatures can be effectively accounted for by Rubisco deactivation and declines in J . Our finding that A n declines with Rubisco deactivation and J supports a coordinated down-regulation of Rubisco and chloroplast electron transport rates to heat stress. We provide a model that, in the absence of CO 2 supply limitations, can predict the response of photosynthesis to short-term increases in leaf temperature. Photosynthesis declines at mild temperatures in terrestrial plants. Here, the authors use published data to show that decline in photosynthetic CO 2 assimilation rate with rising temperatures can be accounted for by Rubisco deactivation and declines in chloroplast electron transport rate.

C₄ plants evolved independently more than 60 times from C₃ ancestors. C₄ photosynthesis is a complex trait and its evolution from the ancestral C₃ photosynthetic pathway involved the modification of the leaf anatomy and the leaf physiology accompanied by changes in the expression of thousands of genes. Under high temperature, high light, and the current CO₂ concentration in the atmosphere, the C₄ pathway is more efficient than C₃ photosynthesis because it increases the CO₂ concentration around the major CO₂ fixating enzyme Rubisco. The oxygenase reaction and, accordingly, photorespiration are largely suppressed. In the present review we describe a scenario for C₄ evolution that not only includes the avoidance of photorespiration as the major driving force for C₄ evolution but also highlights the relevance of changes in the expression of photorespiratory genes in inducing and establishing important phases on the path from C₃ to C₄.

It has been 75 yr since leaf respiratory metabolism in the light (day respiration) was identified as a low-flux metabolic pathway that accompanies photosynthesis. In principle, it provides carbon backbones for nitrogen assimilation and evolves CO2 and thus impacts on plant carbon and nitrogen balances. However, for a long time, uncertainties have remained as to whether techniques used to measure day respiratory efflux were valid and whether day respiration responded to environmental gaseous conditions. In the past few years, significant advances have beenmade using carbon isotopes, ‘omics’ analyses and surveys of respiration rates in mesocosms or ecosystems. There is substantial evidence that day respiration should be viewed as a highly dynamic metabolic pathway that interacts with photosynthesis and photorespiration and responds to atmospheric CO2 mole fraction. The view of leaf day respiration as a constant and/or negligible parameter of net carbon exchange isnow outdated and it should now be regarded as a central actor of plant carbon-use efficiency.

Background Mango is one of the world’s most important tropical fruits. It belongs to the family Anacardiaceae, which includes several other economically important species, notably cashew, sumac and pistachio from other genera. Many species in this family produce family-specific urushiols and related phenols, which can induce contact dermatitis. Results We generate a chromosome-scale genome assembly of mango, providing a reference genome for the Anacardiaceae family. Our results indicate the occurrence of a recent whole-genome duplication (WGD) event in mango. Duplicated genes preferentially retained include photosynthetic, photorespiration, and lipid metabolic genes that may have provided adaptive advantages to sharp historical decreases in atmospheric carbon dioxide and global temperatures. A notable example of an extended gene family is the chalcone synthase (CHS) family of genes, and particular genes in this family show universally higher expression in peels than in flesh, likely for the biosynthesis of urushiols and related phenols. Genome resequencing reveals two distinct groups of mango varieties, with commercial varieties clustered with India germplasms and demonstrating allelic admixture, and indigenous varieties from Southeast Asia in the second group. Landraces indigenous in China formed distinct clades, and some showed admixture in genomes. Conclusions Analysis of chromosome-scale mango genome sequences reveals photosynthesis and lipid metabolism are preferentially retained after a recent WGD event, and expansion of CHS genes is likely associated with urushiol biosynthesis in mango. Genome resequencing clarifies two groups of mango varieties, discovers allelic admixture in commercial varieties, and shows distinct genetic background of landraces.