Photorespiratory compensation: a driver for biological diversity

Tribulus (Zygophyllaceae) as a case study for the evolution of C2 and C4 photosynthesis

C2 photosynthesis is a photosynthetic pathway in which photorespiratory CO2 release and refixation are enhanced in leaf bundle sheath (BS) tissues. The evolution of C2 photosynthesis has been hypothesized to be a major step in the origin of C4 photosynthesis, highlighting the importance of studying C2 evolution. In this study, physiological, anatomical, ultrastructural, and immunohistochemical properties of leaf photosynthetic tissues were investigated in six non-C4 Tribulus species and four C4 Tribulus species. At 42°C, T. cristatus exhibited a photosynthetic CO2 compensation point in the absence of respiration (C*) of 21 µmol mol-1, below the C3 mean C* of 73 µmol mol-1. Tribulus astrocarpus had a C* value at 42°C of 55 µmol mol-1, intermediate between the C3 species and the C2 T. cristatus. Glycine decarboxylase (GDC) allocation to BS tissues was associated with lower C*. Tribulus cristatus and T. astrocarpus allocated 86% and 30% of their GDC to the BS tissues, respectively, well above the C3 mean of 11%. Tribulus astrocarpus thus exhibits a weaker C2 (termed sub-C2) phenotype. Increased allocation of mitochondria to the BS and decreased length-to-width ratios of BS cells, were present in non-C4 species, indicating a potential role in C2 and C4 evolution.

Improving Crop Yield through Increasing Carbon Gain and Reducing Carbon Loss

Photosynthesis is a process where solar energy is utilized to convert atmospheric CO2 into carbohydrates, which forms the basis for plant productivity. The increasing demand for food has created a global urge to enhance yield. Earlier, the plant breeding program was targeting the yield and yield-associated traits to enhance the crop yield. However, the yield cannot be further improved without improving the leaf photosynthetic rate. Hence, in this review, various strategies to enhance leaf photosynthesis were presented. The most promising strategies were the optimization of Rubisco carboxylation efficiency, the introduction of a CO2 concentrating mechanism in C3 plants, and the manipulation of photorespiratory bypasses in C3 plants, which are discussed in detail. Improving Rubisco's carboxylation efficiency is possible by engineering targets such as Rubisco subunits, chaperones, and Rubisco activase enzyme activity. Carbon-concentrating mechanisms can be introduced in C3 plants by the adoption of pyrenoid and carboxysomes, which can increase the CO2 concentration around the Rubisco enzyme. Photorespiration is the process by which the fixed carbon is lost through an oxidative process. Different approaches to reduce carbon and nitrogen loss were discussed. Overall, the potential approaches to improve the photosynthetic process and the way forward were discussed in detail.

Effects of photoperiod on the growth and physiological responses in Ulva prolifera under constant and diurnal temperature difference conditions

Photoperiod and temperature are two main factors in the growth of macroalgae, and changes in photoperiod and diurnal temperature difference exist in natural condition. In order to study the effects of photoperiod and diurnal temperature difference on the growth of green algae Ulva prolifera, we cultured this species under three light/dark cycles (light: dark = 10:14, 12:12 and 16:08) with constant (22 °C for light and dark period, noted as 22-22 °C) and diurnal temperature difference (22 °C and 16 °C for light and dark period, respectively, noted as 22-16 °C) conditions. The results showed that: 1) Compared with 10:14 light/dark cycle, the growth of U. prolifera under 12:12 light/dark cycle was significantly enhanced by 39% and 16% for 22-22 °C and 22-16 °C treatments, respectively, while the increase proportion decreased when the daylength increase from 12 h to 16 h. 2) The enhancement in growth induced by diurnal temperature difference was observed under 10:14 light/dark cycle, but not for 12:12 and 16:08 light/dark cycle treatments. 3) The Chl a content and photosynthetic rate increased under short light period and 22-22 °C conditions, while under 22-16 °C conditions, higher photosynthetic rate was observed under 12:12 light/dark cycle and no significant difference in Chl a content was observed. 4) Under 22-22 °C conditions, compared with 10:14 (L:D) treatment, the expression levels of proteins in light-harvesting complexes, PSII and carbon fixation were down regulated, while the photorespiration and pentose phosphate pathway (PPP) were up regulated by 16:08 light dark cycle. Then we speculate that the higher photosynthetic rate may be one compensation mechanism in short photoperiod, and under long light period condition the up regulations of photorespiration and PPP can be in charge of the decrease in enhancement growth induced by longer daylength.

Atmospheric CO2 decline and the timing of CAM plant evolution

BACKGROUND AND AIMS

CAM photosynthesis is hypothesized to have evolved in atmospheres of low CO2 concentration in recent geological time because of its ability to concentrate CO2 around Rubisco and boost water use efficiency relative to C3 photosynthesis. We assess this hypothesis by compiling estimates of when CAM clades arose using phylogenetic chronograms for 73 CAM clades. We further consider evidence of how atmospheric CO2 affects CAM relative to C3 photosynthesis.

RESULTS

Where CAM origins can be inferred, strong CAM is estimated to have appeared in the past 30 million years in 46 of 48 examined clades, after atmospheric CO2 had declined from high (near 800 ppm) to lower (<450 ppm) values. In turn, 21 of 25 clades containing CAM species (but where CAM origins are less certain) also arose in the past 30 million years. In these clades, CAM is probably younger than the clade origin. We found evidence for repeated weak CAM evolution during the higher CO2 conditions before 30 million years ago, and possible strong CAM origins in the Crassulaceae during the Cretaceous period prior to atmospheric CO2 decline. Most CAM-specific clades arose in the past 15 million years, in a similar pattern observed for origins of C4 clades.

CONCLUSIONS

The evidence indicates strong CAM repeatedly evolved in reduced CO2 conditions of the past 30 million years. Weaker CAM can pre-date low CO2 and, in the Crassulaceae, strong CAM may also have arisen in water-limited microsites under relatively high CO2. Experimental evidence from extant CAM species demonstrates that elevated CO2 reduces the importance of nocturnal CO2 fixation by increasing the contribution of C3 photosynthesis to daily carbon gain. Thus, the advantage of strong CAM would be reduced in high CO2, such that its evolution appears less likely and restricted to more extreme environments than possible in low CO2.

Different photorespiratory mechanisms in conifer leaves, where peroxisomes have intrinsically low catalase activity

Photorespiration is an essential metabolic mechanism associated with photosynthesis; however, little is known about the photorespiratory pathway of conifer gymnosperms. Metabolite analyses of the leaves of 27 tree species showed that the mean glycerate content in conifer leaves was lower than that in angiosperm leaves. We performed experiments where [13 C]-serine was fed to detached shoots of a conifer (Cryptomeria japonica), via the transpiration stream, and compared the labeling patterns of photorespiratory metabolites with those of an angiosperm tree (Populus nigra), because glycerate is produced from serine via hydroxypyruvate in peroxisomes. In P. nigra, hydroxypyruvate, glycerate and glycine were labeled with 13 C, whereas in C. japonica, glycolate and a non-canonical photorespiratory metabolite, formate, were also labeled, suggesting that an H2 O2 -mediated non-enzymatic decarboxylation (NED) reaction occurs in C. japonica. We analyzed changes in the metabolite contents of leaves kept in the dark and leaves exposed to illuminated photorespiration-promoting conditions: a positive relationship between formate and serine levels in C. japonica implied that the active C1 -metabolism pathway synthesizes serine from formate. Leaf gas exchange analyses revealed that CO2 produced through NED was recaptured by chloroplasts. Database analysis of the peroxisomal targeting signal motifs of an H2 O2 -scavenging enzyme, catalase, derived from various species, including nine coniferous species, as well as analyses of peroxisomal fractions isolated from C. japonica and P. nigra leaves indicated that conifer peroxisomes had less catalase activity. These results suggest that NED and the subsequent C1 metabolism are involved in the photorespiratory pathway of conifer leaves, where peroxisomes have intrinsically low catalase activity.

Improving plant heat tolerance through modification of Rubisco activase in C3 plants to secure crop yield and food security in a future warming world

The world's population may reach 10 billion by 2050, but 10% still suffer from food shortages. At the same time, global warming threatens food security by decreasing crop yields, so it is necessary to develop crops with enhanced resistance to high temperatures in order to secure the food supply. In this review, the role of Rubisco activase as an important factor in plant heat tolerance is summarized, based on the conclusions of recent findings. Rubisco activase is a molecular chaperone determining the activation of Rubisco, whose heat sensitivity causes reductions of photosynthesis at high temperatures. Thus, the thermostability of Rubisco activase is considered to be critical for improving plant heat tolerance. It has been shown that the introduction of thermostable Rubisco activase through gene editing into Arabidopsis thaliana and from heat-adapted wild Oryza species or C4Zea mays into Oryza sativa improves Rubisco activation, photosynthesis, and plant growth at high temperatures. We propose that developing a universal thermostable Rubisco activase could be a promising direction for further studies.

Seed size effects on plant establishment under low atmospheric CO2, with implications for seed size evolution

BACKGROUND AND AIMS

Low atmospheric CO2 concentration depresses photosynthesis and resource use efficiency, and therefore can inhibit phases of the life cycle such as seedling establishment. Seed reserves can compensate for photosynthetic inhibition by accelerating seedling growth. We therefore hypothesize that seedlings arising from large seeds show less inhibition from low atmospheric CO2 than young plants from small seeds. Seed size effects on seedling responses to low CO2 may also be enhanced in warm environments, due to greater photorespiration at high temperature.

METHODS

Phaseolus and Vigna seeds differing in mass by over two orders of magnitude were planted and grown for 14 d in growth chambers with CO2 concentrations of 370, 180 or 100 ppm, in thermal regimes of 25 °C/19 °C, 30 °C/24 °C or 35 °C/29 °C (day/night). We measured leaf area expansion, shoot growth and mortality of the seedlings arising from the variously sized seeds at 14 days after planting (14 DAP).

KEY RESULTS

Relative to small-seeded plants, large-seeded genotypes produced greater leaf area and shoot mass at 14 DAP across the range of CO2 treatments in the 25 °C/19 °C and 30 °C/24 °C regimes, and at 100 ppm in the 35 °C/29 °C treatment. The proportional decline in leaf area and seed mass with CO2 reduction was generally greater for seedlings arising from small than from large seeds. Reductions in leaf area due to CO2 reduction increased in the warmer temperature treatments. In the 35 °C/19 °C treatment at 100 ppm CO2, seedling mortality was greater in small- than in large-seeded genotypes, and the small-seeded genotypes were unable to exit the seedling stage by the end of the experiment.

CONCLUSIONS

The results support a hypothesis that seedlings from large seeds grow and establish better than seedlings from small seeds in warm, low CO2 environments. During low CO2 episodes in Earth's history, such as the past 30 million years, large seeds may have been favoured by natural selection in warm environments. With the recent rise in atmospheric CO2 due to human activities, trade-offs between seed size and number may already be affected, such that seed size today may be non-optimal in their natural habitats.

Insights into Regulation of C2 and C4 Photosynthesis in Amaranthaceae/Chenopodiaceae Using RNA-Seq

Amaranthaceae (incl. Chenopodiaceae) shows an immense diversity of C4 syndromes. More than 15 independent origins of C4 photosynthesis, and the largest number of C4 species in eudicots signify the importance of this angiosperm lineage in C4 evolution. Here, we conduct RNA-Seq followed by comparative transcriptome analysis of three species from Camphorosmeae representing related clades with different photosynthetic types: Threlkeldia diffusa (C3), Sedobassia sedoides (C2), and Bassia prostrata (C4). Results show that B. prostrata belongs to the NADP-ME type and core genes encoding for C4 cycle are significantly upregulated when compared with Sed. sedoides and T. diffusa. Sedobassia sedoides and B. prostrata share a number of upregulated C4-related genes; however, two C4 transporters (DIT and TPT) are found significantly upregulated only in Sed. sedoides. Combined analysis of transcription factors (TFs) of the closely related lineages (Camphorosmeae and Salsoleae) revealed that no C3-specific TFs are higher in C2 species compared with C4 species; instead, the C2 species show their own set of upregulated TFs. Taken together, our study indicates that the hypothesis of the C2 photosynthesis as a proxy towards C4 photosynthesis is questionable in Sed. sedoides and more in favour of an independent evolutionary stable state.

The Coevolution of RuBisCO, Photorespiration, and Carbon Concentrating Mechanisms in Higher Plants

Ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (RuBisCO) is the carbon-fixing enzyme present in most photosynthetic organisms, converting CO₂ into organic matter. Globally, photosynthetic efficiency in terrestrial plants has become increasingly challenged in recent decades due to a rapid increase in atmospheric CO₂ and associated changes toward warmer and dryer environments. Well adapted for these new climatic conditions, the C₄ photosynthetic pathway utilizes carbon concentrating mechanisms to increase CO₂ concentrations surrounding RuBisCO, suppressing photorespiration from the oxygenase catalyzed reaction with O₂. The energy efficiency of C₃ photosynthesis, from which the C₄ pathway evolved, is thought to rely critically on an uninterrupted supply of chloroplast CO₂. Part of the homeostatic mechanism that maintains this constancy of supply involves the CO₂ produced as a byproduct of photorespiration in a negative feedback loop. Analyzing the database of RuBisCO kinetic parameters, we suggest that in genera (Flaveria and Panicum) for which both C₃ and C₄ examples are available, the C₄ pathway evolved only from C₃ ancestors possessing much lower than the average carboxylase specificity relative to that of the oxygenase reaction (S _C/O=S _C/S _O), and hence, the higher CO₂ levels required for development of the photorespiratory CO₂ pump (C₂ photosynthesis) essential in the initial stages of C₄ evolution, while in the later stage (final optimization phase in the Flaveria model) increased CO₂ turnover may have occurred, which would have been supported by the higher CO₂ levels. Otherwise, C₄ RuBisCO kinetic traits remain little changed from the ancestral C₃ species. At the opposite end of the spectrum, C₃ plants (from Limonium) with higher than average S _C/O, which may be associated with the ability of increased CO₂, relative to O₂, affinity to offset reduced photorespiration and chloroplast CO₂ levels, can tolerate high stress environments. It is suggested that, instead of inherently constrained by its kinetic mechanism, RuBisCO possesses the extensive kinetic plasticity necessary for adaptation to changes in photorespiration that occur in the homeostatic regulation of CO₂ supply under a broad range of abiotic environmental conditions.

Photorespiration in the context of Rubisco biochemistry, CO2 diffusion and metabolism

Photorespiratory metabolism is essential for plants to maintain functional photosynthesis in an oxygen-containing environment. Because the oxygenation reaction of Rubisco is followed by the loss of previously fixed carbon, photorespiration is often considered a wasteful process and considerable efforts are aimed at minimizing the negative impact of photorespiration on the plant's carbon uptake. However, the photorespiratory pathway has also many positive aspects, as it is well integrated within other metabolic processes, such as nitrogen assimilation and C₁ metabolism, and it is important for maintaining the redox balance of the plant. The overall effect of photorespiratory carbon loss on the net CO₂ fixation of the plant is also strongly influenced by the physiology of the leaf related to CO₂ diffusion. This review outlines the distinction between Rubisco oxygenation and photorespiratory CO₂ release as a basis to evaluate the costs and benefits of photorespiration.

Stomatal and non-stomatal limitations are responsible in down-regulation of photosynthesis in melon plants grown under the saline condition: Application of carbon isotope discrimination as a reliable proxy

Salinity is one of the most severe environmental stresses limiting agricultural crop production worldwide. Photosynthesis is one of the main biochemical processes getting affected by such stress conditions. Here we investigated the stomatal and non-stomatal factors during photosynthesis in two Iranian melon genotypes "Ghobadlu" and "Suski-e-Sabz", as well as the "Galia" F1 cultivar, with an insight into better understanding the physiological mechanisms involved in the response of melon plants to increasing salinity. After plants were established in the greenhouse, they were supplied with nutrient solutions containing three salinity levels (0, 50, or 100 mM NaCl) for 15 and 30 days. With increasing salinity, almost all of the measured traits (e.g. stomatal conductance, transpiration rate, internal to ambient CO₂ concentration ratio (C_i/C_a), Rubisco and nitrate reductase activity, carbon isotope discrimination (Δ¹³C), chlorophyll content, relative water content (RWC), etc.) significantly decreased after 15 and 30 days of treatments. In contrast, the overall mean of water use efficiency (intrinsic and instantaneous WUE), leaf abscisic acid (ABA) and flavonol contents, as well as osmotic potential (Ψ_S), all increased remarkably with increasing stress, across all genotypes. In addition, notable correlations were found between Δ¹³C and leaf gas exchange parameters as well as most of the measured traits (e.g. leaf area, biomass, RWC, Ψ_S, etc.), encouraging the possibility of using Δ¹³C as an important proxy for indirect selection of melon genotypes with higher photosynthetic capacity and higher salinity tolerance. The overall results suggest that both stomatal and non-stomatal limitations play an important role in reduced photosynthesis rate in melon genotypes studied under NaCl stress. This conclusion is supported by the concurrently increased resistance to CO₂ diffusion, and lower Rubisco activity under NaCl treatments at the two sampling dates, and this was revealed by the appearance of lower C_i/C_a ratios and lower Δ¹³C in the leaves of salt-treated plants.

Photosynthetic capacity, leaf respiration and growth in two papaya (Carica papaya) genotypes with different leaf chlorophyll concentrations

Golden genotype of papaya (Carica papaya), named for its yellowish leaves, produces fruits very much appreciated by consumers worldwide. However, its growth and yield are considerably lower than those of other genotypes, such as 'Sunrise Solo', which has intensely green leaves. We undertook an investigation with the goal of evaluating key physiological traits that can affect biomass accumulation of both Golden and Sunrise Solo genotypes. Papaya seeds from two different genotypes with contrasting leaf colour 'Sunrise Solo' and Golden were grown in greenhouse conditions. Plant growth (plant height, leaf number, stem diameter, leaf area, plant dry weight), leaf gas exchanges, leaf carbon balance, RuBisCO oxygenation and carboxylation rates, nitrogen, as well as chlorophyll concentrations and fluorescence variables were assessed. Although no significant differences were observed for photosynthetic rates between genotypes, the accumulation of small differences in photosynthesis, day after day, over a long period, might contribute to some extend to a higher C-budget in Sunrise Solo, higher leaf area and, thus, to higher productivity. Additionally, we consider that physiological processes other than photosynthesis and leaf respiration can be as well involved in lower growth and yield of Golden. One of these aspects could be related to the higher rates of photorespiration observed in Sunrise Solo, which could improve the rate of N assimilation into organic compounds, such as amino acids, thus contributing to the higher biomass production in Sunrise Solo relative to Golden. Further experiments to evaluate the effects of N metabolism on physiology and growth of Golden are required as it has the potential to limit its yield.

Low assimilation efficiency of photorespiratory ammonia in conifer leaves

Glutamine synthetase (GS) localized in the chloroplasts, GS2, is a key enzyme in the assimilation of ammonia (NH₃) produced from the photorespiration pathway in angiosperms, but it is absent from some coniferous species belonging to Pinaceae such as Pinus. We examined whether the absence of GS2 is common in conifers (Pinidae) and also addressed the question of whether assimilation efficiency of photorespiratory NH₃ differs between conifers that may potentially lack GS2 and angiosperms. Search of the expressed sequence tag database of Cryptomeria japonica, a conifer in Cupressaceae, and immunoblotting analyses of leaf GS proteins of 13 species from all family members in Pinidae revealed that all tested conifers exhibited only GS1 isoforms. We compared leaf NH₃ compensation point (γ_NH3) and the increments in leaf ammonium content per unit photorespiratory activity (NH₃ leakiness), i.e. inverse measures of the assimilation efficiency, between conifers (C. japonica and Pinus densiflora) and angiosperms (Phaseolus vulgaris and two Populus species). Both γ_NH3 and NH₃ leakiness were higher in the two conifers than in the three angiosperms tested. Thus, we concluded that the absence of GS2 is common in conifers, and assimilation efficiency of photorespiratory NH₃ is intrinsically lower in conifer leaves than in angiosperm leaves. These results imply that acquisition of GS2 in land plants is an adaptive mechanism for efficient NH₃ assimilation under photorespiratory environments.

Some like it hot: the physiological ecology of C4 plant evolution

The evolution of C₄ photosynthesis requires an intermediate phase where photorespiratory glycine produced in the mesophyll cells must flow to the vascular sheath cells for metabolism by glycine decarboxylase. This glycine flux concentrates photorespired CO₂ within the sheath cells, allowing it to be efficiently refixed by sheath Rubisco. A modest C₄ biochemical cycle is then upregulated, possibly to support the refixation of photorespired ammonia in sheath cells, with subsequent increases in C₄ metabolism providing incremental benefits until an optimized C₄ pathway is established. 'Why' C₄ photosynthesis evolved is largely explained by ancestral C₃ species exploiting photorespiratory CO₂ to improve carbon gain and thus enhance fitness. While photorespiration depresses C₃ performance, it produces a resource (photorespired CO₂) that can be exploited to build an evolutionary bridge to C₄ photosynthesis. 'Where' C₄ evolved is indicated by the habitat of species branching near C₃-to-C₄ transitions on phylogenetic trees. Consistent with the photorespiratory bridge hypothesis, transitional species show that the large majority of > 60 C₄ lineages arose in hot, dry, and/or saline regions where photorespiratory potential is high. 'When' C₄ evolved has been clarified by molecular clock analyses using phylogenetic data, coupled with isotopic signatures from fossils. Nearly all C₄ lineages arose after 25 Ma when atmospheric CO₂ levels had fallen to near current values. This reduction in CO₂, coupled with persistent high temperature at low-to-mid-latitudes, met a precondition where photorespiration was elevated, thus facilitating the evolutionary selection pressure that led to C₄ photosynthesis.

Decrease in the Photosynthetic Performance of Temperate Grassland Species Does Not Lead to a Decline in the Gross Primary Production of the Ecosystem

Plants, under stressful conditions, can proceed to photosynthetic adjustments in order to acclimatize and alleviate the detrimental impacts on the photosynthetic apparatus. However, it is currently unclear how adjustment of photosynthetic processes under environmental constraints by plants influences CO2 gas exchange at the ecosystem-scale. Over a 2-year period, photosynthetic performance of a temperate grassland ecosystem was characterized by conducting frequent chlorophyll fluorescence (ChlF) measurements on three primary grassland species (Lolium perenne L., Taraxacum sp., and Trifolium repens L.). Ecosystem photosynthetic performance was estimated from measurements performed on the three dominant grassland species weighed based on their relative abundance. In addition, monitoring CO2 fluxes was performed by eddy covariance. The highest decrease in photosynthetic performance was detected in summer, when environmental constraints were combined. Dicot species (Taraxacum sp. and T. repens) presented the strongest capacity to up-regulate PSI and exhibited the highest electron transport efficiency under stressful environmental conditions compared with L. perenne. The decline in ecosystem photosynthetic performance did not lead to a reduction in gross primary productivity, likely because increased light energy was available under these conditions. The carbon amounts fixed at light saturation were not influenced by alterations in photosynthetic processes, suggesting photosynthesis was not impaired. Decreased photosynthetic performance was associated with high respiration flux, but both were influenced by temperature. Our study revealed variation in photosynthetic performance of a grassland ecosystem responded to environmental constraints, but alterations in photosynthetic processes appeared to exhibit a negligible influence on ecosystem CO2 fluxes.

Surveying the expanding prokaryotic Rubisco multiverse

The universal, but catalytically modest, CO2-fixing enzyme Rubisco is currently experiencing intense interest by researchers aiming to enhance crop photosynthesis. These efforts are mostly focused on the highly conserved hexadecameric enzyme found in land plants. In comparison, prokaryotic organisms harbor a far greater diversity in Rubisco forms. Recent work towards improving our appreciation of microbial Rubisco properties and harnessing their potential is surveyed. New structural models are providing informative glimpses into catalytic subtleties and diverse oligomeric states. Ongoing characterization is informing us about the conservation of constraints, such as sugar phosphate inhibition and the associated dependence on Rubisco activase helper proteins. Prokaryotic Rubiscos operate under a far wider range of metabolic contexts than the photosynthetic function of higher plant enzymes. Relaxed selection pressures may have resulted in the exploration of a larger volume of sequence space than permitted in organisms performing oxygenic photosynthesis. To tap into the potential of microbial Rubiscos, in vivo selection systems are being used to discover functional metagenomic Rubiscos. Various directed evolution systems to optimize their function have been developed. It is anticipated that this approach will provide access to biotechnologically valuable enzymes that cannot be encountered in the higher plant Rubisco space.

The Diverse AAA+ Machines that Repair Inhibited Rubisco Active Sites

Gaseous carbon dioxide enters the biosphere almost exclusively via the active site of the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). This highly conserved catalyst has an almost universal propensity to non-productively interact with its substrate ribulose 1,5-bisphosphate, leading to the formation of dead-end inhibited complexes. In diverse autotrophic organisms this tendency has been counteracted by the recruitment of dedicated AAA+ (ATPases associated with various cellular activities) proteins that all use the energy of ATP hydrolysis to remodel inhibited Rubisco active sites leading to release of the inhibitor. Three evolutionarily distinct classes of these Rubisco activases (Rcas) have been discovered so far. Green and red-type Rca are mostly found in photosynthetic eukaryotes of the green and red plastid lineage respectively, whereas CbbQO is associated with chemoautotrophic bacteria. Ongoing mechanistic studies are elucidating how the various motors are utilizing both similar and contrasting strategies to ultimately perform their common function of cracking the inhibited Rubisco active site. The best studied mechanism utilized by red-type Rca appears to involve transient threading of the Rubisco large subunit C-terminal peptide, reminiscent of the action performed by Clp proteases. As well as providing a fascinating example of convergent molecular evolution, Rca proteins can be considered promising crop-improvement targets. Approaches aiming to replace Rubisco in plants with improved enzymes will need to ensure the presence of a compatible Rca protein. The thermolability of the Rca protein found in crop plants provides an opportunity to fortify photosynthesis against high temperature stress. Photosynthesis also appears to be limited by Rca when light conditions are fluctuating. Synthetic biology strategies aiming to enhance the autotrophic CO₂ fixation machinery will need to take into consideration the requirement for Rubisco activases as well as their properties.

Exploiting Natural Variation to Discover Candidate Genes Involved in Photosynthesis-Related Traits

Naturally occurring genetic variation in plants can be very useful to dissect the complex regulation of primary metabolism as well as of physiological traits such as photosynthesis and photorespiration. The physiological and genetic mechanisms underlying natural variation in closely related species or accessions may provide important information that can be used to improve crop yield. In this chapter we describe in detail the use of a population of introgression lines (ILs), with the Solanum pennellii IL population as a study case, as a tool for the identification of genomic regions involved in the control of photosynthetic efficiency.

Why are there no C4 forests?

C₄ photosynthesis is absent from the arborescent life form, with the exception of seven Hawaiian Euphorbia species and a few desert shrubs that become arborescent with age. As a consequence, wherever C₃ trees can establish, their height advantage enables them to outcompete low stature C₄ vegetation. Had C₄ photosynthesis been able to evolve in an arborescent life form, forest cover (by C₄ trees) could have been much more extensive than today, with significant consequences for the biosphere. Here, we address why there are so few C₄ trees. Physiological explanations associated with low light performance of C₄ photosynthesis are not supported, because C₄ shade-tolerant species exhibit similar performance as shade-tolerant C₃ species in terms of quantum yield, steady-state photosynthetic and use of sunflecks. Hence, hypothetical C₄ trees could occur in the regeneration niche of forests. Constraints associated with the evolutionary history of the C₄ lineages are more plausible. Most C₄ species are grasses and sedges, which lack meristems needed for arborescence, while most C₄ eudicots are highly specialized for harsh (arid, saline, hot) or disturbed habitats where arborescence may be maladapted. Most C₄ eudicot clades are also young, and have not had sufficient time to radiate beyond the extreme environments where C₄ evolution is favored. In the case of the Hawaiian Euphorbia species, they belong to one of the oldest and most diverse C₄ lineages, which primed this group to evolve arborescence in a low-competition environment that appeared on the remote Hawaiian Islands.

Modelling metabolic evolution on phenotypic fitness landscapes: a case study on C4 photosynthesis

How did the complex metabolic systems we observe today evolve through adaptive evolution? The fitness landscape is the theoretical framework to answer this question. Since experimental data on natural fitness landscapes is scarce, computational models are a valuable tool to predict landscape topologies and evolutionary trajectories. Careful assumptions about the genetic and phenotypic features of the system under study can simplify the design of such models significantly. The analysis of C4 photosynthesis evolution provides an example for accurate predictions based on the phenotypic fitness landscape of a complex metabolic trait. The C4 pathway evolved multiple times from the ancestral C3 pathway and models predict a smooth 'Mount Fuji' landscape accordingly. The modelled phenotypic landscape implies evolutionary trajectories that agree with data on modern intermediate species, indicating that evolution can be predicted based on the phenotypic fitness landscape. Future directions will have to include structural changes of metabolic fitness landscape structure with changing environments. This will not only answer important evolutionary questions about reversibility of metabolic traits, but also suggest strategies to increase crop yields by engineering the C4 pathway into C3 plants.

A novel mechanistic interpretation of instantaneous temperature responses of leaf net photosynthesis

Steady-state rates of leaf CO2 assimilation (A) in response to incubation temperature (T) are often symmetrical around an optimum temperature. A/T curves of C3 plants can thus be fitted to a modified Arrhenius equation, where the activation energy of A close to a low reference temperature is strongly correlated with the dynamic change of activation energy to increasing incubation temperature. We tested how [CO2] < current atmospheric levels and saturating light, or [CO2] at 800 µmol mol(-1) and variable light affect parameters that describe A/T curves, and how these parameters are related to known properties of temperature-dependent thylakoid electron transport. Variation of light intensity and substomatal [CO2] had no influence on the symmetry of A/T curves, but significantly affected their breadth. Thermodynamic and kinetic (physiological) factors responsible for (i) the curvature in Arrhenius plots and (ii) the correlation between parameters of a modified Arrhenius equation are discussed. We argue that the shape of A/T curves cannot satisfactorily be explained via classical concepts assuming temperature-dependent shifts between rate-limiting processes. Instead the present results indicate that any given A/T curve appears to reflect a distinct flux mode, set by the balance between linear and cyclic electron transport, and emerging from the anabolic demand for ATP relative to that for NADPH.

Spreading the news: subcellular and organellar reactive oxygen species production and signalling

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.

Passive CO2 concentration in higher plants

Photorespiratory limitations on C3 photosynthesis are substantial in warm, low CO2 conditions. To compensate, certain plants evolved mechanisms to actively concentrate CO2 around Rubisco using ATP-supported CO2 pumps such as C4 photosynthesis. Plants can also passively accumulate CO2 without additional ATP expenditure by localizing the release of photorespired and respired CO2 around Rubisco that is diffusively isolated from peripheral air spaces. Passive accumulation of photorespired CO2 occurs when glycine decarboxylase is localized to vascular sheath cells in what is termed C2 photosynthesis, and through forming sheaths of chloroplasts around the periphery of mesophyll cells. The peripheral sheaths require photorespired CO2 to re-enter chloroplasts where it can be refixed. Passive accumulation of respiratory CO2 is common in organs such as stems, fruits and flowers, due to abundant heterotrophic tissues and high diffusive resistance along the organ periphery. Chloroplasts within these organs are able to exploit this high CO2 to reduce photorespiration. CO2 concentration can also be enhanced passively by channeling respired CO2 from roots and rhizomes into photosynthetic cells of stems and leaves via lacunae, aerenchyma and the xylem stream. Through passive CO2 concentration, C3 species likely improved their carbon economy and maintained fitness during episodes of low atmospheric CO2.

Manipulating photorespiration to increase plant productivity: recent advances and perspectives for crop improvement

Recycling of the 2-phosphoglycolate generated by the oxygenase reaction of Rubisco requires a complex and energy-consuming set of reactions collectively known as the photorespiratory cycle. Several approaches aimed at reducing the rates of photorespiratory energy or carbon loss have been proposed, based either on screening for natural variation or by means of genetic engineering. Recent work indicates that plant yield can be substantially improved by the alteration of photorespiratory fluxes or by engineering artificial bypasses to photorespiration. However, there is also evidence indicating that, under certain environmental and/or nutritional conditions, reduced photorespiratory capacity may be detrimental to plant performance. Here we summarize recent advances obtained in photorespiratory engineering and discuss prospects for these advances to be transferred to major crops to help address the globally increasing demand for food and biomass production.

Photosynthetic diversity meets biodiversity: the C4 plant example

Physiological diversification reflects adaptation for specific environmental challenges. As the major physiological process that provides plants with carbon and energy, photosynthesis is under strong evolutionary selection that gives rise to variability in nearly all parts of the photosynthetic apparatus. Here, we discuss how plants, notably those using C4 photosynthesis, diversified in response to environmental challenges imposed by declining atmospheric CO2 content in recent geological time. This reduction in atmospheric CO2 increases the rate of photorespiration and reduces photosynthetic efficiency. While plants have evolved numerous mechanisms to compensate for low CO2, the most effective are the carbon concentration mechanisms of C4, C2, and CAM photosynthesis; and the pumping of dissolved inorganic carbon, mainly by algae. C4 photosynthesis enables plants to dominate warm, dry and often salinized habitats, and to colonize areas that are too stressful for most plant groups. Because C4 lineages generally lack arborescence, they cannot form forests. Hence, where they predominate, C4 plants create a different landscape than would occur if C3 plants were to predominate. These landscapes (mostly grasslands and savannahs) present unique selection environments that promoted the diversification of animal guilds able to graze upon the C4 vegetation. Thus, the rise of C4 photosynthesis has made a significant contribution to the origin of numerous biomes in the modern biosphere.

The biodiversity of carbon assimilation

As all plastids that have been investigated so far can be traced back to endosymbiotic uptake of cyanobacteria by heterotrophic host cells, they accordingly show a high similarity regarding photosynthesis, which includes both the photosystems and the biochemical reactions around the CO2 fixation via the Calvin-Bassham cycle. Major differences between the different algal and plant groups may include the presence or absence of carbon concentrating mechanisms, pyrenoids, Rubisco activases, carbonic anhydrases as well as differences in the regulation of the Calvin-Bassham cycle. This review describes the diversity of primary carbon fixation steps in algae and plants and the respective regulatory mechanisms.

On the use of leaf spectral indices to assess water status and photosynthetic limitations in Olea europaea L. during water-stress and recovery

Diffusional limitations to photosynthesis, relative water content (RWC), pigment concentrations and their association with reflectance indices were studied in olive (Olea europaea) saplings subjected to water-stress and re-watering. RWC decreased sharply as drought progressed. Following rewatering, RWC gradually increased to pre-stress values. Photosynthesis (A), stomatal conductance (gs), mesophyll conductance (gm), total conductance (gt), photochemical reflectance index (PRI), water index (WI) and relative depth index (RDI) closely followed RWC. In contrast, carotenoid concentration, the carotenoid to chlorophyll ratio, water content reflectance index (WCRI) and structural independent pigment index (SIPI) showed an opposite trend to that of RWC. Photosynthesis scaled linearly with leaf conductance to CO2; however, A measured under non-photorespiratory conditions (A1%O2) was approximately two times greater than A measured at 21% [O2], indicating that photorespiration likely increased in response to drought. A1%O2 also significantly correlated with leaf conductance parameters. These relationships were apparent in saturation type curves, indicating that under non-photorespiratory conditions, CO2 conductance was not the major limitations to A. PRI was significant correlated with RWC. PRI was also very sensitive to pigment concentrations and photosynthesis, and significantly tracked all CO2 conductance parameters. WI, RDI and WCRI were all significantly correlated with RWC, and most notably to leaf transpiration. Overall, PRI correlated more closely with carotenoid concentration than SIPI; whereas WI tracked leaf transpiration more effectively than RDI and WCRI. This study clearly demonstrates that PRI and WI can be used for the fast detection of physiological traits of olive trees subjected to water-stress.

From proto-Kranz to C4 Kranz: building the bridge to C4 photosynthesis

In this review, we examine how the specialized "Kranz" anatomy of C4 photosynthesis evolved from C3 ancestors. Kranz anatomy refers to the wreath-like structural traits that compartmentalize the biochemistry of C4 photosynthesis and enables the concentration of CO2 around Rubisco. A simplified version of Kranz anatomy is also present in the species that utilize C2 photosynthesis, where a photorespiratory glycine shuttle concentrates CO2 into an inner bundle-sheath-like compartment surrounding the vascular tissue. C2 Kranz is considered to be an intermediate stage in the evolutionary development of C4 Kranz, based on the intermediate branching position of C2 species in 14 evolutionary lineages of C4 photosynthesis. In the best-supported model of C4 evolution, Kranz anatomy in C2 species evolved from C3 ancestors with enlarged bundle sheath cells and high vein density. Four independent lineages have been identified where C3 sister species of C2 plants exhibit an increase in organelle numbers in the bundle sheath and enlarged bundle sheath cells. Notably, in all of these species, there is a pronounced shift of mitochondria to the inner bundle sheath wall, forming an incipient version of the C2 type of Kranz anatomy. This incipient version of C2 Kranz anatomy is termed proto-Kranz, and is proposed to scavenge photorespiratory CO2. By doing so, it may provide fitness benefits in hot environments, and thus represent a critical first stage of the evolution of both the C2 and C4 forms of Kranz anatomy.

The role of photorespiration during the evolution of C4 photosynthesis in the genus Flaveria

C4 photosynthesis represents a most remarkable case of convergent evolution of a complex trait, which includes the reprogramming of the expression patterns of thousands of genes. Anatomical, physiological, and phylogenetic and analyses as well as computational modeling indicate that the establishment of a photorespiratory carbon pump (termed C2 photosynthesis) is a prerequisite for the evolution of C4. However, a mechanistic model explaining the tight connection between the evolution of C4 and C2 photosynthesis is currently lacking. Here we address this question through comparative transcriptomic and biochemical analyses of closely related C3, C3-C4, and C4 species, combined with Flux Balance Analysis constrained through a mechanistic model of carbon fixation. We show that C2 photosynthesis creates a misbalance in nitrogen metabolism between bundle sheath and mesophyll cells. Rebalancing nitrogen metabolism requires anaplerotic reactions that resemble at least parts of a basic C4 cycle. Our findings thus show how C2 photosynthesis represents a pre-adaptation for the C4 system, where the evolution of the C2 system establishes important C4 components as a side effect.

Setaria viridis: a model for C4 photosynthesis

C(4) photosynthesis drives productivity in several major food crops and bioenergy grasses, including maize (Zea mays), sugarcane (Saccharum officinarum), sorghum (Sorghum bicolor), Miscanthus x giganteus, and switchgrass (Panicum virgatum). Gains in productivity associated with C(4) photosynthesis include improved water and nitrogen use efficiencies. Thus, engineering C(4) traits into C(3) crops is an attractive target for crop improvement. However, the lack of a small, rapid cycling genetic model system to study C(4) photosynthesis has limited progress in dissecting the regulatory networks underlying the C(4) syndrome. Setaria viridis is a member of the Panicoideae clade and is a close relative of several major feed, fuel, and bioenergy grasses. It is a true diploid with a relatively small genome of ~510 Mb. Its short stature, simple growth requirements, and rapid life cycle will greatly facilitate genetic studies of the C(4) grasses. Importantly, S. viridis uses an NADP-malic enzyme subtype C(4) photosynthetic system to fix carbon and therefore is a potentially powerful model system for dissecting C(4) photosynthesis. Here, we summarize some of the recent advances that promise greatly to accelerate the use of S. viridis as a genetic system. These include our recent successful efforts at regenerating plants from seed callus, establishing a transient transformation system, and developing stable transformation.