Elevated CO(2) modifies N acquisition of Medicago truncatula by enhancing N fixation and reducing nitrate uptake from soil

Signalling and regulation of plant development by carbon/nitrogen balance

The two most abundant macronutrients in plant cells are carbon (C) and nitrogen (N). Coordination of their cellular metabolism is a fundamental factor in guaranteeing the optimal growth and development of plants. N availability and assimilation profoundly affect plant gene expression and modulate root and stem architecture, thus affecting whole plant growth and crop yield. N status also affects C fixation, as it is an important component of the photosynthetic machinery in leaves. Reciprocally, increasing C supply promotes N uptake and assimilation. There is extensive knowledge of the different mechanisms that plants use for sensing and signalling their nutritional status to regulate the assimilation, metabolism and transport of C and N. However, the crosstalk between C and N pathways has received much less attention. Plant growth and development are greatly affected by suboptimal C/N balance, which can arise from nutrient deficiencies or/and environmental cues. Mechanisms that integrate and respond to changes in this specific nutritional balance have started to arise. This review will examine the specific responses to C/N imbalance in plants by focusing on the main inorganic and organic metabolites involved, how they are sensed and transported, and the interconnection between the early signalling components and hormonal networks that underlies plants' adaptive responses.

Alleviation of gadolinium stress on Medicago by elevated atmospheric CO2 is mediated by changes in carbohydrates, Anthocyanin, and proline metabolism

Rare earth elements (REE) like Gadolinium (Gd), are increasingly used in industry and agriculture and this is concomitant with the increasingly leaking of Gd into the environment. Under a certain threshold concentration, REE can promote plant growth, however, beyond this concentration, they exert negative effects on plant growth. Moreover, the effect of Gd on plants growth and metabolism under a futuristic climate with increasingly atmospheric CO2 has not yet been studied. To this end, we investigated the effect of soil contamination with Gd (150 mg/kg soil) on the growth, carbohydrates, proline, and anthocyanin metabolism of Medicago plants grown under ambient (aCO2, 410 ppm) or elevated CO2 (eCO2, 720 ppm) concentration. Gd negatively affected the growth and photosynthesis of plants and imposed oxidative stress i.e., increased H2O2 and lipid peroxidation (MDA) level. As defense lines, the level and metabolism of osmoprotectants (soluble sugars and proline) and antioxidants (phenolics, anthocyanins, and tocopherols) were increased under Gd treatment. High CO2 positively affected the growth and metabolism of Medicago plants. Moreover, eCO2 mitigated the negative impacts of Gd on Medicago growth. It further induced the levels of osmoprotectants and antioxidants. In line with increased proline and anthocyanins, their metabolic enzymes (e.g. OAT, P5CS, PAL, and CS) were also increased. This study advances our understanding of how Gd adversely affects Medicago plant growth and metabolism. It also sheds light on the biochemical mechanisms underlying the Gd stress-reducing impact of eCO2.

Responses in Nodulated Bean (Phaseolus vulgaris L.) Plants Grown at Elevated Atmospheric CO2

The increase in the concentration of CO₂ in the atmosphere is currently causing metabolomic and physiological changes in living beings and especially in plants. Future climate change may affect crop productivity by limiting the uptake of soil resources such as nitrogen (N) and water. The contribution of legume-rhizobia symbioses to N₂ fixation increases the available biological N reserve. Elevated CO₂ (eCO₂) has been shown to enhance the amount of fixed N₂ primarily by increasing biomass. Greater leaf biomass under eCO₂ levels increases N demand, which can stimulate and increase N₂ fixation. For this reason, bean plants (Phaseolus vulgaris L.) were used in this work to investigate how, in a CO₂-enriched atmosphere, inoculation with rhizobia (Rhizobium leguminosarum) affects different growth parameters and metabolites of carbon and nitrogen metabolism, as well as enzymatic activities of nitrogen metabolism and the oxidative state of the plant, with a view to future scenarios, where the concentration of CO₂ in the atmosphere will increase. The results showed that bean symbiosis with R. leguminosarum improved N₂ fixation, while also decreasing the plant's oxidative stress, and provided the plant with a greater defense system against eCO₂ conditions. In conclusion, the nodulation with rhizobia potentially replaced the chemical fertilization of bean plants (P. vulgaris L.), resulting in more environmentally friendly agricultural practices. However, further optimization of symbiotic activities is needed to improve the efficiency and to also develop strategies to improve the response of legume yields to eCO_2, particularly due to the climate change scenario in which there is predicted to be a large increase in the atmospheric CO₂ concentration.

Control of the rhizobium-legume symbiosis by the plant nitrogen demand is tightly integrated at the whole plant level and requires inter-organ systemic signaling

Symbiotic nodules formed on legume roots with rhizobia fix atmospheric N₂. Bacteria reduce N₂ to NH₄ ⁺ that is assimilated into amino acids by the plant. In return, the plant provides photosynthates to fuel the symbiotic nitrogen fixation. Symbiosis is tightly adjusted to the whole plant nutritional demand and to the plant photosynthetic capacities, but regulatory circuits behind this control remain poorly understood. The use of split-root systems combined with biochemical, physiological, metabolomic, transcriptomic, and genetic approaches revealed that multiple pathways are acting in parallel. Systemic signaling mechanisms of the plant N demand are required for the control of nodule organogenesis, mature nodule functioning, and nodule senescence. N-satiety/N-deficit systemic signaling correlates with rapid variations of the nodules' sugar levels, tuning symbiosis by C resources allocation. These mechanisms are responsible for the adjustment of plant symbiotic capacities to the mineral N resources. On the one hand, if mineral N can satisfy the plant N demand, nodule formation is inhibited, and nodule senescence is activated. On the other hand, local conditions (abiotic stresses) may impair symbiotic activity resulting in plant N limitation. In these conditions, systemic signaling may compensate the N deficit by stimulating symbiotic root N foraging. In the past decade, several molecular components of the systemic signaling pathways controlling nodule formation have been identified, but a major challenge remains, that is, to understand their specificity as compared to the mechanisms of non-symbiotic plants that control root development and how they contribute to the whole plant phenotypes. Less is known about the control of mature nodule development and functioning by N and C nutritional status of the plant, but a hypothetical model involving the sucrose allocation to the nodule as a systemic signaling process, the oxidative pentose phosphate pathway, and the redox status as potential effectors of this signaling is emerging. This work highlights the importance of organism integration in plant biology.

The decline of plant mineral nutrition under rising CO2: physiological and molecular aspects of a bad deal

The elevation of atmospheric CO₂ concentration has a strong impact on the physiology of C3 plants, far beyond photosynthesis and C metabolism. In particular, it reduces the concentrations of most mineral nutrients in plant tissues, posing major threats on crop quality, nutrient cycles, and carbon sinks in terrestrial agro-ecosystems. The causes of the detrimental effect of high CO₂ levels on plant mineral status are not understood. We provide an update on the main hypotheses and review the increasing evidence that, for nitrogen, this detrimental effect is associated with direct inhibition of key mechanisms of nitrogen uptake and assimilation. We also mention promising strategies for identifying genotypes that will maintain robust nutrient status in a future high-CO₂ world.

Interaction of Nitrate Assimilation and Photorespiration at Elevated CO2

It has been shown repeatedly that exposure to elevated atmospheric CO₂ causes an increased C/N ratio of plant biomass that could result from either increased carbon or - in relation to C acquisition - reduced nitrogen assimilation. Possible reasons for diminished nitrogen assimilation are controversial, but an impact of reduced photorespiration at elevated CO₂ has frequently been implied. Using a mutant defective in peroxisomal hydroxy-pyruvate reductase (hpr1-1) that is hampered in photorespiratory turnover, we show that indeed, photorespiration stimulates the glutamine-synthetase 2 (GS) / glutamine-oxoglutarate-aminotransferase (GOGAT) cycle, which channels ammonia into amino acid synthesis. However, mathematical flux simulations demonstrated that nitrate assimilation was not reduced at elevated CO₂, pointing to a dilution of nitrogen containing compounds by assimilated carbon at elevated CO₂. The massive growth reduction in the hpr1-1 mutant does not appear to result from nitrogen starvation. Model simulations yield evidence for a loss of cellular energy that is consumed in supporting high flux through the GS/GOGAT cycle that results from inefficient removal of photorespiratory intermediates. This causes a futile cycling of glycolate and hydroxy-pyruvate. In addition to that, accumulation of serine and glycine as well as carboxylates in the mutant creates a metabolic imbalance that could contribute to growth reduction.

Differential responses of the sunn4 and rdn1-1 super-nodulation mutants of Medicago truncatula to elevated atmospheric CO2

BACKGROUND AND AIMS

Nitrogen fixation in legumes requires tight control of carbon and nitrogen balance. Thus, legumes control nodule numbers via an autoregulation mechanism. 'Autoregulation of nodulation' mutants super-nodulate are thought to be carbon-limited due to the high carbon-sink strength of excessive nodules. This study aimed to examine the effect of increasing carbon supply on the performance of super-nodulation mutants.

METHODS

We compared the responses of Medicago truncatula super-nodulation mutants (sunn-4 and rdn1-1) and wild type to five CO2 levels (300-850 μmol mol-1). Nodule formation and nitrogen fixation were assessed in soil-grown plants at 18 and 42 d after sowing.

KEY RESULTS

Shoot and root biomass, nodule number and biomass, nitrogenase activity and fixed nitrogen per plant of all genotypes increased with increasing CO2 concentration and reached a maximum at 700 μmol mol-1. While the sunn-4 mutant showed strong growth retardation compared with wild-type plants, elevated CO2 increased shoot biomass and total nitrogen content of the rdn1-1 mutant up to 2-fold. This was accompanied by a 4-fold increase in nitrogen fixation capacity in the rdn1-1 mutant.

CONCLUSIONS

These results suggest that the super-nodulation phenotype per se did not limit growth. The additional nitrogen fixation capacity of the rdn1-1 mutant may enhance the benefit of elevated CO2 for plant growth and N2 fixation.

Mitigation of climate change and environmental hazards in plants: Potential role of the beneficial metalloid silicon

In the last decades, the concentration of atmospheric CO₂ and the average temperature have been increasing, and this trend is expected to become more severe in the near future. Additionally, environmental stresses including drought, salinity, UV-radiation, heavy metals, and toxic elements exposure represent a threat for ecosystems and agriculture. Climate and environmental changes negatively affect plant growth, biomass and yield production, and also enhance plant susceptibility to pests and diseases. Silicon (Si), as a beneficial element for plants, is involved in plant tolerance and/or resistance to various abiotic and biotic stresses. The beneficial role of Si has been shown in various plant species and its accumulation relies on the root's uptake capacity. However, Si uptake in plants depends on many biogeochemical factors that may be substantially altered in the future, affecting its functional role in plant protection. At present, it is not clear whether Si accumulation in plants will be positively or negatively affected by changing climate and environmental conditions. In this review, we focused on Si interaction with the most important factors of global change and environmental hazards in plants, discussing the potential role of its application as an alleviation strategy for climate and environmental hazards based on current knowledge.

Effects of human disturbance activities and environmental change factors on terrestrial nitrogen fixation

Biological nitrogen (N) fixation plays an important role in terrestrial N cycling and represents a key driver of terrestrial net primary productivity (NPP). Despite the importance of N fixation in terrestrial ecosystems, our knowledge regarding the controls on terrestrial N fixation remains poor. Here, we conducted a meta-analysis (based on 852 observations from 158 studies) of N fixation across three types of ecosystems with different status of disturbance (no management, restoration [previously disturbed], and disturbance [currently disturbed]) and in response to multiple environmental change factors (warming, elevated carbon dioxide [CO₂ ], increased precipitation, increased drought, increased N deposition, and their combinations). We explored the mechanisms underlying the changes in N fixation by examining the variations in soil physicochemical properties (bulk density, texture, moisture, and pH), plant and microbial characteristics (dominant plant species numbers, plant coverage, and soil microbial biomass), and soil resources (total carbon, total N, total phosphorus (P), inorganic N, and inorganic P). Human disturbance inhibited non-symbiotic N fixation but not symbiotic N fixation. Terrestrial N fixation was stimulated by warming (+152.7%), elevated CO₂ (+19.6%), and increased precipitation (+73.1%) but inhibited by increased drought (-30.4%), N deposition (-31.0%), and combinations of available multiple environmental change factors (-14.5%), the extents of which varied among biomes and ecosystem compartments. Human disturbance reduced the N fixation responses to environmental change factors, which was associated with the changes in soil physicochemical properties (2%-56%, p < .001) and the declines in plant and microbial characteristics (3%-49%, p ≤ .003) and soil resources (6%-48%, p ≤ .03). Overall, our findings reveal for the first time the effects of multiple environmental change factors on terrestrial N fixation and indicate the role of human disturbance activities in inhibiting N fixation, which can improve our understanding, modeling, and prediction of terrestrial N budgets, NPP, and ecosystem feedbacks under global change scenarios.

Potential effects of a high CO2 future on leguminous species

Legumes provide an important source of food and feed due to their high protein levels and many health benefits, and also impart environmental and agronomic advantages as a consequence of their ability to fix nitrogen through their symbiotic relationship with rhizobia. As a result of our growing population, the demand for products derived from legumes will likely expand considerably in coming years. Since there is little scope for increasing production area, improving the productivity of such crops in the face of climate change will be essential. While a growing number of studies have assessed the effects of climate change on legume yield, there is a paucity of information regarding the direct impact of elevated CO2 concentration (e[CO2]) itself, which is a main driver of climate change and has a substantial physiological effect on plants. In this review, we discuss current knowledge regarding the influence of e[CO2] on the photosynthetic process, as well as biomass production, seed yield, quality, and stress tolerance in legumes, and examine how these responses differ from those observed in non-nodulating plants. Although these relationships are proving to be extremely complex, mounting evidence suggests that under limiting conditions, overall declines in many of these parameters could ensue. While further research will be required to unravel precise mechanisms underlying e[CO2] responses of legumes, it is clear that integrating such knowledge into legume breeding programs will be indispensable for achieving yield gains by harnessing the potential positive effects, and minimizing the detrimental impacts, of CO2 in the future.

Carbon sink strength of nodules but not other organs modulates photosynthesis of faba bean (Vicia faba) grown under elevated [CO2 ] and different water supply

Photosynthetic stimulation by elevated [CO₂ ] (e[CO₂ ]) may be limited by the capacity of sink organs to use photosynthates. In many legumes, N₂ -fixing symbionts in root nodules provide an additional sink, so that legumes may be better able to profit from e[CO₂ ]. However, drought not only constrains photosynthesis but also the size and activity of sinks, and little is known about the interaction of e[CO₂ ] and drought on carbon sink strength of nodules and other organs. To compare carbon sink strength, faba bean was grown under ambient (400 ppm) or elevated (700 ppm) atmospheric [CO₂ ] and subjected to well-watered or drought treatments, and then exposed to ¹³ C pulse-labelling using custom-built chambers to track the fate of new photosynthates. Drought decreased ¹³ C uptake and nodule sink strength, and this effect was even greater under e[CO₂ ], and was associated with an accumulation of amino acids in nodules. This resulted in decreased N₂ fixation, and increased accumulation of new photosynthates (¹³ C/sugars) in leaves, which in turn can feed back on photosynthesis. Our study suggests that nodule C sink activity is key to avoid sink limitation in legumes under e[CO₂ ], and legumes may only be able to achieve greater C gain if nodule activity is maintained.

Water availability moderates N2 fixation benefit from elevated [CO2 ]: A 2-year free-air CO2 enrichment study on lentil (Lens culinaris MEDIK.) in a water limited agroecosystem

Increased biomass and yield of plants grown under elevated [CO₂ ] often corresponds to decreased grain N concentration ([N]), diminishing nutritional quality of crops. Legumes through their symbiotic N₂ fixation may be better able to maintain biomass [N] and grain [N] under elevated [CO₂ ], provided N₂ fixation is stimulated by elevated [CO₂ ] in line with growth and yield. In Mediterranean-type agroecosystems, N₂ fixation may be impaired by drought, and it is unclear whether elevated [CO₂ ] stimulation of N₂ fixation can overcome this impact in dry years. To address this question, we grew lentil under two [CO₂ ] (ambient ~400 ppm and elevated ~550 ppm) levels in a free-air CO₂ enrichment facility over two growing seasons sharply contrasting in rainfall. Elevated [CO₂ ] stimulated N₂ fixation through greater nodule number (+27%), mass (+18%), and specific fixation activity (+17%), and this stimulation was greater in the high than in the low rainfall/dry season. Elevated [CO₂ ] depressed grain [N] (-4%) in the dry season. In contrast, grain [N] increased (+3%) in the high rainfall season under elevated [CO₂ ], as a consequence of greater post-flowering N₂ fixation. Our results suggest that the benefit for N₂ fixation from elevated [CO₂ ] is high as long as there is enough soil water to continue N₂ fixation during grain filling.

Stable isotope compounds - production, detection, and application

Stable isotopes are used in wide fields of application from natural tracers in biology, geology and archeology through studies of metabolic fluxes to their application as tracers in quantitative proteomics and structural biology. We review the use of stable isotopes of biogenic elements (H, C, N, O, S, Mg, Se) with the emphasis on hydrogen and its heavy isotope deuterium. We will discuss the limitations of enriching various compounds in stable isotopes when produced in living organisms. Finally, we overview methods for measuring stable isotopes, focusing on methods for detection in single cells in situ and their exploitation in modern biotechnologies.

(15)N methodologies for quantifying the response of N2-fixing associations to elevated [CO2]: A review

Methodologies based on (15)N enrichment (E) and (15)N natural abundance (NA) have been used to obtain quantitative estimates of the response of biological N2 fixation (BNF) of legumes (woody, grain and forage) and actinorhizal plants grown in artificial media or in soil exposed to elevated atmospheric concentrations of carbon dioxide e[CO2] for extended periods of time, in growth rooms, greenhouses, open top chambers or free-air CO2 enrichment (FACE) facilities. (15)N2 has also been used to quantify the response of endophytic and free-living diazotrophs to e[CO2]. The primary criterion of response was the proportional dependence of the N2-fixing system on the atmosphere as a source of N. i.e. the symbiotic dependence (Patm). The unique feature of (15)N-based methods is their ability to provide time-integrated and yield-independent estimates of Patm. In studies conducted in artificial media or in soil using the E methodology there was either no response or a positive response of Patm to e[CO2]. The interpretation of results obtained in artificial media or with (15)N2 is straight forward, not being subject to the assumptions on which the E and NA soil-cultured methods are based. A variety of methods have been used to estimate isotopic fractionation attendant on the NA technique, the so-called 'B value', which attaches a degree of uncertainty to the results obtained. Using the NA technique, a suite of responses of Patm to e[CO2] has been published, from positive to neutral to sometimes negative effects. Several factors which interact with the response of N2-fixing species to e[CO2] were identified.

Nutrient constraints on terrestrial carbon fixation: The role of nitrogen

Carbon dioxide (CO₂) concentrations in the earth's atmosphere are projected to rise from current levels near 400ppm to over 700ppm by the end of the 21st century. Projections over this time frame must take into account the increases in total net primary production (NPP) expected from terrestrial plants, which result from elevated CO₂ (eCO₂) and have the potential to mitigate the impact of anthropogenic CO₂ emissions. However, a growing body of evidence indicates that limitations in soil nutrients, particularly nitrogen (N), the soil nutrient most limiting to plant growth, may greatly constrain future carbon fixation. Here, we review recent studies about the relationships between soil N supply, plant N nutrition, and carbon fixation in higher plants under eCO₂, highlighting key discoveries made in the field, particularly from free-air CO₂ enrichment (FACE) technology, and relate these findings to physiological and ecological mechanisms.

Quantitative RT-PCR Platform to Measure Transcript Levels of C and N Metabolism-Related Genes in Durum Wheat: Transcript Profiles in Elevated [CO2] and High Temperature at Different Levels of N Supply

Only limited public transcriptomics resources are available for durum wheat and its responses to environmental changes. We developed a quantitative reverse transcription-PCR (qRT-PCR) platform for analysing the expression of primary C and N metabolism genes in durum wheat in leaves (125 genes) and roots (38 genes), based on available bread wheat genes and the identification of orthologs of known genes in other species. We also assessed the expression stability of seven reference genes for qRT-PCR under varying environments. We therefore present a functional qRT-PCR platform for gene expression analysis in durum wheat, and suggest using the ADP-ribosylation factor as a reference gene for qRT-PCR normalization. We investigated the effects of elevated [CO(2)] and temperature at two levels of N supply on C and N metabolism by combining gene expression analysis, using our qRT-PCR platform, with biochemical and physiological parameters in durum wheat grown in field chambers. Elevated CO(2) down-regulated the photosynthetic capacity and led to the loss of N compounds, including Rubisco; this effect was exacerbated at low N. Mechanistically, the reduction in photosynthesis and N levels could be associated with a decreased transcription of the genes involved in photosynthesis and N assimilation. High temperatures increased stomatal conductance, and thus did not inhibit photosynthesis, even though Rubisco protein and activity, soluble protein, leaf N, and gene expression for C fixation and N assimilation were down-regulated. Under a future scenario of climate change, the extent to which C fixation capacity and N assimilation are down-regulated will depend upon the N supply.

Long-term non-invasive and continuous measurements of legume nodule activity

Symbiotic nitrogen fixation is a process of considerable economic, ecological and scientific interest. The central enzyme nitrogenase reduces H(+) alongside N2 , and the evolving H2 allows a continuous and non-invasive in vivo measurement of nitrogenase activity. The objective of this study was to show that an elaborated set-up providing such measurements for periods as long as several weeks will produce specific insight into the nodule activity's dependence on environmental conditions and genotype features. A system was developed that allows the air-proof separation of a root/nodule and a shoot compartment. H2 evolution in the root/nodule compartment can be monitored continuously. Nutrient solution composition, temperature, CO2 concentration and humidity around the shoots can concomitantly be maintained and manipulated. Medicago truncatula plants showed vigorous growth in the system when relying on nitrogen fixation. The set-up was able to provide specific insights into nitrogen fixation. For example, nodule activity depended on the temperature in their surroundings, but not on temperature or light around shoots. Increased temperature around the nodules was able to induce higher nodule activity in darkness versus light around shoots for a period of as long as 8 h. Conditions that affected the N demand of the shoots (ammonium application, Mg or P depletion, super numeric nodules) induced consistent and complex daily rhythms in nodule activity. It was shown that long-term continuous measurements of nodule activity could be useful for revealing special features in mutants and could be of importance when synchronizing nodule harvests for complex analysis of their metabolic status.

Growth, photosynthetic acclimation and yield quality in legumes under climate change simulations: an updated survey

Continued emissions of CO2, derived from human activities, increase atmospheric CO2 concentration. The CO2 rise stimulates plant growth and affects yield quality. Effects of elevated CO2 on legume quality depend on interactions with N2-fixing bacteria and mycorrhizal fungi. Growth at elevated CO2 increases photosynthesis under short-term exposures in C3 species. Under long-term exposures, however, plants generally acclimate to elevated CO2 decreasing their photosynthetic capacity. An updated survey of the literature indicates that a key factor, perhaps the most important, that characteristically influences this phenomenon, its occurrence and extent, is the plant source-sink balance. In legumes, the ability of exchanging C for N at nodule level with the N2-fixing symbionts creates an extra C sink that avoids the occurrence of photosynthetic acclimation. Arbuscular mycorrhizal fungi colonizing roots may also result in increased C sink, preventing photosynthetic acclimation. Defoliation (Anthyllis vulneraria, simulated grazing) or shoot cutting (alfalfa, usual management as forage) largely increases root/shoot ratio. During re-growth at elevated CO2, new shoots growth and nodule respiration function as strong C sinks that counteracts photosynthetic acclimation. In the presence of some limiting factor, the legumes response to elevated CO2 is weakened showing photosynthetic acclimation. This survey has identified limiting factors that include an insufficient N supply from bacterial strains, nutrient-poor soils, low P supply, excess temperature affecting photosynthesis and/or nodule activity, a genetically determined low nodulation capacity, an inability of species or varieties to increase growth (and therefore C sink) at elevated CO2 and a plant phenological state or season when plant growth is stopped.

Elevated CO2 alters the feeding behaviour of the pea aphid by modifying the physical and chemical resistance of Medicago truncatula

Elevated CO(2) compromises the resistance of leguminous plants against chewing insects, but little is known about whether elevated CO(2) modifies the resistance against phloem-sucking insects or whether it has contrasting effects on the resistance of legumes that differ in biological nitrogen fixation. We tested the hypothesis that the physical and chemical resistance against aphids would be increased in Jemalong (a wild type of Medicago truncatula) but would be decreased in dnf1 (a mutant without biological nitrogen fixation) by elevated CO(2). The non-glandular and glandular trichome density of Jemalong plants increased under elevated CO(2), resulting in prolonged aphid probing. In contrast, dnf1 plants tended to decrease foliar trichome density under elevated CO(2), resulting in less surface and epidermal resistance to aphids. Elevated CO(2) enhanced the ineffective salicylic acid-dependent defence pathway but decreased the effective jasmonic acid/ethylene-dependent defence pathway in aphid-infested Jemalong plants. Therefore, aphid probing time decreased and the duration of phloem sap ingestion increased on Jemalong under elevated CO(2), which, in turn, increased aphid growth rate. Overall, our results suggest that elevated CO(2) decreases the chemical resistance of wild-type M. truncatula against aphids, and that the host's biological nitrogen fixation ability is central to this effect.