Elevated atmospheric concentrations of carbon dioxide reduce monarch tolerance and increase parasite virulence by altering the medicinal properties of milkweeds

Hosts combat their parasites using mechanisms of resistance and tolerance, which together determine parasite virulence. Environmental factors, including diet, mediate the impact of parasites on hosts, with diet providing nutritional and medicinal properties. Here, we present the first evidence that ongoing environmental change decreases host tolerance and increases parasite virulence through a loss of dietary medicinal quality. Monarch butterflies use dietary toxins (cardenolides) to reduce the deleterious impacts of a protozoan parasite. We fed monarch larvae foliage from four milkweed species grown under either elevated or ambient CO₂, and measured changes in resistance, tolerance, and virulence. The most high‐cardenolide milkweed species lost its medicinal properties under elevated CO₂; monarch tolerance to infection decreased, and parasite virulence increased. Declines in medicinal quality were associated with declines in foliar concentrations of lipophilic cardenolides. Our results emphasize that global environmental change may influence parasite–host interactions through changes in the medicinal properties of plants.     

Genetic variation in expression of defense phenotype may mediate evolutionary adaptation of Asclepias syriaca to elevated CO2

How species interactions may modify the effects of environmental change on evolutionary adaptation is poorly understood. Elevated CO₂ is known to alter plant–herbivore interactions, but the evolutionary consequences for plant populations have received little attention. We conducted an experiment to determine the effects of elevated CO₂ and herbivory by a specialist insect herbivore (Danaus plexippus) on the expression of constitutive and induced plant defense traits in five genotypes of Asclepias syriaca, and assessed the heritability of these traits. We also examined changes in relative fitness among plant genotypes in response to altered CO₂ and herbivory. The expression of plant defense traits varied significantly among genotypes. Elevated CO₂ increased plant growth and physical defenses (toughness and latex), but decreased investment in chemical defenses (cardenolides). We found no effect of elevated CO₂ on plant induction of cardenolides in response to caterpillar herbivory. Elevated CO₂ decreased the expression of chemical defenses (cardenolides) to a different extent depending on plant genotype. Differential effects of CO₂ on plant defense expression, rather than direct effects on relative fitness, may alter A. syriaca adaptation to changing climate.     

Targeting novel chemical and constitutive primed metabolites against Plectosphaerella cucumerina

Priming is a physiological state for protection of plants against a broad range of pathogens, and is achieved through stimulation of the plant immune system. Various stimuli, such as beneficial microbes and chemical induction, activate defense priming. In the present study, we demonstrate that impairment of the high‐affinity nitrate transporter 2.1 (encoded by NRT2.1) enables Arabidopsis to respond more quickly and strongly to Plectosphaerella cucumerina attack, leading to enhanced resistance. The Arabidopsis thaliana mutant lin1 (affected in NRT2.1) is a priming mutant that displays constitutive resistance to this necrotroph, with no associated developmental or growth costs. Chemically induced priming by β–aminobutyric acid treatment, the constitutive priming mutant ocp3 and the constitutive priming present in the lin1 mutant result in a common metabolic profile within the same plant–pathogen interactions. The defense priming significantly affects sugar metabolism, cell‐wall remodeling and shikimic acid derivatives levels, and results in specific changes in the amino acid profile and three specific branches of Trp metabolism, particularly accumulation of indole acetic acid, indole‐3–carboxaldehyde and camalexin, but not the indolic glucosinolates. Metabolomic analysis facilitated identification of three metabolites in the priming fingerprint: galacturonic acid, indole‐3–carboxylic acid and hypoxanthine. Treatment of plants with the latter two metabolites by soil drenching induced resistance against P. cucumerina, demonstrating that these compounds are key components of defense priming against this necrotrophic fungus. Here we demonstrate that indole‐3–carboxylic acid induces resistance by promoting papillae deposition and H₂O₂ production, and that this is independent of PR1, VSP2 and PDF1.2 priming.     

Will chemical defenses become more effective against specialist herbivores under elevated CO2?

Elevated atmospheric CO₂ is known to affect plant–insect herbivore interactions. Elevated CO₂ causes leaf nitrogen to decrease, the ostensible cause of herbivore compensatory feeding. CO₂ may also affect herbivore consumption by altering chemical defenses via changes in plant hormones. We considered the effects of elevated CO₂, in conjunction with soil fertility and damage (simulated herbivory), on glucosinolate concentrations of mustard (Brassica nigra) and collard (B. oleracea var. acephala) and the effects of leaf nitrogen and glucosinolate groups on specialist Pieris rapae consumption. Elevated CO₂ affected B. oleracea but not B. nigra glucosinolates; responses to soil fertility and damage were also species‐specific. Soil fertility and damage also affected B. oleracea glucosinolates differently under elevated CO₂. Glucosinolates did not affect P. rapae consumption at either CO₂ concentration in B. nigra, but had CO₂‐specific effects on consumption in B. oleracea. At ambient CO₂, leaf nitrogen had strong effects on glucosinolate concentrations and P. rapae consumption but only gluconasturtiin was a feeding stimulant. At elevated CO₂, direct effects of leaf nitrogen were weaker, but glucosinolates had stronger effects on consumption. Gluconasturtiin and aliphatic glucosinolates were feeding stimulants and indole glucosinolates were feeding deterrents. These results do not support the compensatory feeding hypothesis as the sole driver of changes in P. rapae consumption under elevated CO₂. Support for hormone‐mediated CO₂ response (HMCR) was mixed; it explained few treatment effects on constitutive or induced glucosinolates, but did explain patterns in SEMs. Further, the novel feeding deterrent effect of indole glucosinolates under elevated CO₂ in B. oleracae underscores the importance of defensive chemistry in CO₂ response. We speculate that P. rapae indole glucosinolate detoxification mechanisms may have been overwhelmed under elevated CO₂ forcing slowed consumption. Specialists may have to contend with hosts with poorer nutritional quality and more effective chemical defenses under elevated CO₂.     

Yield vs. Quality trade‐offs for wheat in response to carbon dioxide and ozone

Although it is established that there exist potential trade‐offs between grain yield and grain quality in wheat exposed to elevated carbon dioxide (CO₂) and ozone (O₃), their underlying causes remain poorly explored. To investigate the processes affecting grain quality under altered CO₂ and O₃, we analysed 57 experiments with CO₂ or O₃ exposure in different exposure systems. The study covered 24 cultivars studied in 112 experimental treatments from 11 countries. A significant growth dilution effect on grain protein was found: a change in grain yield of 10% by O₃ was associated with a change in grain protein yield of 8.1% (R² = 0.96), whereas a change in yield effect of 10% by CO₂ was linked to a change in grain protein yield effect of 7.5% (R² = 0.74). Superimposed on this effect, elevated CO₂, but not O₃, had a significant negative effect on grain protein yield also in the absence of effects on grain yield, indicating that there exists a process by which CO₂ restricts grain protein accumulation, which is absent for O₃. Grain mass, another quality trait, was more strongly affected by O₃ than grain number, whereas the opposite was true for CO₂. Harvest index was strongly and negatively influenced by O₃, but was unaffected by CO₂. We conclude that yield vs. protein trade‐offs for wheat in response to CO₂ and O₃ are constrained by close relationships between effects on grain biomass and less than proportional effects on grain protein. An important and novel finding was that elevated CO₂ has a direct negative effect on grain protein accumulation independent of the yield effect, supporting recent evidence of CO₂‐induced impairment of nitrate uptake/assimilation. Finally, our results demonstrated that processes underlying responses of grain yield vs. quality trade‐offs are very different in wheat exposed to elevated O₃ compared with elevated CO₂.     

Winners always win: growth of a wide range of plant species from low to future high CO2

Evolutionary adaptation to variation in resource supply has resulted in plant strategies that are based on trade‐offs in functional traits. Here, we investigate, for the first time across multiple species, whether such trade‐offs are also apparent in growth and morphology responses to past low, current ambient, and future high CO₂ concentrations. We grew freshly germinated seedlings of up to 28 C₃ species (16 forbs, 6 woody, and 6 grasses) in climate chambers at 160 ppm, 450 ppm, and 750 ppm CO₂. We determined biomass, allocation, SLA (specific leaf area), LAR (leaf area ratio), and RGR (relative growth rate), thereby doubling the available data on these plant responses to low CO₂. High CO₂ increased RGR by 8%; low CO₂ decreased RGR by 23%. Fast growers at ambient CO₂ had the greatest reduction in RGR at low CO₂ as they lost the benefits of a fast‐growth morphology (decoupling of RGR and LAR [leaf area ratio]). Despite these shifts species ranking on biomass and RGR was unaffected by CO₂, winners continued to win, regardless of CO_2. Unlike for other plant resources we found no trade‐offs in morphological and growth responses to CO₂ variation, changes in morphological traits were unrelated to changes in growth at low or high CO₂. Thus, changes in physiology may be more important than morphological changes in response to CO₂ variation.     

Vertebrate defense against parasites: Interactions between avoidance,resistance, and tolerance

Hosts can utilize different types of defense against the effects of parasitism, including avoidance, resistance, and tolerance. Typically, there is tremendous heterogeneity among hosts in these defense mechanisms that may be rooted in the costs associated with defense and lead to trade‐offs with other life‐history traits. Trade‐offs may also exist between the defense mechanisms, but the relationships between avoidance, resistance, and tolerance have rarely been studied. Here, we assessed these three defense traits under common garden conditions in a natural host–parasite system, the trematode eye‐fluke Diplostomum pseudospathaceum and its second intermediate fish host. We looked at host individuals originating from four genetically distinct populations of two closely related salmonid species (Atlantic salmon, Salmo salar and sea trout, Salmo trutta trutta) to estimate the magnitude of variation in these defense traits and the relationships among them. We show species‐specific variation in resistance and tolerance and population‐specific variation in resistance. Further, we demonstrate evidence for a trade‐off between resistance and tolerance. Our results suggest that the variation in host defense can at least partly result from a compromise between different interacting defense traits, the relative importance of which is likely to be shaped by environmental components. Overall, this study emphasizes the importance of considering different components of the host defense system when making predictions on the outcome of host–parasite interactions.     

Plant stomatal closure improves aphid feeding under elevated CO2

Stomata help plants regulate CO₂ absorption and water vapor release in response to various environmental changes, and plants decrease their stomatal apertures and enhance their water status under elevated CO₂. Although the bottom‐up effect of elevated CO₂ on insect performance has been extensively studied, few reports have considered how insect fitness is altered by elevated CO₂‐induced changes in host plant water status. We tested the hypothesis that aphids induce stomatal closure and increase host water potential, which facilitates their passive feeding, and that this induction can be enhanced by elevated CO₂. Our results showed that aphid infestation triggered the abscisic acid (ABA) signaling pathway to decrease the stomatal apertures of Medicago truncatula, which consequently decreased leaf transpiration and helped maintain leaf water potential. These effects increased xylem‐feeding time and decreased hemolymph osmolarity, which thereby enhanced phloem‐feeding time and increased aphid abundance. Furthermore, elevated CO₂ up‐regulated an ABA‐independent enzyme, carbonic anhydrase, which led to further decrease in stomatal aperture for aphid‐infested plants. Thus, the effects of elevated CO₂ and aphid infestation on stomatal closure synergistically improved the water status of the host plant. The results indicate that aphid infestation enhances aphid feeding under ambient CO₂ and that this enhancement is increased under elevated CO₂.     

Genetic variation in plant volatile emission does not result in differential attraction of natural enemies in the field

Volatile organic chemical (VOC) emission by plants may serve as an adaptive plant defense by attracting the natural enemies of herbivores. For plant VOC emission to evolve as an adaptive defense, plants must show genetic variability for the trait. To date, such variability has been investigated primarily in agricultural systems, yet relatively little is known about genetic variation in VOCs emitted by natural populations of native plants. Here, we investigate intraspecific variation in constitutive and herbivore-induced plant VOC emission using the native common milkweed plant (Asclepias syriaca) and its monarch caterpillar herbivore (Danaus plexippus) in complementary field and common garden greenhouse experiments. In addition, we used a common garden field experiment to gauge natural enemy attraction to milkweed VOCs induced by monarch damage. We found evidence of genetic variation in the total constitutive and induced concentrations of VOCs and the composition of VOC blends emitted by milkweed plants. However, all milkweed genotypes responded similarly to induction by monarchs in terms of their relative change in VOC concentration and blend. Natural enemies attacked decoy caterpillars more frequently on damaged than on undamaged milkweed, and natural enemy visitation was associated with higher total VOC concentrations and with VOC blend. Thus, we present evidence that induced VOCs emitted by milkweed may function as a defense against herbivores. However, plant genotypes were equally attractive to natural enemies. Although milkweed genotypes diverge phenotypically in their VOC concentrations and blends, they converge into similar phenotypes with regard to magnitude of induction and enemy attraction.     

Lower incidence and severity of tomato virus in elevated CO2 is accompanied by modulated plant induced defence in tomato

Elevation in atmospheric CO₂ concentration broadly affects plant phenology and physiology, and these effects may alter the performance of plant viruses. The effects of elevated CO₂ on the susceptibility of tomato plants to Tomato yellow leaf curl virus (TYLCV) were examined for two successive years in open top chambers (OTC) in the field. We experimentally tested the hypothesis that elevated CO₂ would reduce the incidence and severity of TYLCV on tomato by altering plant defence strategies. Our results showed that elevated CO₂ decreased TYLCV disease incidence (by 14.6% in 2009 and 11.8% in 2010) and decreased disease severity (by 20.0% in 2009 and 10.4% in 2010). Elevated CO₂ also decreased the level of TYLCV coat protein in tomato leaves. Regardless of virus infection, elevated CO₂ increased plant height and aboveground biomass. Additionally, elevated CO₂ increased the leaf C:N ratio of tomato, but decreased soluble protein content in leaves. Notably, elevated CO₂ increased the salicylic acid (SA) level in uninfected and infected plants. In contrast, elevated CO₂ reduced jasmonic acid (JA) in uninfected plants while it increased JA and abscisic acid (ABA) in virus‐infected plants. Furthermore, combined exogenous SA and JA application enhanced resistance to TYLCV more than application of either SA or JA alone. Our results suggest that the modulated antagonistic relationship between SA and JA under elevated CO₂ makes a great contribution to increased tomato resistance to TYLCV, and the predicted increases in tomato productivity may be enhanced by reduced plant virus susceptibility under projected rising CO₂ conditions.     

Jasmonate‐mediated stomatal closure under elevated CO2 revealed by time‐resolved metabolomics

Foliar stomatal movements are critical for regulating plant water loss and gas exchange. Elevated carbon dioxide (CO₂) levels are known to induce stomatal closure. However, the current knowledge on CO₂ signal transduction in stomatal guard cells is limited. Here we report metabolomic responses of Brassica napus guard cells to elevated CO₂ using three hyphenated metabolomics platforms: gas chromatography‐mass spectrometry (MS); liquid chromatography (LC)‐multiple reaction monitoring‐MS; and ultra‐high‐performance LC‐quadrupole time‐of‐flight‐MS. A total of 358 metabolites from guard cells were quantified in a time‐course response to elevated CO₂ level. Most metabolites increased under elevated CO₂, showing the most significant differences at 10 min. In addition, reactive oxygen species production increased and stomatal aperture decreased with time. Major alterations in flavonoid, organic acid, sugar, fatty acid, phenylpropanoid and amino acid metabolic pathways indicated changes in both primary and specialized metabolic pathways in guard cells. Most interestingly, the jasmonic acid (JA) biosynthesis pathway was significantly altered in the course of elevated CO₂ treatment. Together with results obtained from JA biosynthesis and signaling mutants as well as CO₂ signaling mutants, we discovered that CO₂‐induced stomatal closure is mediated by JA signaling.     

Impacts of 3 years of elevated atmospheric CO2 on rhizosphere carbon flow and microbial community dynamics

Carbon (C) uptake by terrestrial ecosystems represents an important option for partially mitigating anthropogenic CO₂ emissions. Short‐term atmospheric elevated CO₂ exposure has been shown to create major shifts in C flow routes and diversity of the active soil‐borne microbial community. Long‐term increases in CO₂ have been hypothesized to have subtle effects due to the potential adaptation of soil microorganism to the increased flow of organic C. Here, we studied the effects of prolonged elevated atmospheric CO₂ exposure on microbial C flow and microbial communities in the rhizosphere. Carex arenaria (a nonmycorrhizal plant species) and Festuca rubra (a mycorrhizal plant species) were grown at defined atmospheric conditions differing in CO₂ concentration (350 and 700 ppm) for 3 years. During this period, C flow was assessed repeatedly (after 6 months, 1, 2, and 3 years) by ¹³C pulse‐chase experiments, and label was tracked through the rhizosphere bacterial, general fungal, and arbuscular mycorrhizal fungal (AMF) communities. Fatty acid biomarker analyses and RNA‐stable isotope probing (RNA‐SIP), in combination with real‐time PCR and PCR‐DGGE, were used to examine microbial community dynamics and abundance. Throughout the experiment the influence of elevated CO₂ was highly plant dependent, with the mycorrhizal plant exerting a greater influence on both bacterial and fungal communities. Biomarker data confirmed that rhizodeposited C was first processed by AMF and subsequently transferred to bacterial and fungal communities in the rhizosphere soil. Over the course of 3 years, elevated CO₂ caused a continuous increase in the ¹³C enrichment retained in AMF and an increasing delay in the transfer of C to the bacterial community. These results show that, not only do elevated atmospheric CO₂ conditions induce changes in rhizosphere C flow and dynamics but also continue to develop over multiple seasons, thereby affecting terrestrial ecosystems C utilization processes.     

Tree species diversity alters plant defense investment in an experimental forest plantation in southern Mexico

How plant species diversity affects traits conferring herbivore resistance (e.g., chemical defenses), as well as the mechanisms underlying such effects, has received little attention. One potential mechanism for the effect of diversity on plant defenses is that increased plant growth at high diversity could lead to reduced investment in defenses via growth–defense trade‐offs. We measured tree growth (diameter at breast height) and collected leaves to quantify total phenolics in 2.5‐year‐old plants of six tropical tree species (N = 597 plants) in a young experimental plantation in southern Mexico. Selected plants were classified as monocultures or as polycultures represented by mixtures of four of the six species examined. Tree species diversity had a significant negative effect on total phenolics, where polycultures exhibited a 13 percent lower mean concentration than monocultures. However, there was marked variation in the effects of diversity on defenses among tree species, with some species exhibiting strong reductions in phenolic levels in mixtures, whereas others were unresponsive. In addition, tree species diversity had no effect on growth, nor was the negative effect of diversity on chemical defenses mediated by a growth–defense trade‐off. These results demonstrate that tree diversity can alter investment in chemical defenses in long‐lived tree species but that such effect may not always be under strong control by plant endogenous resource allocation trade‐offs. Regardless of the underlying mechanism, these findings have important implications for predicting effects on consumers and ecosystem function.     

Antarctic emerald rockcod have the capacity to compensate for warming when uncoupled from CO2‐acidification

Increases in atmospheric CO₂ levels and associated ocean changes are expected to have dramatic impacts on marine ecosystems. Although the Southern Ocean is experiencing some of the fastest rates of change, few studies have explored how Antarctic fishes may be affected by co‐occurring ocean changes, and even fewer have examined early life stages. To date, no studies have characterized potential trade‐offs in physiology and behavior in response to projected multiple climate change stressors (ocean acidification and warming) on Antarctic fishes. We exposed juvenile emerald rockcod Trematomus bernacchii to three PCO₂ treatments (~450, ~850, and ~1,200 μatm PCO₂) at two temperatures (?1 or 2°C). After 2, 7, 14, and 28 days, metrics of physiological performance including cardiorespiratory function (heart rate [f_H] and ventilation rate [f_V]), metabolic rate (), and cellular enzyme activity were measured. Behavioral responses, including scototaxis, activity, exploration, and escape response were assessed after 7 and 14 days. Elevated PCO₂ independently had little impact on either physiology or behavior in juvenile rockcod, whereas warming resulted in significant changes across acclimation time. After 14 days, f_H, f_V and significantly increased with warming, but not with elevated PCO₂. Increased physiological costs were accompanied by behavioral alterations including increased dark zone preference up to 14%, reduced activity by 12%, as well as reduced escape time suggesting potential trade‐offs in energetics. After 28 days, juvenile rockcod demonstrated a degree of temperature compensation as f_V, , and cellular metabolism significantly decreased following the peak at 14 days; however, temperature compensation was only evident in the absence of elevated PCO₂. Sustained increases in f_V and after 28 days exposure to elevated PCO₂ indicate additive (f_V) and synergistic () interactions occurred in combination with warming. Stressor‐induced energetic trade‐offs in physiology and behavior may be an important mechanism leading to vulnerability of Antarctic fishes to future ocean change.     

Distinct seasonal dynamics of responses to elevated CO2 in two understorey grass species differing in shade‐tolerance

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Light Availability Influences Growth‐Defense Trade‐Offs in Big‐Leaf Mahogany (Swietenia macrophylla King)

We report on findings from a common garden experiment showing that the magnitude of change in the production of phenolic compounds in response to different amounts of light availability is tissue‐specific in saplings of big‐leaf mahogany (Swietenia macrophylla). Moreover, we show that trade‐offs between growth and the production of chemical defenses emerge only under light‐limited conditions. These findings emphasize the need for considering the specificity of plant defensive responses to resource availability, as well as the influence of the abiotic environment (via trade‐offs) on plant defense allocation patterns.     

Carbon dioxide‐ and temperature‐mediated changes in plant defensive compounds alter food utilization of herbivores

Although the impact of elevated carbon dioxide and rising temperature on plants and animals has been extensively documented recently, only limited understanding exists regarding their combined effects. The objective of this research was to address the consequences of using combinations of elevated CO₂ and elevated temperature on a plant's defensive chemistry, and subsequent utilization of the plant as insect food. Our results indicated that elevated CO₂ and increased temperature, for the most part, act independently on the production of defensive compounds in broccoli leaves (Brassica oleracea L. var. italica). CO₂ concentrations had significant effects on the foliar water content, total phenolic compounds, polyphenol oxidase and trypsin inhibitor concentrations. The herbivore Spodoptera litura (Fabricius; Lepidoptera: Noctuidae) responded to changes in the plant secondary chemistry, with larvae consuming more plant materials that had been exposed to elevated CO₂. The food utilization efficiencies of second‐instar larvae were more sensitive to CO₂‐treated foliage than those of the third‐ and fourth‐instar larvae. Temperature did exert a significant effect on food utilization (ECD) by the larvae. Our study will provide important information in future predictions on plant–insect interactions as a result of climate change. The study also demonstrated that since various larval stages might respond differently to climate change, this possibility needs to be considered in future forecasting and monitoring.     

Pea aphid promotes amino acid metabolism both in Medicago truncatula and bacteriocytes to favor aphid population growth under elevated CO2

Rising atmospheric CO₂ levels can dilute the nitrogen (N) resource in plant tissue, which is disadvantageous to many herbivorous insects. Aphids appear to be an exception that warrants further study. The effects of elevated CO₂ (750 ppm vs. 390 ppm) were evaluated on N assimilation and transamination by two Medicago truncatula genotypes, a N‐fixing‐deficient mutant (dnf1) and its wild‐type control (Jemalong), with and without pea aphid (Acyrthosiphon pisum) infestation. Elevated CO₂ increased population abundance and feeding efficiency of aphids fed on Jemalong, but reduced those on dnf1. Without aphid infestation, elevated CO₂ increased photosynthetic rate, chlorophyll content, nodule number, biomass, and pod number for Jemalong, but only increased pod number and chlorophyll content for dnf1. Furthermore, aphid infested Jemalong plants had enhanced activities of N assimilation‐related enzymes (glutamine synthetase, Glutamate synthase) and transamination‐related enzymes (glutamate oxalate transaminase, glutamine phenylpyruvate transaminase), which presumably increased amino acid concentration in leaves and phloem sap under elevated CO₂. In contrast, aphid infested dnf1 plants had decreased activities of N assimilation‐related enzymes and transmination‐related enzymes and amino acid concentrations under elevated CO₂. Furthermore, elevated CO₂ up‐regulated expression of genes relevant to amino acid metabolism in bacteriocytes of aphids associated with Jemalong, but down‐regulated those associated with dnf1. Our results suggest that pea aphids actively elicit host responses that promote amino acid metabolism in both the host plant and in its bacteriocytes to favor the population growth of the aphid under elevated CO₂.     

Anthropogenic increase in carbon dioxide modifies plant–insect interactions

Industrialisation has elevated atmospheric levels of CO₂ from original 280 ppm to current levels at 400 ppm, which is estimated to double by 2050. Although high atmospheric CO₂ levels affect insect interactions with host plants, the impact of global change on plant defences in response to insect attack is not completely understood. Recent studies have made advances in elucidating the mechanisms of the effects of high CO₂ levels in plant–insect interactions. New studies have proposed that gene regulation and phytohormones regulate resource allocation from photosynthesis to plant defences against insects. Biochemical and molecular studies demonstrated that both defensive hormones jasmonic acid (JA) and ethylene (ET) participate in modulating chemical defences against herbivores in plants grown under elevated CO₂ atmosphere rather than changes in C:N ratio. High atmospheric CO₂ levels increase vulnerability to insect damage by down‐regulating both inducive and constitutive chemical defences regulated by JA and ET. However, elevated CO₂ levels increase the JA antagonistic hormone salicylic acid that increases other chemical defences. How plants grown under elevated CO₂ environment allocate primary metabolites from photosynthesis to secondary metabolism would help to understand innate defences and prevent future herbivory in field crops. We present evidence demonstrating that changes in chemical defences in plants grown under elevated CO₂ environment are hormonal regulated and reject the C:N hypothesis. In addition, we discuss current knowledge of the mechanisms that regulate plants defences against insects in elevated CO₂ atmospheres.     

Differential proteomic responses of selectively bred and wild‐type Sydney rock oyster populations exposed to elevated CO2

Previous work suggests that larvae from Sydney rock oysters that have been selectively bred for fast growth and disease resistance are more resilient to the impacts of ocean acidification than nonselected, wild‐type oysters. In this study, we used proteomics to investigate the molecular differences between oyster populations in adult Sydney rock oysters and to identify whether these form the basis for observations seen in larvae. Adult oysters from a selective breeding line (B2) and nonselected wild types (WT) were exposed for 4 weeks to elevated pCO₂ (856 μatm) before their proteomes were compared to those of oysters held under ambient conditions (375 μatm pCO₂). Exposure to elevated pCO₂ resulted in substantial changes in the proteomes of oysters from both the selectively bred and wild‐type populations. When biological functions were assigned, these differential proteins fell into five broad, potentially interrelated categories of subcellular functions, in both oyster populations. These functional categories were energy production, cellular stress responses, the cytoskeleton, protein synthesis and cell signalling. In the wild‐type population, proteins were predominantly upregulated. However, unexpectedly, these cellular systems were downregulated in the selectively bred oyster population, indicating cellular dysfunction. We argue that this reflects a trade‐off, whereby an adaptive capacity for enhanced mitochondrial energy production in the selectively bred population may help to protect larvae from the effects of elevated CO₂, whilst being deleterious to adult oysters.