Sweet problems: insect traits defining the limits to dietary sugar utilisation by the pea aphid, Acyrthosiphon pisum

Avocado cultivar and tree-to-tree leaf compositional differences affect infestation severity of Pseudocysta perseae (Hemiptera: Tingidae)

Avocado lace bug, Pseudocysta perseae (Heidemann) (Hemiptera: Tingidae), is a sap-feeding insect that feeds on the underside of avocado leaves. First observed in 2019, P. perseae has spread throughout the Hawaiian islands, causing premature leaf drop and decrease in avocado yield. Due to Hawai'i's approximately 200 cultivars comprised of all 3 avocado races with extensive racial hybrids, we were able to investigate whether certain cultivars were more prone to experiencing higher P. perseae abundances and infestations compared to others. We conducted longitudinal abundance surveys on Hawai'i Island across several common avocado varieties monitoring changes in P. perseae abundance. These surveys were supplemented with longitudinal infestation severity surveys across 4 avocado lineages (Mexican, Guatemalan, West Indian, and Guatemalan × West Indian hybrid). Additionally, we collected leaves of 'Sharwil', 'Hass', 'Kahalu'u', and 'Nishikawa' cultivars looking at associations between P. perseae abundance and cultivar, herbivory-related biomechanical traits, and soluble sugar content. We found that some cultivars, such as 'Malama', typically experience lower P. perseae abundances compared to cultivars such as 'Kahalu'u', 'Beshore', and 'Sharwil'. Guatemalan × West Indian hybrid trees were also shown to have a higher probability of experiencing more severe P. perseae infestations compared to other lineages. Lastly, soluble sugar content, specifically fructose content, had a positive effect on juvenile P. perseae abundance. These findings suggest that cultivar differences in P. perseae infestations may exist, but tree-to-tree leaf compositional differences, such as soluble sugar content, may be a large driver of variation in P. perseae abundance.

Brown planthoppers manipulate rice sugar transporters to benefit their own feeding

The feeding of piercing-sucking insect herbivores often elicits changes in their host plants that benefit the insect.1 In addition to thwarting a host's defense responses, these phloem-feeding insects may manipulate source-sink signaling so as to increase resources consumed.2,3 To date, the molecular mechanisms underlying herbivore-induced resource reallocation remain less investigated. Brown planthopper (BPH), an important rice pest, feeds on the phloem and oviposits into leaf sheaths. BPH herbivory increases sugar accumulations 5-fold in the phloem sap of leaf sheaths and concurrently induces the expression of two clade III SWEET genes, SWEET13 and SWEET14, in leaf tissues, but not in leaf sheaths of attacked rice plants. Mutations of both genes by genome editing attenuate resistance to BPH without alterations of known chemical and physical defense responses. Moreover, BPH-elicited sugar levels in the phloem sap were significantly reduced in sweet13/14 mutants, which is likely to attenuate BPH feeding behavior on sweet13/14 mutants. In one of the two field seasons tested, the sweet13/14 mutants showed comparable yield to wild types, and in the other season, the mutants demonstrated stronger BPH resistance. These preliminary results suggested that the mutations in these SWEET transporters could enhance BPH resistance without yield penalties. Given that sweet13/14 mutants also exhibit resistance to bacterial blight pathogen, Xanthomonas oryzae pv. oryzae, these SWEET genes could serve as excellent molecular targets for the breeding of resistant rice cultivars.

Effect of the mycotoxins enniatin B and deoxynivalenol on the wheat aphid Sitobion avenae and on the predatory lacewing Chrysoperla carnea

BACKGROUND

Fusarium species are responsible for Fusarium head blight (FHB) in wheat, resulting in yield losses and mycotoxin contamination. Deoxynivalenol (DON) and enniatins (ENNs) are common mycotoxins produced by Fusarium, affecting plant, animal and human health. Although DON's effects have been widely studied, limited research has explored the impact of ENNs on insects. This study examines the influence of DON and enniatin B (ENB), both singularly and in combination, on the wheat aphid Sitobion avenae and one of its predators, the lacewing Chrysoperla carnea.

RESULTS

When exposed to DON (100 mg L-1) or DON + ENB (100 mg L-1), S. avenae exhibited significantly increased mortality compared to the negative control. ENB (100 mg L-1) had no significant effect on aphid mortality. DON-treated aphids showed increasing mortality from 48 to 96 h. A dose-response relationship with DON revealed significant cumulative mortality starting at 25 mg L-1. By contrast, C. carnea larvae exposed to mycotoxins via cuticular application did not show significant differences in mortality when mycotoxins were dissolved in water but exhibited increased mortality with acetone-solubilized DON + ENB (100 mg L-1). Feeding C. carnea with aphids exposed to mycotoxins (indirect exposure) did not impact their survival or predatory activity. Additionally, the impact of mycotoxins on C. carnea was observed only with acetone-solubilized DON + ENB.

CONCLUSIONS

These findings shed light on the complex interactions involving mycotoxins, aphids and their predators, offering valuable insights for integrated pest management strategies. Further research should explore broader ecological consequences of mycotoxin contamination in agroecosystems. © 2024 Society of Chemical Industry.

Non-volatile metabolites mediate plant interactions with insect herbivores

Non-volatile metabolites constitute the bulk of plant biomass. From the perspective of plant-insect interactions, these structurally diverse compounds include nutritious core metabolites and defensive specialized metabolites. In this review, we synthesize the current literature on multiple scales of plant-insect interactions mediated by non-volatile metabolites. At the molecular level, functional genetics studies have revealed a large collection of receptors targeting plant non-volatile metabolites in model insect species and agricultural pests. By contrast, examples of plant receptors of insect-derived molecules remain sparse. For insect herbivores, plant non-volatile metabolites function beyond the dichotomy of core metabolites, classed as nutrients, and specialized metabolites, classed as defensive compounds. Insect feeding tends to elicit evolutionarily conserved changes in plant specialized metabolism, whereas its effect on plant core metabolism varies widely based the interacting species. Finally, several recent studies have demonstrated that non-volatile metabolites can mediate tripartite communication on the community scale, facilitated by physical connections established through direct root-to-root communication, parasitic plants, arbuscular mycorrhizae and the rhizosphere microbiome. Recent advances in both plant and insect molecular biology will facilitate further research on the role of non-volatile metabolites in mediating plant-insect interactions.

Trehalose Biosynthesis Gene otsA Protects against Stress in the Initial Infection Stage of Burkholderia-Bean Bug Symbiosis

Trehalose, a nonreducing disaccharide, functions as a stress protectant in many organisms, including bacteria. In symbioses involving bacteria, the bacteria have to overcome various stressors to associate with their hosts; thus, trehalose biosynthesis may be important for symbiotic bacteria. Here, we investigated the role of trehalose biosynthesis in the Burkholderia-bean bug symbiosis. Expression levels of two trehalose biosynthesis genes, otsA and treS, were elevated in symbiotic Burkholderia insecticola cells, and hence mutant ΔotsA and ΔtreS strains were generated to examine the functions of these genes in symbiosis. An in vivo competition assay with the wild-type strain revealed that fewer ΔotsA cells, but not ΔtreS cells, colonized the host symbiotic organ, the M4 midgut, than wild-type cells. The ΔotsA strain was susceptible to osmotic pressure generated by high salt or high sucrose concentrations, suggesting that the reduced symbiotic competitiveness of the ΔotsA strain was due to the loss of stress resistance. We further demonstrated that fewer ΔotsA cells infected the M4 midgut initially but that fifth-instar nymphs exhibited similar symbiont population size as the wild-type strain. Together, these results demonstrated that the stress resistance role of otsA is important for B. insecticola to overcome the stresses it encounters during passage through the midgut regions to M4 in the initial infection stage but plays no role in resistance to stresses inside the M4 midgut in the persistent stage. IMPORTANCE Symbiotic bacteria have to overcome stressful conditions present in association with the host. In the Burkholderia-bean bug symbiosis, we speculated that a stress-resistant function of Burkholderia is important and that trehalose, known as a stress protectant, plays a role in the symbiotic association. Using otsA, the trehalose biosynthesis gene, and a mutant strain, we demonstrated that otsA confers Burkholderia with competitiveness when establishing a symbiotic association with bean bugs, especially playing a role in initial infection stage. In vitro assays revealed that otsA provides the resistance against osmotic stresses. Hemipteran insects, including bean bugs, feed on plant phloem sap, which may lead to high osmotic pressures in the midguts of hemipterans. Our results indicated that the stress-resistant role of otsA is important for Burkholderia to overcome the osmotic stresses present during the passage through midgut regions to reach the symbiotic organ.

Dichotomous Role of Jasmonic Acid in Modulating Sorghum Defense Against Aphids

The precursors and derivatives of jasmonic acid (JA) contribute to plant protective immunity to insect attack. However, the role of JA in sorghum (Sorghum bicolor) defense against sugarcane aphid (SCA) (Melanaphis sacchari), which is considered a major threat to sorghum production, remains elusive. Sorghum SC265, previously identified as a SCA-resistant genotype among the sorghum nested association mapping founder lines, transiently increased JA at early stages of aphid feeding and deterred aphid settling. Monitoring of aphid feeding behavior using electropenetrography, a technique to unveil feeding process of piercing-sucking insects, revealed that SC265 plants restricted SCA feeding from the phloem sap. However, exogenous application of JA attenuated the resistant phenotype and promoted improved aphid feeding and colonization on SC265 plants. This was further confirmed with sorghum JA-deficient plants, in which JA deficiency promoted aphid settling, however, it also reduced aphid feeding from the phloem sap and curtailed SCA population. Exogenous application of JA caused enhanced feeding and aphid proliferation on JA-deficient plants, suggesting that JA promotes aphid growth and development. SCA feeding on JA-deficient plants altered the sugar metabolism and enhanced the levels of fructose and trehalose compared with wild-type plants. Furthermore, aphid artificial diet containing fructose and trehalose curtailed aphid growth and reproduction. Our findings underscore a previously unknown dichotomous role of JA, which may have opposing effects by deterring aphid settling during the early stage and enhancing aphid proliferative capacity during later stages of aphid colonization on sorghum plants. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Wheat-Fusarium graminearum Interactions Under Sitobion avenae Influence: From Nutrients and Hormone Signals

The English grain aphid Sitobion avenae and phytopathogen Fusarium graminearum are wheat spike colonizers. "Synergistic" effects of the coexistence of S. avenae and F. graminearum on the wheat spikes have been shown in agroecosystems. To develop genetic resistance in diverse wheat cultivars, an important question is how to discover wheat-F. graminearum interactions under S. avenae influence. In recent decades, extensive studies have typically focused on the unraveling of more details on the relationship between wheat-aphids and wheat-pathogens that has greatly contributed to the understanding of these tripartite interactions at the ecological level. Based on the scientific production available, the working hypotheses were synthesized from the aspects of environmental nutrients, auxin production, hormone signals, and their potential roles related to the tripartite interaction S. avenae-wheat-F. graminearum. In addition, this review highlights the relevance of preexposure to the herbivore S. avenae to trigger the accumulation of mycotoxins, which stimulates the infection process of F. graminearum and epidemic of Fusarium head blight (FHB) in the agroecosystems.

Evolutionary and ecological perspectives on the wheat phenotype

Technologies, from molecular genetics to precision agriculture, are outpacing theory, which is becoming a bottleneck for crop improvement. Here, we outline theoretical insights on the wheat phenotype from the perspective of three evolutionary and ecologically important relations-mother-offspring, plant-insect and plant-plant. The correlation between yield and grain number has been misinterpreted as cause-and-effect; an evolutionary perspective shows a striking similarity between crop and fishes. Both respond to environmental variation through offspring number; seed and egg size are conserved. The offspring of annual plants and semelparous fishes, lacking parental care, are subject to mother-offspring conflict and stabilizing selection. Labile reserve carbohydrates do not fit the current model of wheat yield; they can stabilize grain size, but involve trade-offs with root growth and grain number, and are at best neutral for yield. Shifting the focus from the carbon balance to an ecological role, we suggest that labile carbohydrates may disrupt aphid osmoregulation, and thus contribute to wheat agronomic adaptation. The tight association between high yield and low competitive ability justifies the view of crop yield as a population attribute whereby the behaviour of the plant becomes subordinated within that of the population, with implications for genotyping, phenotyping and plant breeding.

The role of plant labile carbohydrates and nitrogen on wheat-aphid relations

Interactions between plants and herbivores are key drivers of evolution and ecosystem complexity. We investigated the role of plant labile carbohydrates and nitrogen on wheat-aphid relations in a 2² factorial combining [CO₂] and nitrogen supply. We measured life history traits (assay 1) and feeding behaviour (assay 2) of bird-cherry oat aphid (Rhopalosiphum padi L.) and English grain aphid (Sitobion avenae F.) forced to feed on single leaf laminae, and reproduction of R. padi in a setting where insects moved freely along the plant (assay 3). Experimental setting impacted aphid traits. Where aphids were constrained to single leaf, high nitrogen reduced their fitness and discouraged phloem feeding. Where aphids could move throughout the plant, high nitrogen enhanced their reproduction. Aphid responses to the interaction between nitrogen and [CO₂] varied with experimental setting. The number of R. padi adults varied tenfold with plant growing conditions and correlated negatively with molar concentration of sugars in stem (assay 3). This finding has two implications. First, the common interpretation that high nitrogen favours insect fitness because protein-rich animal bodies have to build from nitrogen-poor plant food needs expanding to account for the conspicuous association between low nitrogen and high concentration of labile carbohydrates in plant, which can cause osmotic stress in aphids. Second, the function of labile carbohydrates buffering grain growth needs expanding to account for the osmotic role of carbohydrates in plant resistance to aphids.

Resistance and tolerance of ten carrot cultivars to the hawthorn-carrot aphid, Dysaphis crataegi Kalt., in Poland

Damage caused to cultivated carrots by the hawthorn-carrot aphid, Dysaphis crataegi Kalt. (Hemiptera: Aphididae) is one of the factors limiting carrot production in Poland. Planting resistant and tolerant cultivars could reduce yield losses due to the damage caused by this pest. This study was conducted to evaluate the resistance and/or tolerance of 10 carrot genotypes to hawthorn-carrot aphid. Their field resistance was determined under field conditions based on five indicators, namely, mean number of alates (migrants) per plant and mean percentage of plants colonized by them, mean seasonal number of aphids per plant, mean number of aphids per plant and mean percentage of infested plants at peak abundance. Antibiosis experiments were conducted under laboratory conditions and pre-reproductive, reproductive time, fertility, and demographic parameters, represented by the net reproduction rate (R_o), intrinsic rate of increase (r_m) and mean generation time (T), were calculated. Five cultivars, Afro F₁, Nipomo F₁, Samba F₁, White Satin F₁, and Yellowstone showed field resistance. Antibiosis experiments revealed significant differences among the carrot cultivars in the length of the reproductive period, female fecundity in the time equal to the pre-reproduction time, and total progeny of hawthorn-carrot aphid. The intrinsic rate of natural increase (r_m) for apterous aphids varied significantly, ranging between 0.181 (Nipomo F₁) and 0.343 females/female/day (White Satin F₁). Additionally, the estimated net reproductive rate (R₀) was the lowest on Nipomo F₁, and this genotype was determined to be resistant. Our results suggest that a very high density of trichomes on the leaf petioles (71.94 trichomes/cm²) could adversely affect the feeding, bionomy, and demographic parameters of hawthorn-carrot aphid on the cultivar Nipomo F₁. In addition, Napa F₁ and Kongo F₁ demonstrated high tolerance. Considering all the results collectively, four genotypes, Afro F₁, Kongo F₁, Napa F₁ and Nipomo F₁, were relatively resistant/tolerant to the hawthorn-carrot aphid.

Phloem Metabolites of Prunus Sp. Rather than Infection with Candidatus Phytoplasma Prunorum Influence Feeding Behavior of Cacopsylla pruni Nymphs

Phytoplasmas are specialized small bacteria restricted to the phloem tissue and spread by hemipterans feeding on plant sieve tube elements. As for many other plant pathogens, it is known that phytoplasmas alter the chemistry of their hosts. Most research on phytoplasma-plant interactions focused on the induction of plant volatiles and phytohormones. Little is known about the influence of phytoplasma infections on the nutritional composition of phloem and consequences on vector behavior and development. The plum psyllid Cacopsylla pruni transmits 'Candidatus Phytoplasma prunorum', the causing agent of European Stone Fruit Yellows (ESFY). While several Prunus species are susceptible for psyllid feeding, they show different responses to the pathogen. We studied the possible modulation of plant-insect interactions by bacteria-induced changes in phloem sap chemistry. Therefore, we sampled phloem sap from phytoplasma-infected and non-infected Prunus persica and Prunus insititia plants, which differ in their susceptibility to ESFY and psyllid feeding. Furthermore, the feeding behavior and development of C. pruni nymphs was compared on infected and non-infected P. persica and P. insititia plants. Phytoplasma infection did not affect phloem consumption by C. pruni nymphs nor their development time. In contrast, the study revealed significant differences between P. insititia and P. persica in terms of both phloem chemistry and feeding behavior of C. pruni nymphs. Phloem feeding phases were four times longer on P. insititia than on P. persica, resulting in a decreased development time and higher mortality of vector insects on P. persica plants. These findings explain the low infestation rates of peach cultivars with plum psyllids commonly found in field surveys.

Aphid Resistance: An Overlooked Ecological Dimension of Nonstructural Carbohydrates in Cereals

Nonstructural carbohydrates in cereals have been widely investigated from physiological, genetic, and breeding perspectives. Nonstructural carbohydrates may contribute to grain filling, but correlations with yield are inconsistent and sometimes negative. Here we ask if there are hidden functions of nonstructural carbohydrates, advance an ecological dimension to this question, and speculate that high concentration of nonstructural carbohydrates may challenge the osmotic homeostasis of aphids, thus providing a working hypothesis that connects nonstructural carbohydrates with aphid resistance in cereals. In the light of this proposition, the amount and concentration of nonstructural carbohydrates should be regarded as functionally different traits, with amount relevant to the carbon economy of the crop and concentration playing an osmotic role. We conclude with suggestions for experiments to test our hypothesis.

Chemical and growth traits of the peach tree may induce higher infestation rates of the green peach aphid, Myzus persicae (Sulzer)

BACKGROUND

The green peach aphids, Myzus persicae, are a predominant pest in peach orchards as they can alter fruiting and shoot development for several years. This aphid developed resistance against pesticides. Among the alternative control strategies is the reduction of the trees' attractiveness to aphids. In order to identify the plant variables related to plant susceptibility, young peach trees were submitted to various supplies of water and nitrogen, and then artificially infested with aphids. Shoot development, plant water potential and aphid abundance were then monitored on a weekly basis. The apex concentrations in total N, amino acids, soluble sugars and polyphenols were determined at infestation start and infestation peak.

RESULTS

Until infestation peak, the thermal time requests for aphid development were independent of infestation severity. The aphid populations then collapsed more rapidly on the low infested shoots than on the high infested ones. Aphid abundances appeared to be positively related to shoot development (leaf expansion and secondary ramification), to shoot growth (stem length and diameter) and to apex concentrations in amino acids and non-structural carbohydrates (NSC). Polyphenols had the opposite effect.

CONCLUSION

Peach susceptibility to aphids depends on shoot development and apex composition, and could be lowered by decreasing the water and nitrogen inputs. © 2019 Society of Chemical Industry.

New insights into diet breadth of polyphagous and oligophagous aphids on two Arabidopsis ecotypes

We investigated whether plant ecotype might affect aphid performance and behavior. The probing behaviors of the polyphagous aphid Myzus persicae and the oligophagous aphid Brevicoryne brassicae on two ecotypes of Arabidopsis thaliana, WS and Col-0 were recorded using the direct current electrical penetration graph method (DC-EPG). Myzus persicae displayed a significant preference for the WS ecotype but was not greatly disturbed on Col-0, while B. brassicae discriminated between the two A. thaliana ecotypes, feeding less on WS than on Col-0. A Principal Component Analysis of aphid probing behavior data recorded on Col-0 and WS ecotypes showed that the one of M. persicae was positively correlated with the phloem ingestion phases while the one of B. brassicae was more related to nonfeeding phase. The survival of the aphid species was followed during early larval stages on the two ecotypes and a significantly higher mortality was observed of B. brassicae neonates compared to M. persicae, both reared on WS. Moreover, transcriptomic analysis of noninfested plant leaves from both ecotypes was monitored and underlined constitutive differences between Col-0 and WS gene expression that might explain the different aphid behaviors. Among a unigene set comprising 39 042 sequences for A. thaliana, 6% were differently expressed affecting, for example, the secondary metabolites and cell wall pathways: two third upregulated in WS and one third upregulated in Col-0. Thus, the "ecotype" variable should be taken into account when setting up a plant-insect experimental research.

Silencing of sucrose hydrolase causes nymph mortality and disturbs adult osmotic homeostasis in Diaphorina citri (Hemiptera: Liviidae)

Plant piercing sucking insects mainly feed on phloem sap containing a high amount of sucrose. To enhance the absorption of sucrose from the midgut, sucrose hydrolase digests sucrose into glucose and fructose. In this study, a sucrose hydrolase homolog (DcSuh) was identified and targeted in Diaphorina citri, the vector of huanglongbing (HLB), by RNA interference (RNAi). In silico analysis revealed the presence of an Aamy domain in the DcSUH protein, which is characteristic of the glycoside hydrolase family 13 (GH13). Phylogenetic analysis showed DcSuh was closely related to the sucrose hydrolase of other Hemiptera members. The highest gene expression levels of DcSuh was found in the 4th and 5th instar nymphs. dsRNA-mediated RNAi of DcSuh was achieved through topical feeding. Our results showed that application of 0.2 μL of 500 ng μL^-1 (100 ng) dsRNA-DcSuh was sufficient to repress the expression of the targeted gene and cause nymph mortality and reduce adult lifespan. The reduction in gene expression, mortality, and lifespan was dose-dependent. In agreement with the gene expression results, treatment with dsRNA-DcSuh significantly reduced sucrose hydrolase activity in treated nymphs and emerged adults from treated nymphs. Interestingly, some emerged adults from treated nymphs showed a swollen abdomen phenotype, indicating that these insects were under osmotic stress. Although the percentage of swollen abdomens was low, their incidence was significantly correlated with the concentration of applied dsRNA-DcSuh. Metabolomic analyses using GC-MS showed an accumulation of sucrose and a reduction in fructose, glucose and trehalose in treated nymphs, confirming the inhibition of sucrose hydrolase activity. Additionally, most of the secondary metabolites were reduced in the treated nymphs, indicating a reduction in the biological activities in D. citri and that they are under stress. Our findings indicate that sucrose hydrolase might be a potential target for effective RNAi control of D. citri.

Co-inoculum of Beauveria brongniartii and B. bassiana shows in vitro different metabolic behaviour in comparison to single inoculums

The use of entomopathogenic fungi for biocontrol of plant pests is recently receiving an increased interest due to the need of reducing the impact of agricultural practices on the environment. Biocontrol efficacy could be improved by co-inoculation of different microorganisms. However, interactions between the fungal species can trigger or depress the biocontrol activity. Co-inoculation of two entomopathogenic fungi (Beauveria bassiana and B. brongniartii) was performed in vitro to evaluate the effects of their joint behaviour on a range of different carbon sources in comparison to single inoculation. The two species showed a very different metabolic profile by Phenotype MicroArray^TM. B. bassiana showed a broader metabolism than B. brongniartii on a range of substrates. B. brongniartii showed a greater specificity in substrate utilization. Several carbon sources (L-Asparagine, L-Aspartic Acid, L- Glutamic Acid, m- Erythritol, D-Melezitose, D-Sorbitol) triggered the fungal metabolism in the co-inoculum. SSR markers and Real Time qPCR analysis showed that different substrates promoted either the growth of one or the other species, suggesting a form of interaction between the two fungi, related to their different ecological niches. The methodological approach that combines Phenotype MicroArray^TM and SSR genotyping appeared useful to assess the performance and potential competition of co-inoculated entomopathogenic fungi.

Triticum monococcum lines with distinct metabolic phenotypes and phloem-based partial resistance to the bird cherry-oat aphid Rhopalosiphum padi

Crop protection is an integral part of establishing food security, by protecting the yield potential of crops. Cereal aphids cause yield losses by direct damage and transmission of viruses. Some wild relatives of wheat show resistance to aphids but the mechanisms remain unresolved. In order to elucidate the location of the partial resistance to the bird cherry-oat aphid, Rhopalosiphum padi, in diploid wheat lines of Triticum monococcum, we conducted aphid performance studies using developmental bioassays and electrical penetration graphs, as well as metabolic profiling of partially resistant and susceptible lines. This demonstrated that the partial resistance is related to a delayed effect on the reproduction and development of R. padi. The observed partial resistance is phloem based and is shown by an increase in number of probes before the first phloem ingestion, a higher number and duration of salivation events without subsequent phloem feeding and a shorter time spent phloem feeding on plants with reduced susceptibility. Clear metabolic phenotypes separate partially resistant and susceptible lines, with the former having lower levels of the majority of primary metabolites, including total carbohydrates. A number of compounds were identified as being at different levels in the susceptible and partially resistant lines, with asparagine, octopamine and glycine betaine elevated in less susceptible lines without aphid infestation. In addition, two of those, asparagine and octopamine, as well as threonine, glutamine, succinate, trehalose, glycerol, guanosine and choline increased in response to infestation, accumulating in plant tissue localised close to aphid feeding after 24 h. There was no clear evidence of systemic plant response to aphid infestation.

Nitrogen hurdle of host alternation for a polyphagous aphid and the associated changes of endosymbionts

Low proportion of essential amino acids (EAAs) is one of the barriers for animals to use phloem as a diet. Endosymbionts with EAAs synthesis functions are considered crucial for ameliorating the lack of EAAs in insects’ diets. In this study, we transferred the insects from a cabbage-reared Myzus persicae population onto 3 new plant species including eggplant, tobacco and spinach. The performance on these plants was evaluated and the dynamics of endosymbionts in relation to this host alternation were recorded. We found that the EAAs ratio in phloem was largely determined by the concentrations of non-essential amino acids and the higher proportion of EAAs seemed to favor the population establishment on new plant species and the growth of primary endosymbionts inside insects, which indicated that nitrogen quality was an important factor for aphids to infest and spread on new plant hosts.

Cross-tolerance to biotic and abiotic stresses in plants: a focus on resistance to aphid infestation

Plants co-evolved with an enormous variety of microbial pathogens and insect herbivores under daily and seasonal variations in abiotic environmental conditions. Hence, plant cells display a high capacity to respond to diverse stresses through a flexible and finely balanced response network that involves components such as reduction-oxidation (redox) signalling pathways, stress hormones and growth regulators, as well as calcium and protein kinase cascades. Biotic and abiotic stress responses use common signals, pathways and triggers leading to cross-tolerance phenomena, whereby exposure to one type of stress can activate plant responses that facilitate tolerance to several different types of stress. While the acclimation mechanisms and adaptive responses that facilitate responses to single biotic and abiotic stresses have been extensively characterized, relatively little information is available on the dynamic aspects of combined biotic/abiotic stress response. In this review, we consider how the abiotic environment influences plant responses to attack by phloem-feeding aphids. Unravelling the signalling cascades that underpin cross-tolerance to biotic and abiotic stresses will allow the identification of new targets for increasing environmental resilience in crops.

Shifts in Buchnera aphidicola density in soybean aphids (Aphis glycines) feeding on virus-infected soybean

Vertically transmitted bacterial symbionts are common in arthropods. Aphids undergo an obligate symbiosis with Buchnera aphidicola, which provides essential amino acids to its host and contributes directly to nymph growth and reproduction. We previously found that newly adult Aphis glycines feeding on soybean infected with the beetle-transmitted Bean pod mottle virus (BPMV) had significantly reduced fecundity. We hypothesized that the reduced fecundity was attributable to detrimental impacts of the virus on the aphid microbiome, namely Buchnera. To test this, mRNA sequencing and quantitative real-time PCR were used to assay Buchnera transcript abundance and titre in A. glycines feeding on Soybean mosaic virus-infected, BPMV-infected, and healthy soybean for up to 14 days. Our results indicated that Buchnera density was lower and ultimately suppressed in aphids feeding on virus-infected soybean. While the decreased Buchnera titre may be associated with reduced aphid fecundity, additional mechanisms are probably involved. The present report begins to describe how interactions among insects, plants, and plant pathogens influence endosymbiont population dynamics.

The effects of triazophos applied to transgenic Bt rice on the nutritional indexes, Nlvg expression, and population growth of Nilaparvata lugens Stål under elevated CO₂

The brown planthopper, Nilaparvata lugens (Stål) (Hemiptera: Delphacidae), is a typical pest in which population resurgence can be induced by insecticides. Warmer global temperatures, associated with anthropogenic climate change, are likely to have marked ecological effects on terrestrial ecosystems. However, the effects of elevated CO2 (eCO2) concentrations on the resurgence of N. lugens that have been treated with pesticides used for transgenic Bt rice cultivation are not fully understood. The present study investigated changes in the protein content, soluble sugar content, free amino acid level, vitellogenin (Nlvg) mRNA expression, and the population growth of N. lugens on transgenic Bt rice (TT51) following triazaophos foliar spray under conditions of eCO2. The results showed that the protein content in the fat bodies and ovaries of N. lugens adult females in TT51 treated with 40 ppm triazophos under eCO2 was significantly higher than under ambient CO2 (aCO2) and was also higher than that in females feeding on the non-transgenic parent (MH63) under aCO2 at different days after emergence (DAEs). The soluble sugar content and free amino level of adult females in TT51 treated with 40 ppm triazophos under eCO2 was significantly higher than under aCO2 and was also higher than in MH63 under aCO2 at 1 and 3 DAE. The Nlvg mRNA expression level of N. lugens adult females in TT51 treated with 40 ppm triazophos under eCO2 was significantly higher than under aCO2 and was also higher than in MH63 under aCO2 at 1 and 3 DAE. The population number of N. lugens in TT51 treated with 40 ppm triazophos under eCO2 was significantly higher than under aCO2 and was also higher than in MH63 under aCO2. The present findings provide important information for integrated pest management with transgenic varieties and a better understanding of the resurgence mechanism of N. lugens under eCO2.

Phenological synchrony between Scaphoideus titanus (Hemiptera: Cicadellidae) hatchings and grapevine bud break: could this explain the insect's expansion?

Scaphoideus titanus is the invasive vector of the phytoplasma causing the Flavescence dorée in European vineyards. This epidemic is a serious threat to viticulture that has been increasing for more than 60 years in Europe. We studied the effect of synchrony with the plant phenology and the effect of plant-sap quality on the individual fitness. Thus, we conducted laboratory experiments to determine if insect hatchings were synchronized with grapevine bud break. We used two natural populations: one from a cold winter vineyard and one from a mild winter vineyard. In both cases, egg hatching was synchronized with bud break and leaf appearance. The phloem quality of the young and old leaves as a food source was analysed by high-performance liquid chromatography, and the effects on S. titanus growth were evaluated. Phloem composition varied with the grapevine cutting's age but also varied between leaves of different ages from the same plant. The older leaves were less nutritious because they had the highest carbon-to-nitrogen ratio and the lowest content of essential amino acids. Despite diverse phloem qualities, no fitness difference was observed. We found that the synchronization of egg hatchings with bud break is well regulated. However, the nymphs are not affected by the phloem-sap quality, suggesting that S. titanus may accept different food qualities and that egg hatching synchrony could contribute to population expansion in vineyards.

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

Rising atmospheric CO(2) 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 CO(2) (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 CO(2) increased population abundance and feeding efficiency of aphids fed on Jemalong, but reduced those on dnf1. Without aphid infestation, elevated CO(2) 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 CO(2). In contrast, aphid infested dnf1 plants had decreased activities of N assimilation-related enzymes and transmination-related enzymes and amino acid concentrations under elevated CO(2). Furthermore, elevated CO(2) 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 CO(2).

Proteomic profiling of cereal aphid saliva reveals both ubiquitous and adaptive secreted proteins

The secreted salivary proteins from two cereal aphid species, Sitobion avenae and Metopolophium dirhodum, were collected from artificial diets and analysed by tandem mass spectrometry. Protein identification was performed by searching MS data against the official protein set from the current pea aphid (Acyrthosiphon pisum) genome assembly and revealed 12 and 7 proteins in the saliva of S. avenae and M. dirhodum, respectively. When combined with a comparable dataset from A. pisum, only three individual proteins were common to all the aphid species; two paralogues of the GMC oxidoreductase family (glucose dehydrogenase; GLD) and ACYPI009881, an aphid specific protein previously identified as a putative component of the salivary sheath. Antibodies were designed from translated protein sequences obtained from partial cDNA sequences for ACYPI009881 and both saliva associated GLDs. The antibodies detected all parent proteins in secreted saliva from the three aphid species, but could only detect ACYPI009881, and not saliva associated GLDs, in protein extractions from the salivary glands. This result was confirmed by immunohistochemistry using whole and sectioned salivary glands, and in addition, localised ACYPI009881 to specific cell types within the principal salivary gland. The implications of these findings for the origin of salivary components and the putative role of the proteins identified are discussed in the context of our limited understanding of the functional relationship between aphid saliva and the plants they feed on. The mass spectrometry data have been deposited to the ProteomeXchange and can be accessed under the identifier PXD000113.

Relationship between water soluble carbohydrate content, aphid endosymbionts and clonal performance of Sitobion avenae on cocksfoot cultivars

Aphids feed on plant phloem sap, rich in sugars but poor in essential amino acids. However, sugars cause osmotic regulation problems for aphids, which they overcome by hydrolysing the sugars in their gut and polymerising the hydrolysis products into oligosaccharides, excreted with honeydew. Aphids harbour primary bacterial endosymbionts, which supply them with essential amino acids necessary for survival. They also harbour secondary (facultative) endosymbionts (sfS), some of which have a positive impact on life history traits, although it is not yet known whether they also play a role in providing effective tolerance to differing levels of water soluble carbohydrates (WSCs). We investigated the relationship between WSC content of cocksfoot cultivars and performance of clones of the English grain aphid Sitobion avenae F. We evaluated how clone genotype and their sfS modulate performance on these different cultivars. We therefore examined the performance of genetically defined clones of S. avenae, collected from different host plants, harbouring different sfS. The performance was tested on 10 Dactylis glomerata L. cultivars with varying WSC content. D. glomerata is known as a wild host plant for S. avenae and is also commercially planted. We found that high WSCs levels are responsible for the resistance of D. glomerata cultivars to specific S. avenae clones. The minimum level of WSCs conferring resistance to D. glomerata cultivars was 1.7% dw. Cultivars with a WSC content of 2.2% or higher were resistant to S. avenae and did not allow reproduction. Our results further indicate that sfS modulate to some extend host plant cultivar adaptation in S. avenae. This is the first study revealing the importance of WSCs for aphid performance. Cocksfoot cultivars with a high content of WSCs might be therefore considered for aphid control or used for resistance breeding in this and other grass species, including cereals.

The physiology of sterol nutrition in the pea aphid Acyrthosiphon pisum

The phloem sap of fava bean (Vicia faba) plants utilized by the pea aphid Acyrthosiphon pisum contains three sterols, cholesterol, stigmasterol and sitosterol, in a 2:2:1 ratio. To investigate the nutritional value of these sterols, pea aphids were reared on chemically-defined diets containing each sterol at 0.1, 1 and 10μgml(-1) with a sterol-free diet as control. Larval growth rate and aphid lifespan did not vary significantly across the diets, indicating that sterol reserves can buffer some performance indices against a shortfall in dietary sterol over at least one generation. However, lifetime reproductive output was depressed in aphids on diets containing stigmasterol or no sterol, relative to diets supplemented with cholesterol or sitosterol. The cholesterol density of embryos in teneral adults was significantly higher than in the total body; and the number and biomass of embryos in aphids on diets with stigmasterol and no sterols were reduced relative to diets with cholesterol or sitosterol, indicating that the reproductive output of the pea aphid can be limited by the amount and composition of dietary sterol. In a complementary RNA-seq analysis of pea aphids reared on plants and diets with different sterol contents, 7.6% of the 17,417 detected gene transcripts were differentially expressed. Transcript abundance of genes with annotated function in sterol utilization did not vary significantly among treatments, suggesting that the metabolic response to dietary sterol may be mediated primarily at the level of enzyme function or metabolite concentration.

Functional characterization of the adipokinetic hormone in the pea aphid, Acyrthosiphon pisum

Aphids are important plant phloem-sucking pests and detailed knowledge about the hormonal control of their metabolism can potentially contribute to the development of methods for their management. The insect metabolism is predominantly controlled by neuropeptides belonging to the adipokinetic hormone/red pigment-concentrating hormone family (AKH/RPCH). The main goal of this study was to obtain the sequence of AKH transcripts and analyze its expression in all polyphenic female forms of the pea aphid, Acyrthosiphon pisum. The neuropeptide is expressed in the brain of all female forms and in the ovaries of the both (wingless and winged) parthenogenetic forms. The form of active Acypi-AKH decapeptide was confirmed by the LC/MS and +ESI tandem mass spectrometry. The highest relative amount of Acypi-AKH was recorded in winged virginoparae. Furthermore, a potential role of this hormone when directly applied to the aphid was studied as well. Interestingly, no significant increase of trehalose in the wingless virginoparae after application of synthetic Acypi-AKH was detected. Yet this treatment did affect the level of protective polyol (mannitol) and furthermore led to increased activity of the detoxification enzyme glutathione S-transferase. The possible physiological function of AKH in A. pisum under the stress conditions is discussed.

A phloem-sap feeder mixes phloem and xylem sap to regulate osmotic potential

Phloem-sap feeders (Hemiptera) occasionally consume the dilute sap of xylem, a behaviour that has previously been associated with replenishing water balance following dehydration. However, a recent study reported that non-dehydrated aphids ingested xylem sap. Here, we tested the hypothesis that the consumption of xylem sap, which has a low osmolality, is a general response to osmotic stresses other than dehydration. Alate aphids were subjected to different treatments and subsequently transferred onto a plant, where electrical penetration graph (EPG) was used to estimate durations of passive phloem sap consumption and active sucking of xylem sap. The proportion of time aphids fed on xylem sap (i.e., time spent feeding on xylem sap/total time spent feeding on phloem plus xylem sap) was used as a proxy of the solute concentration of the uptake. The proportion of time alate aphids fed on xylem sap increased: (1) with the time spent imbibing an artificial diet containing a solution of sucrose, which is highly concentrated in phloem sap and is mainly responsible for the high osmotic potential of phloem sap; (2) with the osmotic potential of the artificial diet, when osmotic potential excess was not related to sucrose concentration; and (3) when aphids were deprived of primary symbionts, a condition previously shown to lead to a higher haemolymph osmotic potential. All our results converge to support the hypothesis that xylem sap consumption contributes to the regulation of the osmotic potential in phloem-sap feeders.

How the insect immune system interacts with an obligate symbiotic bacterium

The animal immune system provides defence against microbial infection, and the evolution of certain animal-microbial symbioses is predicted to involve adaptive changes in the host immune system to accommodate the microbial partner. For example, the reduced humoral immune system in the pea aphid Acyrthosiphon pisum, including an apparently non-functional immune deficiency (IMD) signalling pathway and absence of peptidoglycan recognition proteins (PGRPs), has been suggested to be an adaptation for the symbiosis with the bacterium Buchnera aphidicola. To investigate this hypothesis, the interaction between Buchnera and non-host cells, specifically cultured Drosophila S2 cells, was investigated. Microarray analysis of the gene expression pattern in S2 cells indicated that Buchnera triggered an immune response, including upregulated expression of genes for antimicrobial peptides via the IMD pathway with the PGRP-LC as receptor. Buchnera cells were readily taken up by S2 cells, but were subsequently eliminated over 1-2 days. These data suggest that Buchnera induces in non-host cells a defensive immune response that is deficient in its host. They support the proposed contribution of the Buchnera symbiosis to the evolution of the apparently reduced immune function in the aphid host.

Hyperthermic aphids: insights into behaviour and mortality

Although the impact of warming on winter limitation of aphid populations is reasonably well understood, the impacts of hot summers and heat wave events are less clear. In this study, we address this question through a detailed analysis of the thermal ecology of three closely related aphid species: Myzus persicae, a widespread, polyphagous temperate zone pest, Myzus polaris, an arctic aphid potentially threatened by climate warming, and, Myzus ornatus, a glasshouse pest that may benefit from warming. The upper lethal limits (ULT(50)) and heat coma temperatures of the aphid species reared at both 15 and 20 degrees C did not differ significantly, suggesting that heat coma is a reliable indicator of fatal heat stress. Heat coma and CT(max) were also measured after aphids were reared at 10 and 25 degrees C for one and three generations. The extent of the acclimation response was not influenced by the number of generations. Acclimation increased CT(max) with rearing temperature for all species. The acclimation temperature also influenced heat coma; this relationship was linear for M. ornatus and M. polaris but non-linear for M. persicae (increased tolerance at 10 and 25 degrees C). Bacteria known generically as secondary symbionts can promote thermal tolerance of aphids, but they were not detected in the aphids studied here. Assays of optimum development temperature were also performed for each species. All data indicate that M. persicae has the greatest tolerance of high temperatures.

Nutrients, not caloric restriction, extend lifespan in Queensland fruit flies (Bactrocera tryoni)

Caloric restriction (CR) has been widely accepted as a mechanism explaining increased lifespan (LS) in organisms subjected to dietary restriction (DR), but recent studies investigating the role of nutrients have challenged the role of CR in extending longevity. Fuelling this debate is the difficulty in experimentally disentangling CR and nutrient effects due to compensatory feeding (CF) behaviour. We quantified CF by measuring the volume of solution imbibed and determined how calories and nutrients influenced LS and fecundity in unmated females of the Queensland fruit fly, Bactocera tryoni (Diptera: Tephritidae). We restricted flies to one of 28 diets varying in carbohydrate:protein (C:P) ratios and concentrations. On imbalanced diets, flies overcame dietary dilutions, consuming similar caloric intakes for most dilutions. The response surface for LS revealed that increasing C:P ratio while keeping calories constant extended LS, with the maximum LS along C:P ratio of 21:1. In general, LS was reduced as caloric intake decreased. Lifetime egg production was maximized at a C:P ratio of 3:1. When given a choice of separate sucrose and yeast solutions, each at one of five concentrations (yielding 25 choice treatments), flies regulated their nutrient intake to match C:P ratio of 3:1. Our results (i) demonstrate that CF can overcome dietary dilutions; (ii) reveal difficulties with methods presenting fixed amounts of liquid diet; (iii) illustrate the need to measure intake to account for CF in DR studies and (iv) highlight nutrients rather than CR as a dominant influence on LS.

A sublethal dose of thiamethoxam causes a reduction in xylem feeding by the bird cherry-oat aphid (Rhopalosiphum padi), which is associated with dehydration and reduced performance

The active ingestion of xylem sap by aphids is hypothesised to be an important mechanism for rehydration. When starved bird cherry-oat aphids (Rhopalosiphum padi) were allowed to feed on wheat (Triticum aestivum) treated with a sublethal dose of the xylem-mobile neonicotinoid thiamethoxam, analysis of feeding behaviours using the electrical penetration graph revealed a reduction in xylem feeding that was reversed on removal of the toxin. To test the importance of xylem-feeding behaviour as a rehydration mechanism, the effects of the sublethal dose of thiamethoxam on aphid water content, honeydew excretion, growth and fecundity were investigated. Body water contents of starved R. padi feeding on wheat treated with thiamethoxam were significantly reduced compared to aphids feeding on wheat treated with distilled water (74.5+/-0.23 and 75.6+/-0.18%, respectively). In addition, the sublethal dose of thiamethoxam had detrimental effects on aphid performance. At reproductive maturity, aphids that had been born on wheat treated with thiamethoxam were significantly smaller (as measured by body plan area; 1.07+/-0.03mm(2)), lighter (0.31+/-0.04mg) and less fecund (2.85+/-0.36nymphs/day) than aphids born on wheat treated with distilled water (1.87+/-0.02mm(2), 0.72+/-0.03mg, 11.28+/-0.58nymphs/day, respectively). Regardless of whether the observed impairment of xylem feeding is due to a neurotoxic or an antifeedant effect, these results have important implications for commercial crop protection as the behaviour-modifying effects of the sublethal dose of thiamethoxam may change the efficacy of this pesticide throughout the growing season.

The secreted salivary proteome of the pea aphid Acyrthosiphon pisum characterised by mass spectrometry

Nine proteins secreted in the saliva of the pea aphid Acyrthosiphon pisum were identified by a proteomics approach using GE-LC-MS/MS and LC-MS/MS, with reference to EST and genomic sequence data for A. pisum. Four proteins were identified by their sequences: a homolog of angiotensin-converting enzyme (an M2 metalloprotease), an M1 zinc-dependant metalloprotease, a glucose-methanol-choline (GMC)-oxidoreductase and a homolog to regucalcin (also known as senescence marker protein 30). The other five proteins are not homologous to any previously described sequence and included an abundant salivary protein (represented by ACYPI009881), with a predicted length of 1161 amino acids and high serine, tyrosine and cysteine content. A. pisum feeds on plant phloem sap and the metalloproteases and regucalcin (a putative calcium-binding protein) are predicted determinants of sustained feeding, by inactivation of plant protein defences and inhibition of calcium-mediated occlusion of phloem sieve elements, respectively. The amino acid composition of ACYPI009881 suggests a role in the aphid salivary sheath that protects the aphid mouthparts from plant defences, and the oxidoreductase may promote gelling of the sheath protein or mediate oxidative detoxification of plant allelochemicals. Further salivary proteins are expected to be identified as more sensitive MS technologies are developed.

Symbiotic bacteria enable insect to use a nutritionally inadequate diet

Animals generally require a dietary supply of various nutrients (vitamins, essential amino acids, etc.) because their biosynthetic capabilities are limited. The capacity of aphids to use plant phloem sap, with low essential amino acid content, has been attributed to their symbiotic bacteria, Buchnera aphidicola, which can synthesize these nutrients; but this has not been demonstrated empirically. We demonstrate here that phloem sap obtained from the severed stylets of pea aphids Acyrthosiphon pisum feeding on Vicia faba plants generally provided inadequate amounts of at least one essential amino acid to support aphid growth. Complementary analyses using aphids reared on chemically defined diets with each amino acid individually omitted revealed that the capacity of the symbiotic bacterium B. aphidicola to synthesize essential amino acids exceeded the dietary deficit of all phloem amino acids except methionine. It is proposed that this shortfall of methionine was met by aphid usage of the non-protein amino acid 5-methylmethionine in the phloem sap. This study provides the first quantitative demonstration that bacterial symbiosis can meet the nutritional demand of plant-reared aphids. It shows how symbiosis with micro-organisms has enabled this group of animals to escape from the constraint of requiring a balanced dietary supply of amino acids.

Nitrogen and carbon relationships between the parasitic weed Orobanche foetida and susceptible and tolerant faba bean lines

The parasitic weed Orobanche foetida (Poiret) is an emergent agronomical problem on faba bean in Tunisia. The Tunisian breeding programs for faba bean resistance to O. foetida have produced several tolerant lines including the line XBJ90.03-16-1-1-1, which limits both parasite attachments to the host roots and growth of the attached parasites. The present study aims to provide a better understanding of the nutritional relationships between the parasite and this tolerant line in comparison with the susceptible Bachaar genotype. Phloem saps of faba bean were harvested using phloem exudation experiments. The major organic compounds potentially transferred from both faba bean genotypes to the parasite were identified as sucrose, raffinose, stachyose, citrate, malate, asparagine (ASN), aspartate (ASP), glutamine, glutamate, serine, alanine and GABA. However, the phloem exudates of the tolerant line were highly deficient in nitrogen when compared to that of the susceptible line. When attached to roots of the tolerant line, the parasite displayed limited activities of soluble invertases in tubercles, and especially in shoots, suggesting that the low performance of the broomrapes attached to the tolerant line resulted from a reduced capacity to utilize the host-derived carbohydrates. On the other hand, the mechanisms involved in the osmotic adjustment and primary metabolism of the parasite did not differ significantly according to the host genotype: mineral cations, especially potassium and calcium, predominated as the major osmotically-active compounds in both tubercles and shoots; shoots accumulated preferentially hexoses as organic solutes although tubercles accumulated preferentially starch and soluble amino acids, especially ASP and ASN. This suggests an important role for a glutamine-dependent asparagine synthetase (EC 6.3.5.4) in the N metabolism of the parasite.

A water-specific aquaporin involved in aphid osmoregulation

The osmotic pressure of plant phloem sap is generally higher than that of insect body fluids. Water cycling from the distal to proximal regions of the gut is believed to contribute to the osmoregulation of aphids and other phloem-feeding insects, with the high flux of water mediated by a membrane-associated aquaporin. A putative aquaporin referred to as ApAQP1 was identified by RT-PCR of RNA isolated from the guts of pea aphids Acyrthosiphon pisum. The ApAQP1 protein has a predicted molecular mass 28.94kDa. Molecular modeling suggests that ApAQP1 has the general aquaporin topology and possesses the conserved pore properties of water-specific aquaporins. When expressed in Xenopus oocytes, ApAQP1 showed the hallmarks of aquaporin-mediated water transport, including an 18-fold increase in the osmotic water permeability of the oolemma, a reduced activation energy, and inhibition of elevated water transport activity by Hg ions. The ApAQP1 transcript was localised to the stomach and distal intestine, and RNAi-mediated knockdown of its expression resulted in elevated osmotic pressure of the haemolymph. Taken together, these data suggest that ApAQP1 contributes to the molecular basis of water cycling in the aphid gut.

Plant-endophyte-herbivore interactions: More than just alkaloids?

A recent paper by Rasmussen et al., (New Phytol 2007; 173:787-97) describes the interactions between Lolium perenne cultivars with contrasting carbohydrate content and the symbiotic fungal endophyte Neotyphodium lolii at different levels of nitrogen supply. In a subsequent study undertaken by Rasmussen et al., (Plant Physiol 2008; 146:1440-53) 66 metabolic variables were analysed in the same material, revealing widespread effects of endophyte infection, N supply and cultivar carbohydrate content on both primary and secondary metabolites. Here, we link insect numerical responses to these metabolic responses using multiple regression analysis.

The insecticidal activity of recombinant garlic lectins towards aphids

The heterodimeric and homodimeric garlic lectins ASAI and ASAII were produced as recombinant proteins in the yeast Pichia pastoris. The proteins were purified as functional dimeric lectins, but underwent post-translational proteolysis. Recombinant ASAII was a single homogenous polypeptide which had undergone C-terminal processing similar to that occurring in planta. The recombinant ASAI was glycosylated and subject to variable and heterogenous proteolysis. Both lectins showed insecticidal effects when fed to pea aphids (Acyrthosiphon pisum) in artificial diet, ASAII being more toxic than ASAI at the same concentration. Acute toxicity (mortality at < or =48 h exposure; similar timescale to starvation) was only apparent at the highest lectin concentrations tested (2.0 mg ml(-)1), but dose-dependent chronic toxicity (mortality at >3d exposure) was observed over the concentration range 0.125-2.0 mg ml(-1). The recombinant lectins caused mortality in both symbiotic and antibiotic-treated aphids, showing that toxicity is not dependent on the presence of the bacterial symbiont (Buchnera aphidicola), or on interaction with symbiont proteins, such as the previously identified lectin "receptor" symbionin. A pull-down assay coupled with peptide mass fingerprinting identified two abundant membrane-associated aphid gut proteins, alanyl aminopeptidase N and sucrase, as "receptors" for lectin binding.

Impact of plant nutrients on the relationship between a herbivorous insect and its symbiotic bacteria

The interactions between herbivorous insects and their symbiotic micro-organisms can be influenced by the plant species on which the insects are reared, but the underlying mechanisms are not understood. Here, we identify plant nutrients, specifically amino acids, as a candidate factor affecting the impact of symbiotic bacteria on the performance of the phloem-feeding aphid Aphis fabae. Aphis fabae grew more slowly on the labiate plant Lamium purpureum than on an alternative host plant Vicia faba, and the negative effect of L. purpureum on aphid growth was consistently exacerbated by the bacterial secondary symbionts Regiella insecticola and Hamiltonella defensa, which attained high densities in L. purpureum-reared aphids. The amino acid content of the phloem sap of L. purpureum was very low; and A. fabae on chemically defined diets of low amino acid content also grew slowly and had elevated secondary symbiont densities. It is suggested that the phloem nutrient profile of L. purpureum promotes deleterious traits in the secondary symbionts and disturbs insect controls over bacterial abundance.

Functional expression and characterisation of a gut facilitative glucose transporter, NlHT1, from the phloem-feeding insect Nilaparvata lugens (rice brown planthopper)

Phloem-sap feeding Hemipteran insects have access to a sucrose-rich diet but are dependent on sucrose hydrolysis and hexose transport for carbon nutrition. A cDNA library from Nilaparvata lugens (rice brown planthopper) was screened for clones encoding potential transmembrane transporters. A selected cDNA, NlHT1, encodes a 53kDa polypeptide with sequence similarity to facilitative hexose transporters of eukaryotes and prokaryotes, including GLUT1, the human erythrocyte hexose transporter. NlHT1 was expressed as a recombinant protein in the methylotropic yeast Pichia pastoris, and was identified in a membrane fraction isolated from transformed yeast cells. Transport experiments using membrane vesicles containing NlHT1 showed that the protein is a saturable, sodium independent transporter, with a relatively low affinity for glucose (K(m) 3.0mM), which can be inhibited by cytochalasin B. Competition experiments with fructose demonstrate NlHT1 is glucose specific. In situ localisation studies revealed that NlHT1 mRNA is expressed in N. lugens gut tissue, mainly in midgut regions, and that expression is absent in hindgut and Malpighian tubules. NlHT1 is therefore likely to play an important role in glucose transport from the gut, and in carbon nutrition in vivo. This is the first report of a facilitative glucose transporter from a phloem-feeding insect pest.

Different levels of transcriptional regulation due to trophic constraints in the reduced genome of Buchnera aphidicola APS

Symbiotic associations involving intracellular microorganisms and animals are widespread, especially for species feeding on poor or unbalanced diets. Buchnera aphidicola, the obligate intracellular bacterium associated with most aphid species, provides its hosts with essential amino acids (EAAs), nutrients in short supply in the plant phloem sap. The Buchnera genome has undergone severe reductions during intracellular evolution. Genes for EAA biosynthesis are conserved, but most of the transcriptional regulatory elements are lost. This work addresses two main questions: is transcription in Buchnera (i) regulated and (ii) scaled to aphid EAA demand? Two microarray experiments were designed for profiling the gene expression in Buchnera. The first one was characterized by a specific depletion of tyrosine and phenylalanine in the aphid diet, and the second experiment combined a global diminution of EAAs in the aphid diet with a sucrose concentration increase to manipulate the aphid growth rate. Aphid biological performance and budget analysis (the balance between EAAs provided by the diet and those synthesized by Buchnera) were performed to quantify the nutritional demand from the aphids toward their symbiotic bacteria. Despite the absence of known regulatory elements, a significant transcriptional regulation was observed at different levels of organization in the Buchnera genome: between genes, within putative transcription units, and within specific metabolic pathways. However, unambiguous evidence for transcriptional changes underpinning the scaling of EAA biosynthesis to aphid demand was not obtained. The phenotypic relevance of the transcriptional response from the reduced genome of Buchnera is addressed.