Functional characterization of two DH44R genes associated with starvation and desiccation in Rhopalosiphum padi (Hemiptera: Aphididae).

CRF-like diuretic hormone receptor (CRF/DHR), also known as DH44R in insects, are G-protein coupled receptors (GPCRs) that play a role in regulating osmotic balance in various insect species. These receptors have the potential to be targeted for the development of insecticides. However, our understanding of the role of DHR genes in aphids, including Rhopalosiphum padi, a major wheat pest, is currently limited. In this study, we isolated and characterized two R. padi DHRs (RpDHR1 and RpDHR2). The expression levels of RpDHR1 increased after starvation and were restored after re-feeding. The expression levels of RpDHR1 gene decreased significantly 24 h after injection of dsRNA targeting the gene. Knockdown of RpDHR1 increased aphid mortality under starvation conditions (24, 36, 48 and 60 h). Under starvation and desiccation condition, the aphid mortality decreased after knockdown of RpDHR1. This is the first study to report the role of DHR genes in the starvation and desiccation response of aphids. The results suggest that RpDHR1 is involved in the resistance of R. padi to starvation and dehydration, making it a potential target for insecticide development. Novel insecticides could be created by utilizing DHR agonists to disrupt the physiological processes of insect pests.

A relaxin receptor gene RpGPCR41 is involved in the resistance of Rhopalosiphum padi to pyrethroids.

Rhopalosiphum padi is a global pest that poses a significant threat to wheat crops and has developed resistance to various insecticides. G protein-coupled receptors (GPCRs), known for their crucial role in signaling and biological processes across insect species, have recently gained attention as a potential target for insecticides. GPCR has the potential to contribute to insect resistance through the regulation of P450 gene expression. However, GPCRs in R. padi remained unexplored until this study. We identified a total of 102 GPCRs in R. padi, including 81 receptors from family A, 10 receptors from family B, 8 receptors from family C, and 3 receptors from family D. Among these GPCR genes, 16 were up-regulated in both lambda-cyhalothrin and bifenthrin-resistant strains of R. padi (LC-R and BIF-R). A relaxin receptor gene, RpGPCR41, showed the highest up-regulated expression in both the resistant strains, with a significant increase of 14.3-fold and 22.7-fold compared to the susceptible strain (SS). RNA interference (RNAi) experiments targeting the relaxin receptor significantly increase the mortality of R. padi when exposed to the LC50 concentration of lambda-cyhalothrin and bifenthrin. The expression levels of five P450 genes (RpCYP6CY8, RpCYP6DC1, RpCYP380B1, RpCYP4CH2, and RpCYP4C1) were significantly down-regulated following knockdown of RpGPCR41 in LC-R and BIF-R strains. Our results highlight the involvement of GPCR gene overexpression in the resistance of R. padi to pyrethroids, providing valuable insights into the mechanisms underlying aphid resistance and a potential target for aphid control.

The Effect of Essential Oils from Asteraceae Plants on Behavior and Selected Physiological Parameters of the Bird Cherry-Oat Aphid.

Essential oils (EOs), including those from the Asteraceae plants, have been shown to have promising insecticidal activity against a wide range of insect pests. Understanding the mechanism of action of EOs is one of the studied aspects. The present study aimed to evaluate the effect of essential oils from Achillea millefolium, Santolina chamaecyparissus, Tagetes patula and Tanacetum vulgare on the settling and probing behavior of the bird cherry-oat aphid (Rhopalosiphum padi L.). In addition, the effect of the oils on the activity of such enzymes as trypsin, pepsin and α- and ß-glucosidase involved in the metabolism of proteins and sugars of the insects was examined. The leaf-choice bioassays demonstrated that the studied EOs limited aphid settling for at least 24 h after the treatment. The application of EOs also inferred with aphid probing behavior by reducing the total probing time and total duration of phloem sap ingestion. Aphids spent more time in the search phase due to an increase in the number and total duration of pathway phases. Moreover, the activity of the studied proteases and glucosidases significantly decreased in R. padi females exposed to the EOs. The enzyme inhibition varied depending on the applied oil and exposure time. Generally, the EOs with stronger deterrent activity also showed higher inhibitory effects. The results suggest that the tested EOs disrupt key digestive processes in R. padi which may be an important factor determining their aphicidal activity.

Chemosensory proteins are associated with thiamethoxam tolerance in bird cherry-oat aphid Rhopalosiphum padi.

Rhopalosiphum padi (L.) is an important cosmopolitan pest of cereal crops. Thiamethoxam is widely used for control R. padi in some regions. Chemosensory proteins (CSPs) are a class of transporter proteins in arthropods which play a key role in various physiological processes including response to insecticide exposure. However, the role of R. padi CSPs (RpCSPs) in insecticide binding and susceptibility has not been well clarified. In this study, we found that the expression levels of RpCSP1, RpCSP4, RpCSP5, RpCSP7, RpCSP10 were dramatically upregulated after exposure to thiamethoxam. Suppression of RpCSP4 and RpCSP5 transcription by RNA interference significantly enhanced the susceptibility of R. padi to thiamethoxam. Molecular docking and fluorescence competitive binding showed that RpCSP4 and RpCSP5 had high binding affinity with thiamethoxam. The present results prove that RpCSP4 and RpCSP5 are related to insecticide resistance through high binding affinity to reduce the toxicity of insecticide.

UGT2B13 and UGT2C1 are involved in lambda-cyhalothrin resistance in Rhopalosiphum padi.

Uridine diphosphate-glucuronosyltransferases (UGTs) are major multifunctional detoxification phase II enzymes involved in the metabolic detoxification of xenobiotics. However, their roles in insecticides resistance are still unclear. In this study, we identified two UGTs genes (UGT2B13 and UGT2C1) in Rhopalosiphum padi, a serious insect pest of wheat worldwide. Bioassays results showed that the resistance ratio of R. padi resistance strain (LC-R) to lambda-cyhalothrin (LC) was 2963.8 fold. The roles of UGT2B13 and UGT2C1 in lambda-cyhalothrin resistance were evaluated. Results indicated that the UGTs contents were significantly increased in the LC resistant strain of R. padi. UGT2B13 and UGT2C1 were significantly overexpressed in the LC-R strain. Transcription levels of UGT2B13 and UGT2C1 were relatively higher in the gut of LC-R strain. RNA interference (RNAi) of UGT2B13 or UGT2C1 significantly decreased the UGTs contents of the LC-R aphids and increased mortality of R. padi exposure to the LC50 concentration of LC. This study provides a new view that UGTs are involved in LC resistance of R. padi. The findings will promote further work to detailed the functions of UGTs in the metabolism resistance of insects to insecticides.

Interspecific differences in thermal tolerance landscape explain aphid community abundance under climate change.

A single critical thermal limit is often used to explain and infer the impact of climate change on geographic range and population abundance. However, it has limited application in describing the temporal dynamic and cumulative impacts of extreme temperatures. Here, we used a thermal tolerance landscape approach to address the impacts of extreme thermal events on the survival of co-existing aphid species (Metopolophium dirhodum, Sitobion avenae and Rhopalosiphum padi). Specifically, we built the thermal death time (TDT) models based on detailed survival datasets of three aphid species with three ages across a broad range of stressful high (34-40 °C) and low (-3∼-11 °C) temperatures to compare the interspecific and developmental stage variations in thermal tolerance. Using these TDT parameters, we performed a thermal risk assessment by calculating the potential daily thermal injury accumulation associated with the regional temperature variations in three wheat-growing sites along a latitude gradient. Results showed that M. dirhodum was the most vulnerable to heat but more tolerant to low temperatures than R. padi and S. avenae. R. padi survived better at high temperatures than Sitobion avenae and M. dirhodum but was sensitive to cold. R. padi was estimated to accumulate higher cold injury than the other two species during winter, while M. dirhodum accrued more heat injury during summer. The warmer site had higher risks of heat injury and the cooler site had higher risks of cold injury along a latitude gradient. These results support recent field observations that the proportion of R. padi increases with the increased frequency of heat waves. We also found that young nymphs generally had a lower thermal tolerance than old nymphs or adults. Our results provide a useful dataset and method for modelling and predicting the consequence of climate change on the population dynamics and community structure of small insects.

Wheat Oxylipins in Response to Aphids, CO2 and Nitrogen Regimes.

Wheat is critical for food security, and is challenged by biotic stresses, chiefly aphids and the viruses they transmit. The objective of this study was to determine whether aphids feeding on wheat could trigger a defensive plant reaction to oxidative stress that involved plant oxylipins. Plants were grown in chambers with a factorial combination of two nitrogen rates (100% N vs. 20% N in Hoagland solution), and two concentrations of CO2 (400 vs. 700 ppm). The seedlings were challenged with Rhopalosiphum padi or Sitobion avenae for 8 h. Wheat leaves produced phytoprostanes (PhytoPs) of the F1 series, and three types of phytofurans (PhytoFs): ent-16(RS)-13-epi-ST-Δ14-9-PhytoF, ent-16(RS)-9-epi-ST-Δ14-10-PhytoF and ent-9(RS)-12-epi-ST-Δ10-13-PhytoF. The oxylipin levels varied with aphids, but not with other experimental sources of variation. Both Rhopalosiphum padi and Sitobion avenae reduced the concentrations of ent-16(RS)-13-epi-ST-Δ14-9-PhytoF and ent-16(RS)-9-epi-ST-Δ14-10-PhytoF in relation to controls, but had little or no effect on PhytoPs. Our results are consistent with aphids affecting the levels of PUFAs (oxylipin precursors), which decreased the levels of PhytoFs in wheat leaves. Therefore, PhytoFs could be postulated as an early indicator of aphid hosting for this plant species. This is the first report on the quantification of non-enzymatic PhytoFs and PhytoPs in wheat leaves in response to aphids.

Wheat as a new host for potato aphid Macrosiphum euphorbiae Thomas (Hemiptera: Aphididae) and construction of its age-stage two-sex life tables.

Wheat (Triticum aestivum L.) is the major global staple food crop that meets the food security demands of various nations across the continents. The recent reduction in wheat production is attributed to several biotic and abiotic factors especially, temperature and rainfall patterns, and pest occurrence. Among insect pests, aphid species are emerging as new pests of economic importance in India and elsewhere. The present investigation identified a new association of Macrosiphum euphorbiae Thomas with the wheat crop. Life table parameters were studied for M. euphorbiae and Rhopalosiphum padi fed on wheat foliage. The total nymphal duration and life cycle duration, respectively, of R. padi (4.76 ± 0.54 and 9.71 ± 1.38 days) and M. euphorbiae (5.84 ± 0.69 and 9.96 ± 1.31 days) were significantly different for these species. The fecundity of the two aphid species was 23.95 ± 8.67 and 11.6 ± 4.10 progeny/female, respectively. Age-specific survival rate (lx), age-specific fecundity (fx), and population age-specific fecundity (mx) were higher in R. Padi compared to M. euphorbiae. Reproductive value (Vxj) was high in R. padi and the duration of reproduction was less, while these parameters showed an opposite trend in M. euphorbiae. The gross reproduction rate (GRR) was found higher in R. Padi (29.17 offspring/adult lifetime) compared to M. euphorbiae (19.58 offspring/adult lifetime). The M. euphorbiae being a pest of solanaceous crops seems to have shifted to a new host, i.e., wheat. This new adaptation strategy to survive for long periods on a wheat crop might pose a serious threat to wheat crop cultivation in near future.

Characterizing serotonin biosynthesis in Setaria viridis leaves and its effect on aphids.

KEY MESSAGE: A combined transcriptomic and metabolic analysis of Setaria viridis leaves responding to aphid infestation was used to identify genes related to serotonin biosynthesis. Setaria viridis (green foxtail), a short life-cycle C4 plant in the Poaceae family, is the wild ancestor of Setaria italica (foxtail millet), a resilient crop that provides good yields in dry and marginal land. Although S. viridis has been studied extensively in the last decade, the molecular mechanisms of insect resistance in this species remain under-investigated. To address this issue, we performed a metabolic analysis of S. viridis and discovered that these plants accumulate the tryptophan-derived compounds tryptamine and serotonin. To elucidate the defensive functions of serotonin, Rhophalosiphum padi (bird cherry-oat aphids) were exposed to this compound, either by exogenous application to the plant medium or with artificial diet bioassays. In both cases, exposure to serotonin increased aphid mortality. To identify genes that are involved in serotonin biosynthesis, we conducted a transcriptome analysis and identified several predicted S. viridis tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H) genes. Two candidate genes were ectopically expressed in Nicotiana tabacum, where SvTDC1 (Sevir.6G066200) had tryptophan decarboxylase activity, and SvT5H1 (Sevir.8G219600) had tryptamine hydroxylase activity. Moreover, the function of the SvTDC1 gene was validated using virus-induced gene silencing in S. italica, which caused a reduction in serotonin levels. This study provides the first evidence of serotonin biosynthesis in Setaria leaves. The biosynthesis of serotonin may play an important role in defense responses and could prove to be useful for developing more pest-tolerant Setaria italica cultivars.

The transcription factor TaMYB31 regulates the benzoxazinoid biosynthetic pathway in wheat.

Benzoxazinoids are specialized metabolites that are highly abundant in staple crops, such as maize and wheat. Although their biosynthesis has been studied for several decades, the regulatory mechanisms of the benzoxazinoid pathway remain unknown. Here, we report that the wheat transcription factor MYB31 functions as a regulator of benzoxazinoid biosynthesis genes. A transcriptomic analysis of tetraploid wheat (Triticum turgidum) tissue revealed the up-regulation of two TtMYB31 homoeologous genes upon aphid and caterpillar feeding. TaMYB31 gene silencing in the hexaploid wheat Triticum aestivum significantly reduced benzoxazinoid metabolite levels and led to susceptibility to herbivores. Thus, aphid progeny production, caterpillar body weight gain, and spider mite oviposition significantly increased in TaMYB31-silenced plants. A comprehensive transcriptomic analysis of hexaploid wheat revealed that the TaMYB31 gene is co-expressed with the target benzoxazinoid-encoded Bx genes under several biotic and environmental conditions. Therefore, we analyzed the effect of abiotic stresses on benzoxazinoid levels and discovered a strong accumulation of these compounds in the leaves. The results of a dual fluorescence assay indicated that TaMYB31 binds to the Bx1 and Bx4 gene promoters, thereby activating the transcription of genes involved in the benzoxazinoid pathway. Our finding is the first report of the transcriptional regulation mechanism of the benzoxazinoid pathway in wheat.

Octopamine receptor genes are involved in the starvation response of Rhopalosiphum padi (Hemiptera: Aphididae).

Insect octopamine (OA) receptors are G-protein coupled receptors (GPCRs) that play essential roles in physiological and behavioural processes. However, there is little information about the function of OA receptors in the aphids' response to stress. From the genome sequence of Rhopalosiphum padi genome sequence, a cosmopolitan cereal pest, we identified six OA receptor genes RpOAMB, RpOctR, RpOctß1R, RpOctß2R, RpOctß3R, RpOctR-like with two, one, one, four, four, seven exons, respectively. All the OA receptors contain seven transmembrane domains, which were the signature of GPCRs. Our results showed that (1) the contents of OA increased significantly after food starvation, (2) the transcription levels of RpOAMB, RpOctR, RpOctß2R and RpOctß3R increased after starvation and were restored after re-feeding, and (3) the expression levels of these four genes decreased significantly 48 h post-injection of dsRNA that targeted the respective genes. Knockdown of RpOctR, RpOctß2R or RpOctß3R genes significantly increased aphid mortality under 24 h starvation conditions. Mortality of R. padi injected with dsRpOctR or dsRpOctß2R was significantly higher than control under 48 h starvation treatments. This is the first report on the role of OA receptors in the starvation response of aphids. The current study provides knowledge for a better understanding the physiological roles of insect OA receptors.

Evaluation and optimization of the protocols for measuring cytochrome P450 activity in aphids.

Insect cytochrome P450 plays major roles in detoxification of phytotoxin and insecticides. However, determination of P450 activity in aphids has variable success and there is no reliable method yet. In this study, we found that homogenizing the green peach aphid, Myzus persicae, in the 96-well microplate resulted in significantly higher P450 activities than those in Eppendorf tube. Homogenizing aphids in Eppendorf tube released uncharacterized compounds that inhibited aphids and pig liver P450 activities, whereas aphids homogenized in the microplate may not be completely ground and thus released fewer such inhibitors. Then, the microplate homogenization method was optimized as follows: one or two aphids were placed in one well of the 96 well-microplate and ground in phosphate buffer using pipette tips for 20 cycles, followed by addition of 7-ethoxycoumarin, and then incubated for 1 h at room temperature, after which glycine buffer-ethanol mixture was added to stop the reaction. This method is also suitable for the pea aphid, Acyrthosiphon pisum, and the bird cherry-oat aphid, Rhopalosiphum padi. These results highlight the importance of considering inhibitory effects of endogenous compounds in insects on their P450 activities and provide one possible method to reduce these inhibitory effects.

Comparative transcriptome and RNA interference reveal CYP6DC1 and CYP380C47 related to lambda-cyhalothrin resistance in Rhopalosiphum padi.

The bird-cherry-oat aphid, Rhopalosiphum padi, is a serious agricultural pest of Triticeae crops, and pyrethroids are the most widely used chemical pesticides for the control of the aphid. Our previous studies found that some R. padi field populations have developed resistance against pyrethroids; an M918L target-site mutation of the voltage gated sodium channel was present in the pyrethroid resistant individuals, while the high-level resistance to lambda-cyhalothrin revealed the presence of other mechanisms in the pest. Here, we conducted genome-wide transcriptional analysis for the lambda-cyhalothrin susceptible (SS) and resistant (LC-RR) strains of R. padi. Results indicated that 2457 genes were differently expressed between the SS and LC-RR strains. In the LC-RR, a total of 1265 and 1192 genes were up- and down-regulated, respectively. KEGG analysis implicated enrichment of P450 involved in insecticide metabolic pathways in the resistant transcriptome. qRT-PCR results confirmed that two P450 genes (CYP6DC1 and CYP380C47) were significantly overexpressed in the LC-RR individuals. Furthermore, RNA interference (RNAi) of CYP6DC1 or CYP380C47 significantly increased mortality of R. padi exposure to lambda-cyhalothrin. These results suggest that the overexpression of CYP6DC1 and CYP380C47 contributed to the lambda-cyhalothrin resistance in the pest. This study provides knowledge for further analyzing the molecular mechanism of resistance to pyrethroids in R. padi.

Grassy Weeds and Corn as Potential Sources of Barley yellow dwarf virus Spread Into Winter Wheat.

Barley yellow dwarf virus (BYDV) is an important vector-borne pathogen of cereals. Although many species of grasses are known to host BYDV, knowledge of their role in virus spread in regional agroecosystems remains limited. Between 2012 and 2016, Idaho winter wheat production was affected by BYDV. BYDV-PAV and the bird cherry-oat aphid (BCOA) (Rhopalosiphum padi L.) vector were commonly present in the affected areas. A series of greenhouse bioassays were performed to examine whether two types of corn (Zea mays L.), dent and sweet, and three commonly found grassy weeds, downy brome (Bromus tectorum L.), green foxtail (Setaria viridis L.), and foxtail barley (Hordeum jubatum L.), can be inoculated with BYDV (species BYDV-PAV) by BCOA and also act as sources of the virus in winter wheat. BCOA successfully transmitted BYDV-PAV to both corn types and all weed species. Virus titers differed between the weed species (P = 0.032) and between corn types (P = 0.001). In transmission bioassays, aphids were able to survive on these host plants during the 5-day acquisition access period and later successfully transmitted BYDV-PAV to winter wheat (var. SY Ovation). Transmission success was positively correlated with the virus titer of the source plant (P < 0.001) and influenced by weed species (P = 0.028) but not corn type. Overall, the results of our inoculation and transmission assays showed that the examined weed species and corn types can be inoculated with BYDV-PAV by BCOA and subsequently act as sources of infections in winter wheat.

Are extreme high temperatures at low or high latitudes more likely to inhibit the population growth of a globally distributed aphid?

Although climate warming can increase both mean temperature and its variability, it is often the effects of climate warming on short periods of extreme temperatures that are expected to have particularly large physiological and ecological consequences. Understanding the vulnerability of organisms at various latitudes to climate extremes is thus critical for understanding warming effects on regional biodiversity conservation and ecosystem management. While previous studies have shown that thermal responses depend on temperature regimes that organisms have previously experienced, this issue has not been considered much when comparing the effects of temperature extremes at different latitudes. To fill this gap, here we manipulated different combinations of amplitude and duration of daily high temperature extremes to simulate conditions at different latitudes. We tested the effects of those regimes on life-history traits and fitness of a globally-distributed aphid species, Rhopalosiphum padi. We compared our results with previous studies to better understand the extent to which these regimes affect conclusions based on comparisons under different mean temperatures. As a consequence of asymmetrical thermal performance curves, we hypothesized that the temperature regimes with higher daily maximum temperatures at higher latitudes would cause strong negative effects. Our results showed that these regimes with thermal extremes caused substantial decreases in life-history traits and fitness relative to the predictions from different mean temperatures. Specifically, the regime with higher daily maximum temperature reflecting a higher mid-latitude location had larger impacts on development, reproduction and population fitness than the regime representing a lower mid-latitude location. These findings have implications for understanding the vulnerability of organisms across latitudes to increasingly frequent extreme heat events under ongoing climate warming.

miR-147b-modulated expression of vestigial regulates wing development in the bird cherry-oat aphid Rhopalosiphum padi.

BACKGROUND: Most aphids exhibit wing polyphenism in which wingless and winged morphs produce depending on the population density and host plant quality. Although the influence of environmental factors on wing polyphenism of aphids have been extensively investigated, molecular mechanisms underlining morph differentiation (i.e. wing development /degeneration), one downstream aspect of the wing polyphenism, has been poorly understood. RESULTS: We examined the expression levels of the twenty genes involved in wing development network, and only vestigial (vg) showed significantly different expression levels in both whole-body and wall-body of third instar nymphs, with 5.4- and 16.14- fold higher expression in winged lines compared to wingless lines, respectively in Rhopalosiphum padi. vg expression was higher in winged lines compared to wingless lines in third, fourth instar nymphs and adults. Larger difference expression was observed in third (21.38-fold) and fourth (20.91-fold) instar nymphs relative to adults (3.12-fold). Suppression of vg using RNAi repressed the wing development of third winged morphs. Furthermore, dual luciferase reporter assay revealed that the miR-147 can target the vg mRNA. Modulation of miR-147b levels by microinjection of its agomir (mimic) decreased vg expression levels and repressed wing development. CONCLUSIONS: Our findings suggest that vg is essential for wing development in R. padi and that miR-147b modulates its expression.

Comparative transcriptomic and metabolic analysis of wild and domesticated wheat genotypes reveals differences in chemical and physical defense responses against aphids.

BACKGROUND: Young wheat plants are continuously exposed to herbivorous insect attack. To reduce insect damage and maintain their growth, plants evolved different defense mechanisms, including the biosynthesis of deterrent compounds named benzoxazinoids, and/or trichome formation that provides physical barriers. It is unclear whether both of these mechanisms are equally critical in providing an efficient defense for wheat seedlings against aphids-an economically costly pest in cereal production. RESULTS: In this study, we compared the transcriptome, metabolome, benzoxazinoids, and trichome density of three selected wheat genotypes, with a focus on differences related to defense mechanisms. We chose diverse wheat genotypes: two tetraploid wheat genotypes, domesticated durum 'Svevo' and wild emmer 'Zavitan,' and one hexaploid bread wheat, 'Chinese Spring.' The full transcriptomic analysis revealed a major difference between the three genotypes, while the clustering of significantly different genes suggested a higher similarity between the two domesticated wheats than between either and the wild wheat. A pathway enrichment analysis indicated that the genes associated with primary metabolism, as well as the pathways associated with defense such as phytohormones and specialized metabolites, were different between the three genotypes. Measurement of benzoxazinoid levels at the three time points (11, 15, and 18 days after germination) revealed high levels in the two domesticated genotypes, while in wild emmer wheat, they were below detection level. In contrast to the benzoxazinoid levels, the trichome density was dramatically higher in the wild emmer than in the domesticated wheat. Lastly, we tested the bird cherry-oat aphid's (Rhopalosiphum padi) performance and found that Chinese Spring is more resistant than the tetraploid genotypes. CONCLUSIONS: Our results show that benzoxazinoids play a more significant defensive role than trichomes. Differences between the abundance of defense mechanisms in the wild and domesticated plants were observed in which wild emmer possesses high physical defenses while the domesticated wheat genotypes have high chemical defenses. These findings provide new insights into the defense adaptations of wheat plants against aphids.

Barley yellow dwarf virus Can Be Inoculated During Brief Intracellular Punctures in Phloem Cells Before the Sieve Element Continuous Salivation Phase.

The distinguished intracellular stylet puncture called phloem-pd (potential drop [pd]) produced by Myzus persicae has been associated with the transmission of the semipersistently transmitted, phloem-limited Beet yellows virus (BYV, Closterovirus). However, the production of intracellular punctures in phloem cells (phloem-pd) by other aphid species and their role in the transmission of persistently transmitted, phloem-limited viruses are still unknown. Previous studies revealed that inoculation of the persistently transmitted, phloem-limited Barley yellow dwarf virus (BYDV, Luteovirus) is associated mainly with the sieve element continuous salivation phase (E1 waveform). However, the role of brief intracellular punctures that occur before the E1 phase in the inoculation of BYDV by aphids is unknown. We aimed to investigate whether the bird cherry-oat aphid Rhopalosiphum padi (Hemiptera: Aphididae) produced a stereotypical phloem-pd and to study its role in the inoculation of BYDV. The feeding behavior of viruliferous R. padi individuals in barley (Hordeum vulgare) was monitored via the electrical penetration graph (EPG) technique. The feeding process was artificially terminated after the observation of specific EPG waveforms: standard-pds, phloem-pd, and E1. Analysis of the EPG recordings revealed the production of a phloem-pd pattern by R. padi, in addition to a short, distinct E1-like pattern (short-E1), both resulting in successful inoculation of BYDV. Also, the transmission efficiency of BYDV was directly proportional to the time spent by aphids in intracellular salivation in phloem cells. Finally, we discussed the main differences between the inoculation process of semipersistent and persistently transmitted phloem-limited viruses by aphids.

Characterisation of bird cherry-oat aphid (Rhopalosiphum padi L.) behaviour and aphid host preference in relation to partially resistant and susceptible wheat landraces.

The bird cherry-oat aphid (Rhopalosiphum padi L.) is a major pest of wheat (Triticum aestivum L.) and can cause up to 30% yield losses. Heritable plant resistance to aphids is both an economically and ecologically sound method for managing aphids. Here we report how the behaviour and performance of R. padi differs on two resistant, one susceptible wheat landrace and a susceptible elite wheat variety. Feeding behaviour differed among the genotypes, with aphids on resistant lines spending longer in the pathway phase and less time phloem feeding. These behaviours suggest that both inter- and intracellular factors encountered during pathway and phloem feeding phases could be linked to the observed aphid resistance. Locomotion and antennal positioning choice tests also revealed a clear preference for susceptible lines. Although feeding studies revealed differences in the first probe indicating that the resistance factors might also be located in the peripheral layers of the plant tissue, scanning electron microscopy revealed no difference in trichrome length and density on the surface of leaves. Aphids are phloem feeders and limiting the nutrient uptake by the aphids may negatively affect their growth and development as shown here in lower weight and survival of nymphs on resistant genotypes and decreased reproductive potential, with lowest mean numbers of nymphs produced by aphids on W064 (54.8) compared to Solstice (71.9). The results indicate that resistant lines markedly alter the behaviour, reproduction and development potential of R. padi and possess both antixenosis and antibiosis type of resistance.

Expression of Thioredoxin/Thioredoxin Reductase System Genes in Aphid-Challenged Maize Seedlings.

Thioredoxins (Trxs) and thioredoxin reductases (TrxRs) encompass a highly complex network involved in sustaining thiol-based redox homeostasis in plant tissues. The purpose of the study was to gain a new insight into transcriptional reprogramming of the several genes involved in functioning of Trx/TrxR system in maize (Zea mays L.) seedlings, exposed to the bird cherry-oat aphid (Rhopalosiphum padi L.) or the rose-grass aphid (Metopolophium dirhodum Walk.) infestation. The biotests were performed on two maize genotypes (susceptible Zlota Karlowa and relatively resistant Waza). The application of real-time qRT-PCR technique allowed to identify a molecular mechanism triggered in more resistant maize plants, linked to upregulation of thioredoxins-encoding genes (Trx-f, Trx-h, Trx-m, Trx-x) and thioredoxin reductase genes (Ftr1, Trxr2). Significant enhancement of TrxR activity in aphid-infested Waza seedlings was also demonstrated. Furthermore, we used an electrical penetration graph (EPG) recordings of M. dirhodum stylet activities in seedlings of the two studied maize varieties. Duration of phloem phase (E1 and E2 models) of rose-grass aphids was about three times longer while feeding in Waza plants, compared to Zlota Karlowa cv. The role of activation of Trx/TrxR system in maintaining redox balance and counteracting oxidative-induced damages of macromolecules in aphid-stressed maize plants is discussed.