Phloem Feeding Research Articles

Differential Costs of Two Distinct Resistance Mechanisms Induced by Different Herbivore Species in Arabidopsis

Plants respond to herbivory with the induction of resistance, mediated by distinct phytohormonal signaling pathways and their interactions. Phloem feeders are known to induce plant resistance via the salicylic acid pathway, whereas biting-chewing herbivores induce plant resistance mainly via the jasmonate pathway. Here, we show that a specialist caterpillar (biting-chewing herbivore) and a specialist aphid (phloem feeder) differentially induce resistance against Pieris brassicae caterpillars in Arabidopsis (Arabidopsis thaliana) plants. Caterpillar feeding induces resistance through the jasmonate signaling pathway that is associated with the induction of kaempferol 3,7-dirhamnoside, whereas aphid feeding induces resistance via a novel mechanism involving sinapoyl malate. The role of sinapoyl malate is confirmed through the use of a mutant compromised in the biosynthesis of this compound. Caterpillar-induced resistance is associated with a lower cost in terms of plant growth reduction than aphid-induced resistance. A strong constitutive resistance against P. brassicae caterpillars in combination with a strong growth attenuation in plants of a transfer DNA (T-DNA) insertion mutant of WRKY70 (wrky70) suggest that the WRKY70 transcription factor, a regulator of downstream responses mediated by jasmonate-salicylic acid signaling cross talk, is involved in the negative regulation of caterpillar resistance and in the tradeoff between growth and defense. In conclusion, different mechanisms of herbivore-induced resistance come with different costs, and a functional WRKY70 transcription factor is required for the induction of low-cost resistance.

Negative impact of drought stress on a generalist leaf chewer and a phloem feeder is associated with, but not explained by an increase in herbivore-induced indole glucosinolates

Plants are constantly exposed to multiple biotic and abiotic stresses, such as drought and herbivory. However, plant responses to these stresses have usually been studied in isolation. Here, we take a multidisciplinary approach addressing ecological and chemical aspects of plant responses to generalist herbivores and several intensities of drought. We hypothesize that in brassicaceous plants, the effects of drought stress on herbivores can be explained by an increase in indole glucosinolates. Four-week-old Arabidopsis thaliana plants were drought stressed for one week or watered as normal. Three types of drought stress were compared: (1) no watering for 1 week and then rewatered to saturation (low drought); (2) no watering for 1 week and then rewatered to 60% of soil water content (high drought); (3) watering every other day to 60% of soil water content (continuous drought). All three types of drought stress negatively affected both the larval mass of the leaf chewer Mamestra brassicae and the population growth of the phloem feeder Myzus persicae. This was associated with increased levels of herbivore-induced indole glucosinolates compared to infested control plants. Interestingly, the levels of total indole glucosinolates did not change in uninfested plants, except for the indole 4-methoxy-glucobrassicin that was induced by continuous drought. Two-choice experiments also showed that caterpillars of M. brassicae, but not aphids, avoided drought-stressed plants only after feeding on them, but not by visual/olfactory cues. However, on a knockout mutant blocked in the production of indole glucosinolates (cyp79B2 cyp79B3), the effect of drought on herbivore performance was similar to that on wild-type plants. The results of this study show that drought stress induced higher levels of indole glucosinolates; however, these levels were not responsible for higher resistance to generalist herbivores in drought-stressed plants.

The plant vigor hypothesis applies to a holoparasitic plant on a drought-stressed host

Parasitic plants extract resources from host vascular tissues but their responses to environmental fluctuation experienced by the host are poorly studied. Three frequently-cited hypotheses for effects of environmental stress on plant resistance to herbivores predict decreased, increased, or fluctuation in herbivore performance in response to drought stress. We tested which hypothesis best accounts for how drought stress applied to a perennial herb affects growth of the holoparasite, Cuscuta gronovii Willd. ex Roem. & Schult. (common dodder), an obligate shoot parasite. Verbesina alternifolia (L.) Britton ex Kearney (wingstem) supporting single, young C. gronovii were exposed to continuous, pulsed, or no water stress for 32 days and then dry mass of each parasite was determined. Consistent with the plant vigor hypothesis, C. gronovii grew significantly better on well-watered hosts. Continuous and pulsed drought stress of the host resulted in similar growth reduction relative to no drought stress. In addition to reducing absolute growth of the holoparasite, continuous and pulsed drought stress reduced the growth of the holoparasite relative to host growth. Although functionally similar to insect phloem feeders, growth of holoparasites such as C. gronovii is constrained by source–sink relations. Our results suggest drought stress experienced by a host weakens source strength and reduces uptake by the holoparasite acting as a sink.

Role of the major antigenic membrane protein in phytoplasma transmission by two insect vector species.

BackgroundPhytoplasmas are bacterial plant pathogens (class Mollicutes), transmitted by phloem feeding leafhoppers, planthoppers and psyllids in a persistent/propagative manner. Transmission of phytoplasmas is under the control of behavioral, environmental and geographical factors, but molecular interactions between membrane proteins of phytoplasma and vectors may also be involved. The aim of the work was to provide experimental evidence that in vivo interaction between phytoplasma antigenic membrane protein (Amp) and vector proteins has a role in the transmission process. In doing so, we also investigated the topology of the interaction at the gut epithelium and at the salivary glands, the two barriers encountered by the phytoplasma during vector colonization.MethodsExperiments were performed on the ‘Candidatus Phytoplasma asteris’ chrysanthemum yellows strain (CYP), and the two leafhopper vectors Macrosteles quadripunctulatus Kirschbaum and Euscelidius variegatus Kirschbaum.To specifically address the interaction of CYP Amp at the gut epithelium barrier, insects were artificially fed with media containing either the recombinant phytoplasma protein Amp, or the antibody (A416) or both, and transmission, acquisition and inoculation efficiencies were measured.An abdominal microinjection protocol was employed to specifically address the interaction of CYP Amp at the salivary gland barrier. Phytoplasma suspension was added with Amp or A416 or both, injected into healthy E. variegatus adults and then infection and inoculation efficiencies were measured.An internalization assay was developed, consisting of dissected salivary glands from healthy E. variegatus exposed to phytoplasma suspension alone or together with A416 antibody. The organs were then either observed in confocal microscopy or subjected to DNA extraction and phytoplasma quantification by qPCR, to visualize and quantify possible differences among treatments in localization/presence/number of CYP cells.ResultsArtificial feeding and abdominal microinjection protocols were developed to address the two barriers separately. The in vivo interactions between Amp of ‘Candidatus Phytoplasma asteris’ Chrysanthemum yellows strain (CYP) and vector proteins were studied by evaluating their effects on phytoplasma transmission by Euscelidius variegatus and Macrosteles quadripunctulatus leafhoppers. An internalization assay was developed, consisting of dissected salivary glands from healthy E. variegatus exposed to phytoplasma suspension alone or together with anti-Amp antibody. To visualize possible differences among treatments in localization/presence of CYP cells, the organs were observed in confocal microscopy. Pre-feeding of E. variegatus and M. quadripunctulatus on anti-Amp antibody resulted in a significant decrease of acquisition efficiencies in both species. Inoculation efficiency of microinjected E. variegatus with CYP suspension and anti-Amp antibody was significantly reduced compared to that of the control with phytoplasma suspension only. The possibility that this was due to reduced infection efficiency or antibody-mediated inhibition of phytoplasma multiplication was ruled out. These results provided the first indirect proof of the role of Amp in the transmission process.ConclusionProtocols were developed to assess the in vivo role of the phytoplasma native major antigenic membrane protein in two phases of the vector transmission process: movement through the midgut epithelium and colonization of the salivary glands. These methods will be useful also to characterize other phytoplasma-vector combinations. Results indicated for the first time that native CYP Amp is involved in vivo in specific crossing of the gut epithelium and salivary gland colonization during early phases of vector infection.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-015-0522-5) contains supplementary material, which is available to authorized users.

Cool Headed Individuals Are Better Survivors: Non-Consumptive and Consumptive Effects of a Generalist Predator on a Sap Feeding Insect.

Non-consumptive effects (NCEs) of predators are part of the complex interactions among insect natural enemies and prey. NCEs have been shown to significantly affect prey foraging and feeding. Leafhopper's (Auchenorrhyncha) lengthy phloem feeding bouts may play a role in pathogen transmission in vector species and also exposes them to predation risk. However, NCEs on leafhoppers have been scarcely studied, and we lack basic information about how anti-predator behaviour influences foraging and feeding in these species. Here we report a study on non-consumptive and consumptive predator-prey interactions in a naturally co-occurring spider–leafhopper system. In mesocosm arenas we studied movement patterns during foraging and feeding of the leafhopper Psammotettix alienus in the presence of the spider predator Tibellus oblongus. Leafhoppers delayed feeding and fed much less often when the spider was present. Foraging movement pattern changed under predation risk: movements became more frequent and brief. There was considerable individual variation in foraging movement activity. Those individuals that increased movement activity in the presence of predators exposed themselves to higher predation risk. However, surviving individuals exhibited a ‘cool headed’ reaction to spider presence by moving less than leafhoppers in control trials. No leafhoppers were preyed upon while feeding. We consider delayed feeding as a “paradoxical” antipredator tactic, since it is not necessarily an optimal strategy against a sit-and-wait generalist predator.

Inhibition of α-glucosidase activity by N-deoxynojirimycin analogs in several insect phloem sap feeders.

Secondary metabolites and synthetic iminosugars that structurally resemble monosaccharides are potent inhibitors of α-glucosidase activity. The enzyme is core in cleaving sucrose in phloem feeding insects and it also plays a crucial role of reducing osmotic stress via the formation of oligosaccharides. Inhibition of hydrolysis by iminosugars should result in nutritional deficiencies and/or disruption of normal osmoregulation. Deoxynojirimycin (DNJ) and 2 N-alkylated analogs [N-butyl DNJ (NB-DNJ) and N-nonyl DNJ (NN-DNJ)] were the major iminosugars used throughout the study. The extensive experiments conducted with α-glucosidase of the whitefly Bemisia tabaci indicated the competitive nature of inhibition and that the hydrophilic DNJ is a potent inhibitor in comparison to the more hydrophobic NB-DNJ and NN-DNJ compounds. The same inhibitory pattern was observed with the psyllid Cacopsylla bidens α-glucosidase. In contrast to the above pattern, enzymes of the aphids, Myzus persicae and Aphis gossypii were more sensitive to the hydrophobic iminosugars as compared to DNJ. In vivo experiments in which adult B. tabaci were fed dietary iminosugars, show that the hydrophilic DNJ was far less toxic than the lipophilic NB-DNJ and NN-DNJ. It is proposed that this pattern is attributed to the better accessibility of the hydrophobic NN-DNJ to the α-glucosidase membrane-bound compartment in the midgut. Based on the inhibitory effects of certain polyhydroxy N-alkylated iminosugars, α-glucosidase of phloem feeding hemipterans could serve as an attractive target site for developing novel pest control agents.

Differential expression patterns among heat-shock protein genes and thermal responses in the whitefly Bemisia tabaci (MEAM 1)

There is convincing evidence that heat-shock proteins (HSP) are upregulated by stress conditions in insects; however, the relative contribution of each HSP gene to the heat-shock response remains unclear. Here we considered the whitefly Bemisia tabaci (MEAM 1), a phloem feeder and invasive species whose molecular stress response is an important mechanism for overcoming heat stress. We assessed the expression of the hsp23, 40, 70 and 90 genes at the mRNA level when submitted to heat shocks of 40 and 44°C/1h (control at 25°C). For this, we evaluated a set of available and suitable reference genes in order to perform data normalization using the real-time polymerase chain reaction (qRT-PCR) technique, and then confirmed the production of HSP70 protein based on Western blot. Results were compared with the hardening capacity of B. tabaci, measured by fitness components as a response to heat shocks, using 40°C as the induction temperature. Three of the four genes (hsp23, 70 and 90) were upregulated by heat stress at mRNA, showing differential expression patterns. Hsp70 expression was confirmed at the protein level. Hardening significantly increased fitness following heat stress, suggesting that HSPs may contribute to hardening capacity in B. tabaci. Potential role of each gene in the heat-shock response for whiteflies is discussed.

Quantitative 1H NMR metabolome profiling of Thai Jasmine rice (Oryza sativa) reveals primary metabolic response during brown planthopper infestation

Brown planthopper (BPH; Nilaparvata lugens) is a phloem feeding insect which is one of the most serious threats to rice crops in many countries throughout Asia. 1H NMR spectroscopy, combined with chemometrics, was used to analyze the polar metabolome from leaf extracts of Thai Jasmine rice (brown planthopper (BPH)-susceptible KD) and its BPH resistant isogenic lines (BPH-resistant IL7 and BPH-resistant+ IL308 varieties) with and without BPH infestation at various time points (days 1, 2, 3, 4 and 8). Physiological changes of the rice isogenic lines were different based on the quantitative trait loci of BPH resistance. Multivariate models were capable of distinguishing between the susceptible and the resistant rice varieties throughout the infestation. The concentration of 10 metabolites were significantly altered (p < 0.05) between the infested and the control groups of each examined rice variety. Metabolic pathway analysis suggested that BPH infestation could perturb transamination during the early stages of infestation (days 1–3) for all rice varieties. In addition, the IL7 and IL308 varieties responded earlier (day 3) than the KD variety (day 8) by perturbing amino acid metabolism, shikimate and gluconeogenesis pathways. By day 8 of the infestation, the KD cultivar responded by activating the amino acid-mediated-de novo pathway whereas the IL308 variety activated the purine and pyrimidine compound-mediated-salvage pathway for nucleotide biosynthesis. This study has identified, for the first time, several potential metabolic pathways for acclimatization and defense mechanisms against BPH infestation. These findings provide a valuable, first insight into BPH resistance mechanisms in Thai Jasmine rice.

Electrical penetration graphs indicate that tricin is a key secondary metabolite of rice, inhibiting phloem feeding of brown planthopper, Nilaparvata lugens

AbstractTricin (5,7,4′‐trihydroxy‐3′,5′‐dimethoxyflavone) is a valuable secondary metabolite which is widely present in gramineous plants, including cultivated rice (Oryza sativa L.) (Poaceae). It can defend the rice plant against damage by the brown planthopper (BPH), Nilaparvata lugens (Stål) (Hemiptera: Delphacidae), one of the most important pests of rice. This study was conducted to elucidate the mechanisms of action of tricin on BPH feeding behavior. BPH feeding behavior in resistant (Rathu Heenati, RHT) and susceptible (Taichuang native 1, TN1) rice varieties and artificial diets was monitored using the electrical penetration graph (EPG) technique. Tricin concentrations in leaves of varieties RHT and TN1 were quantitatively analyzed by liquid chromatography, coupled to tandem mass spectrometric techniques. Six (NP and N1‐5) and four (NP, N1, N2, and N4) types of waveforms occurred during feeding on rice plants and artificial diets, respectively. The tricin concentration of rice varieties was correlated with total and average durations of N4. Moreover, EPG data indicated that tricin significantly increased the duration of non‐probing and pathway periods and strongly inhibited phloem ingestion (N4). The inhibition was strongly dose dependent, resulting in complete suppression of activity in the phloem region when the tricin concentration was increased to 1 g l−1. This study revealed that tricin disturbed the feeding behavior of BPH mainly by increasing the non‐probe period and inhibiting phloem ingestion. We confirmed the hypothesis that tricin is a ‘stylet probing stimulant’ of rice planthoppers as proposed in previous studies. The information on the ecological effect of tricin from this study may be useful to clarify the resistance mechanism against BPH of RHT and other tricin‐containing rice varieties.

Towards an understanding of how phloem amino acid composition shapes elevated CO2‐induced changes in aphid population dynamics

1. The performance of foliage feeders tends to decrease under elevated CO2, but the responses of phloem‐feeding insects have been much more equivocal. As phloem tissues are less accessible than whole‐plant tissues, much less is known about how phloem composition is altered under elevated CO2 and the mechanisms driving changes in aphid performance.2. In this study, the plant mechanisms underlying the performance of Rhopalosiphum padi aphids on Hordeum vulgare (barley) grown under ambient (390 ppm) and elevated (700 ppm) CO2 were examined. We used aphid stylectomy to sample pure phloem from plants in CO2‐controlled conditions and high‐performance liquid chromatography to analyse phloem samples for amino acid concentrations.3. Aphid abundance significantly increased by 127% under elevated CO2. Consequently, plant biomass decreased under elevated CO2 in trials with herbivores present, possibly due to the increased herbivore load, but increased when aphids were absent. The intrinsic rate of population increase (rm) was significantly higher under elevated CO2; however, there were no statistically significant effects on aphid fecundity or development time. The concentration of individual amino acids tended to increase, although these increases were statistically significant in only a few cases. A principal components analysis revealed that the relative abundance (mol %) of those amino acids considered essential for aphids tended to increase under elevated CO2.4. These results indicate that CO2 may affect nutrient translocation in plants in ways that are contrary to predictions about nitrogen metabolite responses to CO2. Such plant biochemical responses may underlie observations of improved phloem feeder performance under elevated CO2.

Development of an artificial diet and feeding system for juvenile stages of the Asian citrus psyllid, Diaphorina citri

AbstractThe Asian citrus psyllid (ACP), Diaphorina citri Kuwayama (Hemiptera: Psyllidae), is the main vector for the plant pathogen Candidatus Liberibacter asiaticus Jagoueix et al. (CLas). To date, attempts to develop an artificial diet for ACP have focused solely on the adult life stage, ignoring the juvenile stages. We developed a feeding system and artificial diet that is compatible with the juvenile stages of ACP and tested growth rates when exposed to varying concentrations of sucrose and amino acids. We found a surprisingly high tolerance for high sucrose concentrations in ACP when compared to known tolerances of sucrose concentrations in other phloem‐feeding insects. This is indicative that ACP may be physiologically adapted to a broad range of sucrose concentrations and osmotic stresses. The growth rate of juvenile ACP was optimized when amino acids were in a global concentration of 150 mM, with higher concentrations not appreciably impacting growth. This finding corresponds with the optimal amino acid concentrations required in other phloem feeders, notably the pea aphid. The development of this feeding system, with the diet optimized for growth, will allow for future experiments to be undertaken into the uptake of CLas by the psyllid and into the nutritional requirements of ACP. This feeding system will also allow for physiological comparisons among the phloem‐feeding insects.

The structural sheath protein of aphids is required for phloem feeding

Aphids produce two types of saliva that mediate their interactions with plants. Watery saliva is secreted during cell penetration and ingestion, whereas gel saliva is secreted during stylet movement through the apoplast where it forms a sheath around the stylet to facilitate penetration and seal puncture sites on cell membranes. In order to study the function of the sheath when aphids interact with plants, we used RNA interference (RNAi) to silence the aphid structural sheath protein (SHP) in the pea aphid Acyrthosiphon pisum. The injection of 50 ng of double stranded RNA completely disrupted sheath formation, as confirmed by scanning electron microscopy. Aphid behavior was monitored using the electrical penetration graph technique, revealing that disrupted sheath formation prevented efficient long-term feeding from sieve tubes, with a silencing effect on reproduction but not survival. We propose that sealing the stylet penetration site in the sieve tube plasma membrane is part of a two-step mechanism to suppress sieve-tube occlusion by preventing calcium influx into the sieve tube lumen. The SHP is present in several aphid species and silencing has a similar impact to aphid-resistant plants, suggesting that SHP is an excellent target for RNAi-mediated pest control.

RNAi-Based Strategy for Asian Citrus Psyllid (Diaphorina citri) Control: A Method to Reduce the Spread of Citrus Greening Disease

Citrus greening disease is a serious bacterial disease of citrus worldwide and is vectored by the Asian citrus pysllid (Diaphorina Citri).&nbsp; The only effective control strategy includes vigorous control of the psyllid, primarily through heavy reliance on pesticides. As a more sustainable and environmentally friendly method of psyllid control, we evaluated a RNA interference (RNAi) approach based on psyllid oral uptake of dsRNA molecules that target specific psyllid genes.&nbsp; This approach is based on the finding that cellular uptake of dsRNAs, that match the sequence of essential genes, results in down regulation of those genes and can lead to cell/organism death.&nbsp; These dsRNA molecules were introduced into the psyllids through feeding on citrus engineered to express the dsRNA using a Citrus tristeza virus as a paratransgenesis vector.&nbsp; Increased toxicity was observed when adult psyllids were fed on citrus producing dsRNA targeting either gut protease genes.&nbsp; No increased psyllid toxicity was observed in psyllids fed on citrus producing green fluorescent protein (GFP) dsRNA.&nbsp; Toxicity related to specific psyllid gene knockout will be discussed.&nbsp; These results suggest that RNAi-based control may be a viable alternative to current pesticide use for control of psyllids and all phloem feeding pests.

Identification and expression analysis of aquaporins in the potato psyllid, Bactericera cockerelli.

Aquaporin (AQPs) proteins transport water and uncharged low molecular-weight solutes across biological membranes. Six to 8 AQP genes have been identified in many insect species, but presently only three aquaporins have been characterized in phloem feeding insects. The objective of this study was to identify candidate AQPs in the potato psyllid, Bactericera cockerelli. Herein, we identified four candidate aquaporin cDNAs in B. cockerelli transcriptome. Phylogenetic analysis showed that candidate BcAQP2-like had high similarity to PRIP aquaporins; while candidates BcAQP4-like, BcAQP5-like and BcAQP9-like clustered within clade B. In particular, candidates BcAQP4-like and BcAQP5-like clustered with functionally validated insect aquaglyceroporin proteins. Expression analyses using RT-qPCR showed that all candidates were expressed in all life stages and tissues. Candidates BcAQP4-like and BcAQP5-like were highly expressed in bacteriocytes, while BcAQP9-like appeared to be expressed at high levels in whole body but not in the assayed tissues. This study is the first global attempt to identify putative aquaporins in a phloem feeding insect.

Identification of three potential insect vectors of Xylella fastidiosa in southern Italy.

In order to identify potential vectors of Xylella fastidiosa in olive orchards in Puglia (southern Italy), Hemiptera insects were collected from October to December, 2013, in olive orchards with high incidences of X. fastidiosa associated with “rapid decline” symptoms. The study focused on species in the Auchenorrhyncha (sharpshooter leafhoppers and froghoppers or spittlebugs), a group that includes known vectors of X. fastidiosa . Adults of three species, i.e. Philaenus spumarius L. (Aphrophoridae), Neophilaenus campestris Fallen (Aphrophoridae) and Euscelis lineolatus Brulle (Cicadellidae) were captured, from which total DNA was extracted and assayed by PCR using three sets of specific primers designed for X. fastidiosa detection. Results of PCR showed that 38 out of a total of 84 tested insects were positive for X. fastidiosa , i.e. eight (of 20) P. spumarius , 14 (of 18) N. campestris and 16 (of 46) E. lineolatus . PCR amplicons of the RNA polymerase sigma-70 factor gene from six specimens (two of each insect species) were sequenced. The sequences obtained were 99.3‒99.4% identical. BlastN analyses demonstrated these sequences to be similar to those of X. fastidiosa isolates from olive OL-X and OL-G reported from Puglia, whereas they displayed distant molecular identity (always less than 98%) with X. fastidiosa subspecies from other countries. The detection of X. fastidiosa in P. spumarius and, for the first time, in N. campestris and E. lineolatus (which, unlike the others, is a phloem feeder), indicates potential vectoring roles of these insects for the spread of the bacterium in Puglia. Further investigations and specific infectivity trials are required to definitively determine the roles of these insects as effective vectors of this pathogen.

Constitutive overexpression of the pollen specific gene SKS13 in leaves reduces aphid performance on Arabidopsis thaliana.

BackgroundPlants have developed a variety of mechanisms to counteract aphid attacks. They activate their defences by changing the expression of specific genes. Previously we identified an activation tag mutant of Arabidopsis thaliana on which Myzus persicae population development was reduced. Activation tag mutants are gain-of-function in which the expression of a gene is increased by the insertion of the Cauliflower mosaic virus 35S enhancer that acts on the natural promoter. By further characterizing this previously identified mutant we identified a gene that reduces performance of M. persicae and also provided clues about the mechanism involved.ResultsWe show that SKU5 SIMILAR 13 (SKS13), a gene whose expression in wild type plants is restricted to pollen and non-responsive to M. persicae attack, is overexpressed in the A. thaliana mutant showing reduced performance of M. persicae. Monitoring M. persicae feeding behaviour on SKS13 overexpressing plants indicated that M. persicae have difficulties feeding from the phloem. The constitutive expression of SKS13 results in accumulation of reactive oxygen species, which is possibly regulated through the jasmonic acid pathway. The enhanced resistance is not aphid species specific as also the population development of Brevicoryne brassicae was affected.ConclusionsWe demonstrate that constitutive expression in leaves of the pollen-specific gene SKS13 can enhance plant defence, resulting in a reduction of M. persicae population development and also decreases the transmission of persistent viruses. Overexpression of SKS13 in A. thaliana also affects B. brassicae and possibly other phloem feeding insects as well. Identifying genes that can enhance plant defence against insects will be important to open up new avenues for the development of insect resistant crop plants.

Phytohormone mediation of interactions between herbivores and plant pathogens.

Induced plant defenses against either pathogens or herbivore attackers are regulated by phytohormones. These phytohormones are increasingly recognized as important mediators of interactions between organisms associated with plants. In this review, we discuss the role of plant defense hormones in sequential tri-partite interactions among plants, pathogenic microbes, and herbivorous insects, based on the most recent literature. We discuss the importance of pathogen trophic strategy in the interaction with herbivores that exhibit different feeding modes. Plant resistance mechanisms also affect plant quality in future interactions with attackers. We discuss exemplary evidence for the hypotheses that (i) biotrophic pathogens can facilitate chewing herbivores, unless plants exhibit effector-triggered immunity, but (ii) facilitate or inhibit phloem feeders. (iii) Necrotrophic pathogens, on the other hand, can inhibit both phloem feeders and chewers. We also propose herbivore feeding mode as predictor of effects on pathogens of different trophic strategies, providing evidence for the hypotheses that (iv) phloem feeders inhibit pathogen attack by increasing SA induction, whereas (v) chewing herbivores tend not to affect necrotrophic pathogens, while they may either inhibit or facilitate biotrophic pathogens. Putting these hypotheses to the test will increase our understanding of phytohormonal regulation of plant defense to sequential attack by plant pathogens and insect herbivores. This will provide valuable insight into plant-mediated ecological interactions among members of the plant-associated community.

Suppression of Plant Defenses by a Myzus persicae (Green Peach Aphid) Salivary Effector Protein

The complex interactions between aphids and their host plant are species-specific and involve multiple layers of recognition and defense. Aphid salivary proteins, which are released into the plant during phloem feeding, are a likely mediator of these interactions. In an approach to identify aphid effectors that facilitate feeding from host plants, eleven Myzus persicae (green peach aphid) salivary proteins and the GroEL protein of Buchnera aphidicola, a bacterial endosymbiont of this aphid species, were expressed transiently in Nicotiana tabacum (tobacco). Whereas two salivary proteins increased aphid reproduction, expression of three other aphid proteins and GroEL significantly decreased aphid reproduction on N. tabacum. These effects were recapitulated in stable transgenic Arabidopsis thaliana plants. Further experiments with A. thaliana expressing Mp55, a salivary protein that increased aphid reproduction, showed lower accumulation of 4-methoxyindol-3-ylmethylglucosinolate, callose and hydrogen peroxide in response to aphid feeding. Mp55-expressing plants also were more attractive for aphids in choice assays. Silencing Mp55 gene expression in M. persicae using RNA interference approaches reduced aphid reproduction on N. tabacum, A. thaliana, and N. benthamiana. Together, these results demonstrate a role for Mp55, a protein with as-yet-unknown molecular function, in the interaction of M. persicae with its host plants.

Relationship between an exotic phloem feeder and balsam fir (Abies balsamea L. (Mill.)) foliar chemistry: implications for two native defoliators

The balsam woolly adelgid (Adelges piceae) is a gout-inducing hemipteran native to the silver fir forests of Europe. Introduced to eastern North America approximately 100 years ago, it is now found in most balsam fir forests in Atlantic Canada. When A. piceae feed, they trigger a reaction in the host branch that alters both xylem and phloem morphology. We conducted a field survey to examine the relationship between A. piceae gout density and balsam fir foliar chemistry and shoot growth in naturally unthinned and precommercially thinned stands. A. piceae gout density negatively affected branch growth and was related to changes in the chemistry of older, but not current-year foliage. Older foliage experienced decreases in camphene and bornyl acetate, while foliar concentrations of camphene, myrcene, phenolics, potassium and water differed between thinned and unthinned stands. Foliar chemistry was also influenced by interactions between thinning and A. piceae gout density in old foliage. This study suggests that changes in balsam fir associated with A. piceae gout density may force native defoliators that feed in highly gouted trees to adapt to diets of different chemical compositions and that thinning may alter these interactions.

Transcriptomic analyses reveal the adaptive features and biological differences of guts from two invasive whitefly species.

BackgroundThe gut of phloem feeding insects is critical for nutrition uptake and xenobiotics degradation. However, partly due to its tiny size, genomic information for the gut of phloem feeding insects is limited.ResultsIn this study, the gut transcriptomes of two species of invasive whiteflies in the Bemisia tabaci complex, Middle East Asia Minor 1 (MEAM1) and Mediterranean (MED), were analyzed using the Illumina sequencing. A total of 12,879 MEAM1 transcripts and 11,246 MED transcripts were annotated with a significant Blastx hit. In addition, 7,000 and 5,771 gut specific genes were respectively identified for MEAM1 and MED. Functional analyses on these gut specific genes demonstrated the important roles of gut in metabolism of insecticides and secondary plant chemicals. To reveal the molecular difference between guts of MEAM1 and MED, a comparison between gut transcriptomes of the two species was conducted and 3,910 pairs of orthologous genes were identified. Based on the ratio of nonsynonymous and synonymous substitutions, 15 genes were found evolving under positive selection. Many of those genes are predicted to be involved in metabolism and insecticide resistance. Furthermore, many genes related to detoxification were expressed at an elevated level in the gut of MED compared to MEAM1, which might be responsible for the MED’s higher resistance to insecticides and environmental stresses.ConclusionThe sequencing of MED and MEAM1 gut transcriptomes and extensive comparisons of MEAM1 and MED gut transcripts provide substantial sequence information for revealing the role of gut in whiteflies.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-370) contains supplementary material, which is available to authorized users.