Biochemistry and molecular biology of Arabidopsis-aphid interactions

Resistance to both aphids and nematodes in tobacco plants expressing a Bacillus thuringiensis crystal protein

BACKGROUND

Bacillus thuringiensis (Bt) and its crystal toxin or δ-endotoxins (Cry) offer great potential for the efficient control of crop pests. A vast number of pests can potentially infect the same host plant, either simultaneously or sequentially. However, no effective Bt-Cry protein has been reported to control both aphids and plant parasitic nematodes due to its highly specific activity.

RESULTS

Our study indicated that the Cry5Ba2 protein was toxic to the green peach aphid Myzus persicae, which had a median lethal concentration (LC50 ) of 9.7 ng μL-1 and fiducial limits of 3.1-34.6 ng μL-1 . Immunohistochemical localization of Cry5Ba2 revealed that it could bind to the apical tip of microvilli in midgut regions. Moreover, transgenic tobacco plants expressing Cry5Ba2 exhibited significant resistance to Myzus persicae, as evidenced by reduced insect survival and impaired fecundity, and also intoxicated the Meloidogyne incognita as indicated by a decrease in galls and progeny reproduction.

CONCLUSION

In sum, we identified a new aphicidal Bt toxin resource that could simultaneously control both aboveground and belowground pests, thus extending the application range of Bt-based strategy for crop protection. © 2024 Society of Chemical Industry.

Temporal transcriptomic profiling elucidates sorghum defense mechanisms against sugarcane aphids

BACKGROUND

The sugarcane aphid (SCA; Melanaphis sacchari) has emerged as a key pest on sorghum in the United States that feeds from the phloem tissue, drains nutrients, and inflicts physical damage to plants. Previously, it has been shown that SCA reproduction was low and high on sorghum SC265 and SC1345 plants, respectively, compared to RTx430, an elite sorghum male parental line (reference line). In this study, we focused on identifying the defense-related genes that confer resistance to SCA at early and late time points in sorghum plants with varied levels of SCA resistance.

RESULTS

We used RNA-sequencing approach to identify the global transcriptomic responses to aphid infestation on RTx430, SC265, and SC1345 plants at early time points 6, 24, and 48 h post infestation (hpi) and after extended period of SCA feeding for 7 days. Aphid feeding on the SCA-resistant line upregulated the expression of 3827 and 2076 genes at early and late time points, respectively, which was relatively higher compared to RTx430 and SC1345 plants. Co-expression network analysis revealed that aphid infestation modulates sorghum defenses by regulating genes corresponding to phenylpropanoid metabolic pathways, secondary metabolic process, oxidoreductase activity, phytohormones, sugar metabolism and cell wall-related genes. There were 187 genes that were highly expressed during the early time of aphid infestation in the SCA-resistant line, including genes encoding leucine-rich repeat (LRR) proteins, ethylene response factors, cell wall-related, pathogenesis-related proteins, and disease resistance-responsive dirigent-like proteins. At 7 days post infestation (dpi), 173 genes had elevated expression levels in the SCA-resistant line and were involved in sucrose metabolism, callose formation, phospholipid metabolism, and proteinase inhibitors.

CONCLUSIONS

In summary, our results indicate that the SCA-resistant line is better adapted to activate early defense signaling mechanisms in response to SCA infestation because of the rapid activation of the defense mechanisms by regulating genes involved in monolignol biosynthesis pathway, oxidoreductase activity, biosynthesis of phytohormones, and cell wall composition. This study offers further insights to better understand sorghum defenses against aphid herbivory.

Compensatory phenolic induction dynamics in aspen after aphid infestation

Condensed tannins (CTs) are polyphenolics and part of the total phenolic (TP) pool that shape resistance in aspen (Populus tremula). CTs are negatively associated with pathogens, but their resistance properties against herbivores are less understood. CTs shape resistance to pathogens and chewing herbivores and could also shape resistance to aphids. Being chemical pools that are highly variable it can further be questioned whether CT-shaped resistance is better described by constitutive levels, by the induced response potential, or by both. Here, aspen genotypes were propagated and selected to represent a range of inherent abilities to produce and store foliar CTs; the plantlets were then exposed to Chaitophorus aphid infestation and to mechanical (leaf rupture) damage, and the relative abundance of constitutive and induced CTs was related to aphid fitness parameters. As expected, aphid fecundity was negatively related to CT-concentrations of the aphid infested plants although more consistently related to TPs. While TPs increased in response to damage, CT induction was generally low and it even dropped below constitutive levels in more CT-rich genotypes, suggesting that constitutive CTs are more relevant measurements of resistance compared to induced CT-levels. Relating CT and TP dynamics with phenolic low molecular compounds further suggested that catechin (the building block of CTs) increased in response to aphid damage in amounts that correlated negatively with CT-induction and positively with constitutive CT-levels and aphid fecundity. Our study portrays dynamic phenolic responses to two kinds of damage detailed for major phenylpropanoid classes and suggests that the ability of a genotype to produce and store CTs may be a measurement of resistance, caused by other, more reactive, phenolic compounds such as catechin. Rupture damage however appeared to induce catechin levels oppositely supporting that CTs may respond differently to different kinds of damage.

Ecological convergence of secondary phytochemicals along elevational gradients

Biologists still strive to identify the ecological and evolutionary drivers of phytochemical variation that mediate biotic interactions. We hypothesized that plant species growing at sites characterized by high herbivore pressure would converge to produce highly toxic blends of secondary metabolites, independent of phylogenetic constraints. To address the role of shared evolutionary history and ecological niches in driving variation in plant phytochemistry, we combined targeted metabolomics with insect herbivore bioassays and with a set of growth-related traits of several Cardamine species growing along the entire elevational gradient of the Alps. We observed that Cardamine phytochemical profiles grouped according to previously established growth form categorizations within specific abiotic conditions, independently of phylogenetic relationship. We also showed that novel indices summarizing functional phytochemical diversity better explain plant resistance against chewing and sap-feeding herbivores than classic diversity indices. We conclude that multiple functional axes of phytochemical diversity should be integrated with the functional axis of plant growth forms to study phenotypic convergence along large-scale ecological gradients.

Transcriptome analysis provides insights into the mechanisms underlying wheat cultivar Shumai126 responding to stripe rust

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a destructive fungal disease of wheat globally. Breeding resistance cultivars is one of the most cost-effective methods to control Pst. Shumai126 (SM126), a high-yielding commercial wheat cultivar, showed strong stripe rust resistance for more than ten years. However, the molecular mechanisms and the responsive genes underlying the SM126 resistance to Pst have not been explored yet. In the present study, RNA-seq was used to analyze changes in the transcriptome at different time points of Pst infection in seedling leaves of SM126. In total, 520, 148 and 1439 differentially expressed genes (DEGs) were found to be up- or down-regulated after Pst infection at 1, 3, and 7 days post inoculation, respectively. The majority of DEGs exhibited transient expression patterns during Pst infection at different time points. GO and KEGG enrichment analysis revealed that many biological processes, such as photosynthesis, flavonoid biosynthesis, oxidative phosphorylation, MAPK signaling pathway, and phenylalanine metabolism are involved in SM126 response to Pst. Expression of genes involved in the plant-pathogen interaction pathway was detected and some key genes showed differential expression. DEGs encoded R proteins and transcription factors were also identified. Our study suggests the gene resources in SM126 related to stripe rust response could be valuable for understanding the mechanisms involved in stripe rust resistance and improvement of wheat resistance to Pst.

Aphid infestation differently affects the defences of nitrate-fed and nitrogen-fixing Medicago truncatula and alters symbiotic nitrogen fixation

Legumes can meet their nitrogen requirements through root nodule symbiosis, which could also trigger plant systemic resistance against pests. The pea aphid Acyrthosiphon pisum, a legume pest, can harbour different facultative symbionts (FS) influencing various traits of their hosts. It is therefore worth determining if and how the symbionts of the plant and the aphid modulate their interaction. We used different pea aphid lines without FS or with a single one (Hamiltonella defensa, Regiella insecticola, Serratia symbiotica) to infest Medicago truncatula plants inoculated with Sinorhizobium meliloti (symbiotic nitrogen fixation, SNF) or supplemented with nitrate (non-inoculated, NI). The growth of SNF and NI plants was reduced by aphid infestation, while aphid weight (but not survival) was lowered on SNF compared to NI plants. Aphids strongly affected the plant nitrogen fixation depending on their symbiotic status, suggesting indirect relationships between aphid- and plant-associated microbes. Finally, all aphid lines triggered expression of Pathogenesis-Related Protein 1 (PR1) and Proteinase Inhibitor (PI), respective markers for salicylic and jasmonic pathways, in SNF plants, compared to only PR1 in NI plants. We demonstrate that the plant symbiotic status influences plant-aphid interactions while that of the aphid can modulate the amplitude of the plant's defence response.

Basal differences in the transcriptional profiles of tomato leaves associated with the presence/absence of the resistance gene Mi-1 and changes in these differences after infestation by the whitefly Bemisia tabaci

The tomato Mi-1 gene mediates plant resistance to whitefly Bemisia tabaci, nematodes, and aphids. Other genes are also required for this resistance, and a model of interaction between the proteins encoded by these genes was proposed. Microarray analyses were used previously to identify genes involved in plant resistance to pests or pathogens, but scarcely in resistance to insects. In the present work, the GeneChip™ Tomato Genome Array (Affymetrix®) was used to compare the transcriptional profiles of Motelle (bearing Mi-1) and Moneymaker (lacking Mi-1) cultivars, both before and after B. tabaci infestation. Ten transcripts were expressed at least twofold in uninfested Motelle than in Moneymaker, while other eight were expressed half or less. After whitefly infestation, differences between cultivars increased to 14 transcripts expressed more in Motelle than in Moneymaker and 14 transcripts less expressed. Half of these transcripts showed no differential expression before infestation. These results show the baseline differences in the tomato transcriptomic profile associated with the presence or absence of the Mi-1 gene and provide us with valuable information on candidate genes to intervene in either compatible or incompatible tomato-whitefly interactions.

The plastidial metabolite 2-C-methyl-D-erythritol-2,4-cyclodiphosphate modulates defence responses against aphids

Feeding by insect herbivores such as caterpillars and aphids induces plant resistance mechanisms that are mediated by the phytohormones jasmonic acid (JA) and salicylic acid (SA). These phytohormonal pathways often crosstalk. Besides phytohormones, methyl-D-erythriol-2,4-cyclodiphosphate (MEcPP), the penultimate metabolite in the methyl-D-erythritol-4-phosphate pathway, has been speculated to regulate transcription of nuclear genes in response to biotic stressors such as aphids. Here, we show that MEcPP uniquely enhances the SA pathway without attenuating the JA pathway. Arabidopsis mutant plants that accumulate high levels of MEcPP (hds3) are highly resistant to the cabbage aphid (Brevicoryne brassicae), whereas resistance to the large cabbage white caterpillar (Pieris brassicae) remains unaltered. Thus, MEcPP is a distinct signalling molecule that acts beyond phytohormonal crosstalk to induce resistance against the cabbage aphid in Arabidopsis. We dissect the molecular mechanisms of MEcPP mediating plant resistance against the aphid B. brassicae. This shows that MEcPP induces the expression of genes encoding enzymes involved in the biosynthesis of several primary and secondary metabolic pathways contributing to enhanced resistance against this aphid species. A unique ability to regulate multifaceted molecular mechanisms makes MEcPP an attractive target for metabolic engineering in Brassica crop plants to increase resistance to cabbage aphids.

Plant Genes Benefitting Aphids-Potential for Exploitation in Resistance Breeding

Aphids are phloem sap-feeding insects common as pests in various crops. Here we review 62 omics studies of aphid/plant interactions to search for indications of how aphids may manipulate the plants to make them more suitable as hosts, i.e. more susceptible. Our aim is to try to reveal host plant susceptibility (S) genes, knowledge which can be exploited for making a plant more resistant to its pest by using new plant breeding techniques to knock out or down such S genes. S genes may be of two types, those that are involved in reducing functional plant defense and those involved in further increasing plant factors that are positive to the aphid, such as facilitated access to food or improved nutritional quality. Approximately 40% of the omics studies we have reviewed indicate how aphids may modify their host to their advantage. To exploit knowledge obtained so far, we suggest knocking out/down candidate aphid S genes using CRISPR/Cas9 or RNAi techniques in crops to evaluate if this will be sufficient to keep the aphid pest at economically viable levels without severe pleiotropic effects. As a complement, we also propose functional studies of recessively inherited resistance previously discovered in some aphid-crop combinations, to potentially identify new types of S genes that later could be knocked out or down also in other crops to improve their resistance to aphids.

Abscisic and Jasmonic Acids Contribute to Soybean Tolerance to the Soybean Aphid (Aphis glycines Matsumura)

Plant resistance can provide effective, economical, and sustainable pest control. Tolerance to the soybean aphid has been identified and confirmed in the soybean KS4202. Although its resistance mechanisms are not fully understood, evidence suggests that enhanced detoxification of reactive oxygen species (ROS) is an active system under high aphid infestation. We further explored tolerance by evaluating the differences in constitutive and aphid-induced defenses in KS4202 through the expression of selected defense-related transcripts and the levels of the phytohormones abscisic acid (ABA), jasmonic acid (JA), JA-isoleucine (JA-Ile), cis-(+)-12-oxo-phytodienoic acid (OPDA), and salicylic acid (SA) over several time points. Higher constitutive levels of ABA and JA, and basal expression of ABA- and JA-related transcripts were found in the tolerant genotype. Conversely, aphid-induced defenses in KS4202 were expressed as an upregulation of peroxidases under prolonged aphid infestation (>7 days). Our results point at the importance of phytohormones in constitutive defense in KS4202 tolerance to the soybean aphid. Understanding the underlying mechanisms of tolerance will assist breeding for soybean with these traits, and perhaps help extend the durability of Rag (Resistance to Aphis glycines)-mediated resistance genes.

The Arabidopsis RNA Polymerase II Carboxyl Terminal Domain (CTD) Phosphatase-Like1 (CPL1) is a biotic stress susceptibility gene

Crop breeding for improved disease resistance may be achieved through the manipulation of host susceptibility genes. Previously we identified multiple Arabidopsis mutants known as enhanced stress response1 (esr1) that have defects in a KH-domain RNA-binding protein and conferred increased resistance to the root fungal pathogen Fusarium oxysporum. Here, screening the same mutagenized population we discovered two further enhanced stress response mutants that also conferred enhanced resistance to F. oxysporum. These mutants also have enhanced resistance to a leaf fungal pathogen (Alternaria brassicicola) and an aphid pest (Myzus persicae), but not to the bacterial leaf pathogen Pseudomonas syringae. The causal alleles in these mutants were found to have defects in the ESR1 interacting protein partner RNA Polymerase II Carboxyl Terminal Domain (CTD) Phosphatase-Like1 (CPL1) and subsequently given the allele symbols cpl1-7 and cpl1-8. These results define a new role for CPL1 as a pathogen and pest susceptibility gene. Global transcriptome analysis and oxidative stress assays showed these cpl1 mutants have increased tolerance to oxidative stress. In particular, components of biotic stress responsive pathways were enriched in cpl1 over wild-type up-regulated gene expression datasets including genes related to defence, heat shock proteins and oxidative stress/redox state processes.

Aphid (Myzus persicae) feeding on the parasitic plant dodder (Cuscuta australis) activates defense responses in both the parasite and soybean host

Dodders (Cuscuta spp.) are shoot holoparasites, whose haustoria penetrate host tissues to enable fusion between the parasite and host vascular systems, allowing Cuscuta to extract water, nutrients and other molecules from hosts. Aphids are piercing-sucking herbivores that use specialized stylets to feed on phloem sap. Aphids are known to feed on Cuscuta, but how Cuscuta and its host plant respond to aphids attacking the parasite was unknown. Phytohormone quantification, transcriptomic analysis and bioassays were performed to determine the responses of Cuscuta australis and its soybean (Glycine max) hosts to the feeding of green peach aphid (GPA; Myzus persicae) on C. australis. Decreased salicylic acid levels and 172 differentially expressed genes (DEGs) were found in GPA-attacked C. australis, and the soybean hosts exhibited increased jasmonic acid contents and 1015 DEGs, including > 100 transcription factor genes. Importantly, GPA feeding on C. australis increased the resistance of the soybean host to subsequent feeding by the leafworm Spodoptera litura and soybean aphid Aphis glycines, resulting in 21% decreased leafworm mass and 41% reduced aphid survival rate. These data strongly suggest that GPA feeding on Cuscuta induces a systemic signal, which is translocated to hosts and activates defense against herbivores.

Soil microbial species loss affects plant biomass and survival of an introduced bacterial strain, but not inducible plant defences

BACKGROUND AND AIMS

Plant growth-promoting rhizobacteria (PGPR) strains can influence plant-insect interactions. However, little is known about the effect of changes in the soil bacterial community in general and especially the loss of rare soil microbes on these interactions. Here, the influence of rare soil microbe reduction on induced systemic resistance (ISR) in a wild ecotype of Arabidopsis thaliana against the aphid Myzus persicae was investigated.

METHODS

To create a gradient of microbial abundances, soil was inoculated with a serial dilution of a microbial community and responses of Arabidopsis plants that originated from the same site as the soil microbes were tested. Plant biomass, transcription of genes involved in plant defences, and insect performance were measured. In addition, the effects of the PGPR strain Pseudomonas fluorescens SS101 on plant and insect performance were tested under the influence of the various soil dilution treatments.

KEY RESULTS

Plant biomass showed a hump-shaped relationship with soil microbial community dilution, independent of aphid or Pseudomonas treatments. Both aphid infestation and inoculation with Pseudomonas reduced plant biomass, and led to downregulation of PR1 (salicylic acid-responsive gene) and CYP79B3 (involved in synthesis of glucosinolates). Aphid performance and gene transcription were unaffected by soil dilution.

CONCLUSIONS

Neither the loss of rare microbial species, as caused by soil dilution, nor Pseudomonas affect the resistance of A. thaliana against M. persicae. However, both Pseudomonas survival and plant biomass respond to rare species loss. Thus, loss of rare soil microbial species can have a significant impact on both above- and below-ground organisms.

Probing behavior of aposymbiotic green peach aphid (Myzus persicae) on susceptible Solanum tuberosum and resistant Solanum stoloniferum plants

The green peach aphid, Myzus persicae Sulzer (Hemiptera: Aphididae) is one of the potato important pests; it is the most efficient vector of potato viruses. Myzus persicae harbors the endosymbiotic bacteria Buchnera aphidicola which supplements their diet. There is increasing evidence that B. aphidicola is involved in plant-aphid interactions and we previously demonstrated that B. aphidicola disruption (aposymbiosis) affected the probing behavior of M. persicae on radish plants, delaying host plant acceptance. In this work, we evaluated the effect of aposymbiosis on the probing behavior of M. persicae on 2 Solanum species with different compatibility with M. persicae, Solanum tuberosum (susceptible) and Solanum stoloniferum (resistant) with the electrical penetration graph technique (EPG). To disrupt B. aphidicola, rifampicin was administered to aphids through artificial diets. Aposymbiotic aphids, on both plant species, showed increased pathway activities, mechanical problems with the stylets, and delayed salivation in the phloem. The extended time in derailed stylet mechanics affected the occurrence of most other probing activities; it delayed the time to the first phloem phase and prevented ingestion from the phloem. The effect of aposymbiosis was more evident in the compatible interaction of M. persicae-S. tuberosum, than in the incompatible interaction with S. stoloniferum, which generated the M. persicae-S. tuberosum interaction to become incompatible. These results confirm that B. aphidicola is involved in the plant-aphid interaction in relation to plant acceptance, presumably through a role in stylets penetration in the plant.

Trans-methylation reactions in plants: focus on the activated methyl cycle

Trans-methylation reactions are vital in basic metabolism, epigenetic regulation, RNA metabolism, and posttranslational control of protein function and therefore fundamental in determining the physiological processes in all living organisms. The plant kingdom is additionally characterized by the production of secondary metabolites that undergo specific hydroxylation, oxidation and methylation reactions to obtain a wide array of different chemical structures. Increasing research efforts have started to reveal the enzymatic pathways underlying the biosynthesis of complex metabolites in plants. Further engineering of these enzymatic machineries offers significant possibilities in the development of bio-based technologies, but necessitates deep understanding of their potential metabolic and regulatory interactions. Trans-methylation reactions are tightly coupled with the so-called activated methyl cycle (AMC), an essential metabolic circuit that maintains the trans-methylation capacity in all living cells. Tight regulation of the AMC is crucial in ensuring accurate trans-methylation reactions in different subcellular compartments, cell types, developmental stages and environmental conditions. This review addresses the organization and posttranslational regulation of the AMC and elaborates its critical role in determining metabolic regulation through modulation of methyl utilization in stress-exposed plants.

Aphid Colonization Affects Potato Root Exudate Composition and the Hatching of a Soil Borne Pathogen

Plants suffer multiple, simultaneous biotic threats from both above and below ground. These pests and/or pathogens are commonly studied on an individual basis and the effects of above-ground pests on below-ground pathogens are poorly defined. Root exudates from potato plants (Solanum tuberosum L.) were analyzed to characterize the top-down plant-mediated interactions between a phloem-sucking herbivore (Myzus persicae) and a sedentary, endoparasitic nematode (Globodera pallida). Increasing inocula of the aphid, M. persicae, reduced the root mass of potato plants. Exudates collected from these roots induced significantly lower hatching of second-stage juveniles from G. pallida eggs over a 28-day period, than those from uninfested control plants. Inhibition of hatch was significantly positively correlated with size of aphid inoculum. Diminished hatching was partially recovered after treatment with root exudate from uninfested potato plants indicating that the effect on hatching is reversible but cannot be fully recovered. Glucose and fructose content was reduced in root exudates from aphid-infested potato plants compared to controls and these sugars were found to induce hatching of G. pallida, but not to the same degree as potato root exudates (PRE). Supplementing aphid-infested PRE with sugars did not recover the hatching potential of the treatment, suggesting that additional compounds play an important role in egg hatch. The first gene upregulated in the closely related potato cyst nematode Globodera rostochiensis post-exposure to host root exudate, Neprilysin-1, was confirmed to be upregulated in G. pallida cysts after exposure to PRE and was also upregulated by the sugar treatments. Significantly reduced upregulation of Gpa-nep-1 was observed in cysts treated with root exudates from potato plants infested with greater numbers of aphids. Our data suggest that aphid infestation of potato plants affects the composition of root exudates, with consequential effects on the hatching and gene expression of G. pallida eggs. This work shows that an above-ground pest can indirectly impact the rhizosphere and reveals secondary effects for control of an economically important below-ground pathogen.

The Protease Inhibitor CI2c Gene Induced by Bird Cherry-Oat Aphid in Barley Inhibits Green Peach Aphid Fecundity in Transgenic Arabidopsis

Aphids are phloem feeders that cause large damage globally as pest insects. They induce a variety of responses in the host plant, but not much is known about which responses are promoting or inhibiting aphid performance. Here, we investigated whether one of the responses induced in barley by the cereal aphid, bird cherry-oat aphid (Rhopalosiphum padi L.) affects aphid performance in the model plant Arabidopsis thaliana L. A barley cDNA encoding the protease inhibitor CI2c was expressed in A. thaliana and aphid performance was studied using the generalist green peach aphid (Myzus persicae Sulzer). There were no consistent effects on aphid settling or preference or on parameters of life span and long-term fecundity. However, short-term tests with apterous adult aphids showed lower fecundity on three of the transgenic lines, as compared to on control plants. This effect was transient, observed on days 5 to 7, but not later. The results suggest that the protease inhibitor is taken up from the tissue during probing and weakly inhibits fecundity by an unknown mechanism. The study shows that a protease inhibitor induced in barley by an essentially monocot specialist aphid can inhibit a generalist aphid in transgenic Arabidopsis.

Brevicoryne brassicae aphids interfere with transcriptome responses of Arabidopsis thaliana to feeding by Plutella xylostella caterpillars in a density-dependent manner

Plants are commonly attacked by multiple herbivorous species. Yet, little is known about transcriptional patterns underlying plant responses to multiple insect attackers feeding simultaneously. Here, we assessed transcriptomic responses of Arabidopsis thaliana plants to simultaneous feeding by Plutella xylostella caterpillars and Brevicoryne brassicae aphids in comparison to plants infested by P. xylostella caterpillars alone, using microarray analysis. We particularly investigated how aphid feeding interferes with the transcriptomic response to P. xylostella caterpillars and whether this interference is dependent on aphid density and time since aphid attack. Various JA-responsive genes were up-regulated in response to feeding by P. xylostella caterpillars. The additional presence of aphids, both at low and high densities, clearly affected the transcriptional plant response to caterpillars. Interestingly, some important modulators of plant defense signalling, including WRKY transcription factor genes and ABA-dependent genes, were differentially induced in response to simultaneous aphid feeding at low or high density compared with responses to P. xylostella caterpillars feeding alone. Furthermore, aphids affected the P. xylostella-induced transcriptomic response in a density-dependent manner, which caused an acceleration in plant response against dual insect attack at high aphid density compared to dual insect attack at low aphid density. In conclusion, our study provides evidence that aphids influence the caterpillar-induced transcriptional response of A. thaliana in a density-dependent manner. It highlights the importance of addressing insect density to understand how plant responses to single attackers interfere with responses to other attackers and thus underlines the importance of the dynamics of transcriptional plant responses to multiple herbivory.

Plant-Mediated Systemic Interactions Between Pathogens, Parasitic Nematodes, and Herbivores Above- and Belowground

Plants are important mediators of interactions between aboveground (AG) and belowground (BG) pathogens, arthropod herbivores, and nematodes (phytophages). We highlight recent progress in our understanding of within- and cross-compartment plant responses to these groups of phytophages in terms of altered resource dynamics and defense signaling and activation. We review studies documenting the outcome of cross-compartment interactions between these phytophage groups and show patterns of cross-compartment facilitation as well as cross-compartment induced resistance. Studies involving soilborne pathogens and foliar nematodes are scant. We further highlight the important role of defense signaling loops between shoots and roots to activate a full resistance complement. Moreover, manipulation of such loops by phytophages affects systemic interactions with other plant feeders. Finally, cross-compartment-induced changes in root defenses and root exudates extend systemic defense loops into the rhizosphere, enhancing or reducing recruitment of microbes that induce systemic resistance but also affecting interactions with root-feeding phytophages.

Interactive Effects of Cabbage Aphid and Caterpillar Herbivory on Transcription of Plant Genes Associated with Phytohormonal Signalling in Wild Cabbage

Plants are commonly attacked by a variety of insect herbivores and have developed specific defenses against different types of attackers. At the molecular level, herbivore-specific signalling pathways are activated by plants in response to attackers with different feeding strategies. Feeding by leaf-chewing herbivores predominantly activates jasmonic acid (JA)-regulated defenses, whereas feeding by phloem-sucking herbivores generally activates salicylic acid (SA)-regulated defenses. When challenged sequentially by both phloem-sucking and leaf-chewing herbivores, SA-JA antagonism may constrain the plant's ability to timely and adequately divert defense to the second herbivore that requires activation of a different defensive pathway. We investigated the effect of the temporal sequence of infestation by the aphid Brevicoryne brassicae and three caterpillar species, Plutella xylostella, Pieris brassicae, and Mamestra brassicae, on the interaction between JA and SA signal-transduction pathways in three wild cabbage populations. We found no support for SA-JA antagonism, irrespective of the temporal sequence of herbivore introduction or the identity of the caterpillar species based on the transcript levels of the JA- and SA-regulated marker genes LOX and PR-1, respectively, at the examined time points, 6, 24, and 48 h. In general, infestation with aphids alone had little effect on the transcript levels of the two marker genes, whereas the three caterpillar species upregulated not only LOX but also PR-1. Transcriptional changes were different for plants from the three different natural cabbage populations.

AtWRKY22 promotes susceptibility to aphids and modulates salicylic acid and jasmonic acid signalling

Aphids induce many transcriptional perturbations in their host plants, but the signalling cascades responsible and the effects on plant resistance are largely unknown. Through a genome-wide association (GWA) mapping study in Arabidopsis thaliana, we identified WRKY22 as a candidate gene associated with feeding behaviour of the green peach aphid, Myzus persicae The transcription factor WRKY22 is known to be involved in pathogen-triggered immunity, and WRKY22 gene expression has been shown to be induced by aphids. Assessment of aphid population development and feeding behaviour on knockout mutants and overexpression lines showed that WRKY22 increases susceptibility to M. persicae via a mesophyll-located mechanism. mRNA sequencing analysis of aphid-infested wrky22 knockout plants revealed the up-regulation of genes involved in salicylic acid (SA) signalling and down-regulation of genes involved in plant growth and cell-wall loosening. In addition, mechanostimulation of knockout plants by clip cages up-regulated jasmonic acid (JA)-responsive genes, resulting in substantial negative JA-SA crosstalk. Based on this and previous studies, WRKY22 is considered to modulate the interplay between the SA and JA pathways in response to a wide range of biotic and abiotic stimuli. Its induction by aphids and its role in suppressing SA and JA signalling make WRKY22 a potential target for aphids to manipulate host plant defences.

Abscisic acid deficiency increases defence responses against Myzus persicae in Arabidopsis

Comparison of Arabidopsis thaliana (Arabidopsis) gene expression induced by Myzus persicae (green peach aphid) feeding, aphid saliva infiltration and abscisic acid (ABA) treatment showed a significant positive correlation. In particular, ABA-regulated genes are over-represented among genes that are induced by M. persicae saliva infiltration into Arabidopsis leaves. This suggests that the induction of ABA-related gene expression could be an important component of the Arabidopsis-aphid interaction. Consistent with this hypothesis, M. persicae populations induced ABA production in wild-type plants. Furthermore, aphid populations were smaller on Arabidopsis aba1-1 mutants, which cannot synthesize ABA, and showed a significant preference for wild-type plants compared with the mutant. Total free amino acids, which play an important role in aphid nutrition, were not altered in the aba1-1 mutant line, but the levels of isoleucine (Ile) and tryptophan (Trp) were differentially affected by aphids in wild-type and mutant plants. Recently, indole glucosinolates have been shown to promote aphid resistance in Arabidopsis. In this study, 4-methoxyindol-3-ylmethylglucosinolate was more abundant in the aba1-1 mutant than in wild-type Arabidopsis, suggesting that the induction of ABA signals that decrease the accumulation of defence compounds may be beneficial for aphids.

Novel Genes Affecting the Interaction between the Cabbage Whitefly and Arabidopsis Uncovered by Genome-Wide Association Mapping

Plants have evolved a variety of ways to defend themselves against biotic attackers. This has resulted in the presence of substantial variation in defense mechanisms among plants, even within a species. Genome-wide association (GWA) mapping is a useful tool to study the genetic architecture of traits, but has so far only had limited exploitation in studies of plant defense. Here, we study the genetic architecture of defense against the phloem-feeding insect cabbage whitefly (Aleyrodes proletella) in Arabidopsis thaliana. We determined whitefly performance, i.e. the survival and reproduction of whitefly females, on 360 worldwide selected natural accessions and subsequently performed GWA mapping using 214,051 SNPs. Substantial variation for whitefly adult survival and oviposition rate (number of eggs laid per female per day) was observed between the accessions. We identified 39 candidate SNPs for either whitefly adult survival or oviposition rate, all with relatively small effects, underpinning the complex architecture of defense traits. Among the corresponding candidate genes, i.e. genes in linkage disequilibrium (LD) with candidate SNPs, none have previously been identified as a gene playing a role in the interaction between plants and phloem-feeding insects. Whitefly performance on knock-out mutants of a number of candidate genes was significantly affected, validating the potential of GWA mapping for novel gene discovery in plant-insect interactions. Our results show that GWA analysis is a very useful tool to gain insight into the genetic architecture of plant defense against herbivorous insects, i.e. we identified and validated several genes affecting whitefly performance that have not previously been related to plant defense against herbivorous insects.

Proteomic analysis of Arabidopsis thaliana (L.) Heynh responses to a generalist sucking pest (Myzus persicae Sulzer)

Herbivorous insects can cause severe cellular changes to plant foliage following infestations, depending on feeding behaviour. Here, a proteomic study was conducted to investigate the influence of green peach aphid (Myzus persicae Sulzer) as a polyphagous pest on the defence response of Arabidopsis thaliana (L.) Heynh after aphid colony establishment on the host plant (3 days). Analysis of about 574 protein spots on 2-DE gels revealed 31 differentially expressed protein spots. Twenty out of these 31 differential proteins were selected for analysis by mass spectrometry. In 12 of the 20 analysed spots, we identified seven and nine proteins using MALDI-TOF-MS and LC-ESI-MS/MS, respectively. Of the analysed spots, 25% contain two proteins. Different metabolic pathways were modulated in Arabidopsis leaves according to aphid feeding: most corresponded to carbohydrate, amino acid and energy metabolism, photosynthesis, defence response and translation. This paper has established a survey of early alterations induced in the proteome of Arabidopsis by M. persicae aphids. It provides valuable insights into the complex responses of plants to biological stress, particularly for herbivorous insects with sucking feeding behaviour.

Feeding on Leaves of the Glucosinolate Transporter Mutant gtr1gtr2 Reduces Fitness of Myzus persicae

As aphids are a pest on various crops worldwide, a better understanding of the interaction between aphids and plant host defenses is required. The green peach aphid (Myzus persicae) feeds on a variety of plant species, including the model plant Arabidopsis thaliana (Arabidopsis), in which glucosinolates function as a major part of the chemical defense. Several studies have shown that glucosinolates play a role in interactions between Arabidopsis and the green peach aphid. In this work, we used a recently identified Arabidopsis glucosinolate transporter mutant (gtr1gtr2 dKO), with altered glucosinolate content in the vasculature, to investigate the role of defense compound transport in aphid infestation. By monitoring aphid performance on caged leaves and analyzing glucosinolates in leaf tissue and phloem sap, as well as inside aphids, we examined if a change in spatial distribution of glucosinolates within a leaf influences aphid performance. Based on reduced glucosinolate content in the phloem sap of the transporter mutant, we hypothesized that aphids would perform better on gtr1gtr2 dKO leaves compared to WT. Unexpectedly, aphids performed poorly on gtr1gtr2 dKO leaves. Our data suggest that higher glucosinolate content in tissues surrounding the phloem of the double transporter mutant may play a role in reducing aphid performance on this genotype.

Aphid-induced Defences in Chilli Affect Preferences of the Whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae)

The sweetpotato whitefly (WF), Bemisia tabaci, is a major pest that damages a wide range of vegetable crops in Malaysia. WF infestation is influenced by a variety of factors, including previous infestation of the host plant by other insect pests. This study investigated the effects of previous infestation of host chilli plants by the green peach aphid (Myzus persicae) on the olfactory behavioural response of B. tabaci, using free-choice bioassay with a Y-tube olfactometer. We analysed volatile organic compounds (VOCs) emitted by non-infested and M. persicae-infested chilli plants using solid-phase microextraction and gas chromatography–mass spectrometry. Our results showed that female WFs preferred non-infested to pre-infested plants. Collection and analysis of volatile compounds emitted by infested plants confirmed that there were significant increases in the production of monoterpenes (cymene; 1,8-cineole), sesquiterpenes (β–cadinene, α-copaene), and methyl salicylate (MeSA) compared to non-infested plants. Our results suggest that host plant infestation by aphids may induce production of secondary metabolites that deter B. tabaci from settling on its host plants. These results provide important information for understanding WF host selection and dispersal among crops, and also for manipulating WF behaviour to improve IPM in chilli.

Characterization of Arabidopsis Transcriptional Responses to Different Aphid Species Reveals Genes that Contribute to Host Susceptibility and Non-host Resistance

Aphids are economically important pests that display exceptional variation in host range. The determinants of diverse aphid host ranges are not well understood, but it is likely that molecular interactions are involved. With significant progress being made towards understanding host responses upon aphid attack, the mechanisms underlying non-host resistance remain to be elucidated. Here, we investigated and compared Arabidopsis thaliana host and non-host responses to aphids at the transcriptional level using three different aphid species, Myzus persicae, Myzus cerasi and Rhopalosiphum pisum. Gene expression analyses revealed a high level of overlap in the overall gene expression changes during the host and non-host interactions with regards to the sets of genes differentially expressed and the direction of expression changes. Despite this overlap in transcriptional responses across interactions, there was a stronger repression of genes involved in metabolism and oxidative responses specifically during the host interaction with M. persicae. In addition, we identified a set of genes with opposite gene expression patterns during the host versus non-host interactions. Aphid performance assays on Arabidopsis mutants that were selected based on our transcriptome analyses identified novel genes contributing to host susceptibility, host defences during interactions with M. persicae as well to non-host resistance against R. padi. Understanding how plants respond to aphid species that differ in their ability to infest plant species, and identifying the genes and signaling pathways involved, is essential for the development of novel and durable aphid control in crop plants.

Aphids are phloem-feeding insects that cause feeding damage and transmit plant viruses to many crops. While most aphid species are restricted to one or few host plants, some aphids can infest a wide range of plant species. These insects spend a considerable time on non-hosts, where they probe the leaf tissue and secrete saliva, but for unknown reasons are unable to ingest phloem sap. This suggests that aphids interact with non-host plants at the molecular level, but potentially do not suppress plant defences and/or promote the release of nutrients. We compared gene expression of plants during host and non-host interactions with aphids to identify genes involved in immunity. We found significant overlap in the plant responses to aphids regardless of the type of interaction. Despite this, we identified a set of genes specifically affected during host or non-host interactions with specific aphid species. In addition, we showed that several of these genes contribute to host and/or non-host immunity. These findings are important, as they advance our understanding of the plant cellular processes involved in host and non-host responses against insect pests. Understanding mechanisms of host and non-host resistance to plant parasites is essential for development of novel control strategies.

Rag Virulence Among Soybean Aphids (Hemiptera: Aphididae) in Wisconsin

Soybean aphid, Aphis glycines Matsumura, a pest of soybean, Glycine max (L.) Merr., and native of Asia, invaded North America sometime before 2000 and rapidly became the most significant insect pest of soybean in the upper Midwest. Plant resistance, a key component of integrated pest management, has received significant attention in the past decade, and several resistance (Rag) genes have been identified. However, the efficacy of Rag (Resistance to Aphis glycines) genes in suppressing aphid abundance has been challenged by the occurrence of soybean aphids capable of overcoming Rag gene-mediated resistance. Although the occurrence of these Rag virulent biotypes poses a serious threat to effective and sustainable management of soybean aphid, little is known about the current abundance of biotypes in North America. The objective of this research was to determine the distribution of Rag virulent soybean aphids in Wisconsin. Soybean aphids were collected from Wisconsin during the summers of 2012 and 2013, and assayed for Rag1, Rag2, and Rag1+2 virulence using no-choice tests in a greenhouse. One clone from Monroe County in 2012 reacted like biotype 4, three clones in different counties in 2013 responded like biotype 2, and eight others expressed varying degrees of Rag virulence. Rag virulence in 2013 was observed in aphids from 33% of the sampled sites and was accounted for by just 4.5% of sampled clones, although this is likely a conservative estimate. No-choice test results are discussed in light of current questions on the biology, ecology, and population genetics of soybean aphid.

Plant-insect interactions under bacterial influence: ecological implications and underlying mechanisms

Plants and insects have been co-existing for more than 400 million years, leading to intimate and complex relationships. Throughout their own evolutionary history, plants and insects have also established intricate and very diverse relationships with microbial associates. Studies in recent years have revealed plant- or insect-associated microbes to be instrumental in plant-insect interactions, with important implications for plant defences and plant utilization by insects. Microbial communities associated with plants are rich in diversity, and their structure greatly differs between below- and above-ground levels. Microbial communities associated with insect herbivores generally present a lower diversity and can reside in different body parts of their hosts including bacteriocytes, haemolymph, gut, and salivary glands. Acquisition of microbial communities by vertical or horizontal transmission and possible genetic exchanges through lateral transfer could strongly impact on the host insect or plant fitness by conferring adaptations to new habitats. Recent developments in sequencing technologies and molecular tools have dramatically enhanced opportunities to characterize the microbial diversity associated with plants and insects and have unveiled some of the mechanisms by which symbionts modulate plant-insect interactions. Here, we focus on the diversity and ecological consequences of bacterial communities associated with plants and herbivorous insects. We also highlight the known mechanisms by which these microbes interfere with plant-insect interactions. Revealing such mechanisms in model systems under controlled environments but also in more natural ecological settings will help us to understand the evolution of complex multitrophic interactions in which plants, herbivorous insects, and micro-organisms are inserted.

Proteomic analysis shows that stress response proteins are significantly up-regulated in resistant diploid wheat (Triticum monococcum) in response to attack by the grain aphid (Sitobion avenae)

The grain aphid Sitobion avenae (F.) is a major pest of wheat, acting as a virus vector as well as causing direct plant damage. Commonly grown wheat varieties in the UK have only limited resistance to this pest. The present study was carried out to investigate the potential of a diploid wheat line (ACC20 PGR1755), reported as exhibiting resistance to S. avenae, to serve as a source of resistance genes. The diploid wheat line was confirmed as partially resistant, substantially reducing the fecundity, longevity and growth rate of the aphid. Proteomic analysis showed that approximately 200 protein spots were reproducibly detected in leaf extracts from both the resistant line and a comparable susceptible line (ACC5 PGR1735) using two-dimensional gel electrophoresis and image comparison software. Twenty-four spots were significantly up-regulated (>2-fold) in the resistant line after 24 h of aphid feeding (13 and 11 involved in local and systemic responses, respectively). Approximately 50 % of all differentially expressed protein spots were identified by a combination of database searching with MS and MS/MS data, revealing that the majority of proteins up-regulated by aphid infestation were involved in metabolic processes (including photosynthesis) and transcriptional regulation. However, in the resistant line only, several stress response proteins (including NBS-LRR-like proteins) and oxidative stress response proteins were identified as up-regulated in response to aphid feeding, as well as proteins involved in DNA synthesis/replication/repair. This study indicates that the resistant diploid line ACC20 PGR1755 may provide a valuable resource in breeding wheat for resistance to aphids.

Density-dependent interference of aphids with caterpillar-induced defenses in Arabidopsis: involvement of phytohormones and transcription factors

In nature, plants are exposed to attacks by multiple herbivore species at the same time. To cope with these attacks, plants regulate defenses with the production of hormones such as salicylic acid (SA) and jasmonic acid (JA). Because herbivore densities are dynamic in time, this may affect plant-mediated interactions between different herbivores attacking at the same time. In Arabidopsis thaliana, feeding by Brevicoryne brassicae aphids interferes with induced defenses against Plutella xylostella caterpillars. This is density dependent: at a low aphid density, the growth rate of P. xylostella was increased, whereas caterpillars feeding on plants colonized by aphids at a high density have a reduced growth rate. Growth of P. xylostella larvae was unaffected on sid2-1 or on dde2-2 mutant plants when feeding simultaneously with a low or high aphid density. This shows that aphid interference with caterpillar-induced defenses requires both SA and JA signal transduction pathways. Transcriptional analysis revealed that simultaneous feeding by caterpillars and aphids at a low density induced the expression of the SA transcription factor gene WRKY70 whereas expression of WRKY70 was lower in plants induced with both caterpillars and a high aphid density. Interestingly, the expression of the JA transcription factor gene MYC2 was significantly higher in plants simultaneously attacked by aphids at a high density and caterpillars. These results indicate that a lower expression level of WRKY70 leads to significantly higher MYC2 expression through SA-JA cross-talk. Thus, plant-mediated interactions between aphids and caterpillars are density dependent and involve phytohormonal cross-talk and differential activation of transcription factors.

The phytoplasmal virulence factor TENGU causes plant sterility by downregulating of the jasmonic acid and auxin pathways

Despite plants infected by pathogens are often unable to produce offspring, it remains unclear how sterility is induced in host plants. In this study, we demonstrate that TENGU, a phytoplasmal virulence peptide known as a dwarfism inducer, acts as an inducer of sterility. Transgenic expression of TENGU induced both male and female sterility in Arabidopsis thaliana flowers similar to those observed in double knockout mutants of auxin response factor 6 (ARF6) and ARF8, which are known to regulate floral development in a jasmonic acid (JA)-dependent manner. Transcripts of ARF6 and ARF8 were significantly decreased in both tengu-transgenic and phytoplasma-infected plants. Furthermore, JA and auxin levels were actually decreased in tengu-transgenic buds, suggesting that TENGU reduces the endogenous levels of phytohormones by repressing ARF6 and ARF8, resulting in impaired flower maturation. TENGU is the first virulence factor with the effects on plant reproduction by perturbation of phytohormone signaling.

Different metabolic and genetic responses in citrus may explain relative susceptibility to Tetranychus urticae

BACKGROUND

Life history parameters of the phytophagous spider mite Tetranychus urticae in citrus depend on the rootstock where the cultivar is grafted. To unveil the mechanisms responsible for this effect, the authors have carried out comparative experiments of T. urticae performance on two citrus rootstocks, the highly T. urticae-sensitive Cleopatra mandarin and the more tolerant sour orange.

RESULTS

Sour orange showed reduced leaf damage symptoms, supported lower mite populations and reduced oviposition rates compared with Cleopatra mandarin. Hormonal, metabolomic and gene expression analyses of the main defence pathways suggest a relevant role of the oxylipin and the flavonoid pathways in the response against T. urticae. Sour orange showed an increased activity of the JA pathway, which was hardly active in the most susceptible rootstock. Moreover, treatments with the LOX inhibitor Phenidone abolished the enhanced tolerance of sour orange. Therefore, oxylipin-dependent defence seems to be rootstock dependent. The metabolomic analysis showed the importance of the flavonoid pathway, which is implicated in the interaction between plants and their environment.

CONCLUSION

The findings suggest that sour-orange enhanced tolerance to spider mites can be sustained by a combination of pre-existing and induced responses depending on high levels of flavonoids and a fast and effective activation of the oxylipin pathway. © 2013 Society of Chemical Industry.

Exploring the nitrogen ingestion of aphids--a new method using electrical penetration graph and (15)N labelling

Studying plant-aphid interactions is challenging as aphid feeding is a complex process hidden in the plant tissue. Here we propose a combination of two well established methods to study nutrient acquisition by aphids focusing on the uptake of isotopically labelled nitrogen (¹⁵N). We combined the Electrical Penetration Graph (EPG) technique that allows detailed recording of aphid feeding behaviour and stable isotope ratio mass spectrometry (IRMS) to precisely measure the uptake of nitrogen. Bird cherry-oat aphids Rhopalosiphum padi L. (Hemiptera, Aphididae) fed for 24 h on barley plants (Hordeum vulgare L., cultivar Lina, Poaceae) that were cultivated with a ¹⁵N enriched nutrient solution. The time aphids fed in the phloem was strongly positive correlated with their ¹⁵N uptake. All other single behavioural phases were not correlated with ¹⁵N enrichment in the aphids, which corroborates their classification as non-feeding EPG phases. In addition, phloem-feeding and ¹⁵N enrichment of aphids was divided into two groups. One group spent only short time in the phloem phase and was unsuccessful in nitrogen acquisition, while the other group displayed longer phloem-feeding phases and was successful in nitrogen acquisition. This suggests that several factors such as the right feeding site, time span of feeding and individual conditions play a role for the aphids to acquire nutrients successfully. The power of this combination of methods for studying plant-aphid interactions is discussed.

Plant immunity in plant-aphid interactions

Aphids are economically important pests that cause extensive feeding damage and transmit viruses. While some species have a broad host range and cause damage to a variety of crops, others are restricted to only closely related plant species. While probing and feeding aphids secrete saliva, containing effectors, into their hosts to manipulate host cell processes and promote infestation. Aphid effector discovery studies pointed out parallels between infection and infestation strategies of plant pathogens and aphids. Interestingly, resistance to some aphid species is known to involve plant resistance proteins with a typical NB-LRR domain structure. Whether these resistance proteins indeed recognize aphid effectors to trigger ETI remains to be elucidated. In addition, it was recently shown that unknown aphid derived elicitors can initiate reactive oxygen species (ROS) production and callose deposition and that these responses were dependent on BAK1 (BRASSINOSTERIOD INSENSITIVE 1-ASSOCIATED RECEPTOR KINASE 1) which is a key component of the plant immune system. In addition, BAK-1 contributes to non-host resistance to aphids pointing to another parallel between plant-pathogen and - aphid interactions. Understanding the role of plant immunity and non-host resistance to aphids is essential to generate durable and sustainable aphid control strategies. Although insect behavior plays a role in host selection and non-host resistance, an important observation is that aphids interact with non-host plants by probing the leaf surface, but are unable to feed or establish colonization. Therefore, we hypothesize that aphids interact with non-host plants at the molecular level, but are potentially not successful in suppressing plant defenses and/or releasing nutrients.

Induced senescence promotes the feeding activities and nymph development of Myzus persicae (Hemiptera: Aphididae) on potato plants

The effect of dark-induced senescence on Solanum tuberosum L. (Solanales: Solanaceae) plants was assessed on the feeding behavior and performance of the green peach aphid, Myzus persicae Sulzer (Hemiptera: Aphididae). Senescence was induced by covering the basal part of the plant with a black cloth for 5 d, avoiding the light passage, but keeping the apical buds uncovered. The basal part of control plants was covered with a white nonwoven cloth. The degree of senescence was determined by measuring the chlorophyll content of the covered leaves. The performance and feeding behavior of M. persicae were studied on the uncovered nonsenescent apical leaves. The aphid's performance was evaluated by measuring nymphal mortality and prereproductive time. Aphid feeding behavior was monitored by the electrical penetration graph technique. In plants with dark-induced senescence, the aphids showed a reduction in their prereproductive time. Aphids also spent more time ingesting sap from the phloem than in control plants and performed more test probes after the first sustained ingestion of phloem sap. These data suggest that M. persicae's phloem activities and nymph development benefit from the nutritional enrichment of phloem sap, derived from dark-induced senescence on potato plants. The induced senescence improved plant acceptance by M. persicae through an increase in sap ingestion that likely resulted in a reduction in developmental time.

Mp10 and Mp42 from the aphid species Myzus persicae trigger plant defenses in Nicotiana benthamiana through different activities

Aphids are phloem-feeding insects that, like other plant parasites, deliver effectors inside their host to manipulate host responses. The Myzus persicae (green peach aphid) candidate effectors Mp10 and Mp42 were previously found to reduce aphid fecundity upon intracellular transient overexpression in Nicotiana benthamiana. We performed functional analyses of these proteins to investigate whether they activate defenses through similar activities. We employed a range of functional characterization experiments based on intracellular transient overexpression in N. benthamiana to determine the subcellular localization of Mp10 and Mp42 and investigate their role in activating plant defense signaling. Mp10 and Mp42 showed distinct subcellular localization in planta, suggesting that they target different host compartments. Also, Mp10 reduced the levels of Agrobacterium-mediated overexpression of proteins. This reduction was not due to an effect on Agrobacterium viability. Transient overexpression of Mp10 but not Mp42 activated jasmonic acid and salicylic acid signaling pathways and decreased susceptibility to the hemibiotrophic plant pathogen Phytophthora capsici. We found that two candidate effectors from the broad-host-range aphid M. persicae can trigger aphid defenses through different mechanisms. Importantly, we found that some (candidate) effectors such as Mp10 interfere with Agrobacterium-based overexpression assays, an important tool to study effector activity and function.

Plutella xylostella (L.) infestations at varying temperatures induce the emission of specific volatile blends by Arabidopsis thaliana (L.) Heynh

The effect of combined abiotic and biotic factors on plant volatile organic compound (VOC) emissions is poorly understood. This study evaluated the VOC emissions produced by Arabidopsis thaliana (L.) Col-0 subjected to 3 temperature regimes (17, 22, and 27°C) in the presence and absence of Plutella xylostella larvae over 2 time intervals (0-4 and 4-8 h), in comparison to control plants. The analyses of VOCs emitted by Arabidopsis plants were made by headspace solid phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS). It was found that certain volatile groups (e.g., alcohols, ketones, aldehydes, and terpenes) are induced by both single factors (temperature or larval infestation) and combined factors (temperature and larvae interactions), whereas other volatile groups (e.g., isothiocyanates [ITCs] and nitrile) were specific to the experimental conditions. ITCs (mainly 4-methylpentyl isothiocyanate) were emitted from plants subjected to larval infestation at 17 and 27°C after the 2 time intervals. The proportions of sulfides (mainly dimethyl disulfide) and 4-(methylthio) butanenitrile were significantly higher on herbivore-infested plants at 22°C compared to the other treatments. Overall, our findings indicate that changes in all experimental conditions caused significant changes to the VOC emissions of Arabidopsis plants. Therefore, the interaction between temperature and larval feeding may represent an important factor determining the variability of volatile emissions by plants subjected to multiple simultaneous factors.

Plant virus differentially alters the plant's defense response to its closely related vectors

BACKGROUND

The whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae), is one of the most widely distributed agricultural pests. In recent years, B. tabaci Q has invaded China, and Q has displaced B in many areas now. In a number of regions of the world, invasion by B and/or Q has been followed by outbreaks of tomato yellow leaf curl virus (TYLCV). Our previous study showed TYLCV directly and indirectly modified the feeding behavior of B. tabaci in favor of Q rather than B.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we quantified the salicylic acid (SA) titers and relative gene expression of SA in tomato leaves that were infested with viruliferous or non-viruliferous B and Q. We also measured the impacts of exogenous SA on the performance of B and Q, including the effects on ovary development. SA titer was always higher in leaves that were infested with viruliferous B than with viruliferous Q, whereas the SA titer did not differ between leaves infested with non-viruliferous B and Q. The relative gene expression of SA signaling was increased by feeding of viruliferous B but was not increased by feeding of viruliferous Q. The life history traits of B and Q were adversely affected on SA-treated plants. On SA-treated plants, both B and Q had lower fecundity, shorter longevity, longer developmental time and lower survival rate than on untreated plants. Compared with whiteflies feeding on control plants, those feeding on SA-treated plants had fewer oocytes and slower ovary development. On SA-treated plants, viruliferous B had fewer oocytes than viruliferous Q.

CONCLUSIONS/SIGNIFICANCE

These results indicate that TYLCV tends to induce SA-regulated plant defense against B but SA-regulated plant defense against Q was reduced. In other words, Q may have a mutualistic relationship with TYLCV that results in the reduction of the plant's defense response.

Activation of the Phenylpropanoid pathway in Nicotiana tabacum improves the performance of the whitefly Bemisia tabaci via reduced jasmonate signaling

BACKGROUND

Phloem-feeding insects can manipulate plant-induced resistance and are able to suppress effective jasmonic acid/ethylene (JA/ET) defenses by the induction of inefficient salicylic acid (SA) based responses. As a result, activation of the phenylpropanoid biosynthesis pathway in transgenic plants is anticipated to cause complex interactions between phloem-feeding insects and their host plants due to predicted contradiction between two defense forces: the toxicity of various phenylpropanoids and the accumulation of SA via a branch of the activated pathway.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we investigated the effect of activating the phenylpropanoids pathway in Nicotiana tabacum, by over-expression of the PAP1 transcription factor, on the whitefly Bemisia tabaci, a phloem-feeding insect model. Our performance assays indicated that the over-expression made the transgenic plants a more suitable host for B. tabaci than wild-type (WT) plants, although these plants accumulated significantly higher levels of flavonoids. Transcription analyses of indicator genes in the SA (PR1a) and JA/ET (ERF1, COI1 and AOC) pathways followed by quantification of the SA and JA hormone levels, indicated that B. tabaci infestation periods longer than 8 hours, caused higher levels of activity of SA signaling in transgenic plants and higher levels of JA/ET signaling in WT plants.

CONCLUSIONS/SIGNIFICANCE

Taken together, these results emphasize the important role JA/ET-induced defenses play in protecting plants from successful infestation by B. tabaci and likely other phloem-feeding insects. It also indicates the necessity of phloem feeders to suppress these defenses for efficient utilization of plant hosts. Our data also indicate that the defensive chemistry produced by the phenylpropanoids pathway has only a minor effect on the insect fitness.

Identification of genes involved in wild crucifer Rorippa indica resistance response on mustard aphid Lipaphis erysimi challenge

Mustard aphid, Lipaphis erysimi (L.) Kaltenbach is a perpetual annual threat in the cultivation of rapeseed- mustard (Brassica spp.) crop in tropical and sub-tropical climate. Cultivated Brassica germplasm has failed so far to provide any source of resistance. Wild germplasm is a potential source of resistance against many threatening herbivores. On wild germplasm screening, we noted that the wild crucifer Rorippa indica (L.) Hiern confers resistance against L. erysimi. In the present study L. erysimi challenged transcriptome of R. indica was compared to un-infested R. indica sample to get a molecular insight about the aphid resistance mechanism and identify the candidate defense response genes. Cloning, sequencing and in silico sequence analysis of complimentary DNA amplified fragment length polymorphism identified 116 differentially expressed transcript derived fragments revealed thirty candidates which are from different functional categories including redox regulation, signalling, photosynthesis, structure, metabolism, defense response as well as a few of unknown function. Twenty four identifications were then studied by quantitative real time RT PCR analysis at 6, 12, 24 and 48 hour time point post infestation to understand the early-to-late defense response through their relative gene expression profiles. Seventeen fragments showed significant up or down regulation at p<0.05 level. The response was influenced by different phytohormonal signalling pathways simultaneously. The candidate defense response expressed sequence tags specifically for the resistance genes identified in this study have implication in building desired mustard aphid resistance in susceptible rapeseed-mustard plants in future. This is the first molecular report on crucifer defense response against mustard aphid L. erysimi.

Inoculation of tomato plants with rhizobacteria enhances the performance of the phloem-feeding insect Bemisia tabaci

In their natural environment, plants experience multiple biotic interactions and respond to this complexity in an integrated manner. Therefore, plant responses to herbivory are flexible and depend on the context and complexity in which they occur. For example, plant growth promoting rhizobacteria (PGPR) can enhance plant growth and induce resistance against microbial pathogens and herbivorous insects by a phenomenon termed induced systemic resistance (ISR). In the present study, we investigated the effect of tomato (Solanum lycopersicum) pre-inoculation with the PGPR Pseudomonas fluorescens WCS417r, on the performance of the generalist phloem-feeding insect Bemisia tabaci. Based on the ability of P. fluorescens WCS417r to prime for ISR against generalists chewing insects and necrotrophic pathogens, we hypothesized that pre-inoculated plants will strongly resist B. tabaci infestation. In contrast, we discovered that the pre-inoculation treatment increased the tomato plant suitability for B. tabaci which was emphasized both by faster developmental rate and higher survivability of nymph stages on pre-inoculated plants. Our molecular and chemical analyses suggested that the phenomenon is likely to be related to: (I) the ability of the bacteria to reduce the activity of the plant induced defense systems; (II) a possible manipulation by P. fluorescens of the plant quality (in terms of suitability for B. tabaci) through an indirect effect on the rhizosphere bacterial community. The contribution of our study to the pattern proposed for other belowground rhizobacteria and mycorrhizal fungi and aboveground generalist phloem-feeders is discussed.

Source-to-sink transport of sugar and regulation by environmental factors

Source-to-sink transport of sugar is one of the major determinants of plant growth and relies on the efficient and controlled distribution of sucrose (and some other sugars such as raffinose and polyols) across plant organs through the phloem. However, sugar transport through the phloem can be affected by many environmental factors that alter source/sink relationships. In this paper, we summarize current knowledge about the phloem transport mechanisms and review the effects of several abiotic (water and salt stress, mineral deficiency, CO2, light, temperature, air, and soil pollutants) and biotic (mutualistic and pathogenic microbes, viruses, aphids, and parasitic plants) factors. Concerning abiotic constraints, alteration of the distribution of sugar among sinks is often reported, with some sinks as roots favored in case of mineral deficiency. Many of these constraints impair the transport function of the phloem but the exact mechanisms are far from being completely known. Phloem integrity can be disrupted (e.g., by callose deposition) and under certain conditions, phloem transport is affected, earlier than photosynthesis. Photosynthesis inhibition could result from the increase in sugar concentration due to phloem transport decrease. Biotic interactions (aphids, fungi, viruses…) also affect crop plant productivity. Recent breakthroughs have identified some of the sugar transporters involved in these interactions on the host and pathogen sides. The different data are discussed in relation to the phloem transport pathways. When possible, the link with current knowledge on the pathways at the molecular level will be highlighted.

Vitamin C and the abscisic acid-insensitive 4 transcription factor are important determinants of aphid resistance in Arabidopsis

AIMS

Aphids, like other insects, are probably unable to synthesize vitamin C (ascorbic acid), which is therefore an essential dietary nutrient that has to be obtained from the host plant. Plant responses to aphids involve hormones such as salicylic acid (SA), jasmonic acid (JA), and abscisic acid (ABA), but hormone/redox interactions remain poorly characterized. We therefore investigated hormone/redox signaling in the response of Arabidopsis thaliana to infestation by the aphid Myzus persicae, focusing on the interactions between ascorbic acid and ABA, together with the influence of altered ascorbate and ABA signaling on the SA- and JA-dependent pathways.

RESULTS

Whole-genome microarray analysis revealed highly dynamic transcriptional responses to aphid infestation with extensive differences between transcript profiles of infested and systemic leaves, revealing aphid-dependent effects on the suites of transcripts involved in the redox, SA, and ABA responses. Central roles for ascorbate, ABA-insensitive 4 (ABI4), and oxidative signal-inducible 1 in plant resistance to aphids were demonstrated by altered fecundity on respective mutants. However, ABA had a negative effect on aphid resistance, as did ABI4 or redox-responsive transcription factor 1. The decrease in aphid fecundity observed in mutants defective in ascorbate accumulation (vtc2) was absent from abi4vtc2 double mutants that are also deficient in ABA signaling (abi4). Aphid-dependent transcriptome responses reveal a role for ascorbate-regulated receptor-like kinases in plant defenses against aphids.

INNOVATION

Vitamin C deficiency enhances plant resistance to aphids through redox signaling pathways rather than dietary requirements.

CONCLUSION

ABI4 is a linchpin of redox regulation of the innate immune response to aphids.

Comparative transcriptome analysis of Gossypium hirsutum L. in response to sap sucking insects: aphid and whitefly

Background

Cotton (Gossypium hirsutum L.) is a major fiber crop that is grown worldwide; it faces extensive damage from sap-sucking insects, including aphids and whiteflies. Genome-wide transcriptome analysis was performed to understand the molecular details of interaction between Gossypium hirsutum L. and sap-sucking pests, namely Aphis gossypii (Aphid) and Bemisia tabacci (Whiteflies). Roche’s GS-Titanium was used to sequence transcriptomes of cotton infested with aphids and whiteflies for 2 h and 24 h.

Results

A total of 100935 contigs were produced with an average length of 529 bp after an assembly in all five selected conditions. The Blastn of the non-redundant (nr) cotton EST database resulted in the identification of 580 novel contigs in the cotton plant. It should be noted that in spite of minimal physical damage caused by the sap-sucking insects, they can change the gene expression of plants in 2 h of infestation; further change in gene expression due to whiteflies is quicker than due to aphids. The impact of the whitefly 24 h after infestation was more or less similar to that of the aphid 2 h after infestation. Aphids and whiteflies affect many genes that are regulated by various phytohormones and in response to microbial infection, indicating the involvement of complex crosstalk between these pathways. The KOBAS analysis of differentially regulated transcripts in response to aphids and whiteflies indicated that both the insects induce the metabolism of amino acids biosynthesis specially in case of whiteflies infestation at later phase. Further we also observed that expression of transcript related to photosynthesis specially carbon fixation were significantly influenced by infestation of Aphids and Whiteflies.

Conclusions

A comparison of different transcriptomes leads to the identification of differentially and temporally regulated transcripts in response to infestation by aphids and whiteflies. Most of these differentially expressed contigs were related to genes involved in biotic, abiotic stresses and enzymatic activities related to hydrolases, transferases, and kinases. The expression of some marker genes such as the overexpressors of cationic peroxidase 3, lipoxygenase I, TGA2, and non-specific lipase, which are involved in phytohormonal-mediated plant resistance development, was suppressed after infestation by aphids and whiteflies, indicating that insects suppressed plant resistance in order to facilitate their infestation. We also concluded that cotton shares several pathways such as phagosomes, RNA transport, and amino acid metabolism with Arabidopsis in response to the infestation by aphids and whiteflies.

Comparative analysis of Solanum stoloniferum responses to probing by the green peach aphid Myzus persicae and the potato aphid Macrosiphum euphorbiae

Plants protect themselves against aphid attacks by species-specific defense mechanisms. Previously, we have shown that Solanum stoloniferum Schlechtd has resistance factors to Myzus persicae Sulzer (Homoptera: Aphididae) at the epidermal/mesophyll level that are not effective against Macrosiphum euphorbiae Thomas (Homoptera: Aphididae). Here, we compare the nymphal mortality, the pre-reproductive development time, and the probing behavior of M. persicae and M. euphorbiae on S. stoloniferum and Solanum tuberosum L. Furthermore, we analyze the changes in gene expression in S. stoloniferum 96 hours post infestation by either aphid species. Although the M. euphorbiae probing behavior shows that aphids encounter more probing constrains on phloem activities-longer probing and salivation time- on S. stoloniferum than on S. tuberosum, the aphids succeeded in reaching a sustained ingestion of phloem sap on both plants. Probing by M. persicae on S. stoloniferum plants resulted in limited feeding only. Survival of M. euphorbiae and M. persicae was affected on young leaves, but not on senescent leaves of S. stoloniferum. Infestation by M. euphorbiae changed the expression of more genes than M. persicae did. At the systemic level both aphids elicited a weak response. Infestation of S. stoloniferum plants with a large number of M. persicae induced morphological changes in the leaves, leading to the development of pustules that were caused by disrupted vascular parenchyma and surrounding tissue. In contrast, an infestation by M. euphorbiae had no morphological effects. Both plant species can be regarded as good host for M. euphorbiae, whereas only S. tuberosum is a good host for M. persicae and S. stoloniferum is not. Infestation of S. stoloniferum by M. persicae or M. euphorbiae changed the expression of a set of plant genes specific for each of the aphids as well as a set of common genes.

Long distance root-shoot signalling in plant-insect community interactions

Plants mediate interactions between insects, including leaf- and root-feeders; yet the underlying mechanisms and connection with ecological theory remain unresolved. In this review, based on novel insights into long-distance (i.e., leaf-leaf, root-shoot) defence signalling, we explore the role of phytohormones in driving broad-scale patterns of aboveground-belowground interactions that can be extrapolated to general plant-insect relationships. We propose that the outcome of intra-feeding guild interactions is generally negative due to induction of similar phytohormonal pathways, whereas between-guild interactions are often positive due to negative signal crosstalk. However, not all outcomes could be explained by feeding guild; we argue that future studies should target ecologically representative plant-insect systems, distinguish subguilds, and include plant growth hormones to improve our understanding of plant-mediated interactions.