A single gene, AIN, in Medicago truncatula mediates a hypersensitive response to both bluegreen aphid and pea aphid, but confers resistance only to bluegreen aphid

A butterfly egg-killing hypersensitive response in Brassica nigra is controlled by a single locus, PEK, containing a cluster of TIR-NBS-LRR receptor genes

Knowledge of plant recognition of insects is largely limited to a few resistance (R) genes against sap-sucking insects. Hypersensitive response (HR) characterizes monogenic plant traits relying on R genes in several pathosystems. HR-like cell death can be triggered by eggs of cabbage white butterflies (Pieris spp.), pests of cabbage crops (Brassica spp.), reducing egg survival and representing an effective plant resistance trait before feeding damage occurs. Here, we performed genetic mapping of HR-like cell death induced by Pieris brassicae eggs in the black mustard Brassica nigra (B. nigra). We show that HR-like cell death segregates as a Mendelian trait and identified a single dominant locus on chromosome B3, named PEK (Pieris  egg- killing). Eleven genes are located in an approximately 50 kb region, including a cluster of genes encoding intracellular TIR-NBS-LRR (TNL) receptor proteins. The PEK locus is highly polymorphic between the parental accessions of our mapping populations and among B. nigra reference genomes. Our study is the first one to identify a single locus potentially involved in HR-like cell death induced by insect eggs in B. nigra. Further fine-mapping, comparative genomics and validation of the PEK locus will shed light on the role of these TNL receptors in egg-killing HR.

Genomic architecture of resistance to latania scale (H. lataniae) in kiwifruit (A. chinensis var. chinensis)

BACKGROUND

Latania scale (Hemiberlesia lataniae Signoret) is an armoured scale insect known to cause damage to kiwifruit plants and fruit, which ultimately reduces crop values and creates post-harvest export and quarantine issues. Resistance to H. lataniae does exist in some commercial cultivars of kiwifruit. However, some of the commercial cultivars bred in New Zealand have not inherited alleles for resistance to H. lataniae carried by their parents. To elucidate the architecture of resistance in the parents and develop molecular markers to assist breeding, these experiments analysed the inheritance of resistance to H. lataniae from families related to commercial cultivars.

RESULTS

The first experiment identified a 15.97 Mb genomic region of interest for resistance to H. lataniae in rtGBS data of 3.23 to 19.20 Mb on chromosome 10. A larger population was then QTL mapped, which confirmed the region of interest as the sole locus contributing to H. lataniae resistance. inDel markers mapping the region of low recombination under the QTL peak further narrowed the region associated with H. lataniae resistance to a 5.73 Mb region.

CONCLUSIONS

The kiwifruit populations and genomic methods used in this study identify the same non-recombinant region of chromosome 10 which confers resistance of A. chinensis var. chinensis to H. lataniae. The markers developed to target the H. lataniae resistance loci will reduce the amount of costly and time-consuming phenotyping required for breeding H. lataniae scale resistance into new kiwifruit cultivars.

Genome wide identification and evolutionary analysis of vat like NBS-LRR genes potentially associated with resistance to aphids in cotton

Nucleotide Binding Site - Leucine Rich Repeat (NBS-LRR) genes play a significant role in plant defense against biotic stresses and are an integral part of signal transduction pathways. Vat gene has been well reported for their role in resistance to Aphis gossypii and viruses transmitted by them. Despite their importance, Vat like NBS-LRR resistance genes have not yet been identified and studied in cotton species. This study report hundreds of orthologous Vat like NBS-LRR genes from the genomes of 18 cotton species through homology searches and the distribution of those identified genes were tend to be clustered on different chromosome. Especially, in a majority of the cases, Vat like genes were located on chromosome number 13 and they all shared two conserved NBS-LRR domains, one disease resistant domain and several repeats of LRR on the investigated cotton Vat like proteins. Gene ontology study on Vat like NBS-LRR genes revealed the molecular functions viz., ADP and protein binding. Phylogenetic analysis also revealed that Vat like sequences of two diploid species, viz., G. arboreum and G. anomalum, were closely related to the sequences of the tetraploids than all other diploids. The Vat like genes of G. aridum and G. schwendimanii were distantly related among diploids and tetraploids species. Various hormones and defense related cis-acting regulatory elements were identified from the 2 kb upstream sequences of the Vat like genes implying their defensive response towards the biotic stresses. Interestingly, G. arboreum and G. trilobum were found to have more regulatory elements than larger genomes of tetraploid cotton species. Thus, the present study provides the evidence for the evolution of Vat like genes in defense mechanisms against aphids infestation in cotton genomes and allows further characterization of candidate genes for developing aphid and aphid transmitted viruses resistant crops through cotton breeding.

Research Progress and Prospect of Alfalfa Resistance to Pathogens and Pests

Alfalfa is one of the most important legume forages in the world and contributes greatly to the improvement of ecosystems, nutrition, and food security. Diseases caused by pathogens and pests severely restrict the production of alfalfa. Breeding resistant varieties is the most economical and effective strategy for the control of alfalfa diseases and pests, and the key to breeding resistant varieties is to identify important resistance genes. Plant innate immunity is the theoretical basis for identifying resistant genes and breeding resistant varieties. In recent years, the framework of plant immunity theory has been gradually formed and improved, and considerable progress has been made in the identification of alfalfa resistance genes and the revelation of the related mechanisms. In this review, we summarize the basic theory of plant immunity and identify alfalfa resistance genes to different pathogens and insects and resistance mechanisms. The current situation, problems, and future prospects of alfalfa resistance research are also discussed. Breeding resistant cultivars with effective resistance genes, together with other novel plant protection technologies, will greatly improve alfalfa production.

Identification and Characterization of Resistance of Three Aphid Species on Contrasting Alfalfa Cultivars

Simple Summary

Aphis craccivora Koch (cowpea aphid, CPA), Acyrthosiphon pisum Harris (pea aphid, PA) and Therioaphis trifolii Buckton (spotted alfalfa aphid, SAA) are the three species of devastating pests on alfalfa in China. A study was conducted in the laboratory for identification and characterization of resistance to these three aphids among 16 of the main alfalfa cultivars planted in China. Resistance was indicated by antibiosis, antixenosis, and measuring feeding behavior using EPG (electrical penetration graph). The results indicated that different alfalfa cultivars have significantly different resistance levels to a particular species of aphid, and the same alfalfa variety also has different resistance to the three aphid species. Specifically, we evaluated the resistance of different alfalfa cultivars to CPA, which can help us for further study on the defense mechanism against CPA and for better management of this pest.

Abstract

Aphids on alfalfa (Medicago sativa) including Aphis craccivora Koch (cowpea aphid, CPA), Acyrthosiphon pisum Harris (pea aphid, PA) and Therioaphis trifolii Buckton (spotted alfalfa aphid, SAA) cause significant yield losses worldwide. In this experiment, the development of these three species of aphids on 16 alfalfa cultivars was compared. The results showed that the plant cultivar had a significant influence on the development of aphids as there are significant differences in the body weight of aphids reared on different alfalfa cultivars. In addition, antibiosis between the alfalfa cultivars Pegasis and Gannong NO.9 and the three species of aphids was evaluated by measuring aphid body weight and fecundity. Antixenosis was measured using choice tests, and feeding behavior was quantified using electrical penetration graphs (EPG). The Pegasis cultivar was observed to have both antibiosis and antixenosis effects with CPA, but was susceptible to PA and SAA compared with the Gannong NO.9 cultivar. CPA had less mean body weight, less fecundity, and shorter feeding time on the Pegasis cultivar, and preferred to settle on Gannong NO.9 cultivar. In contrast, Gannong NO.9 exhibits antibiosis and antixenosis to PA and SAA compared with Pegasis, as shown by lower body weight, lower fecundity and chose to settle less often, but EPG data showed that PA and SAA showed no different significance in feeding behavior between Pegasis and Gannong NO.9.

Phloem: At the center of action in plant defense against aphids

The location of the phloem deep inside the plant, the high hydrostatic pressure in the phloem, and the composition of phloem sap, which is rich in sugar with a high C:N ratio, allows phloem sap feeding insects to occupy a unique ecological niche. The anatomy and physiology of aphids, a large group of phytophagous insects that use their mouthparts, which are modified into stylets, to consume large amounts of phloem sap, has allowed aphids to successfully exploit this niche, however, to the detriment of agriculture and horticulture. The ability to reproduce asexually, a short generation time, the development of resistance to commonly used insecticides, and their ability to vector viral diseases makes aphids among the most damaging pests of plants. Here we review how plants utilize their ability to occlude sieve elements and accumulate antibiotic and antinutritive factors in the phloem sap to limit aphid infestation. In addition, we summarize progress on understanding how plants perceive aphids to activate defenses in the phloem.

Investigation of Metabolic Resistance to Soybean Aphid (Aphis glycines Matsumura) Feeding in Soybean Cultivars

Simple Summary

This project examined the interaction between soybean aphids and Ontario-grown soybean cultivars to determine which leaf metabolites were most associated with aphid resistance. Tolerance and resistance were determined by measuring the growth and reproduction of aphids and leaf feeding damage over 10-day and 4-week infestation periods. Chromatographic techniques were used for the analysis of legume-specific plant natural products and isoflavonoids, and high-resolution mass spectrometry was used for the identification of free amino acids in aphid-resistant, tolerant, and susceptible soybean cultivars. There was a low correlation between isoflavonoid leaf concentrations and aphid resistance in the soybean varieties studied; however, the aphid-resistant cultivars were determined to have lower free amino acid concentrations, indicating that lower nutrient quality could be responsible for the resistance observed. Identifying these cultivars is important for managing aphid populations and provides an additional tool for soybean integrated pest management.

Abstract

Soybean aphid (Aphis glycines) is a major soybean (Glycine max) herbivore pest in many soybean growing regions. High numbers of aphids on soybean can cause severe reductions in yield. The management of soybean aphids includes monitoring, insecticide applications when required, and the use of resistant cultivars. Soybean aphid-resistant soybean varieties are associated with genes that confer one or more categories of resistance to soybean aphids, including antibiosis (affects survival, growth, and fecundity), antixenosis (affects behaviour such as feeding), and tolerance (plant can withstand greater damage without economic loss). The genetic resistance of soybean to several herbivores has been associated with isoflavonoid phytoalexins; however, this correlation has not been observed in soybean varieties commonly grown in southern Ontario, Canada. Isoflavonoids in the leaves of 18 cultivars in the early growth stage were analyzed by HPLC and the concentration by fresh weight was used to rate the potential resistance to aphids. Greenhouse and growth cabinet trials determined that the cultivars with greater resistance to aphids were Harosoy 63 and OAC Avatar. The most susceptible cultivar was Maple Arrow, whereas Pagoda and Conrad were more tolerant to aphid feeding damage. Overall, there was a low correlation between the number of aphids per leaf, feeding damage, and leaf isoflavonoid levels. Metabolite profiling by high-resolution LC-MS determined that the most resistant cultivar had on average lower levels of certain free amino acids (Met, Tyr, and His) relative to the most susceptible cultivar. This suggests that within the tested cultivars, nutritional quality stimulates aphid feeding more than isoflavonoids negatively affect aphid feeding or growth. These findings provide a better understanding of soybean host plant resistance and suggest ways to improve soybean resistance to aphid feeding through the breeding or metabolic engineering of leaf metabolites.

A functional genomics approach to dissect spotted alfalfa aphid resistance in Medicago truncatula

Aphids are virus-spreading insect pests affecting crops worldwide and their fast population build-up and insecticide resistance make them problematic to control. Here, we aim to understand the molecular basis of spotted alfalfa aphid (SAA) or Therioaphis trifolii f. maculata resistance in Medicago truncatula, a model organism for legume species. We compared susceptible and resistant near isogenic Medicago lines upon SAA feeding via transcriptome sequencing. Expression of genes involved in defense and stress responses, protein kinase activity and DNA binding were enriched in the resistant line. Potentially underlying some of these changes in gene expression was the finding that members of the MYB, NAC, AP2 domain and ERF transcription factor gene families were differentially expressed in the resistant versus susceptible lines. A TILLING population created in the resistant cultivar was screened using exome capture sequencing and served as a reverse genetics tool to functionally characterise genes involved in the aphid resistance response. This screening revealed three transcription factors (a NAC, AP2 domain and ERF) as important regulators in the defence response, as a premature stop-codon in the resistant background led to a delay in aphid mortality and enhanced plant susceptibility. This combined functional genomics approach will facilitate the future development of pest resistant crops by uncovering candidate target genes that can convey enhanced aphid resistance.

A Genome-Wide Association Study for Resistance to the Insect Pest Leptocybe invasa in Eucalyptus grandis Reveals Genomic Regions and Positional Candidate Defense Genes

The galling insect, Leptocybe invasa, causes significant losses in plantations of various Eucalyptus species and hybrids, threatening its economic viability. We applied a genome-wide association study (GWAS) to identify single-nucleotide polymorphism (SNP) markers associated with resistance to L. invasa. A total of 563 insect-challenged Eucalyptus grandis trees, from 61 half-sib families, were genotyped using the EUChip60K SNP chip, and we identified 15,445 informative SNP markers in the test population. Multi-locus mixed-model (MLMM) analysis identified 35 SNP markers putatively associated with resistance to L. invasa based on four discreet classes of insect damage scores: (0) not infested, (1) infested showing evidence of oviposition but no gall development, (2) infested with galls on leaves, midribs or petioles and (3) stunting and lethal gall formation. MLMM analysis identified three associated genomic regions on chromosomes 3, 7 and 8 jointly explaining 17.6% of the total phenotypic variation. SNP analysis of a validation population of 494 E. grandis trees confirmed seven SNP markers that were also detected in the initial association analysis. Based on transcriptome profiles of resistant and susceptible genotypes from an independent experiment, we identified several putative candidate genes in associated genomic loci including Nucleotide-binding ARC- domain (NB-ARC) and toll-interleukin-1-receptor-Nucleotide binding signal- Leucine rich repeat (TIR-NBS-LRR) genes. Our results suggest that Leptocybe resistance in E. grandis may be influenced by a few large-effect loci in combination with minor effect loci segregating in our test and validation populations.

Ethylene Is Not Essential for R-Gene Mediated Resistance but Negatively Regulates Moderate Resistance to Some Aphids in Medicago truncatula

Ethylene is important for plant responses to environmental factors. However, little is known about its role in aphid resistance. Several types of genetic resistance against multiple aphid species, including both moderate and strong resistance mediated by R genes, have been identified in Medicago truncatula. To investigate the potential role of ethylene, a M. truncatula ethylene- insensitive mutant, sickle, was analysed. The sickle mutant occurs in the accession A17 that has moderate resistance to Acyrthosiphon kondoi, A. pisum and Therioaphis trifolii. The sickle mutant resulted in increased antibiosis-mediated resistance against A. kondoi and T. trifolii but had no effect on A. pisum. When sickle was introduced into a genetic background carrying resistance genes, AKR (A. kondoi resistance), APR (A. pisum resistance) and TTR (T. trifolii resistance), it had no effect on the strong aphid resistance mediated by these genes, suggesting that ethylene signaling is not essential for their function. Interestingly, for the moderate aphid resistant accession, the sickle mutant delayed leaf senescence following aphid infestation and reduced the plant biomass losses caused by both A. kondoi and T. trifolii. These results suggest manipulation of the ethylene signaling pathway could provide aphid resistance and enhance plant tolerance against aphid feeding.

Aphid growth and reproduction on plants with altered sterol profiles: Novel insights using Arabidopsis mutant and overexpression lines

Sterols are essential membrane components and are critical for many physiological processes in all eukaryotes. Insects and other arthropods are sterol auxotrophs that typically rely on a dietary source of sterols. Herbivorous insects generally obtain sterols from plants and then metabolize them into cholesterol, the dominant sterol in most insects. However, there is significant variation in phytosterol structure, and not all phytosterols are equally suitable for insects. In the current study, we used seven Arabidopsis thaliana lines that display altered sterol profiles due to mutations in the sterol biosynthetic pathway or to overexpression of key enzymes of the pathway, and investigated how plant sterol profiles affected green peach aphid (Myzus persicae) growth and reproduction. We also characterized the sterol profile of aphids reared on these Arabidopsis genotypes. Aphids on two mutant lines (14R/fk and ste1-1) that accumulated biosynthetic sterol intermediates (Δ^8,14-sterols, and Δ⁷-sterols, respectively) all showed significantly reduced growth and reproduction. Aphids on SMT2^COSUP plants (which have decreased β-sitosterol but increased campesterol) also displayed significantly reduced growth and reproduction. However, aphids on SMT2^OE plants (which have increased β-sitosterol but decreased campesterol) performed similarly to aphids on wild-type plants. Finally, Arabidopsis plants that had an overproduction of sterols (CD-HMGR^OE) or decreased sterol esters (psat1-2) had no impact on aphid performance. Two noteworthy results come from the aphid sterol profile study. First, β-sitosterol, cholesterol and stigmasterol were recovered in all aphids. Second, we did not detect Δ^8,14-sterols in aphids reared on 14R/fk plants. We discuss the implications of our findings, including how aphid sterol content does not appear to reflect plant leaf sterol profiles. We also discuss the potential of modifying plant sterol profiles to control insect herbivore pests, including aphids.

An RNAi supplemented diet as a reverse genetics tool to control bluegreen aphid, a major pest of legumes

Aphids are important agricultural pests causing major yield losses worldwide. Since aphids can rapidly develop resistance to chemical insecticides there is an urgent need to find alternative aphid pest management strategies. Despite the economic importance of bluegreen aphid (Acyrthosiphon kondoi), very few genetic resources are available to expand our current understanding and help find viable control solutions. An artificial diet is a desirable non-invasive tool to enable the functional characterisation of genes in bluegreen aphid and discover candidate target genes for future use in RNA interference (RNAi) mediated crop protection against aphids. To date no artificial diet has been developed for bluegreen aphid, so we set out to develop a suitable diet by testing and optimising existing diets. Here, we describe an artificial diet for rearing bluegreen aphid and also provide a proof of concept for the supplementation of the diet with RNAi molecules targeting the salivary gland transcript C002 and gap gene hunchback, resulting in bluegreen aphid mortality which has not yet been documented in this species. Managing this pest, for example via RNAi delivery through artificial feeding will be a major improvement to test bluegreen aphid candidate target genes for future pest control and gain significant insights into bluegreen aphid gene function.

Additive and epistatic interactions between AKR and AIN loci conferring bluegreen aphid resistance and hypersensitivity in Medicago truncatula

Aphids, including the bluegreen aphid (BGA; Acyrthosiphon kondoi), are important pests in agriculture. Two BGA resistance genes have been identified in the model legume Medicago truncatula, namely AKR (Acyrthosiphon kondoi resistance) and AIN (Acyrthosiphon induced necrosis). In this study, progeny derived from a cross between a resistant accession named Jester and a highly susceptible accession named A20 were used to study the interaction between the AKR and AIN loci with respect to BGA performance and plant response to BGA infestation. These studies demonstrated that AKR and AIN have additive effects on the BGA resistance phenotype. However, AKR exerts dominant suppression epistasis on AIN-controlled macroscopic necrotic lesions. Nevertheless, both AKR and AIN condition production of H2O2 at the BGA feeding site. Electrical penetration graph analysis demonstrated that AKR prevents phloem sap ingestion, irrespective of the presence of AIN. Similarly, the jasmonic acid defense signaling pathway is recruited by AKR, irrespective of AIN. This research identifies an enhancement of aphid resistance through gene stacking, and insights into the interaction of distinct resistance genes against insect pests.

Influence of Abscisic Acid-Biosynthesis Inhibitor Fluridone on the Feeding Behavior and Fecundity of Nilaparvata lugens

Fluridone (FLU) was a pyrrolidone herbicide that was used for selective weeding in wheat, rice, corn and pasture and was also a biosynthesis inhibitor of abscisic acid (ABA), a significant plant hormone. ABA-promoted callose deposition facilitates rice resistance to pests but whether FLU had the opposite influence was unknown. The effects of FLU on the feeding behavior of the brown planthopper (Nilaparvata lugens Stål; BPH), after feeding with rice plants treated with FLU, were studied, using an electrical penetration graph (EPG). For susceptible rice cultivar (TN1), the duration for which BPH sucked phloem sap (N4 wave duration) after 15 μmol/L of FLU treatment was longer than that of the control but decreased after 30 and 60 μmol/L FLU treatments. Fecundity of BPH treated with 15 μmol/L FLU had no significant change, while the deposition area of callose was significantly decreased. For moderately-resistant rice cultivar (IR42), no differences in BPH feeding behavior and fecundity were observed but the deposition area of callose declined after treated with 15 μmol/L of FLU. These findings suggested that a low concentration of FLU (15 μmol/L) promoted BPH feeding behavior in TN1 but not in IR42 and the response in IR42 appeared to be more complicated, which provided supplementary evidence that ABA promoted plant resistance to BPH.

Plant-arthropod interactions: who is the winner?

Herbivorous arthropods have interacted with plants for millions of years. During feeding they release chemical cues that allow plants to detect the attack and mount an efficient defense response. A signaling cascade triggers the expression of hundreds of genes, which encode defensive proteins and enzymes for synthesis of toxic metabolites. This direct defense is often complemented by emission of volatiles that attract beneficial parasitoids. In return, arthropods have evolved strategies to interfere with plant defenses, either by producing effectors to inhibit detection and downstream signaling steps, or by adapting to their detrimental effect. In this review, we address the current knowledge on the molecular and chemical dialog between plants and herbivores, with an emphasis on co-evolutionary aspects.

Medicago truncatula: Genetic and Genomic Resources

Medicago truncatula was chosen by the legume community, along with Lotus japonicus, as a model plant to study legume biology. Since then, numerous resources and tools have been developed for M. truncatula. These include, for example, its genome sequence, core ecotype collections, transformation/regeneration methods, extensive mutant collections, and a gene expression atlas. This review aims to describe the different genetic and genomic tools and resources currently available for M. truncatula. We also describe how these resources were generated and provide all the information necessary to access these resources and use them from a practical point of view. © 2017 by John Wiley & Sons, Inc.

How plants handle multiple stresses: hormonal interactions underlying responses to abiotic stress and insect herbivory

Adaptive plant responses to specific abiotic stresses or biotic agents are fine-tuned by a network of hormonal signaling cascades, including abscisic acid (ABA), ethylene, jasmonic acid (JA) and salicylic acid. Moreover, hormonal cross-talk modulates plant responses to abiotic stresses and defenses against insect herbivores when they occur simultaneously. How such interactions affect plant responses under multiple stresses, however, is less understood, even though this may frequently occur in natural environments. Here, we review our current knowledge on how hormonal signaling regulates abiotic stress responses and defenses against insects, and discuss the few recent studies that attempted to dissect hormonal interactions occurring under simultaneous abiotic stress and herbivory. Based on this we hypothesize that drought stress enhances insect resistance due to synergistic interactions between JA and ABA signaling. Responses to flooding or waterlogging involve ethylene signaling, which likely reduces plant resistance to chewing herbivores due to its negative cross-talk with JA. However, the outcome of interactions between biotic and abiotic stress signaling is often plant and/or insect species-dependent and cannot simply be predicted based on general knowledge on the involvement of signaling pathways in single stress responses. More experimental data on non-model plant and insect species are needed to reveal general patterns and better understand the molecular mechanisms allowing plants to optimize their responses in complex environments.

Genetic Mapping of a Major Resistance Gene to Pea Aphid (Acyrthosipon pisum) in the Model Legume Medicago truncatula

Resistance to the Australian pea aphid (PA; Acyrthosiphon pisum) biotype in cultivar Jester of the model legume Medicago truncatula is mediated by a single dominant gene and is phloem-mediated. The genetic map position for this resistance gene, APR (Acyrthosiphon pisum resistance), is provided and shows that APR maps 39 centiMorgans (cM) distal of the A. kondoi resistance (AKR) locus, which mediates resistance to a closely related species of the same genus bluegreen aphid (A. kondoi). The APR region on chromosome 3 is dense in classical nucleotide binding site leucine-rich repeats (NLRs) and overlaps with the region harbouring the RAP1 gene which confers resistance to a European PA biotype in the accession Jemalong A17. Further screening of a core collection of M. truncatula accessions identified seven lines with strong resistance to PA. Allelism experiments showed that the single dominant resistance to PA in M. truncatula accessions SA10481 and SA1516 are allelic to SA10733, the donor of the APR locus in cultivar Jester. While it remains unclear whether there are multiple PA resistance genes in an R-gene cluster or the resistance loci identified in the other M. truncatula accessions are allelic to APR, the introgression of APR into current M. truncatula cultivars will provide more durable resistance to PA.

QTLs for Woolly Poplar Aphid (Phloeomyzus passerinii L.) Resistance Detected in an Inter-Specific Populus deltoides x P. nigra Mapping Population

The genus Populus represents one of the most economically important groups of forest trees. It is composed by approximately 30 species used for wood and non-wood products, phytoremediation and biomass. Poplar is subjected to several biological and environmental threats although, compared to annual crops, we know far less about the genetic bases of biotic stress resistance. Woolly poplar aphid (Phloeomyzus passerinii) is considered a main pest of cultivated poplars in European and American countries. In this work we present two high density linkage maps in poplar obtained by a genotyping by sequencing (GBS) approach and the identification of QTLs involved in Ph. passerinii resistance. A total of 5,667 polymorphic markers (5,606 SNPs and 61 SSRs) identified on expressed sequences have been used to genotype 131 plants of an F1 population P ×canadensis obtained by an interspecific mate between Populus deltoides (resistant to woolly poplar aphid) and Populus nigra (susceptible to woolly poplar aphid). The two linkage maps, obtained following the two-way pseudo-testcross mapping strategy, have been used to investigate the genetic bases of woolly poplar aphid resistance. One major QTL and two QTLs with minor effects (mapped on LGV, LGXVI and LG XIX) explaining the 65.8% of the genetic variance observed in the progeny in response to Ph. passerinii attack were found. The high density coverage of functional markers allowed the identification of three genes belonging to disease resistance pathway as putative candidates for P. deltoides resistance to woolly poplar aphid. This work is the first report on genetic of woolly poplar aphid genetic resistance and the resistant loci associated markers identified represent a valuable tool in resistance poplar breeding programs.

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

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

Spider mites suppress tomato defenses downstream of jasmonate and salicylate independently of hormonal crosstalk

Plants respond to herbivory by mounting a defense. Some plant-eating spider mites (Tetranychus spp.) have adapted to plant defenses to maintain a high reproductive performance. From natural populations we selected three spider mite strains from two species, Tetranychus urticae and Tetranychus evansi, that can suppress plant defenses, using a fourth defense-inducing strain as a benchmark, to assess to which extent these strains suppress defenses differently. We characterized timing and magnitude of phytohormone accumulation and defense-gene expression, and determined if mites that cannot suppress defenses benefit from sharing a leaf with suppressors. The nonsuppressor strain induced a mixture of jasmonate- (JA) and salicylate (SA)-dependent defenses. Induced defense genes separated into three groups: 'early' (expression peak at 1 d postinfestation (dpi)); 'intermediate' (4 dpi); and 'late', whose expression increased until the leaf died. The T. evansi strains suppressed genes from all three groups, but the T. urticae strain only suppressed the late ones. Suppression occurred downstream of JA and SA accumulation, independently of the JA-SA antagonism, and was powerful enough to boost the reproductive performance of nonsuppressors up to 45%. Our results show that suppressing defenses not only brings benefits but, within herbivore communities, can also generate a considerable ecological cost when promoting the population growth of a competitor.

Enhanced aphid detoxification when confronted by a host with elevated ROS production

Reactive oxygen species (ROS) plays an important role in plant defense responses against bacteria, fungi and insect pests. Most recently, we have demonstrated that loss of Arabidopsis thaliana BOTRYTIS-INDUCED KINASE1 (BIK1) function releases its suppression of aphid-induced H2O2 production and cell death, rendering the bik1 mutant more resistant to green peach aphid (Myzus persicae) than wild-type plants. However, little is known regarding how ROS-related gene expression is correlated with bik1-mediated resistance to aphids, or whether these aphids biochemically respond to the oxidative stress. Here, we show that the bik1 mutant exhibited elevated basal expression of ROS-generating and -responsive genes, but not ROS-metabolizing genes. Conversely, we detected enhanced detoxification enzymatic activities in aphids reared on bik1 plants compared to those on wild-type plants, suggesting that aphids counter the oxidative stress associated with bik1 through elevated metabolic resistance.

Pea aphid biotype performance on diverse Medicago host genotypes indicates highly specific virulence and resistance functions

Aphid-plant interactions depend on genotypes of both organisms, which determine the two-way molecular exchange that leads to compatible or incompatible outcomes. The underlying genes are mostly unknown, making it difficult to predict likelihood of aphid success or host resistance, and hampering crop genetic improvement. Here we screened eight pea aphid clonal genotypes collected from diverse legume hosts, on a species-wide panel of Medicago truncatula (Mt) genotypes. Aphid virulence was measured by survival, fecundity and growth rate, together with scores for chlorosis and necrosis as host response indicators. Outcomes were highly dependent on the specific aphid-host genotype combinations. Only one Mt line was fully resistant against all clones. Aphid-induced host chlorosis and necrosis varied greatly, but correlated with resistance only in a few combinations. Bi-clustering analysis indicated that all aphid clones could be distinguished by their performance profiles across the host genotypes tested, with each clone being genetically differentiated and potentially representing a distinct biotype. Clones originating from Medicago sativa ranged from highly virulent to almost completely avirulent on both Medicago species, indicating that some were well adapted, whereas others were most likely migrants. Comparisons of closely related pairs of Australian Mt genotypes differing in aphid resistance revealed no enhanced resistance to European pea aphid clones. Based on the extensive variation in pea aphid adaptation even on unfamiliar hosts, most likely reflecting multiple biotype-specific gene-for-gene interactions, we conclude that robust defences require an arsenal of appropriate resistance genes.

Engineering plants for aphid resistance: current status and future perspectives

The current status of development of transgenic plants for improved aphid resistance, and the pros and cons of different strategies are reviewed and future perspectives are proposed. Aphids are major agricultural pests that cause significant yield losses of crop plants each year. Excessive dependence on insecticides for aphid control is undesirable because of the development of insecticide resistance, the potential negative effects on non-target organisms and environmental pollution. Transgenic plants engineered for resistance to aphids via a non-toxic mode of action could be an efficient alternative strategy. In this review, the distribution of major aphid species and their damages on crop plants, the so far isolated aphid-resistance genes and their applications in developments of transgenic plants for improved aphid resistance, and the pros and cons of these strategies are reviewed and future perspectives are proposed. Although the transgenic plants developed through expressing aphid-resistant genes, manipulating plant secondary metabolism and plant-mediated RNAi strategy have been demonstrated to confer improved aphid resistance to some degree. So far, no aphid-resistant transgenic crop plants have ever been commercialized. This commentary is intended to be a helpful insight into the generation and future commercialization of aphid-resistant transgenic crops in a global context.

BOTRYTIS-INDUCED KINASE1 Modulates Arabidopsis Resistance to Green Peach Aphids via PHYTOALEXIN DEFICIENT4

BOTRYTIS-INDUCED KINASE1 (BIK1) plays important roles in induced defense against fungal and bacterial pathogens in Arabidopsis (Arabidopsis thaliana). Its tomato (Solanum lycopersicum) homolog is required for host plant resistance to a chewing insect herbivore. However, it remains unknown whether BIK1 functions in plant defense against aphids, a group of insects with a specialized phloem sap-feeding style. In this study, the potential role of BIK1 was investigated in Arabidopsis infested with the green peach aphid (Myzus persicae). In contrast to the previously reported positive role of intact BIK1 in defense response, loss of BIK1 function adversely impacted aphid settling, feeding, and reproduction. Relative to wild-type plants, bik1 displayed higher aphid-induced hydrogen peroxide accumulation and more severe lesions, resembling a hypersensitive response (HR) against pathogens. These symptoms were limited to the infested leaves. The bik1 mutant showed elevated basal as well as induced salicylic acid and ethylene accumulation. Intriguingly, elevated salicylic acid levels did not contribute to the HR-like symptoms or to the heightened aphid resistance associated with the bik1 mutant. Elevated ethylene levels in bik1 accounted for an initial, short-term repellence. Introducing a loss-of-function mutation in the aphid resistance and senescence-promoting gene PHYTOALEXIN DEFICIENT4 (PAD4) into the bik1 background blocked both aphid resistance and HR-like symptoms, indicating bik1-mediated resistance to aphids is PAD4 dependent. Taken together, Arabidopsis BIK1 confers susceptibility to aphid infestation through its suppression of PAD4 expression. Furthermore, the results underscore the role of reactive oxygen species and cell death in plant defense against phloem sap-feeding insects.

Plant resistance to aphid feeding: behavioral, physiological, genetic and molecular cues regulate aphid host selection and feeding

Aphids damage major world food and fiber crops through direct feeding and transmission of plant viruses. Fortunately, the development of many aphid-resistant crop plants has provided both ecological and economic benefits to food production. Plant characters governing aphid host selection often dictate eventual plant resistance or susceptibility to aphid herbivory, and these phenotypic characters have been successfully used to map aphid resistance genes. Aphid resistance is often inherited as a dominant trait, but is also polygenic and inherited as recessive or incompletely dominant traits. Most aphid-resistant cultivars exhibit constitutively expressed defenses, but some cultivars exhibit dramatic aphid-induced responses, resulting in the overexpression of large ensembles of putative aphid resistance genes. Two aphid resistance genes have been cloned. Mi-1.2, an NBS-LRR gene from wild tomato, confers resistance to potato aphid and three Meloidogyne root-knot nematode species, and Vat, an NBS-LRR gene from melon, controls resistance to the cotton/melon aphid and to some viruses. Virulence to aphid resistance genes of plants occurs in 17 aphid species--more than half of all arthropod biotypes demonstrating virulence. The continual appearance of aphid virulence underscores the need to identify new sources of resistance of diverse sequence and function in order to delay or prevent biotype development.

Plant-aphid interactions with a focus on legumes

Sap-sucking insects such as aphids cause substantial yield losses in agriculture by draining plant nutrients as well as vectoring viruses. The main method of control in agriculture is through the application of insecticides. However, aphids rapidly evolve mechanisms to detoxify these, so there is a need to develop durable plant resistance to these damaging insect pests. The focus of this review is on aphid interactions with legumes, but work on aphid interactions with other plants, particularly Arabidopsis and tomato is also discussed. This review covers advances on the plant side of the interaction, including the identification of major resistance genes and quantitative trait loci conferring aphid resistance in legumes, basal and resistance gene mediated defence signalling following aphid infestation and the role of specialised metabolites. On the aphid side of the interaction, this review covers what is known about aphid effector proteins and aphid detoxification enzymes. Recent advances in these areas have provided insight into mechanisms underlying resistance to aphids and the strategies used by aphids for successful infestations and have significant impacts for the delivery of durable resistance to aphids in legume crops.

Characterization and genetic dissection of resistance to spotted alfalfa aphid (Therioaphis trifolii) in Medicago truncatula

Aphids cause significant yield losses in agricultural crops worldwide. Medicago truncatula, a model legume, cultivated pasture species in Australia and close relative of alfalfa (Medicago sativa), was used to study the defence response against Therioaphis trifolii f. maculate [spotted alfalfa aphid (SAA)]. Aphid performance and plant damage were compared among three accessions. A20 is highly susceptible, A17 has moderate resistance, and Jester is strongly resistant. Subsequent analyses using A17 and A20, reciprocal F1s and an A17×A20 recombinant inbred line (RIL) population revealed that this moderate resistance is phloem mediated and involves antibiosis and tolerance but not antixenosis. Electrical penetration graph analysis also identified a novel waveform termed extended potential drop, which occurred following SAA infestation of M. truncatula. Genetic dissection using the RIL population revealed three quantitative trait loci on chromosomes 3, 6, and 7 involved in distinct modes of aphid defence including antibiosis and tolerance. An antibiosis locus resides on linkage group 3 (LG3) and is derived from A17, whereas a plant tolerance and antibiosis locus resides on LG6 and is derived from A20, which exhibits strong temporary tolerance. The loci identified reside in regions harbouring classical resistance genes, and introgression of these loci in current medic cultivars may help provide durable resistance to SAA, while elucidation of their molecular mechanisms may provide valuable insight into other aphid-plant interactions.

Genetic mapping of a novel gene for soybean aphid resistance in soybean (Glycine max [L.] Merr.) line P203 from China

The soybean aphid (SA: Aphis glycines Matsumura) is a worldwide pest of soybean (Glycine max [L.] Merr.). The objectives of this study were to identify the type of aphid resistance and the resistance phenotype in soybean line 'P203', and to map the relative position of the gene involved. Compared with cultivars 'P746' and 'Dongnong 47', P203 was demonstrated to possess antixenosis resistance. P203 prevented aphids from reproducing in a choice test, but the resistance level decreased significantly in a no-choice test at 11 and 21 days after infestation. Analysis of 273 Dongnong 47/P203 F2 plants and confirmed using 260 F2:3 families revealed that a single dominant gene from P203 was positioned between marker loci Sat_377 and Satt409 on chromosome 8. The gene was further mapped to a 1.57 Mb interval flanked by marker loci BARCSOYSSR_08_1451 and BARCSOYSSR_08_1527. We developed five new SSR markers in the target interval and the resistance locus mapped between new markers SSR_08_75 and SSR_08_88 with the genetic distance of 1.1 and 1.0 cM corresponding to a physical distance of 192 kb on the Williams 82 8X draft genome assembly (Glyma1.01). A single serine/threonine protein kinase gene is present in this region, suggesting that the SA resistance mechanism in P203 may be different from those previously reported. Therefore, the resistance gene could very well be novel, and could be valuable in soybean aphid resistance breeding programs.

Natural variation in maize aphid resistance is associated with 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one glucoside methyltransferase activity

Plants differ greatly in their susceptibility to insect herbivory, suggesting both local adaptation and resistance tradeoffs. We used maize (Zea mays) recombinant inbred lines to map a quantitative trait locus (QTL) for the maize leaf aphid (Rhopalosiphum maidis) susceptibility to maize Chromosome 1. Phytochemical analysis revealed that the same locus was also associated with high levels of 2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one glucoside (HDMBOA-Glc) and low levels of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one glucoside (DIMBOA-Glc). In vitro enzyme assays with candidate genes from the region of the QTL identified three O-methyltransferases (Bx10a-c) that convert DIMBOA-Glc to HDMBOA-Glc. Variation in HDMBOA-Glc production was attributed to a natural CACTA family transposon insertion that inactivates Bx10c in maize lines with low HDMBOA-Glc accumulation. When tested with a population of 26 diverse maize inbred lines, R. maidis produced more progeny on those with high HDMBOA-Glc and low DIMBOA-Glc. Although HDMBOA-Glc was more toxic to R. maidis than DIMBOA-Glc in vitro, BX10c activity and the resulting decline of DIMBOA-Glc upon methylation to HDMBOA-Glc were associated with reduced callose deposition as an aphid defense response in vivo. Thus, a natural transposon insertion appears to mediate an ecologically relevant trade-off between the direct toxicity and defense-inducing properties of maize benzoxazinoids.

Resistance to sap-sucking insects in modern-day agriculture

Plants and herbivores have co-evolved in their natural habitats for about 350 million years, but since the domestication of crops, plant resistance against insects has taken a different turn. With the onset of monoculture-driven modern agriculture, selective pressure on insects to overcome resistances has dramatically increased. Therefore plant breeders have resorted to high-tech tools to continuously create new insect-resistant crops. Efforts in the past 30 years have resulted in elucidation of mechanisms of many effective plant defenses against insect herbivores. Here, we critically appraise these efforts and - with a focus on sap-sucking insects - discuss how these findings have contributed to herbivore-resistant crops. Moreover, in this review we try to assess where future challenges and opportunities lay ahead. Of particular importance will be a mandatory reduction in systemic pesticide usage and thus a greater reliance on alternative methods, such as improved plant genetics for plant resistance to insect herbivores.

Toward understanding the role of aphid effectors in plant infestation

In recent years, immense progress has been made toward understanding the functions of effectors from a range of plant pathogens, such as oomycetes, fungi, bacteria, and nematodes. Like plant pathogens, aphids form close associations with host plants, featuring signal exchange between the two organisms. While feeding and probing, aphids deliver effector proteins mixed with saliva directly into the host-stylet interface. With the increasing availability of aphid genome and transcriptome sequence data, aphid effector biology is emerging as a new and exciting area of research. In this review, we provide an overview of recent advances in the aphid effector biology field and highlight some of the current questions.

Perception of insect feeding by plants

The recognition of phytophagous insects by plants induces a set of very specific responses aimed at deterring tissue consumption and reprogramming metabolism and development of the plant to tolerate the herbivore. The recognition of insects by plants requires the plant's ability to perceive chemical cues generated by the insects and to distinguish a particular pattern of tissue disruption. Relatively little is known about the molecular basis of insect perception by plants and the signalling mechanisms directly associated with this perception. Importantly, the insect feeding behaviour (piercing-sucking versus chewing) is a decisive determinant of the plant's defence response, and the mechanisms used to perceive insects from different feeding guilds may be distinct. During insect feeding, components of the saliva of chewing or piercing-sucking insects come into contact with plant cells, and elicitors or effectors present in this insect-derived fluid are perceived by plant cells to initiate the activation of specific signalling cascades. Although receptor-ligand interactions controlling insect perception have yet not been molecularly described, a significant number of regulatory components acting downstream of receptors and involved in the activation of defence responses against insects has been reported. Some of these regulators mediate changes in the phytohormone network, while others directly control gene expression or the redox state of the cell. These processes are central in the orchestration of plant defence responses against insects.

Identification and characterization of resistance to cowpea aphid (Aphis craccivora Koch) in Medicago truncatula

BACKGROUND

Cowpea aphid (CPA; Aphis craccivora) is the most important insect pest of cowpea and also causes significant yield losses in other legume crops including alfalfa, beans, chickpea, lentils, lupins and peanuts. In many of these crops there is no natural genetic resistance to this sap-sucking insect or resistance genes have been overcome by newly emerged CPA biotypes.

RESULTS

In this study, we screened a subset of the Medicago truncatula core collection of the South Australian Research and Development Institute (SARDI) and identified strong resistance to CPA in a M. truncatula accession SA30199, compared to all other M. truncatula accessions tested. The biology of resistance to CPA in SA30199 plants was characterised compared to the highly susceptible accession Borung and showed that resistance occurred at the level of the phloem, required an intact plant and involved a combination of antixenosis and antibiosis. Quantitative trait loci (QTL) analysis using a F2 population (n = 150) from a cross between SA30199 and Borung revealed that resistance to CPA is controlled in part by a major quantitative trait locus (QTL) on chromosome 2, explaining 39% of the antibiosis resistance.

CONCLUSIONS

The identification of strong CPA resistance in M. truncatula allows for the identification of key regulators and genes important in this model legume to give effective CPA resistance that may have relevance for other legume crops. The identified locus will also facilitate marker assisted breeding of M. truncatula for increased resistance to CPA and potentially other closely related Medicago species such as alfalfa.

Identification of distinct quantitative trait loci associated with defence against the closely related aphids Acyrthosiphon pisum and A. kondoi in Medicago truncatula

Aphids are a major family of plant insect pests. Medicago truncatula and Acyrthosiphon pisum (pea aphid, PA) are model species with a suite of resources available to help dissect the mechanism underlying plant-aphid interactions. A previous study focused on monogenic and relatively strong resistance in M. truncatula to PA and other aphid species. In this study a moderate resistance to PA was characterized in detail in the M. truncatula line A17 and compared with the highly susceptible line A20 and the more resistant line Jester. The results show that PA resistance in A17 involves both antibiosis and tolerance, and that resistance is phloem based. Quantitative trait locus (QTL) analysis using a recombinant inbred line (RIL) population (n=114) from a cross between A17 and A20 revealed that one locus, which co-segregated with AIN (Acyrthosiphon-induced necrosis) on chromosome 3, is responsible for the reduction of aphid biomass (indicator of antibiosis) for both PA and bluegreen aphid (BGA, A. kondoi), albeit to a lesser degree for PA than BGA. Interestingly, two independent loci on chromosomes 5 and 3 were identified for the plant biomass reduction (indicator of plant tolerance) by PA and BGA, respectively, demonstrating that the plant's tolerance response to these two closely related aphid species is distinct. Together with previously identified major resistant (R) genes, the QTLs identified in this study are powerful tools to understand fully the spectrum of plant defence against sap-sucking insects and provide opportunities for breeders to generate effective and sustainable strategies for aphid control.

Molecular bases of plant resistance to arthropods

Arthropod-resistant crops provide significant ecological and economic benefits to global agriculture. Incompatible interactions involving resistant plants and avirulent pest arthropods are mediated by constitutively produced and arthropod-induced plant proteins and defense allelochemicals synthesized by resistance gene products. Cloning and molecular mapping have identified the Mi-1.2 and Vat arthropod resistance genes as CC-NBS-LRR (coiled coil-nucleotide binding site-leucine rich repeat) subfamily NBS-LRR resistance proteins, as well as several resistance gene analogs. Genetic linkage mapping has identified more than 100 plant resistance gene loci and linked molecular markers used in cultivar development. Rice and sorghum arthropod-resistant cultivars and, to a lesser extent, raspberry and wheat cultivars are components of integrated pest management (IPM) programs in Asia, Australia, Europe, and North America. Nevertheless, arthropod resistance in most food and fiber crops has not been integrated due primarily to the application of synthetic insecticides. Plant and arthropod genomics provide many opportunities to more efficiently develop arthropod-resistant plants, but integration of resistant cultivars into IPM programs will succeed only through interdisciplinary collaboration.

Exploiting natural variation to identify insect-resistance genes

Herbivorous insects are widespread and often serious constraints to crop production. The use of insect-resistant crops is a very effective way to control insect pests in agriculture, and the development of such crops can be greatly enhanced by knowledge on plant resistance mechanisms and the genes involved. Plants have evolved diverse ways to cope with insect attack that has resulted in natural variation for resistance towards herbivorous insects. Studying the molecular genetics and transcriptional background of this variation has facilitated the identification of resistance genes and processes that lead to resistance against insects. With the development of new technologies, molecular studies are not restricted to model plants anymore. This review addresses the need to exploit natural variation in resistance towards insects to increase our knowledge on resistance mechanisms and the genes involved. We will discuss how this knowledge can be exploited in breeding programmes to provide sustainable crop protection against insect pests. Additionally, we discuss the current status of genetic research on insect-resistance genes. We conclude that insect-resistance mechanisms are still unclear at the molecular level and that exploiting natural variation with novel technologies will contribute greatly to the development of insect-resistant crop varieties.

Identification of potential early regulators of aphid resistance in Medicago truncatula via transcription factor expression profiling

*Resistance to aphids has been identified in a number of plant species, yet the molecular mechanisms underlying aphid resistance remain largely unknown. *Using high-throughput quantitative real-time PCR technology, the transcription profiles of 752 putative Medicago truncatula transcription factor genes were analysed in a pair of susceptible and resistant closely related lines of M. truncatula following 6 and 12 h of bluegreen aphid (Acyrthosiphon kondoi) infestation. *Eighty-two transcription factor genes belonging to 30 transcription factor families were responsive to bluegreen aphid infestation. More transcription factor genes were responsive in the resistant interaction than in the susceptible interaction; of the 36 genes that were induced at 6 and/or 12 h, 32 were induced only in the resistant interaction. Bluegreen aphid-induced expression of a subset of these genes was correlated with the presence of AKR, a single dominant gene conferring resistance to bluegreen aphids. Similar transcription factor expression patterns of this subset were associated with bluegreen aphid resistance in other M. truncatula genetic backgrounds, as well as with resistance to pea aphid (Acyrthosiphon pisum). *Our results suggest that these transcription factors are among the early aphid-responsive genes in resistant plants, and may play important roles in resistance to multiple aphid species.

Resistance to aphid vectors of virus disease

The majority of plant viruses rely on vectors for their transmission and completion of their life cycle. These vectors comprise a diverse range of life forms including insects, nematodes, and fungi with the most common of these being insects. The geographic range of many of these vectors is continually expanding due to climate change. The viruses that they carry are therefore also expanding their range to exploit novel and naïve plant hosts. There are many forms of naturally occurring vector resistance ranging from broad nonhost resistance to more specific types of inducible resistance. Understanding and exploiting the many and varied forms of natural resistance to virus vectors is therefore extremely important for current and future agricultural production systems. To demonstrate the range and extent of these resistance mechanisms, this chapter will primarily focus on aphids to highlight key developments appropriate to plant-insect-virus interactions.