Induced epidermal permeability modulates resistance and susceptibility of wheat seedlings to herbivory by Hessian fly larvae

Comparative Hessian Fly Larval Transcriptomics Provides Novel Insight into Host and Nonhost Resistance

The Hessian fly is a destructive pest of wheat. Employing additional molecular strategies can complement wheat's native insect resistance. However, this requires functional characterization of Hessian-fly-responsive genes, which is challenging because of wheat genome complexity. The diploid Brachypodium distachyon (Bd) exhibits nonhost resistance to Hessian fly and displays phenotypic/molecular responses intermediate between resistant and susceptible host wheat, offering a surrogate genome for gene characterization. Here, we compared the transcriptomes of Biotype L larvae residing on resistant/susceptible wheat, and nonhost Bd plants. Larvae from susceptible wheat and nonhost Bd plants revealed similar molecular responses that were distinct from avirulent larval responses on resistant wheat. Secreted salivary gland proteins were strongly up-regulated in all larvae. Genes from various biological pathways and molecular processes were up-regulated in larvae from both susceptible wheat and nonhost Bd plants. However, Bd larval expression levels were intermediate between larvae from susceptible and resistant wheat. Most genes were down-regulated or unchanged in avirulent larvae, correlating with their inability to establish feeding sites and dying within 4-5 days after egg-hatch. Decreased gene expression in Bd larvae, compared to ones on susceptible wheat, potentially led to developmentally delayed 2nd-instars, followed by eventually succumbing to nonhost resistance defense mechanisms.

Insect derived extra oral GH32 plays a role in susceptibility of wheat to Hessian fly

The Hessian fly is an obligate parasite of wheat causing significant economic damage, and triggers either a resistant or susceptible reaction. However, the molecular mechanisms of susceptibility leading to the establishment of the larvae are unknown. Larval survival on the plant requires the establishment of a steady source of readily available nutrition. Unlike other insect pests, the Hessian fly larvae have minute mandibles and cannot derive their nutrition by chewing tissue or sucking phloem sap. Here, we show that the virulent larvae produce the glycoside hydrolase MdesGH32 extra-orally, that localizes within the leaf tissue being fed upon. MdesGH32 has strong inulinase and invertase activity aiding in the breakdown of the plant cell wall inulin polymer into monomers and converting sucrose, the primary transport sugar in plants, to glucose and fructose, resulting in the formation of a nutrient-rich tissue. Our finding elucidates the molecular mechanism of nutrient sink formation and establishment of susceptibility.

Differential localization of Hessian fly candidate effectors in resistant and susceptible wheat plants

Hessian fly Mayetiola destructor is a notorious pest of wheat. Previous studies suggest that Hessian fly uses effector-based mechanisms to attack wheat plants during parasitism, but no direct evidence has been reported to support this postulation. Here, we produced recombinant proteins for five Family-1 candidate effectors and antibodies. Indirect immunostaining and western blots were carried out to examine the localization of Hessian fly Family-1 proteins in plant and insect tissues. Confocal images revealed that Family-1 putative effectors were exclusively produced in the basal region of larval salivary glands, which are directly linked to the mandibles' ducts for effector injection. The five Family-1 proteins were detected in infested host plants on western blots. Indirect immunostaining of sectioned host tissues around the feeding site revealed strikingly different localization patterns between resistant and susceptible plants. In susceptible plants, the Family-1 proteins penetrated from the feeding cell into deep tissues, indicative of movement between cells during nutritive cell formation. In contrast, the Hessian fly proteins were primarily limited to the initially attacked cells in resistant plants. The limitation of effectors' spread in resistant plants was likely due to wall strengthening and rapid hypersensitive cell death. Cell death was found in Nicotiana benthamiana in association with hypersensitive reaction triggered by the Family-1 effector SSGP-1A2. Our finding represents a significant progress in visualizing insect effectors in host tissues and mechanisms of plant resistance and susceptibility to gall midge pests.

Phenotypic and molecular characterization of Hessian fly resistance in diploid wheat, Aegilops tauschii

BACKGROUND

The Hessian fly (Mayetiola destructor), belonging to the gall midge family (Cecidomyiidae), is a devastating pest of wheat (Triticum aestivum) causing significant yield losses. Despite identification and characterization of numerous Hessian fly-responsive genes and associated biological pathways involved in wheat defense against this dipteran pest, their functional validation has been challenging. This is largely attributed to the large genome, polyploidy, repetitive DNA, and limited genetic resources in hexaploid wheat. The diploid progenitor Aegilops tauschii, D-genome donor of modern-day hexaploid wheat, offers an ideal surrogate eliminating the need to target all three homeologous chromosomes (A, B and D) individually, and thereby making the functional validation of candidate Hessian fly-responsive genes plausible. Furthermore, the well-annotated sequence of Ae. tauschii genome and availability of genetic resources amenable to manipulations makes the functional assays less tedious and time-consuming. However, prior to utilization of this diploid genome for downstream studies, it is imperative to characterize its physical and molecular responses to Hessian fly.

RESULTS

In this study we screened five Ae. tauschii accessions for their response to the Hessian fly biotypes L and vH13. Two lines were identified that exhibited a homozygous resistance response to feeding by both Hessian fly biotypes. Studies using physical measurements and neutral red staining showed that the resistant Ae. tauschii accessions resembled hexaploid wheat in their phenotypic responses to Hessian fly, that included similarities in larval developmental stages, leaf and plant growth, and cell wall permeability. Furthermore, molecular responses, characterized by gene expression profiling using quantitative real-time PCR, in select resistant Ae. tauschii lines also revealed similarities with resistant hexaploid wheat.

CONCLUSIONS

Phenotypic and molecular characterization of Ae. tauschii to Hessian fly infestation revealed resistant accessions that shared similarities to hexaploid wheat. Resembling the resistant hexaploid wheat, the Ae. tauschii accessions mount an early defense strategy involving defense proteins including lectins, secondary metabolites and reactive oxygen species (ROS) radicals. Our results reveal the suitability of the diploid progenitor for use as an ideal tool for functional genomics research in deciphering the wheat-Hessian fly molecular interactions.

Spectral characterization of wheat functional trait responses to Hessian fly: Mechanisms for trait-based resistance

Insect herbivores can manipulate host plants to inhibit defenses. Insects that induce plant galls are excellent examples of these interactions. The Hessian fly (HF, Mayetiola destructor) is a destructive pest of wheat (Triticum spp.) that occurs in nearly all wheat producing globally. Under compatible interactions (i.e., successful HF establishment), HF larvae alter host tissue physiology and morphology for their benefit, manifesting as the development of plant nutritive tissue that feeds the larva and ceases plant cell division and elongation. Under incompatible interactions (i.e., unsuccessful HF establishment), plants respond to larval feeding by killing the larva, permitting normal plant development. We used reflectance spectroscopy to characterize whole-plant functional trait responses during both compatible and incompatible interactions and related these findings with morphological and gene expression observations from earlier studies. Spectral models successfully characterized wheat foliar traits, with mean goodness of fit statistics of 0.84, 0.85, 0.94, and 0.69 and percent root mean square errors of 22, 10, 6, and 20%, respectively, for nitrogen and carbon concentrations, leaf mass per area, and total phenolic content. We found that larvae capable of generating compatible interactions successfully manipulated host plant chemical and morphological composition to create a more hospitable environment. Incompatible interactions resulted in lower host plant nutritional quality, thicker leaves, and higher phenolic levels. Spectral measurements successfully characterized wheat responses to compatible and incompatible interactions, providing an excellent example of the utility of Spectral phenotyping in quantifying responses of specific plant functional traits associated with insect resistance.

Multiple molecular defense strategies in Brachypodium distachyon surmount Hessian fly (Mayetiola destructor) larvae-induced susceptibility for plant survival

The Hessian fly is a destructive pest of wheat causing severe economic damage. Numerous genes and associated biological pathways have been implicated in defense against Hessian fly. However, due to limited genetic resources, compounded with genome complexity, functional analysis of the candidate genes are challenging in wheat. Physically, Brachypodium distachyon (Bd) exhibits nonhost resistance to Hessian fly, and with a small genome size, short life cycle, vast genetic resources and amenability to transformation, it offers an alternate functional genomic model for deciphering plant-Hessian fly interactions. RNA-sequencing was used to reveal thousands of Hessian fly-responsive genes in Bd one, three, and five days after egg hatch. Genes encoding defense proteins, stress-regulating transcription factors, signaling kinases, and secondary metabolites were strongly up-regulated within the first 24 hours of larval feeding indicating an early defense, similar to resistant wheat. Defense was mediated by a hypersensitive response that included necrotic lesions, up-regulated ROS-generating and -scavenging enzymes, and H₂O₂ production. Suppression of cell wall-associated proteins and increased cell permeability in Bd resembled susceptible wheat. Thus, Bd molecular responses shared similarities to both resistant and susceptible wheat, validating its suitability as a model genome for undertaking functional studies of candidate Hessian fly-responsive genes.

Modulation of nonessential amino acid biosynthetic pathways in virulent Hessian fly larvae (Mayetiola destructor), feeding on susceptible host wheat (Triticum aestivum)

Compatible interactions between wheat (Triticum aestivum), and its dipteran pest Hessian fly (Hf, Mayetiola destructor) result in successful establishment of larval feeding sites rendering the host plant susceptible. Virulent larvae employ an effector-based feeding strategy to reprogram the host physiology resulting in formation of a protein- and sugar-rich nutritive tissue beneficial to developing larvae. Previous studies documented increased levels of nonessential amino acids (NAA; that need not be received through insect diet) in the susceptible wheat in response to larval feeding, suggesting importance of plant-derived NAA in larval nutrition. Here, we investigated the modulation of genes from NAA biosynthetic pathways (NAABP) in virulent Hf larvae. Transcript profiling for 16 NAABP genes, annotated from the recently assembled Hf genome, was carried out in the feeding first-, and second-instars and compared with that of the first-instar neonate (newly hatched, migrating, assumed to be non-feeding) larvae. While Tyr, Gln, Glu, and Pro NAABP genes transcript abundance declined in the feeding instars as compared to the neonates, those for Ala, and Ser increased in the feeding larval instars, despite higher levels of these NAA in the susceptible host plant. Asp, Asn, Gly and Cys NAABP genes exhibited variable expression profiles in the feeding first- and second-instars. Our results indicate that while Hf larvae utilize the plant-derived NAA, de novo synthesis of several NAA may be necessary to: (i) provide larvae with the requisite amount for sustaining growth before nutritive tissue formation and, (ii) overcome any inadequate amounts in the host plant, post-nutritive tissue formation.

Plant-Herbivore Interactions: A Case of an Extreme Generalist, the Two-Spotted Spider Mite Tetranychus urticae

Plant-herbivore interactions evolved over long periods of time, resulting in an elaborate arms race between interacting species. While specialist herbivores evolved specific strategies to cope with the defenses of a limited number of hosts, our understanding of how generalist herbivores deal with the defenses of a plethora of diverse host plants is largely unknown. Understanding the interaction between a plant host and a generalist herbivore requires an understanding of the plant's mechanisms aimed at defending itself and the herbivore's mechanisms intended to counteract diverse defenses. In this review, we use the two-spotted spider mite (TSSM), Tetranychus urticae (Koch) as an example of a generalist herbivore, as this chelicerate pest has a staggering number of plant hosts. We first establish that the ability of TSSM to adapt to marginal hosts underlies its polyphagy and agricultural pest status. We then highlight our understanding of direct plant defenses against spider mite herbivory and review recent advances in uncovering mechanisms of spider mite adaptations to them. Finally, we discuss the adaptation process itself, as it allows TSSM to overcome initially effective plant defenses. A high-quality genome sequence and developing genetic tools, coupled with an ease of mite experimental selection to new hosts, make TSSM an outstanding system to study the evolution of host range, mechanisms of pest xenobiotic resistance and plant-herbivore interactions. In addition, knowledge of plant defense mechanisms that affect mite fitness are of practical importance, as it can lead to development of new control strategies against this important agricultural pest. In parallel, understanding mechanisms of mite counter adaptations to these defenses is required to maintain the efficacy of these control strategies in agricultural practices.

Avirulence gene mapping in the Hessian fly (Mayetiola destructor) reveals a protein phosphatase 2C effector gene family

The genetic tractability of the Hessian fly (HF, Mayetiola destructor) provides an opportunity to investigate the mechanisms insects use to induce plant gall formation. Here we demonstrate that capacity using the newly sequenced HF genome by identifying the gene (vH24) that elicits effector-triggered immunity in wheat (Triticum spp.) seedlings carrying HF resistance gene H24. vH24 was mapped within a 230-kb genomic fragment near the telomere of HF chromosome X1. That fragment contains only 21 putative genes. The best candidate vH24 gene in this region encodes a protein containing a secretion signal and a type-2 serine/threonine protein phosphatase (PP2C) domain. This gene has an H24-virulence associated insertion in its promoter that appears to silence transcription of the gene in H24-virulent larvae. Candidate vH24 is a member of a small family of genes that encode secretion signals and PP2C domains. It belongs to the fraction of genes in the HF genome previously predicted to encode effector proteins. Because PP2C proteins are not normally secreted, our results suggest that these are PP2C effectors that HF larvae inject into wheat cells to redirect, or interfere, with wheat signal transduction pathways.

Plant Photosynthetic Responses During Insect Effector-Triggered Plant Susceptibility and Immunity

Gall-inducing insects are known for altering source-sink relationships within plants. Changes in photosynthesis may contribute to this phenomenon. We investigated photosynthetic responses in wheat [Triticum aestivum L. (Poaceae: Triticeae)] seedlings attacked by the Hessian fly [Mayetiola destructor (Say) (Diptera: Cecidomyiidae], which uses a salivary effector-based strategy to induce a gall nutritive tissue in susceptible plants. Resistant plants have surveillance systems mediated by products of Resistance (R) genes. Detection of a specific salivary effector triggers downstream responses that result in a resistance that kills neonate larvae. A 2 × 2 factorial design was used to study maximum leaf photosynthetic assimilation and stomatal conductance rates. The plant treatments were-resistant or susceptible wheat lines expressing or not expressing the H13 resistance gene. The insect treatments were-no attack (control) or attack by larvae killed by H13 gene-mediated resistance. Photosynthesis was measured for the second and third leaves of the seedling, the latter being the only leaf directly attacked by larvae. We predicted effector-based attack would trigger increases in photosynthetic rates in susceptible but not resistant plants. For susceptible plants, attack was associated with increases (relative to controls) in photosynthesis for the third but not the second leaf. For resistant plants, attack was associated with increases in photosynthesis for both the second and third leaves. Mechanisms underlying the increases appeared to differ. Resistant plants exhibited responses suggesting altered source-sink relationships. Susceptible plants exhibited responses suggesting a mechanism other than altered source-sink relationships, possibly changes in water relations that contributed to increased stomatal conductance.

Insect effectors and gene-for-gene interactions with host plants

Within the context of the four-phase model of plant immunity, gene-for-gene interactions have gained new relevance. Genes conferring resistance to the Asian rice gall midge (Orseolia oryzae) and the small brown planthopper (Nilaparvata lugens) have been cloned in rice (Oryza sativa). Mutations in insect avirulence genes that defeat plant resistance have been identified and cloned. Results are consistent with both the gene-for-gene hypothesis and the new model of plant immunity. Insect resistance genes encode proteins with nucleotide binding sites and leucine-rich repeats. Insects use effectors that elicit effector-triggered immunity. At least seven-percent of Hessian fly genes are effector encoding.

Differential expression of candidate salivary effector proteins in field collections of Hessian fly, Mayetiola destructor

Evidence is emerging that some proteins secreted by gall-forming parasites of plants act as effectors responsible for systemic changes in the host plant, such as galling and nutrient tissue formation. A large number of secreted salivary gland proteins (SSGPs) that are the putative effectors responsible for the physiological changes elicited in susceptible seedling wheat by Hessian fly, Mayetiola destructor (Say), larvae have been documented. However, how the genes encoding these candidate effectors might respond under field conditions is unknown. The goal of this study was to use microarray analysis to investigate variation in SSGP transcript abundance amongst field collections from different geographical regions (southeastern USA, central USA, and the Middle East). Results revealed significant variation in SSGP transcript abundance amongst the field collections studied. The field collections separated into three distinct groups that corresponded to the wheat classes grown in the different geographical regions as well as to recently described Hessian fly populations. These data support previous reports correlating Hessian fly population structure with micropopulation differences owing to agro-ecosystem parameters such as cultivation of regionally adapted wheat varieties, deployment of resistance genes and variation in climatic conditions.

Pivoting from Arabidopsis to wheat to understand how agricultural plants integrate responses to biotic stress

In this review, we argue for a research initiative on wheat's responses to biotic stress. One goal is to begin a conversation between the disparate communities of plant pathology and entomology. Another is to understand how responses to a variety of agents of biotic stress are integrated in an important crop. We propose gene-for-gene interactions as the focus of the research initiative. On the parasite's side is an Avirulence (Avr) gene that encodes one of the many effector proteins the parasite applies to the plant to assist with colonization. On the plant's side is a Resistance (R) gene that mediates a surveillance system that detects the Avr protein directly or indirectly and triggers effector-triggered plant immunity. Even though arthropods are responsible for a significant proportion of plant biotic stress, they have not been integrated into important models of plant immunity that come from plant pathology. A roadblock has been the absence of molecular evidence for arthropod Avr effectors. Thirty years after this evidence was discovered in a plant pathogen, there is now evidence for arthropods with the cloning of the Hessian fly's vH13 Avr gene. After reviewing the two models of plant immunity, we discuss how arthropods could be incorporated. We end by showing features that make wheat an interesting system for plant immunity, including 479 resistance genes known from agriculture that target viruses, bacteria, fungi, nematodes, insects, and mites. It is not likely that humans will be subsisting on Arabidopsis in the year 2050. It is time to start understanding how agricultural plants integrate responses to biotic stress.

Hessian fly larval feeding triggers enhanced polyamine levels in susceptible but not resistant wheat

BACKGROUND

Hessian fly (Mayetiola destructor), a member of the gall midge family, is one of the most destructive pests of wheat (Triticum aestivum) worldwide. Probing of wheat plants by the larvae results in either an incompatible (avirulent larvae, resistant plant) or a compatible (virulent larvae, susceptible plant) interaction. Virulent larvae induce the formation of a nutritive tissue, resembling the inside surface of a gall, in susceptible wheat. These nutritive cells are a rich source of proteins and sugars that sustain the developing virulent Hessian fly larvae. In addition, on susceptible wheat, larvae trigger a significant increase in levels of amino acids including proline and glutamic acid, which are precursors for the biosynthesis of ornithine and arginine that in turn enter the pathway for polyamine biosynthesis.

RESULTS

Following Hessian fly larval attack, transcript abundance in susceptible wheat increased for several genes involved in polyamine biosynthesis, leading to higher levels of the free polyamines, putrescine, spermidine and spermine. A concurrent increase in polyamine levels occurred in the virulent larvae despite a decrease in abundance of Mdes-odc (ornithine decarboxylase) transcript encoding a key enzyme in insect putrescine biosynthesis. In contrast, resistant wheat and avirulent Hessian fly larvae did not exhibit significant changes in transcript abundance of genes involved in polyamine biosynthesis or in free polyamine levels.

CONCLUSIONS

The major findings from this study are: (i) although polyamines contribute to defense in some plant-pathogen interactions, their production is induced in susceptible wheat during interactions with Hessian fly larvae without contributing to defense, and (ii) due to low abundance of transcripts encoding the rate-limiting ornithine decarboxylase enzyme in the larval polyamine pathway the source of polyamines found in virulent larvae is plausibly wheat-derived. The activation of the host polyamine biosynthesis pathway during compatible wheat-Hessian fly interactions is consistent with a model wherein the virulent larvae usurp the polyamine biosynthesis machinery of the susceptible plant to acquire nutrients required for their own growth and development.

Transcriptome comparative profiling of barley eibi1 mutant reveals pleiotropic effects of HvABCG31 gene on cuticle biogenesis and stress responsive pathways

Wild barley eibi1 mutant with HvABCG31 gene mutation has low capacity to retain leaf water, a phenotype associated with reduced cutin deposition and a thin cuticle. To better understand how such a mutant plant survives, we performed a genome-wide gene expression analysis. The leaf transcriptomes between the near-isogenic lines eibi1 and the wild type were compared using the 22-k Barley1 Affymetrix microarray. We found that the pleiotropic effect of the single gene HvABCG31 mutation was linked to the co-regulation of metabolic processes and stress-related system. The cuticle development involved cytochrome P450 family members and fatty acid metabolism pathways were significantly up-regulated by the HvABCG31 mutation, which might be anticipated to reduce the levels of cutin monomers or wax and display conspicuous cuticle defects. The candidate genes for responses to stress were induced by eibi1 mutant through activating the jasmonate pathway. The down-regulation of co-expressed enzyme genes responsible for DNA methylation and histone deacetylation also suggested that HvABCG31 mutation may affect the epigenetic regulation for barley development. Comparison of transcriptomic profiling of barley under biotic and abiotic stresses revealed that the functions of HvABCG31 gene to high-water loss rate might be different from other osmotic stresses of gene mutations in barley. The transcriptional profiling of the HvABCG31 mutation provided candidate genes for further investigation of the physiological and developmental changes caused by the mutant.

Flexible resource allocation during plant defense responses

Plants are organisms composed of modules connected by xylem and phloem transport streams. Attack by both insects and pathogens elicits sometimes rapid defense responses in the attacked module. We have also known for some time that proteins are often reallocated away from pathogen-infected tissues, while the same infection sites may draw carbohydrates to them. This has been interpreted as a tug of war in which the plant withdraws critical resources to block microbial growth while the microbes attempt to acquire more resources. Sink-source regulated transport among modules of critical resources, particularly carbon and nitrogen, is also altered in response to attack. Insects and jasmonate can increase local sink strength, drawing carbohydrates that support defense production. Shortly after attack, carbohydrates may also be drawn to the root. The rate and direction of movement of photosynthate or signals in phloem in response to attack is subject to constraints that include branching, degree of connection among tissues, distance between sources and sinks, proximity, strength, and number of competing sinks, and phloem loading/unloading regulators. Movement of materials (e.g., amino acids, signals) to or from attack sites in xylem is less well understood but is partly driven by transpiration. The root is an influential sink and may regulate sink-source interactions and transport above and below ground as well as between the plant and the rhizosphere and nearby, connected plants. Research on resource translocation in response to pathogens or herbivores has focused on biochemical mechanisms; whole-plant research is needed to determine which, if any, of these plant behaviors actually influence plant fitness.

A genome-wide survey of small interfering RNA and microRNA pathway genes in a galling insect

Deployment of resistance (R) genes is the most effective control for Hessian fly, Mayetiola destructor (Say); however, deployment of R genes results in an increased frequency of pest genotypes that display virulence to them. RNA interference (RNAi) is a useful reverse genetics tool for studying such insect virulence pathways, but requires a systemic phenotype, which is not found in all species. In an effort to correlate our observed weak RNAi phenotype in M. destructor with a genetic basis, we have aggregated and compared RNAi related genes across M. destructor, three other insect species, and the nematode Caenorhabditis elegans. We report here the annotation of the core genes in the small interfering RNA (siRNA) and microRNA (miRNA) pathways in M. destructor. While most of the miRNA pathway genes were highly conserved across the species studied, the siRNA pathway genes showed increased relative variability in comparison to the miRNA pathway. In particular, the Piwi/Argonaute/Zwille (PAZ) domain of Dicer-2 (DCR-2) had the least amount of sequence similarity of any domain among species surveyed, with a trend of increased conservation in those species with amenable systemic RNAi. A homolog of the systemic interference defective-1 (Sid-1) gene of C. elegans was also not annotated in the M. destructor genome. Indeed, it is of interest that a Sid-1 homolog has not been detected in any dipteran species to date. We hypothesize the sequence architecture of the PAZ domain in the M. destructor DCR-2 protein is related to reduced efficacy of this enzyme and this taken together with the lack of a Sid-1 homolog may account for the weak RNAi response observed to date in this species as well as other dipteran species.

Obviation of wheat resistance to the Hessian fly through systemic induced susceptibility

Unlike most documented plant-insect interactions, Hessian fly-resistance [Mayetiola destructor (Say)] in wheat (Triticum aestivum L.) is initiated by a gene-for-gene recognition event in which plants carrying a specific R gene recognize salivary effectors encoded by a corresponding larval avirulence gene. However, dual infestation resulting from oviposition by virulent insects from 5 d before to 3 d after oviposition by avirulent insects on the same host plant, lead to systemic induced susceptibility, obviation of resistance, and ultimately the survival of both virulent and genetically avirulent progeny to adulthood. Simultaneous oviposition allowed greater survival of avirulent progeny than ovipositions separated by larger intervals. Because of the induction of plant resistance, hatch of avirulent larvae before virulent was more detrimental to rate of development than hatch of virulent before avirulent larvae. Obviation of resistance was not localized to the leaf being attacked by the virulent larvae, but also functioned across spatial distance into younger leaves. This research suggests that virulent Hessian fly larvae directly suppress the defense response of wheat, thus providing a refuge for avirulent genotypes, preserving diversity in field populations and increasing durability of deployed resistance genes.

Gall midges (Hessian flies) as plant pathogens

Gall midges constitute an important group of plant-parasitic insects. The Hessian fly (HF; Mayetiola destructor), the most investigated gall midge, was the first insect hypothesized to have a gene-for-gene interaction with its host plant, wheat (Triticum spp.). Recent investigations support that hypothesis. The minute larval mandibles appear to act in a manner that is analogous to nematode stylets and the haustoria of filamentous plant pathogens. Putative effector proteins are encoded by hundreds of genes and expressed in the HF larval salivary gland. Cultivar-specific resistance (R) genes mediate a highly localized plant reaction that prevents the survival of avirulent HF larvae. Fine-scale mapping of HF avirulence (Avr) genes provides further evidence of effector-triggered immunity (ETI) against HF in wheat. Taken together, these discoveries suggest that the HF, and other gall midges, may be considered biotrophic, or hemibiotrophic, plant pathogens, and they demonstrate the potential that the wheat-HF interaction has in the study of insect-induced plant gall formation.