Differential expression of two cytochrome P450 genes in compatible and incompatible Hessian fly/wheat interactions

A chromosome-level genome assembly of the orange wheat blossom midge, Sitodiplosis mosellana Géhin (Diptera: Cecidomyiidae) provides insights into the evolution of a detoxification system

The orange wheat blossom midge Sitodiplosis mosellana Géhin (Diptera: Cecidomyiidae), an economically important pest, has caused serious yield losses in most wheat-growing areas worldwide in the past half-century. A high-quality chromosome-level genome for S. mosellana was assembled using PacBio long read, Illumina short read, and Hi-C sequencing technologies. The final genome assembly was 180.69 Mb, with contig and scaffold N50 sizes of 998.71 kb and 44.56 Mb, respectively. Hi-C scaffolding reliably anchored 4 pseudochromosomes, accounting for 99.67% of the assembled genome. In total, 12,269 protein-coding genes were predicted, of which 91% were functionally annotated. Phylogenetic analysis indicated that S. mosellana and its close relative, the swede midge Contarinia nasturtii, diverged about 32.7 MYA. The S. mosellana genome showed high chromosomal synteny with the genome of Drosophila melanogaster and Anopheles gambiae. The key gene families involved in the detoxification of plant secondary chemistry were analyzed. The high-quality S. mosellana genome data will provide an invaluable resource for research in a broad range of areas, including the biology, ecology, genetics, and evolution of midges, as well as insect–plant interactions and coevolution.

Expression Patterns, Molecular Characterization, and Response to Host Stress of CYP Genes from Phenacoccus solenopsis (Hemiptera: Pseudococcidae)

The quarantine insect pest Phenacoccus solenopsis (Hemiptera: Pseudococcidae) has a broad host range and is distributed worldwide. Each year, P. solenopsis causes significant crop losses. The detoxification of various xenobiotic compounds involves the cytochrome P450 monooxygenase (CYP) superfamily of enzymes. However, the functions of CYPs in P. solenopsis are poorly understood. In the present study, P. solenopsis was reared from the egg to the adult stage on three host plants: Tomato, cotton, and hibiscus. Thirty-seven P. solenopsis CYP genes were identified and their phylogenetic relationships were analyzed. Eleven CYP genes (PsCYP4NT1, PsCYP4G219, PsCYP6PZ1, PsCYP6PZ5, PsCYP301B1, PsCYP302A1, PsCYP305A22, PsCYP315A1, PsCYP353F1, PsCYP3634A1, and PsCYP3635A2) were selected for quantitative real-time PCR analysis. The results demonstrated marked differences in CYP expression levels in P. solenopsis grown on different host plants. The results will aid the molecular characterization of CYPs and will increase our understanding of CYP expression patterns in P. solenopsis during development and growth on different hosts.

Genes Expressed Differentially in Hessian Fly Larvae Feeding in Resistant and Susceptible Plants

The Hessian fly, Mayetiola destructor, is a destructive pest of wheat worldwide and mainly controlled by deploying resistant cultivars. In this study, we investigated the genes that were expressed differentially between larvae in resistant plants and those in susceptible plants through RNA sequencing on the Illumina platform. Informative genes were 11,832, 14,861, 15,708, and 15,071 for the comparisons between larvae in resistant versus susceptible plants for 0.5, 1, 3, and 5 days, respectively, after larvae had reached the feeding site. The transcript abundance corresponding to 5401, 6902, 8457, and 5202 of the informative genes exhibited significant differences (p ≤ 0.05) in the respective paired comparisons. Overall, genes involved in nutrient metabolism, RNA and protein synthesis exhibited lower transcript abundance in larvae from resistant plants, indicating that resistant plants inhibited nutrient metabolism and protein production in larvae. Interestingly, the numbers of cytochrome P450 genes with higher transcript abundance in larvae from resistant plants were comparable to, or higher than those with lower transcript abundance, indicating that toxic chemicals from resistant plants may have played important roles in Hessian fly larval death. Our study also identified several families of genes encoding secreted salivary gland proteins (SSGPs) that were expressed at early stage of 1^st instar larvae and with more genes with higher transcript abundance in larvae from resistant plants. Those SSGPs are candidate effectors with important roles in plant manipulation.

Feeding on resistant rice leads to enhanced expression of defender against apoptotic cell death (OoDAD1) in the Asian rice gall midge

BACKGROUND

The Asian rice gall midge (Orseolia oryzae) is a destructive insect pest of rice. Gall midge infestation in rice triggers either compatible or incompatible interactions leading to survival or mortality of the feeding maggots, respectively. In incompatible interactions, generation of plant allelochemicals/defense molecules and/or inability of the maggots to continue feeding on the host initiate(s) apoptosis within the maggots. Unraveling these molecular events, triggered within the maggots as a response to feeding on resistant hosts, will enable us to obtain a better understanding of host resistance. The present study points towards the likely involvement of a defender against apoptotic cell death gene (DAD1) in the insect in response to the host defense.

RESULTS

The cDNA coding for the DAD1 orthologue in the rice gall midge (OoDAD1) consisted of 339 nucleotides with one intron of 85 bp and two exons of 208 and 131 nucleotides. The deduced amino acid sequence of OoDAD1 showed a high degree of homology (94.6%) with DAD1 orthologue from the Hessian fly (Mayetiola destructor)--a major dipteran pest of wheat. Southern hybridization analysis indicated that OoDAD1 was present as a single copy in the genomes of the Asian rice gall midge biotypes (GMB) 1, 4 and 4 M. In the interactions involving GMB4 with Jaya (susceptible rice host) the expression level of OoDAD1 in feeding maggots gradually increased to 3-fold at 96 hai (hours after infestation) and peaked to 3.5-fold at 96 hai when compared to that at 24 hai. In contrast, expression in maggots feeding on RP2068 (resistant host) showed a steep increase of more than 8-fold at 24 hai and this level was sustained at 48, 72 and 96 hai when compared with the level in maggots feeding on Jaya at 24 hai. Recombinant OoDAD1, expressed in E. coli cells, when injected into rice seedlings induced a hypersensitive response (HR) in the resistant rice host, RP2068, but not in the susceptible rice variety, Jaya.

CONCLUSIONS

The results indicate that the expression of OoDAD1 is triggered in the feeding maggots probably due to the host resistance response and therefore, is likely an important molecule in the initial stages of the interaction between the midge and its rice host.

Transcriptome analysis of the salivary glands of potato leafhopper, Empoasca fabae

The potato leafhopper, Empoasca fabae, is a pest of economic crops in the United States and Canada, where it causes damage known as hopperburn. Saliva, along with mechanical injury, leads to decreases in gas exchange rates, stunting and chlorosis. Although E. fabae saliva is known to induce plant responses, little knowledge exists of saliva composition at the molecular level. We subjected the salivary glands of E. fabae to Roche 454-pyrosequencing which resulted significant number (30,893) of expressed sequence tags including 2805 contigs and 28,088 singletons. A high number of sequences (78%) showed similarity to other insect species in GenBank, including Triboliumcastaneum, Drosophilamelanogaster and Acrythosiphonpisum. KEGG analysis predicted the presence of pathways for purine and thiamine metabolic, biosynthesis of secondary metabolites, drug metabolism, and lysine degradation. Pfam analysis showed a high number of cellulase and carboxylesterase protein domains. Expression analysis of candidate genes (alpha amylase, lipase, pectin lyase, etc.) among different tissues revealed tissue-specific expression of digestive enzymes in E. fabae. This is the first study to characterize the sialotranscriptome of E. fabae and the first for any species in the family of Cicadellidae. Due to the status of these insects as economic pests, knowledge of which genes are active in the salivary glands is important for understanding their impact on host plants.

Effects of antinutrient proteins on Hessian fly (Diptera: Cecidomyiidae) larvae

One strategy to enhance the durability of Hessian fly resistance (R) genes in wheat is to combine them with transgenes for resistance. To identify potential transgenes for resistance a protocol for rapidly screening the proteins they encode for efficacy toward resistance is required. However, the Hessian fly is an obligate parasite of wheat and related grasses. Consequently, no protocol for in vitro delivery of antinutrient or toxic proteins to feeding larvae is available. We report here the development of a Hessian fly in plantatranslocation (HIT) feeding assay and the evaluation of eight lectins and the Bowman-Birk serine proteinase inhibitor for potential in transgenic resistance. Of the antinutrient proteins evaluated, Galanthus nivalis L. agglutinin (GNA), commonly termed snowdrop lectin, was the most efficacious. Ingestion of GNA caused a significant reduction in growth of Hessian fly larvae, disruption of midgut microvilli, and changes in transcript level of genes involved in carbohydrate metabolism, digestion, detoxification, and stress response. These effects of GNA are discussed from the perspective of larval Hessian fly physiology.

Compatible interaction with its rice host leads to enhanced expression of the gamma subunit of oligosaccharyl transferase in the Asian rice gall midge, Orseolia oryzae

The Asian rice gall midge, Orseolia oryzae, is a fast evolving, damaging pest of rice. Understanding the underlying molecular mechanism of interaction between the gall midge and rice will help in devising strategies to control and manage the pest. The present study aims to identify rice-responsive genes in the gall midge that aid pest survival. The abundance of transcripts coding for enzymes related to glycosylation, in a cDNA library prepared from maggots of the rice gall midge feeding on susceptible hosts, indicated their probable involvement in the gall midge-rice interaction. Hence, a full-length transcript for a gamma subunit of the oligosaccharyl transferase gene (OoOST) from the gall midge was cloned and characterized. It has 72% similarity to its orthologue cloned from Aedes aegypti. Tissue-specific analysis of the expression of OoOST revealed an increase (> sevenfold) in the transcripts of the gene in the salivary glands of maggots in susceptible plants when compared with the transcript level in the salivary glands of maggots feeding on resistant hosts. Using quantitative PCR, performed on different developmental stages of the maggots in two susceptible and two resistant hosts, we observed similar expression patterns (i.e. overexpression in the compatible interaction). These results indicate the involvement of OoOST in maggot survival and establishment in the susceptible host. In order to identify polymorphism in the gene, OoOST was cloned from three gall midge biotypes GMB1, GMB4 and GMB4M.

Metabolic Resistance in Bed Bugs

Blood-feeding insects have evolved resistance to various insecticides (organochlorines, pyrethroids, carbamates, etc.) through gene mutations and increased metabolism. Bed bugs (Cimex lectularius) are hematophagous ectoparasites that are poised to become one of the major pests in households throughout the United States. Currently, C. lectularius has attained a high global impact status due to its sudden and rampant resurgence. Resistance to pesticides is one factor implicated in this phenomenon. Although much emphasis has been placed on target sensitivity, little to no knowledge is available on the role of key metabolic players (e.g., cytochrome P450s and glutathione S-transferases) towards pesticide resistance in C. lectularius. In this review, we discuss different modes of resistance (target sensitivity, penetration resistance, behavioral resistance, and metabolic resistance) with more emphasis on metabolic resistance.

Tissue-specific transcriptomics of the exotic invasive insect pest emerald ash borer (Agrilus planipennis)

BACKGROUND

The insect midgut and fat body represent major tissue interfaces that deal with several important physiological functions including digestion, detoxification and immune response. The emerald ash borer (Agrilus planipennis), is an exotic invasive insect pest that has killed millions of ash trees (Fraxinus spp.) primarily in the Midwestern United States and Ontario, Canada. However, despite its high impact status little knowledge exists for A. planipennis at the molecular level.

METHODOLOGY AND PRINCIPAL FINDINGS

Newer-generation Roche-454 pyrosequencing was used to obtain 126,185 reads for the midgut and 240,848 reads for the fat body, which were assembled into 25,173 and 37,661 high quality expressed sequence tags (ESTs) for the midgut and the fat body of A. planipennis larvae, respectively. Among these ESTs, 36% of the midgut and 38% of the fat body sequences showed similarity to proteins in the GenBank nr database. A high number of the midgut sequences contained chitin-binding peritrophin (248)and trypsin (98) domains; while the fat body sequences showed high occurrence of cytochrome P450s (85) and protein kinase (123) domains. Further, the midgut transcriptome of A. planipennis revealed putative microbial transcripts encoding for cell-wall degrading enzymes such as polygalacturonases and endoglucanases. A significant number of SNPs (137 in midgut and 347 in fat body) and microsatellite loci (317 in midgut and 571 in fat body) were predicted in the A. planipennis transcripts. An initial assessment of cytochrome P450s belonging to various CYP clades revealed distinct expression patterns at the tissue level.

CONCLUSIONS AND SIGNIFICANCE

To our knowledge this study is one of the first to illuminate tissue-specific gene expression in an invasive insect of high ecological and economic consequence. These findings will lay the foundation for future gene expression and functional studies in A. planipennis.

Decreased detoxification genes and genome size make the human body louse an efficient model to study xenobiotic metabolism

The human body louse, Pediculus humanus humanus, has one of the smallest insect genomes, containing ∼10 775 annotated genes. Annotation of detoxification [cytochrome P450 monooxygenase (P450), glutathione-S-transferase (GST), esterase (Est) and ATP-binding cassette transporter (ABC transporter)] genes revealed that they are dramatically reduced in P. h. humanus compared to other insects except for Apis mellifera. There are 37 P450, 13 GST and 17 Est genes present in P. h. humanus, approximately half the number found in Drosophila melanogaster and Anopheles gambiae. The number of putatively functional ABC transporter genes in P. h. humanus and Ap. mellifera are the same (36) but both have fewer than An. gambiae (44) or Dr. melanogaster (65). The reduction of detoxification genes in P. h. humanus may be a result of this louse's simple life history, in which it does not encounter a wide variety of xenobiotics. Neuronal component genes are highly conserved across different insect species as expected because of their critical function. Although reduced in number, P. h. humanus still retains at least a minimum repertoire of genes known to confer metabolic or toxicokinetic resistance to xenobiotics (eg Cyp3 clade P450s, Delta GSTs, B clade Ests and B/C subfamily ABC transporters), suggestive of its high potential for resistance development.

Ultrastructural changes in the midguts of Hessian fly larvae feeding on resistant wheat

The focus of the present study was to compare ultrastructure in the midguts of larvae of the Hessian fly, Mayetiola destructor (Say), under different feeding regimens. Larvae were either fed on Hessian fly-resistant or -susceptible wheat, and each group was compared to starved larvae. Within 3h of larval Hessian fly feeding on resistant wheat, midgut microvilli were disrupted, and after 6h, microvilli were absent. The disruption in microvilli in larvae feeding on resistant wheat were similar to those reported for midgut microvilli of European corn borer, Ostrinia nubilasis (Hubner), larvae fed a diet containing wheat germ agglutinin. Results from the present ultrastructural study, coupled with previous studies documenting expression of genes encoding lectin and lectin-like proteins is rapidly up-regulated in resistant wheat to larval Hessian fly, are indications that the midgut is a target of plant resistance compounds. In addition, the midgut of the larval Hessian fly is apparently unique among other dipterans in that no peritrophic membrane was observed. Ultrastructural changes in the midgut are discussed from the prospective of their potential affects on the gut physiology of Hessian fly larvae and the mechanism of antibiosis in the resistance of wheat to Hessian fly attack.

Metabolic enzymes associated with xenobiotic and chemosensory responses in Nasonia vitripennis

The numbers of glutathione S-transferase, cytochrome P450 and esterase genes in the genome of the hymenopteran parasitoid Nasonia vitripennis are about twice those found in the genome of another hymenopteran, the honeybee Apis mellifera. Some of the difference is associated with clades of these families implicated in xenobiotic resistance in other insects and some is in clades implicated in hormone and pheromone metabolism. The data support the hypothesis that the eusocial behaviour of the honeybee and the concomitant homeostasis of the nest environment may obviate the need for as many gene/enzyme systems associated with xenobiotic metabolism as are found in other species, including N. vitripennis, that are thought to encounter a wider range of potentially toxic xenobiotics in their diet and habitat.

Characterization and expression analysis of a gene encoding a secreted lipase-like protein expressed in the salivary glands of the larval Hessian fly, Mayetiola destructor (Say)

In a salivary gland transcriptomics study we identified a cDNA with a full-length open reading frame for a gene (MdesL1) encoding a lipase-like protein expressed in the salivary glands of the larval Hessian fly, Mayetiola destructor (Say). Fluorescent in situ hybridization on salivary polytenes positioned MdesL1 on the long arm of Autosome 1. BLASTp and conserved domain searches revealed the deduced amino acid sequence contained a lipase superfamily domain with similarity to lipases and phospholipases from other insects. A secretion signal peptide was identified at the amino terminus of the deduced amino acid sequence. Analysis of the transcript of MdesL1 in larval Hessian fly tissues by quantitative real-time PCR (qPCR) revealed the greatest abundance was in salivary glands. Analysis of transcript levels during development showed the greatest level was detected in feeding 1st-instar and early 2nd-instar larvae. Transcript levels increased dramatically over time in larvae feeding on susceptible wheat but were detected at low levels in larvae feeding on resistant wheat. These data suggest the protein encoded by MdesL1 is likely secreted into host-plant cells during larval feeding and could be involved in extra-oral digestion and changes in host-cell permeability or in generating a second messenger in a host-cell-signaling cascade.

Transcriptomic profiles of Drosophila melanogaster third instar larval midgut and responses to oxidative stress

Oligoarray analysis was used to determine the number and nature of genes expressed in third instar Drosophila melanogaster larval midguts. The majority of transcripts were associated with protein synthesis and metabolism. Serine proteases were the main proteolytic enzymes detected. Some 40% of the cytochrome P450 genes and 74% of the glutathione S transferases (GSTs) in the genome of D. melanogaster were observed to be expressed in the midgut by oligoarray analysis. We also identified potential transcription factor binding motifs (TFBMs) of P450s, GSTs and carboxylesterases. Many of the midgut-expressed GST genes contained candidate TFBMs homologous to TFBMs in mammals that have been associated with responses to oxidative stress. We also investigated the response of GSTs in the midgut to dietary H2O2, which showed a dosage-based differential response.

Tissue and developmental expression of a gene from Hessian fly encoding an ABC-active-transporter protein: implications for Malpighian tubule function during interactions with wheat

We report on the transcriptional patterns of a putative white (w) gene encoding an ABC-active-transporter protein during development in Hessian fly, Mayetiola destructor. The deduced amino acid sequence for the Hessian fly white showed 74-77% similarities to white/ATP-binding-cassette proteins and 52-57% similarities to scarlet/ATP-binding-cassette proteins from other dipterans. Conserved ATP-binding motifs and transmembrane alpha-helix segments were identified in the Hessian fly white protein further supporting its function as an ABC-active-transporter similar to the Drosophila white protein. Spatial analysis of transcript levels for white in larval Hessian fly tissues by quantitative real-time PCR revealed the greatest level of transcript in the Malpighian tubules, while analysis of temporal expression during development revealed the highest transcript levels in late 2nd- and early 3rd-instar larvae. Analysis of transcript levels for white in Hessian fly larvae feeding on susceptible and resistant wheat showed greater levels of the transcript in larvae feeding on resistant plants. We speculate the increased transcript level for white in larvae feeding on resistant wheat could be correlated with stress and increased Malpighian tubule activity associated with the metabolism and detoxification of toxic substrates generated either endogenously or encountered exogenously from the host plant.

Molecular characterization and responsive expression of a defender against apoptotic cell death homologue from the Hessian fly, Mayetiola destructor

Apoptosis or programmed cell death is an active process occurring in multicellular organisms to maintain growth and development. The Hessian fly, Mayetiola destructor, is rapidly emerging as a model insect species to study insect-plant interactions and to decipher some exceptional physiological phenomena. In this study, we report the characterization and expression profiles of a putative Hessian fly defender against apoptotic cell death (DAD1) homologue designated MdesDAD1. The deduced amino acid sequence of MdesDAD1 revealed significant similarity (75% identity, 9e-42) to other insect and non-insect DAD1 sequences. Phylogenetic analysis grouped MdesDAD1 within a sub-clade consisting of other insect DAD1 homologues. Quantitative analysis indicated constitutive levels of MdesDAD1 mRNA in all the tissues examined but an altered expression pattern during development, wherein the highest mRNA levels observed were prior to pupation. Most interestingly, MdesDAD1 transcript was found to be up-regulated during incompatible (larvae reared on resistant wheat) Hessian fly/wheat interactions compared to compatible (larvae reared on susceptible wheat) interactions. These results suggest MdesDAD1 to have a putative role in the inhibition of unwanted apoptosis triggered during development and in incompatible Hessian fly/wheat interactions. The results obtained provide clues to plausible insect and host-plant factors that could be responsible for the induction of MdesDAD1.

Antioxidant defense response in a galling insect

Herbivorous insect species are constantly challenged with reactive oxygen species (ROS) generated from endogenous and exogenous sources. ROS produced within insects because of stress and prooxidant allelochemicals produced by host plants in response to herbivory require a complex mode of antioxidant defense during insect/plant interactions. Some insect herbivores have a midgut-based defense against the suite of ROS encountered. Because the Hessian fly (Mayetiola destructor) is the major insect pest of wheat worldwide, and an emerging model for all gall midges, we investigated its antioxidant responses during interaction with its host plant. Quantitative data for two phospholipid glutathione peroxidases (MdesPHGPX-1 and MdesPHGPX-2), two catalases (MdesCAT-1 and MdesCAT-2), and two superoxide dismutases (MdesSOD-1 and MdesSOD-2) revealed high levels of all of the mRNAs in the midgut of larvae on susceptible wheat (compatible interaction). During development of the Hessian fly on susceptible wheat, a differential expression pattern was observed for all six genes. Analysis of larvae on resistant wheat (incompatible interaction) compared with larvae on susceptible wheat showed increased levels of mRNAs in larvae on resistant wheat for all of the antioxidant genes except MdesSOD-1 and MdesSOD-2. We postulate that the increased mRNA levels of MdesPHGPX-1, MdesPHGPX-2, MdesCAT-1, and MdesCAT-2 reflect responses to ROS encountered by larvae while feeding on resistant wheat seedlings and/or ROS generated endogenously in larvae because of stress/starvation. These results provide an opportunity to understand the cooperative antioxidant defense responses in the Hessian fly/wheat interaction and may be applicable to other insect/plant interactions.

cDNA cloning and transcriptional expression of a peritrophin-like gene in the Hessian fly, Mayetiola destructor [Say]

One of the well-studied components of the insect gut is the peritrophic matrix (PM). This semipermeable structure primarily functions in digestion, and protection against invasive microorganisms and mechanical damage. We report the cDNA cloning and transcription profiles of a peritrophin-A like gene (designated MdesPERI-A1) in the Hessian fly Mayetiola destructor. The predicted amino acid sequence of MdesPERI-A1 revealed a putative secretion signal peptide at its amino terminus, similarity to peritrophins from other insects including dipterans, and the presence of two chitin binding domains each containing six cysteine residues. Quantitative expression analysis of MdesPERI-A1 mRNA in different larval tissues revealed the transcript to be predominantly present in the midgut (597.9-fold) compared to other tissues assayed including salivary glands and fat bodies. Spatial expression patterns during development showed a peak expression of MdesPERI-A1 in the feeding second-instars (146-fold) and a decline in expression in the pupal and adult stages. Transcription profiling of MdesPERI-A1 during compatible (larvae on susceptible plants) and incompatible (larvae on resistant plants) interactions with wheat revealed a greater level (1.7-fold) of MdesPERI-A1 transcript in larvae on resistant plants in the initial time point examined. However, MdesPERI-A1 expression declined in larvae on resistant plants at the later time points.

Expression patterns of antibacterial genes in the Hessian fly

We report on the transcriptional patterns of three antibacterial genes, a defensin (MdesDEF-1), a diptericin (MdesDIP-1) and a lysozyme (MdesLYS-1), during development in Hessian fly, Mayetiola destructor. Quantitative analysis by real-time PCR of mRNA levels in different tissues revealed a predominance of the transcripts for all three genes in the midgut, while analysis during development revealed greatest abundance in mRNA during the 3rd-instar. An evaluation of the midgut lumen revealed the presence of a diverse bacterial flora in larvae maintained on susceptible wheat. Further, the titer of bacteria in the midgut increased approximately 250-fold from the 1st-instar through the 2nd-instar. However, no detectable titer of bacteria was observed from the midgut lumen of larvae maintained on resistant plants. PCR amplicons produced using primers designed to conserved regions of the Pseudomonas 16S rRNA gene supported taxonomic identification for some of the bacteria comprising the midgut flora as belonging to the genus Pseudomonas. Analysis of mRNA for the Hessian fly antibacterial genes in larvae feeding on susceptible and resistant plants revealed an increase in the transcript level for MdesDEF-1 in 1st-instar larvae on susceptible plants, while the transcript levels for MdesDIP-1 and MdesLYS-1 were constant. Results suggest the transcriptional patterns of the Hessian fly antibacterial genes observed could be associated with the developing midgut bacterial flora present in larvae feeding on susceptible wheat as well as microbial challenge encountered at other stages in development.

Gene-for-gene defense of wheat against the Hessian fly lacks a classical oxidative burst

Genetic similarities between plant interactions with microbial pathogens and wheat interactions with Hessian fly larvae prompted us to investigate defense and counterdefense mechanisms. Plant oxidative burst, a rapid increase in the levels of active oxygen species (AOS) within the initial 24 h of an interaction with pathogens, commonly is associated with defenses that are triggered by gene-for-gene recognition events similar to those involving wheat and Hessian fly larvae. RNAs encoded by Hessian fly superoxide dismutase (SOD) and catalase (CAT) genes, involved in detoxification of AOS, increased in first-instar larvae during both compatible and incompatible interactions. However, mRNA levels of a wheat NADPH oxidase (NOX) gene that generates superoxide (O2-) did not increase. In addition, inhibiting wheat NOX enzyme with diphenyleneiodonium did not result in increased survival of avirulent larvae. However, nitro blue tetrazolium staining indicated that basal levels of O2- are present in both uninfested and infested wheat tissue. mRNA encoded by wheat genes involved in detoxification of the cellular environment, SOD, CAT, and glutathione-S-transferase did not increase in abundance. Histochemical staining with 3,3-diaminobenzidine revealed no increases in wheat hydrogen peroxide (H2O2) during infestation that were correlated with the changes in larval SOD and CAT mRNA. However, treatment with 2',7'-dichlorofluorescin demonstrated the presence of basal levels of H2O2 in the elongation zone of both infested and uninfested plants. The accumulation of a wheat flavanone 3-hydroxylase mRNA did show some parallels with larval gene mRNA profiles. These results suggested that larvae encounter stresses imposed by mechanisms other than an oxidative burst in wheat seedlings.