A novel wheat gene encoding a putative chitin-binding lectin is associated with resistance against Hessian fly

Plant Secondary Metabolites: The Weapons for Biotic Stress Management

The rise in global temperature also favors the multiplication of pests and pathogens, which calls into question global food security. Plants have developed special coping mechanisms since they are sessile and lack an immune system. These mechanisms use a variety of secondary metabolites as weapons to avoid obstacles, adapt to their changing environment, and survive in less-than-ideal circumstances. Plant secondary metabolites include phenolic compounds, alkaloids, glycosides, and terpenoids, which are stored in specialized structures such as latex, trichomes, resin ducts, etc. Secondary metabolites help the plants to be safe from biotic stressors, either by repelling them or attracting their enemies, or exerting toxic effects on them. Modern omics technologies enable the elucidation of the structural and functional properties of these metabolites along with their biosynthesis. A better understanding of the enzymatic regulations and molecular mechanisms aids in the exploitation of secondary metabolites in modern pest management approaches such as biopesticides and integrated pest management. The current review provides an overview of the major plant secondary metabolites that play significant roles in enhancing biotic stress tolerance. It examines their involvement in both indirect and direct defense mechanisms, as well as their storage within plant tissues. Additionally, this review explores the importance of metabolomics approaches in elucidating the significance of secondary metabolites in biotic stress tolerance. The application of metabolic engineering in breeding for biotic stress resistance is discussed, along with the exploitation of secondary metabolites for sustainable pest management.

Impact of Heat Stress on Expression of Wheat Genes Responsive to Hessian Fly Infestation

Heat stress compromises wheat (Triticum aestivium) resistance to Hessian fly (HF, Mayetiola destructor (Say)). This study aimed to investigate the impact of heat stress on transcript expression of wheat genes associated with resistance to HF infestation under normal and heat-stressed conditions. To this end, ‘Molly’, a wheat cultivar containing the resistance gene H13, was subjected to HF infestation, heat stress, and the combination of HF infestation and heat stress. Our RNA-Seq approach identified 21 wheat genes regulated by HF infestation under normal temperatures (18 °C) and 155 genes regulated by HF infestation when plants were exposed to 35 °C for 6 h. Three differentially expressed genes (DEGs) from the RNA-Seq analysis were selected to validate the gene function of these DEGs using the RT-qPCR approach, indicating that these DEGs may differentially contribute to the expression of wheat resistance during the early stage of wheat–HF interaction under various stresses. Moreover, the jasmonate ZIM domain (JAZ) gene was also significantly upregulated under these treatments. Our results suggest that the genes in heat-stressed wheat plants are more responsive to HF infestation than those in plants growing under normal temperature conditions, and these genes in HF-infested wheat plants are more responsive to heat stress than those in plants without infestation.

Biostimulant impacts of Glutacetine® and derived formulations (VNT1 and VNT4) on the bread wheat grain proteome

Nitrogen (N) fertilizer is essential to ensure grain yield and quality in bread wheat. Improving N use efficiency is therefore crucial for wheat grain protein quality. In the present work, we analysed the effects on the winter wheat grain proteome of biostimulants containing Glutacetine® or two derived formulations (VNT1 and 4) when mixed with urea-ammonium-nitrate fertilizer. A large-scale quantitative proteomics analysis of two wheat flour fractions produced a dataset of 4369 identified proteins. Quantitative analysis revealed 9, 39 and 96 proteins with a significant change in abundance after Glutacetine®, VNT1 and VNT4 treatments, respectively, with a common set of 11 proteins that were affected by two different biostimulants. The major effects impacted proteins involved in (i) protein synthesis regulation (mainly ribosomal and binding proteins), (ii) defence and responses to stresses (including chitin-binding protein, heat shock 70 kDa protein 1 and glutathione S-transferase proteins), (iii) storage functions related to gluten protein alpha-gliadins and starch synthase and (iv) seed development with proteins implicated in protease activity, energy machinery, and the C and N metabolism pathways. Altogether, our study showed that Glutacetine®, VNT1 and VNT4 biostimulants positively affected protein composition related to grain quality. Data are available via ProteomeXchange with identifier PXD021513. SIGNIFICANCE: We performed a large-scale quantitative proteomics study of the total protein extracts from flour samples to determine the effect of Glutacetine®-based biostimulants treatment on the protein composition of bread wheat grain. To our knowledge, only a few studies in the literature have applied proteomic approaches to study bread wheat grains and in particular to investigate the effect of biostimulants on the grain proteome of this cereal crop. In addition, most approaches used fractional extraction of proteins to target reserve proteins followed electrophoresis which leads to low identification rate of proteins. We identified and quantified a large protein dataset of 4369 proteins and determined ontological class of proteins affected by biostimulants treatments. Our proteomics investigation revealed the important role of these new biostimulants in achieving significant changes in protein synthesis regulation, storage functions, protease activity, energy machinery, C and N metabolism pathways and responses to biotic and abiotic stresses in grain.

Insects-plants-pathogens: Toxicity, dependence and defense dynamics

In a natural ecosystem, the pathogen-plant-insect relationship has diverse implications for each other. The pathogens as well as insect-pests consume plant tissues as their feed that mostly results in damage. In turn, plant species have evolved specialized defense system to not only protect themselves but reduce the damage also. Such tripartite interactions involve toxicity, metabolic modulations, resistance etc. among all participants of interaction. These attributes result in selection pressure among participants. Coevolution of such traits reveals need to focus and unravel multiple hidden aspects of insect-plant-pathogen interactions. The definite modulations during plant responses to biotic stress and the operating defense network against herbivores are vital to research areas. Different types of plant pathogens and herbivores are tackled with various changes in plants, e.g. changes in genes expression, glucosinolate metabolism detoxification, signal transduction, cell wall modifications, Ca²⁺dependent signaling. It is essential to clarify which chemical in plants can work as a defense signal or weapon in plant-pathogen-herbivore interactions. In spite of increased knowledge regarding signal transduction pathways regulating growth-defense balance, much more is needed to unveil the coordination of growth rate with metabolic modulations in bi-trophic interactions. Here, we addressed plant-pathogen-insect interaction for toxicity as well as dependnce along with plant defense dynamics against pathogens and insects with broad range effects at the physio-biochemical and molecular level. We have reviewed interfaces in plant-pathogen-insect research to show pulsating regulation of plant immunity for attuning survival and ecological equilibrium. An improved understanding of the systematic foundation of growth-defense stability has vital repercussions for enhancing crop yield, including insights into uncoupling of host-parasite tradeoffs for ecological and environmental sustainability.

Mayetiola destructor (Diptera: Cecidmyiidae) host preference and survival on small grains with respect to leaf reflectance and phytohormone concentrations

The Hessian fly Mayetiola destructor (Diptera: Cecidmyiidae) is a major pest of wheat, globally. We conducted a series of laboratory choice and no-choice assays to quantify Hessian fly host preference for barley (cv. Champion), oat (cv. Cayuse), susceptible (cv. Alturas), and resistant (cv. Hollis) wheat. In addition, larval survivorship and adult emergence were compared among the evaluated host plants. We then examined whether insect preference for a host can be explained by differences in plant spectral reflectance. Further, larval survivorship and adult emergence were compared among host plants in relation to phytohormone concentrations. Hessian flies laid more eggs on wheat compared to either oat or barley. Spectral reflectance measurements of leaves were similar between susceptible and resistant wheat cultivars but different from those of barley and oat. Our results suggested that higher reflectance in the near-infrared range and lower reflectance in the visible range may be used by females for host selection. Hessian fly larvae were unable to develop into the pupal stage on resistant wheat and oat. No significant difference in larval survivorship was detected between the susceptible wheat and barley. However, adult emergence was significantly higher on barley than the susceptible wheat. Phytohormonal evaluations revealed that salicylic acid (SA) may be an important contributor to plant defense response to larval feeding as relatively higher concentrations of SA were present in oat and resistant wheat. While resistance in the resistant wheat is achieved only through antibiosis, both antibiosis and antixenosis were in effect rendering oat as a non-host for Hessian flies.

The Hessian fly recessive resistance gene h4 mapped to chromosome 1A of the wheat cultivar 'Java' using genotyping-by-sequencing

The recessive Hessian fly resistance gene h4 and flanking SNP markers were located to a 642 kb region in chromosome 1A of the wheat cultivar 'Java.' Hessian fly (HF), Mayetiola destructor, is one of the most destructive insect pests in wheat worldwide. The wheat cultivar 'Java' was reported to carry a recessive gene (h4) for HF resistance; however, its chromosome location has not been determined. To map the HF resistance gene in Java, two populations of recombinant inbred lines (RILs) were developed from 'Bobwhite' × Java and 'Overley' × Java, respectively, and were phenotyped for responses to infestation of HF Great Plains biotype. Analysis of phenotypic data from the F₁ and the RIL populations confirmed that one recessive gene conditioned HF resistance in Java. Two linkage maps were constructed using single-nucleotide polymorphism (SNP) markers generated by genotyping-by-sequencing (GBS). The h4 gene was mapped to the distal end of the short arm of chromosome 1A, which explained 60.4 to 70.5% of the phenotypic variation for HF resistance in the two populations. The GBS-SNPs in the h4 candidate interval were converted into Kompetitive Allele-Specific Polymerase Chain Reaction (KASP) markers to eliminate the missing data points in GBS-SNPs. Using the revised maps with KASP markers, h4 was further located to a 642 kb interval (6,635,984-7,277,935 bp). The two flanking KASP markers, KASP3299 and KASP1871, as well as four other closely linked KASP markers, may be useful for pyramiding h4 with other HF resistance genes in breeding.

Analyzing Molecular Basis of Heat-Induced Loss-of-Wheat Resistance to Hessian Fly (Diptera: Cecidomyiidae) Infestation Using RNA-Sequencing

Heat stress compromises wheat resistance to Hessian fly (HF, Mayetiola destructor (Say)) (Diptera: Cecidomyiidae) infestation. The objective of this research is to analyze the molecular basis of heat-induced loss of wheat resistance to HF infestation using RNA Sequencing (RNA-seq). To this end, two resistant wheat cultivars 'Molly' and 'Caldwell' containing the resistance genes H13 and H6, respectively, were infested with an avirulent HF biotype GP and treated with different temperatures to examine the impact of heat stress on their resistance phenotypes. Tissue samples collected from HF feeding sites in Molly plants were subjected to RNA-seq analysis to determine the effect of heat stress on transcript expression of genes in wheat plants. Our results indicate that resistance to HF infestation in Caldwell is more sensitive to heat stress than that in Molly, and that heat stress down-regulates most genes involved in primary metabolism and biosynthesis of lignin and cuticular wax, but up-regulate most or all genes involved in auxin and 12-oxo-phytodienoic acid (OPDA) signaling pathways. Our results and previous reports suggest that heat stress may impair the processes in wheat plants that produce and mobilize chemical resources needed for synthesizing defensive compounds, weaken cell wall and cuticle defense, decrease OPDA signaling, but increase auxin signaling, leading to the suppressed resistance and activation of susceptibility.

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.

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.

Three MYB genes co-regulate the phloem-based defence against English grain aphid in wheat

Plant phloem-based defence (PBD) against phloem-feeding insects is characteristic of the sieve occlusion by phloem lectins and β-1,3-glucan callose, both of which are produced under regulation by ethylene and MYB transcription factors. Wheat PBD requires β-1,3-glucan synthase-like proteins GSL2, GSL10, and GSL12, and may also require insect-resistant mannose-binding lectins Hfr-1 and Wci-1, which can accumulate in the phloem upon aphid feeding. This study elucidates whether any of the 73 MYB genes identified previously in the common wheat Triticum aestivum genome plays a role in wheat PBD activation with regard to the GSLs and lectins. Wheat MYB genes TaMYB19, TaMYB29, and TaMYB44 are highly activated in response to infestation of English grain aphid, and their silencing facilitates aphid feeding on wheat phloem and represses wheat PBD responses. Repressed PBD is shown to decrease aphid-induced callose deposition in wheat leaf epidermis and decrease aphid-induced expression of genes GSL2, GSL10, GSL12, Hfr-1, and Wci-1 in wheat leaf tissues. Based on single gene silencing effects, TaMYB19, TaMYB29, and TaMYB44 contribute 55-82% of PBD responses. However, the contributions of TaMYB genes to PBD are eliminated by ethylene signalling inhibitors, while simultaneous silencing of the three TaMYB genes cancels the tested PBD responses. Therefore, TaMYB19, TaMYB29, and TaMYB44 are co-regulators of wheat PBD and execute this function through crosstalk with the ethylene signalling pathway.

Large-scale identification of wheat genes resistant to cereal cyst nematode Heterodera avenae using comparative transcriptomic analysis

Background

Cereal cyst nematode Heterodera avenae, an important soil-borne pathogen in wheat, causes numerous annual yield losses worldwide, and use of resistant cultivars is the best strategy for control. However, target genes are not readily available for breeding resistant cultivars. Therefore, comparative transcriptomic analyses were performed to identify more applicable resistance genes for cultivar breeding.

Methods

The developing nematodes within roots were stained with acid fuchsin solution. Transcriptome assemblies and redundancy filteration were obtained by Trinity, TGI Clustering Tool and BLASTN, respectively. Gene Ontology annotation was yielded by Blast2GO program, and metabolic pathways of transcripts were analyzed by Path_finder. The ROS levels were determined by luminol-chemiluminescence assay. The transcriptional gene expression profiles were obtained by quantitative RT-PCR.

Results

The RNA-sequencing was performed using an incompatible wheat cultivar VP1620 and a compatible control cultivar WEN19 infected with H. avenae at 24 h, 3 d and 8 d. Infection assays showed that VP1620 failed to block penetration of H. avenae but disturbed the transition of developmental stages, leading to a significant reduction in cyst formation. Two types of expression profiles were established to predict candidate resistance genes after developing a novel strategy to generate clean RNA-seq data by removing the transcripts of H. avenae within the raw data before assembly. Using the uncoordinated expression profiles with transcript abundance as a standard, 424 candidate resistance genes were identified, including 302 overlapping genes and 122 VP1620-specific genes. Genes with similar expression patterns were further classified according to the scales of changed transcript abundances, and 182 genes were rescued as supplementary candidate resistance genes. Functional characterizations revealed that diverse defense-related pathways were responsible for wheat resistance against H. avenae. Moreover, phospholipase was involved in many defense-related pathways and localized in the connection position. Furthermore, strong bursts of reactive oxygen species (ROS) within VP1620 roots infected with H. avenae were induced at 24 h and 3 d, and eight ROS-producing genes were significantly upregulated, including three class III peroxidase and five lipoxygenase genes.

Conclusions

Large-scale identification of wheat resistance genes were processed by comparative transcriptomic analysis. Functional characterization showed that phospholipases associated with ROS production played vital roles in early defense responses to H. avenae via involvement in diverse defense-related pathways as a hub switch. This study is the first to investigate the early defense responses of wheat against H. avenae, not only provides applicable candidate resistance genes for breeding novel wheat cultivars, but also enables a better understanding of the defense mechanisms of wheat against H. avenae.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2037-8) contains supplementary material, which is available to authorized users.

Toxic proteins in plants

Plants have evolved to synthesize a variety of noxious compounds to cope with unfavorable circumstances, among which a large group of toxic proteins that play a critical role in plant defense against predators and microbes. Up to now, a wide range of harmful proteins have been discovered in different plants, including lectins, ribosome-inactivating proteins, protease inhibitors, ureases, arcelins, antimicrobial peptides and pore-forming toxins. To fulfill their role in plant defense, these proteins exhibit various degrees of toxicity towards animals, insects, bacteria or fungi. Numerous studies have been carried out to investigate the toxic effects and mode of action of these plant proteins in order to explore their possible applications. Indeed, because of their biological activities, toxic plant proteins are also considered as potentially useful tools in crop protection and in biomedical applications, such as cancer treatment. Genes encoding toxic plant proteins have been introduced into crop genomes using genetic engineering technology in order to increase the plant's resistance against pathogens and diseases. Despite the availability of ample information on toxic plant proteins, very few publications have attempted to summarize the research progress made during the last decades. This review focuses on the diversity of toxic plant proteins in view of their toxicity as well as their mode of action. Furthermore, an outlook towards the biological role(s) of these proteins and their potential applications is discussed.

Precisely mapping a major gene conferring resistance to Hessian fly in bread wheat using genotyping-by-sequencing

Background

One of the reasons hard red winter wheat cultivar ‘Duster’ (PI 644016) is widely grown in the southern Great Plains is that it confers a consistently high level of resistance to biotype GP of Hessian fly (Hf). However, little is known about the genetic mechanism underlying Hf resistance in Duster. This study aimed to unravel complex structures of the Hf region on chromosome 1AS in wheat by using genotyping-by-sequencing (GBS) markers and single nucleotide polymorphism (SNP) markers.

Results

Doubled haploid (DH) lines generated from a cross between two winter wheat cultivars, ‘Duster’ and ‘Billings’ , were used to identify genes in Duster responsible for effective and consistent resistance to Hf. Segregation in reaction of the 282 DH lines to Hf biotype GP fit a one-gene model. The DH population was genotyped using 2,358 markers developed using the GBS approach. A major QTL, explaining 88% of the total phenotypic variation, was mapped to a chromosome region that spanned 178 cM and contained 205 GBS markers plus 1 SSR marker and 1 gene marker, with 0.86 cM per marker in genetic distance. The analyses of GBS marker sequences and further mapping of SSR and gene markers enabled location of the QTL-containing linkage group on the short arm of chromosome 1A. Comparative mapping of the common markers for the gene for QHf.osu-1A^d in Duster and the Hf-resistance gene for QHf.osu-1A⁷⁴ in cultivar ‘2174’ showed that the two Hf resistance genes are located on the same chromosome arm 1AS, only 11.2 cM apart in genetic distance. The gene at QHf.osu-1A^d in Duster has been delimited within a 2.7 cM region.

Conclusion

Two distinct resistance genes exist on the short arm of chromosome 1A as found in the two hard red winter cultivars, 2174 and Duster. Whereas the Hf resistance gene in 2174 is likely allelic to one or more of the previously mapped resistance genes (H9, H10, H11, H16, or H17) in wheat, the gene in Duster is novel and confers a more consistent phenotype than 2174 in response to biotype GP infestation in controlled-environment assays.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1297-7) contains supplementary material, which is available to authorized users.

Insecticidal activity of plant lectins and potential application in crop protection

Lectins constitute a complex group of proteins found in different organisms. These proteins constitute an important field for research, as their structural diversity and affinity for several carbohydrates makes them suitable for numerous biological applications. This review addresses the classification and insecticidal activities of plant lectins, providing an overview of the applicability of these proteins in crop protection. The likely target sites in insect tissues, the mode of action of these proteins, as well as the use of lectins as biotechnological tools for pest control are also described. The use of initial bioassays employing artificial diets has led to the most recent advances in this field, such as plant breeding and the construction of fusion proteins, using lectins for targeting the delivery of toxins and to potentiate expected insecticide effects. Based on the data presented, we emphasize the contribution that plant lectins may make as tools for the development of integrated insect pest control strategies.

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.

Treatment of Amaranthus cruentus with chemical and biological inducers of resistance has contrasting effects on fitness and protection against compatible Gram positive and Gram negative bacterial pathogens

Amaranthus cruentus (Ac) plants were treated with the synthetic systemic acquired resistance (SAR) inducer benzothiadiazole (BTH), methyl jasmonate (MeJA) and the incompatible pathogen, Pseudomonas syringae pv. syringae (Pss), under greenhouse conditions. The treatments induced a set of marker genes in the absence of pathogen infection: BTH and Pss similarly induced genes coding for pathogenesis-related and antioxidant proteins, whereas MeJA induced the arginase, LOX2 and amarandin 1 genes. BTH and Pss were effective when tested against the Gram negative pathogen Ps pv. tabaci (Pst), which was found to have a compatible interaction with grain amaranth. The resistance response appeared to be salicylic acid-independent. However, resistance against Clavibacter michiganensis subsp. michiganensis (Cmm), a Gram positive tomato pathogen also found to infect Ac, was only conferred by Pss, while BTH increased susceptibility. Conversely, MeJA was ineffective against both pathogens. Induced resistance against Pst correlated with the rapid and sustained stimulation of the above genes, including the AhPAL2 gene, which were expressed both locally and distally. The lack of protection against Cmm provided by BTH, coincided with a generalized down-regulation of defense gene expression and chitinase activity. On the other hand, Pss-treated Ac plants showed augmented expression levels of an anti-microbial peptide gene and, surprisingly, of AhACCO, an ethylene biosynthetic gene associated with susceptibility to Cmm in tomato, its main host. Pss treatment had no effect on productivity, but compromised growth, whereas MeJA reduced yield and harvest index. Conversely, BTH treatments led to smaller plants, but produced significantly increased yields. These results suggest essential differences in the mechanisms employed by biological and chemical agents to induce SAR in Ac against bacterial pathogens having different infection strategies. This may determine the outcome of a particular plant-pathogen interaction, leading to resistance or susceptibility, as in Cmm-challenged Ac plants previously induced with Pss or BTH, respectively.

Allergen ligands in the initiation of allergic sensitization

As investigations into the innate immune responses that lead to allergic sensitization become better defined, there is a need to determine how allergens could interact with pattern recognition receptors that bind non-proteinaceous moieties. Many important allergens are not covalently bound to lipid or carbohydrate, but have structures belonging to lipid, glycan and glycolipid-binding families. These include ML-domain proteins, lipopolysaccharide-binding/cell permeability-increasing proteins, von Ebner gland lipocalins, salivary lipocalins/major urinary proteins, plant pathogenesis-related proteins PR-5 and -10, uteroglobins, non-specific lipid transfer proteins, large lipid transfer proteins and proteins with chitin and other carbohydrate-binding modules. The binding expected is overviewed with regard to importance of the allergens and their ability to elicit responses proposed from experimental models. The evidence compiled showing that allergens from the same source sensitize for different types of adaptive immune responses supports the concept that individual allergens within these sources have their own distinctive interactions with innate immunity.

Genetic association of OPR genes with resistance to Hessian fly in hexaploid wheat

BACKGROUND

Hessian fly (Mayetiola destructor) is one of the most destructive pests of wheat. The genes encoding 12-oxo-phytodienoic acid reductase (OPR) and lipoxygenase (LOX) play critical roles in insect resistance pathways in higher plants, but little is known about genes controlling resistance to Hessian fly in wheat.

RESULTS

In this study, 154 F6:8 recombinant inbred lines (RILs) generated from a cross between two cultivars, 'Jagger' and '2174' of hexaploid wheat (2n = 6 × =42; AABBDD), were used to map genes associated with resistance to Hessian fly. Two QTLs were identified. The first one was a major QTL on chromosome 1A (QHf.osu-1A), which explained 70% of the total phenotypic variation. The resistant allele at this locus in cultivar 2174 could be orthologous to one or more of the previously mapped resistance genes (H9, H10, H11, H16, and H17) in tetraploid wheat. The second QTL was a minor QTL on chromosome 2A (QHf.osu-2A), which accounted for 18% of the total phenotypic variation. The resistant allele at this locus in 2174 is collinear to an Yr17-containing-fragment translocated from chromosome 2N of Triticum ventricosum (2n = 4 × =28; DDNN) in Jagger. Genetic mapping results showed that two OPR genes, TaOPR1-A and TaOPR2-A, were tightly associated with QHf.osu-1A and QHf.osu-2A, respectively. Another OPR gene and three LOX genes were mapped but not associated with Hessian fly resistance in the segregating population.

CONCLUSIONS

This study has located two major QTLs/genes in bread wheat that can be directly used in wheat breeding programs and has also provided insights for the genetic association and disassociation of Hessian fly resistance with OPR and LOX genes in wheat.

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.

A virus responds instantly to the presence of the vector on the host and forms transmission morphs

Many plant and animal viruses are spread by insect vectors. Cauliflower mosaic virus (CaMV) is aphid-transmitted, with the virus being taken up from specialized transmission bodies (TB) formed within infected plant cells. However, the precise events during TB-mediated virus acquisition by aphids are unknown. Here, we show that TBs react instantly to the presence of the vector by ultra-rapid and reversible redistribution of their key components onto microtubules throughout the cell. Enhancing or inhibiting this TB reaction pharmacologically or by using a mutant virus enhanced or inhibited transmission, respectively, confirming its requirement for efficient virus-acquisition. Our results suggest that CaMV can perceive aphid vectors, either directly or indirectly by sharing the host perception. This novel concept in virology, where viruses respond directly or via the host to the outside world, opens new research horizons, that is, investigating the impact of ‘perceptive behaviors’ on other steps of the infection cycle.

DOI:http://dx.doi.org/10.7554/eLife.00183.001

Viruses are infectious agents that can replicate only inside a living host cell. When a virus infects an animal or plant, it introduces its own genetic material and tricks the host cells into producing viral proteins that can be used to assemble new viruses. An essential step in the life cycle of any virus is transmission to a new host: understanding this process can be crucial in the fight against viral epidemics.

Many viruses use living organisms, or vectors, to move between hosts. In the case of plant viruses such as cauliflower mosaic virus, the vectors are often aphids. When an aphid sucks sap out of a leaf, virus particles already present in the leaf become attached to its mouth, and these viruses can be transferred to the next plant that the insect feeds on. However, in order for cauliflower mosaic virus particles to become attached to the aphid, structures called transmission bodies must form beforehand in the infected plant cells. These structures are known to contain helper proteins that bind the viruses to the mouth of the aphid, but the precise role of the transmission body has remained obscure.

Now Martinière et al. show that the transmission body is in fact a dynamic structure that reacts to the presence of aphids and, in so doing, boosts the efficiency of viral transmission. In particular, they show that the action of an aphid feeding on an infected leaf triggers a rapid and massive influx of a protein called tubulin into the transmission body. The transmission body then bursts open, dispersing helper protein-virus particle complexes throughout the cell, where they become more accessible to aphids. This series of events increases viral transmission rates twofold to threefold.

The results show that a virus can detect insect vectors, likely by using the sensory system of its host, and trigger a response that boosts viral uptake and thus transmission. This is a novel concept in virology. It will be important to discover whether similar mechanisms are used by other viruses, including those that infect animals and humans.

DOI:http://dx.doi.org/10.7554/eLife.00183.002

Mechanisms of plant defense against insect herbivores

Plants respond to herbivory through various morphological, biochemicals, and molecular mechanisms to counter/offset the effects of herbivore attack. The biochemical mechanisms of defense against the herbivores are wide-ranging, highly dynamic, and are mediated both by direct and indirect defenses. The defensive compounds are either produced constitutively or in response to plant damage, and affect feeding, growth, and survival of herbivores. In addition, plants also release volatile organic compounds that attract the natural enemies of the herbivores. These strategies either act independently or in conjunction with each other. However, our understanding of these defensive mechanisms is still limited. Induced resistance could be exploited as an important tool for the pest management to minimize the amounts of insecticides used for pest control. Host plant resistance to insects, particularly, induced resistance, can also be manipulated with the use of chemical elicitors of secondary metabolites, which confer resistance to insects. By understanding the mechanisms of induced resistance, we can predict the herbivores that are likely to be affected by induced responses. The elicitors of induced responses can be sprayed on crop plants to build up the natural defense system against damage caused by herbivores. The induced responses can also be engineered genetically, so that the defensive compounds are constitutively produced in plants against are challenged by the herbivory. Induced resistance can be exploited for developing crop cultivars, which readily produce the inducible response upon mild infestation, and can act as one of components of integrated pest management for sustainable crop production.

Insecticidal activity of wheat Hessian fly responsive proteins HFR-1 and HFR-3 towards a non-target wheat pest, cereal aphid (Sitobion avenae F.)

The interaction between Hessian fly (Mayetiola destructor) and wheat (Triticum aestivum) involves a gene-for-gene resistance mechanism. The incompatible interaction leading to resistance involves up-regulation of several Hfr (Hessian fly responsive) genes encoding proteins with potential insecticidal activity. The encoded proteins HFR-1, HFR-2 and HFR-3 all possess lectin-like domains. HFR-1 and HFR-3 were produced as recombinant proteins using Escherichia coli and Pichia pastoris, respectively as expression hosts. Purified recombinant proteins were assayed for insecticidal effects towards cereal aphid (Sitobion avenae), an insect to which wheat shows only tolerance. Both HFR-1 and HFR-3 were found to be insecticidal towards S. avenae when fed in artificial diet. Although HFR-3 has sequence similarity and similar chitin-binding activity to wheat germ agglutinin (WGA), the latter protein was almost non-toxic to S. avenae. HFR-3 binds strongly to aphid midguts after ingestion, whereas WGA binds but does not persist over a feed-chase period. Quantitative PCR showed that Hfr-3 mRNA does not increase in level after cereal aphid infestation. The results suggest that the lack of effective resistance to cereal aphid in wheat is not due to an absence of genes encoding suitable insecticidal proteins, but results from a failure to up-regulate gene expression in response to aphid attack.

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.

Fine genetic mapping of greenbug aphid-resistance gene Gb3 in Aegilops tauschii

The greenbug, Schizaphis graminum (Rondani), is an important aphid pest of small grain crops especially wheat (Triticum aestivum L., 2n = 6x = 42, genomes AABBDD) in many parts of the world. The greenbug-resistance gene Gb3 originated from Aegilops tauschii Coss. (2n = 2x = 14, genome D(t)D(t)) has shown consistent and durable resistance against prevailing greenbug biotypes in wheat fields. We previously mapped Gb3 in a recombination-rich, telomeric bin of wheat chromosome arm 7DL. In this study, high-resolution genetic mapping was carried out using an F(2:3) segregating population derived from two Ae. tauschii accessions, the resistant PI 268210 (original donor of Gb3 in the hexaploid wheat germplasm line 'Largo') and susceptible AL8/78. Molecular markers were developed by exploring bin-mapped wheat RFLPs, SSRs, ESTs and the Ae. tauschii physical map (BAC contigs). Wheat EST and Ae. tauschii BAC end sequences located in the deletion bin 7DL3-0.82-1.00 were used to design STS (sequence tagged site) or CAPS (Cleaved Amplified Polymorphic Sequence) markers. Forty-five PCR-based markers were developed and mapped to the chromosomal region spanning the Gb3 locus. The greenbug-resistance gene Gb3 now was delimited in an interval of 1.1 cM by two molecular markers (HI067J6-R and HI009B3-R). This localized high-resolution genetic map with markers closely linked to Gb3 lays a solid foundation for map based cloning of Gb3 and marker-assisted selection of this gene in wheat breeding.

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.

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.

Plant lectins as defense proteins against phytophagous insects

One of the most important direct defense responses in plants against the attack by phytophagous insects is the production of insecticidal peptides or proteins. One particular class of entomotoxic proteins present in many plant species is the group of carbohydrate-binding proteins or lectins. During the last decade a lot of progress was made in the study of a few lectins that are expressed in response to herbivory by phytophagous insects and the insecticidal properties of plant lectins in general. This review gives an overview of lectins with high potential for the use in pest control strategies based on their activity towards pest insects. In addition, potential target sites for lectins inside the insect and the mode of action are discussed. In addition, the effect of plant lectins on non-target organisms such as beneficial insects as well as on human/animal consumers is discussed. It can be concluded that some insecticidal lectins are useful tools that can contribute to the development of integrated pest management strategies with minimal effect(s) on non-target organisms.

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

Salivary secretions of neonate Hessian fly larvae initiate a two-way exchange of molecules with their wheat host. Changes in properties of the leaf surface allow larval effectors to enter the plant where they trigger plant processes leading to resistance and delivery of defence molecules, or susceptibility and delivery of nutrients. To increase understanding of the host plant's response, the timing and characteristics of the induced epidermal permeability were investigated. Resistant plant permeability was transient and limited in area, persisting just long enough to deliver defence molecules before gene expression and permeability reverted to pre-infestation levels. The abundance of transcripts for GDSL-motif lipase/hydrolase, thought to contribute to cuticle reorganization and increased permeability, followed the same temporal profile as permeability in resistant plants. In contrast, susceptible plants continued to increase in permeability over time until the entire crown of the plant became a nutrient sink. Permeability increased with higher infestation levels in susceptible but not in resistant plants. The ramifications of induced plant permeability on Hessian fly populations are discussed.

Molecular characterization and functional analysis of elite genes in wheat and its related species

The tribe Triticeae includes major cereal crops (bread wheat, durum wheat, triticale, barley and rye), as well as abundant forage and lawn grasses. Wheat and its wild related species possess numerous favourable genes for yield improvement, grain quality enhancement, biotic and abiotic stress resistance, and constitute a giant gene pool for wheat improvement. In recent years, significant progress on molecular characterization and functional analysis of elite genes in wheat and its related species have been achieved. In this paper, we review the cloned functional genes correlated with grain quality, biotic and abiotic stress resistance, photosystem and nutrition utilization in wheat and its related species.

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.

Changes in properties of wheat leaf cuticle during interactions with Hessian fly

Infestation of wheat by Hessian fly larvae causes a variety of physical and biochemical modifications of the host plant. Changes occur in cuticle permeability, lipid composition and gene transcript abundance, and these responses differ substantially between resistant and susceptible wheat lines. Staining assays revealed that susceptible plants exhibited a generalized increase in leaf sheath epidermal permeability during infestation; whereas, epidermal permeability was only minimally affected in resistant plants. Furthermore, temporal profiling using gas chromatographic methods revealed that changes in cuticle lipid (wax and cutin) composition correlated well with differing levels of epidermal permeability in susceptible and resistant plants. Temporal analysis of cuticle-associated gene mRNA levels, by quantitative real-time PCR, indicated a relationship between transcript abundance and changes in cuticle lipid profiles of resistant and susceptible plants. Results suggest that conserving cuticle integrity via induction of specific wax constituents and maintenance of cutin amounts, determined by the accumulation of cuticle-associated transcripts, could be important components of wheat resistance to Hessian fly larvae.

Plant-insect interactions: what can we learn from plant lectins?

Many plant lectins have high anti-insect potential. Although the effects of most lectins are only moderately influencing development or population growth of the insect, some lectins have strong insecticidal properties. In addition, some studies report a deterrent activity towards feeding and oviposition behavior. Transmission of plant lectins to the next trophic level has been investigated for several tritrophic interactions. Effects of lectins with different sugar specificities can vary substantially with the insect species under investigation and with the experimental setup. Lectin binding in the insect is an essential step in exerting a toxic effect. Attempts have been made to study the interactions of lectins in several insect tissues and to identify lectin-binding receptors. Ingested lectins generally bind to parts of the insect gut. Furthermore, some lectins such as the Galanthus nivalus agglutinin (GNA) cross the gut epithelium into the hemolymph and other tissues. Recently, several candidate lectin-binding receptors have been isolated from midgut extracts. To date little is known about the exact mechanism for insecticidal activity of plant lectins. However, insect glycobiology is an emerging research field and the recent technological advances in the analysis of lectin carbohydrate specificities and insect glycobiology will certainly lead to new insights in the interactions between plant lectins and insects, and to a better understanding of the molecular mechanisms involved.

Reactive oxygen species are involved in plant defense against a gall midge

Reactive oxygen species (ROS) play a major role in plant defense against pathogens, but evidence for their role in defense against insects is still preliminary and inconsistent. In this study, we examined the potential role of ROS in defense of wheat (Triticum aestivum) and rice (Oryza sativa) against Hessian fly (Mayetiola destructor) larvae. Rapid and prolonged accumulation of hydrogen peroxide (H(2)O(2)) was detected in wheat plants at the attack site during incompatible interactions. Increased accumulation of both H(2)O(2) and superoxide was detected in rice plants during nonhost interactions with the larvae. No increase in accumulation of either H(2)O(2) or superoxide was observed in wheat plants during compatible interactions. A global analysis revealed changes in the abundances of 250 wheat transcripts and 320 rice transcripts encoding proteins potentially involved in ROS homeostasis. A large number of transcripts encoded class III peroxidases that increased in abundance during both incompatible and nonhost interactions, whereas the levels of these transcripts decreased in susceptible wheat during compatible interactions. The higher levels of class III peroxidase transcripts were associated with elevated enzymatic activity of peroxidases at the attack site in plants during incompatible and nonhost interactions. Overall, our data indicate that class III peroxidases may play a role in ROS generation in resistant wheat and nonhost rice plants during response to Hessian fly attacks.

Transcript profiles of two wheat lipid transfer protein-encoding genes are altered during attack by Hessian fly larvae

A sequence encoding a putative type-1 lipid transfer protein from wheat (Triticum aestivum L. em Thell) was identified through 'GeneCalling', an mRNA profiling technology. The mRNA for the Hfr-LTP (Hessian fly-responsive lipid transfer protein) gene decreased in abundance (196-fold) in susceptible wheat plants over the first eight days of attack by virulent Hessian fly larvae (Mayetiola destructor Say). Hfr-LTP encodes a putative protein containing eight cysteine residues that are conserved among plant LTPs and are responsible for correct protein folding through formation of disulfide bridges. Twelve hydrophobic amino acids in addition to arginine, glycine, proline, serine, threonine and tyrosine, plus an LTP signature sequence were present in conserved positions. A highly conserved signal peptide sequence was also present. Although attack by one virulent larva was sufficient to cause a decrease in Hfr-LTP mRNA abundance, higher infestation levels led to near silencing of the gene. Hfr-LTP transcript levels were not affected by other biotic factors (feeding by bird cherry-oat aphid, Rhopalosiphum padi L., and fall armyworm larvae, Spodoptera frugiperda Smith) or abiotic factors tested (mechanical wounding or treatment with abscisic acid, methyl jasmonate, or salicylic acid). Comparison to a previously described Hessian fly-responsive wheat LTP gene, TaLTP3, confirmed an initial increase in TaLTP3 mRNA in resistant plants. However, when quantified through eight days after egg hatch, responsiveness to infestation level and a marked decrease in susceptible plant TaLTP3 mRNA abundance were detected, as was seen for Hfr-LTP. Possible functions of LTP gene products in wheat-Hessian fly interactions are discussed.

Transcriptional signatures in response to wheat germ agglutinin and starvation in Drosophila melanogaster larval midgut

One function of plant lectins such as wheat germ agglutinin is to serve as defences against herbivorous insects. The midgut is one critical site affected by dietary lectins. We observed marked cellular, structural and gene expression changes in the midguts of Drosophila melanogaster third instar larvae that were fed wheat germ agglutinin. Some of these changes were similar to those observed in the midguts of starved D. melanogaster. Dietary wheat germ agglutinin caused shortening, branching, swelling, distortion and in some cases disintegration of the midgut microvilli. Starvation was accompanied primarily by shortening of the microvilli. Microarray analyses revealed that dietary wheat germ agglutinin evoked differential expression of 61 transcripts; seven of these were also differentially expressed in starved D. melanogaster. The differentially transcribed gene clusters in wheat germ agglutinin-fed larvae were associated with (1) cytoskeleton organization; (2) digestive enzymes; (3) detoxification reactions; and (4) energy metabolism. Four possible transcription factor binding motifs were associated with the differentially expressed genes. One of these exhibited substantial similarity to MyoD, a transcription factor binding motif associated with cellular structures in mammals. These results are consistent with the hypothesis that wheat germ agglutinin caused a starvation-like effect and structural changes of midgut cells of D. melanogaster third-instar larvae.

Virulent Hessian fly larvae manipulate the free amino acid content of host wheat plants

Gall-forming insects induce host plants to form specialized structures (galls) that provide immature life stages of the insect access to host plant nutrients and protection from natural enemies. Feeding by larvae of the Hessian fly (Mayetiola destructor Say) causes susceptible host wheat plants to produce a gall-like nutritive tissue that supports larval growth and development. To determine if changes in host plant free amino acid levels are associated with virulent Biotype L Hessian fly larval feeding, we quantified free amino acid levels in crown tissues of susceptible Newton wheat plants 1, 4, and 7 days after Hessian fly egg hatch. Hessian fly-infested susceptible plants were more responsive than resistant plants or uninfested controls, showing higher concentrations of alanine, glutamic acid, glycine, phenylalanine, proline, and serine 4 days after egg hatch. This 4-day post-hatch time point corresponds to the maturation of nutritive tissue cells in susceptible plants and the onset of rapid larval growth. By 7 days after egg hatch, when virulent second instars are actively feeding on the contents of nutritive tissue cells, the aromatic amino acids phenylalanine and tyrosine were more abundant compared to uninfested controls, but the levels of other free amino acids were no longer elevated. Changes in free amino acid abundance described in this report were associated with increased levels of mRNA encoded by wheat genes involved in amino acid synthesis and transport.

Functional characterization of HFR1, a high-mannose N-glycan-specific wheat lectin induced by Hessian fly larvae

We previously cloned and characterized a novel jacalin-like lectin gene from wheat (Triticum aestivum) plants that responds to infestation by Hessian fly (Mayetiola destructor) larvae, a major dipteran pest of this crop. The infested resistant plants accumulated higher levels of Hfr-1 (for Hessian fly-responsive gene 1) transcripts compared with uninfested or susceptible plants. Here, we characterize the soluble and active recombinant His(6)-HFR1 protein isolated from Escherichia coli. Functional characterization of the protein using hemagglutination assays revealed lectin activity. Glycan microarray-binding assays indicated strong affinity of His(6)-HFR1 to Manalpha1-6(Manalpha1-3)Man trisaccharide structures. Resistant wheat plants accumulated high levels of HFR1 at the larval feeding sites, as revealed by immunodetection, but the avirulent larvae were deterred from feeding and consumed only small amounts of the lectin. Behavioral studies revealed that avirulent Hessian fly larvae on resistant plants exhibited prolonged searching and writhing behaviors as they unsuccessfully attempted to establish feeding sites. During His(6)-HFR1 feeding bioassays, Drosophila melanogaster larvae experienced significant delays in growth and pupation, while percentage mortality increased with progressively higher concentrations of His(6)-HFR1 in the diet. Thus, HFR1 is an antinutrient to dipteran larvae and may play a significant role in deterring Hessian fly larvae from feeding on resistant wheat plants.

Gene expression of different wheat genotypes during attack by virulent and avirulent Hessian fly (Mayetiola destructor) larvae

Wheat and its relatives possess a number of resistance (R) genes specific for the Hessian fly (HF) [Mayetiola destructor (Say)]. HF populations overcome R gene resistance by evolving virulence. Virulent HF larvae manipulate the plant to produce a nutritionally enhanced feeding tissue and, probably, also suppress plant defense responses. Using two wheat R genes, H9 and H13, and three HF strains (biotypes) differing in virulence for H9 and H13, we conducted a genome-wide transcriptional analysis of gene expression during compatible interactions with virulent larvae and incompatible interactions with avirulent larvae. During both types of interactions, a large number of genes (>1,000) showed alterations in gene expression. Analysis of genes with known functions revealed that major targets for differential regulation were genes that encoded defense proteins or enzymes involved in the phenylpropanoid, cell wall, and lipid metabolism pathways. A combination of the enhancement of antibiosis defense, the evasion of nutrient metabolism induction, and the fortification and expansion of the cell wall are likely the collective mechanism for host-plant resistance observed during incompatible interactions. To overcome this resistance, virulent larvae appeared to suppress antibiosis defense while inducing nutrient metabolism, weakening cell wall, and inhibiting plant growth.

Molecular genetic improvement of cereals: transgenic wheat (Triticum aestivum L.)

Only modest progress has been made in the molecular genetic improvement of wheat following the production of the first transgenic plants in 1992, made possible by the development of efficient, long-term regenerable embryogenic cultures derived from immature embryos and use of the biolistics method for the direct delivery of DNA into regenerable cells. Transgenic lines expressing genes that confer resistance to environmentally friendly non-selective herbicides, and pests and pathogens have been produced, in addition to lines with improved bread-making and nutritional qualities; some of these are ready for commercial production. Reduction of losses caused by weeds, pests and pathogens in such plants not only indirectly increases available arable land and fresh water supplies, but also conserves energy and natural resources. Nevertheless, the work carried out thus far can be considered only the beginning, as many difficult tasks lie ahead and much remains to be done. The challenge now is to produce higher-yielding varieties that are more nutritious, and are resistant or tolerant to a wide variety of biotic as well as abiotic stresses (especially drought, salinity, heavy metal toxicity) that currently cause substantial losses in productivity. How well we will meet this challenge for wheat, and indeed for other cereal and non-cereal crops, will depend largely on establishing collaborative partnerships between breeders, molecular biologists, biotechnologists and industry, and on how effectively they make use of the knowledge and insights gained from basic studies in plant biology and genetics, the sequencing of plant/cereal genomes, the discovery of synteny in cereals, and the availability of DNA-based markers and increasingly detailed chromosomal maps.