Mobilization of lipids and fortification of cell wall and cuticle are important in host defense against Hessian fly

Untargeted metabolomics profiling of oat (Avena sativa L.) and wheat (Triticum aestivum L.) infested with wheat stem sawfly (Cephus cinctus Norton) reveals differences associated with plant defense and insect nutrition

Introduction

Wheat stem sawfly (WSS), Cephus cinctus Norton, is a major pest of common bread wheat (Triticum aestivum L.) and other cultivated cereals in North America. Planting of cultivars with solid stems has been the primary management strategy to prevent yield loss due to WSS infestation, however expression of this phenotype can vary depending on environmental conditions and solid stems hinder biological control of WSS via braconid parasitoids Bracon cephi (Gahan) and Bracon lissogaster Muesebeck. In the hollow stems of oat (Avena sativa L.), WSS larvae experience 100% mortality before they reach late instars, but the mechanisms for this observed resistance have not been characterized.

Objective

The objective of this study was to explore additional sources of resistance outside of the historic solid stem phenotype.

Methods

Here, we use an untargeted metabolomics approach to examine the response of the metabolome of two cultivars of oat and four cultivars of spring wheat to infestation by WSS. Using liquid chromatography-mass spectrometry (LC-MS), differentially expressed metabolites were identified between oat and wheat which were associated with the phenylpropanoid pathway, phospholipid biosynthesis and signaling, the salicylic acid signaling pathway, indole-3-acetic acid (IAA) degradation, and biosynthesis of 1,4-benzoxazin-3-ones (Bxs). Several phospho- and galacto- lipids were found in higher abundance in oat, and with the exception of early stem solidness cultivar Conan, both species experienced a decrease in abundance once infested. In all wheat cultivars except Conan, an increase in abundance was observed for Bxs HMDBOA-glc and DIBOA-β-D-glucoside after infestation, indicating that this pathway is involved in wheat response to infestation in both solid and hollow stemmed cultivars. Differences between species in compounds involved in IAA biosynthesis, degradation and inactivation suggest that wheat may respond to infestation by inactivating IAA or altering the IAA pool in stem tissue.

Conclusion

We propose that the species differences found here likely affect the survival of WSS larvae and may also be associated with differences in stem architecture at the molecular level. Our findings suggest pathways to focus on for future studies in elucidating plant response to WSS infestation.

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.

Dynamics of Zea mays transcriptome in response to a polyphagous herbivore, Spodoptera litura

Zea mays defense response is well-crafted according to the physical and chemical weapons utilized by their invaders during the coevolutionary period. Maize plants employ diversified defense strategies and alter the spatiotemporal distribution of several classes of defensive compounds to affect insect herbivore performance. However, only little knowledge is available about the defense orchestration of maize in response to Spodoptera litura, a voracious Noctuidae pest. In order to decipher the defense status of Zea mays (African tall variety) against S. litura, a comparative feeding bioassay was executed, which revealed reduced performance of the herbivore on maize. In order to understand the molecular mechanism behind maize tolerance against S. litura, a microarray-based genome-wide expression analysis was performed. The comparative analysis displayed 792 differentially expressed genes (DEGs), wherein 357 genes were upregulated and 435 genes were downregulated at fold change ≥ 2 and p value ≤ 0.05. The upregulated genes were identified and categorized as defense-related, oxidative stress-related, transcription regulatory genes, protein synthesis genes, phytohormone-related, and primary and secondary metabolism-related. In contrast, downregulated genes were mainly associated with plant growth and development, indicating a balance of growth and defense response and utilization of a highly evolved C-diversion response were noticed. Maize plants showed better tolerance against herbivory and maintained its fitness using a combinatorial strategy. This peculiar response of Zea mays against S. litura offers an excellent possibility of managing polyphagous pests by spicing up the plant's defensive response with tolerance mechanism.

Gall-Inducing Parasites: Convergent and Conserved Strategies of Plant Manipulation by Insects and Nematodes

Gall-inducing insects and nematodes engage in sophisticated interactions with their host plants. These parasites can induce major morphological and physiological changes in host roots, leaves, and other tissues. Sedentary endoparasitic nematodes, root-knot and cyst nematodes in particular, as well as gall-inducing and leaf-mining insects, manipulate plant development to form unique organs that provide them with food from feeding cells. Sometimes, infected tissues may undergo a developmental switch resulting in the formation of aberrant and spectacular structures (clubs or galls). We describe here the complex interactions between these plant-reprogramming sedentary endoparasites and their infected hosts, focusing on similarities between strategies of plant manipulation. We highlight progress in our understanding of the host plant response to infection and focus on the nematode and insect molecules secreted in planta. We suggest thatlooking at similarities may identify convergent and conserved strategies and shed light on the promise they hold for the development of new management strategies in agriculture and forestry.

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

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

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.

A Horizontal Gene Transfer Led to the Acquisition of a Fructan Metabolic Pathway in a Gall Midge

Animals are thought to use only glucose polymers (glycogen) as energy reserve, whereas both glucose (starch) and fructose polymers (fructans) are used by microbes and plants. Here, it is reported that the gall midge Mayetiola destructor, and likely other herbivorous animal species, gained the ability to utilize dietary fructans directly as storage polysaccharides by a single horizontal gene transfer (HGT) of bacterial levanase/inulinase gene followed by gene expansion and differentiation. Multiple genes encoding levanases/inulinases have their origin in a single HGT event from a bacterium and they show high expression levels and enzymatic activities in different tissues of the gall midge, including nondigestive fat bodies and eggs, both of which contained significant amounts of fructans. This study provides evidence that animals can also use fructans as energy reserve by incorporating bacterial genes in their genomes.

Comparative transcriptome analyses provide novel insights into the differential response of Pigeonpea (Cajanus cajan L.) and its wild relative (Cajanus platycarpus (Benth.) Maesen) to herbivory by Helicoverpa armigera (Hübner)

Deeper insights into the resistance response of Cajanus platycarpus were obtained based on comparative transcriptomics under Helicoverpa armigera infestation. Devastation by pod borer, Helicoverpa armigera is one of the major factors for stagnated productivity in Pigeonpea. Despite possessing a multitude of desirable traits including pod borer resistance, wild relatives of Cajanus spp. have remained under-utilized due to linkage drag and cross-incompatibility. Discovery and deployment of genes from them can provide means to tackle key pests like H. armigera. Transcriptomic differences between Cajanus platycarpus and Cajanus cajan during different time points (0, 18, 38, 96 h) of pod borer infestation were elucidated in this study. For the first ever time, we demonstrated captivating variations in their response; C. platycarpus apparently being reasonably agile with effectual transcriptomic reprogramming to deter the insect. Deeper insights into the differential response were obtained by identification of significant GO-terms related to herbivory followed by combined KEGG and ontology analyses. C. platycarpus portrayed a multilevel response with cardinal involvement of SAR, redox homeostasis and reconfiguration of primary metabolites leading to a comprehensive defense response. The credibility of RNA-seq analyses was ascertained by transient expression of selected putative insect resistance genes from C. platycarpus viz., chitinase (CHI4), Alpha-amylase/subtilisin inhibitor (IAAS) and Flavonoid 3_5 hydroxylase (C75A1) in Nicotiana benthamiana followed by efficacy analysis against H. armigera. qPCR validated results of the study provided innovative insights and useful leads for development of durable pod borer resistance.

Transcriptome Analysis of Chinese Chestnut (Castanea mollissima Blume) in Response to Dryocosmus kuriphilus Yasumatsu Infestation

Chinese chestnut (Castanea mollissima Blume) can be infested by Dryocosmus kuriphilus Yasumatsu, resulting in gall formation and yield losses. Research on the control of gall wasps using genomics approaches is rarely reported. We used RNA-seq to investigate the dynamic changes in the genes of a chestnut species (C. mollissima B.) during four gall-formation stages caused by D. kuriphilus. A total of 21,306 genes were annotated by BLAST in databases. Transcriptome comparison between different gall-formation stages revealed many genes that were differentially expressed compared to the control. Among these, 2410, 7373, 6294, and 9412 genes were differentially expressed in four gall-formation stages: initiation stage (A), early growth stage (B), late growth stage (C), and maturation stage (D), respectively. Annotation analysis indicated that many metabolic processes (e.g., phenylpropanoid biosynthesis, secondary metabolism, plant⁻pathogen interaction) were affected. Interesting genes encoding putative components of signal transduction, stress response, and transcription factors were also differentially regulated. These genes might play important roles in response to D. kuriphilus gall formation. These new data on the mechanism by which D. kuriphilus infests chestnuts could help improve chestnut resistance.

Development of SNP assays for hessian fly response genes, Hfr-1 and Hfr-2, for marker-assisted selection in wheat breeding

Background

The Hessian fly response genes, Hfr-1 and Hfr-2, have been reported to be significantly induced in a Hessian fly attack. Nothing is known about the allelic variants of these two genes in susceptible (S) and resistant (R) wheat cultivars.

Results

Basic local alignment search tool (BLAST) analysis of Hessian fly response genes have identified three alleles of Hessian fly response gene 1 (Hfr-1) on chromosome 4AL and 7DS, and 10 alleles of Hessian fly response gene 2 (Hfr-2) on chromosome 2BS, 2DL, 4BS, 4BL, 5AL and 5BL. Resequencing exons of Hfr-1 and Hfr-2 have identified a single nucleotide polymorphism (SNP) in the lectin domain of each gene that segregates some R sources from S cultivars. Two SNP assays have been developed. The SNP883_Hfr-1 assay characterizes a ‘G/A’ SNP in Hfr-1, which differentiates 14 Hessian fly R cultivars from S ones. The SNP1294_Hfr-2 assay differentiates 12 R cultivars from S ones. Each of the two SNPs identified in Hfr-1 and Hfr-2 is ‘G/A’ and resulted in an amino acid change from isoleucine to valine in the lectin domain of the proteins of the alleles in the R cultivars. In addition to the genotype profiles of Hfr-1 and Hfr-2, generated for a set of 249 wheat cultivars which included a set of 39 R cultivars, this study has genotyped the Hessian fly response gene, HfrDrd, and the H32 gene for the wheat germplasm. Resistant cultivars from different origins with one, two, three or four resistance (R) genes in various combinations/permutations have been identified.

Conclusion

This study has identified allelic differences in two Hessian fly response genes, Hfr-1 and Hfr-2, between S and R cultivars and developed one SNP assay for each of the genes. These two SNP assays for Hfr-1 and Hfr-2, together with the published assays for HfrDrd and the H32 gene, can be used for the selection and incorporation of one or more of these 4 R genes identified in the different R sources in wheat breeding programs.

Electronic supplementary material

The online version of this article (10.1186/s12863-018-0659-y) contains supplementary material, which is available to authorized users.

Metabolic dynamics and physiological adaptation of Panax ginseng during development

The dynamics of metabolites from leaves to roots of Panax ginseng during development has revealed the tissue-specific and year-specific metabolic networks. Being an essential Oriental medicinal plant, ginseng (Panax ginseng Meyer) is a slow-growing perennial herb-accumulating pharmaceutically active metabolites such as ginsenosides in roots during growth. However, little is known about how ginseng plants survive in the harsh environments such as winter cold and summer heat for a longer period and accumulates those active metabolites as the plant grows. To understand the metabolic kinetics in both source and sink organs such as leaves and roots of ginseng plant, respectively, and to assess the changes in ginsenosides biosynthesis during ginseng growth, we investigated the metabolic profiles from leaves and roots of 1-, 4-, and 6-year-old field-grown ginseng plants. Using an integrated non-targeted metabolomic approach, we identified in total 348 primary and secondary metabolites, which provided us for the first time a global metabolomic assessment of ginseng during growth, and morphogenesis. Strikingly, the osmoprotectants and oxidized chemicals were highly accumulated in 4- and 6-year-old ginseng leaves suggested that ginseng develop a wide range of metabolic strategies to adapt unfavorable conditions as they mature. In 6-year-old plants, ginsenosides were decreased in leaves but increased in roots up to 1.2- to sixfold, supporting the view that there is a long-distance transport of ginsenosides from leaves to roots as ginseng plants mature. Our findings provide insights into the metabolic kinetics during the development of ginseng plant and this could complement the pharmacological importance of ginseng and its compounds according to their age.

TaWRKY70 transcription factor in wheat QTL-2DL regulates downstream metabolite biosynthetic genes to resist Fusarium graminearum infection spread within spike

A semi-comprehensive metabolomics was used to identify the candidate metabolites and genes to decipher mechanisms of resistance in wheat near-isogenic lines (NILs) containing QTL-2DL against Fusarium graminearum (Fg). Metabolites, with high fold-change in abundance, belonging to hydroxycinnamic acid amides (HCAAs): such as coumaroylagmatine, coumaroylputrescine and Fatty acids: phosphatidic acids (PAs) were identified as resistance related induced (RRI) metabolites in rachis of resistant NIL (NIL-R), inoculated with Fg. A WRKY like transcription factor (TF) was identified within the QTL-2DL region, along with three resistance genes that biosynthesized RRI metabolites. Sequencing and in-silico analysis of WRKY confirmed it to be wheat TaWRKY70. Quantitative real time-PCR studies showed a higher expression of TaWRKY70 in NIL-R as compared to NIL-S after Fg inoculation. Further, the functional validation of TaWRKY70 based on virus induced gene silencing (VIGS) in NIL-R, not only confirmed an increased fungal biomass but also decreased expressions of downstream resistance genes: TaACT, TaDGK and TaGLI1, along with decreased abundances of RRI metabolites biosynthesized by them. Among more than 200 FHB resistance QTL identified in wheat, this is the first QTL from which a TF was identified, and its downstream target genes as well as the FHB resistance functions were deciphered.

Metabolic and transcriptomic changes induced in host during hypersensitive response mediated resistance in rice against the Asian rice gall midge

BACKGROUND

An incompatible interaction between rice (Oryza sativa) and the Asian rice gall midge (AGM, Orseolia oryzae Wood-Mason), that is usually manifested through a hypersensitive response (HR), represents an intricate relationship between the resistant host and its avirulent pest. We investigated changes in the transcriptome and metabolome of the host (indica rice variety: RP2068-18-3-5, RP), showing HR when attacked by an avirulent gall midge biotype (GMB1), to deduce molecular and biochemical bases of such a complex interaction. Till now, such an integrated analysis of host transcriptome and metabolome has not been reported for any rice-insect interaction.

RESULTS

Transcript and metabolic profiling data revealed more than 7000 differentially expressed genes and 80 differentially accumulated metabolites, respectively, in the resistant host. Microarray data revealed deregulation of carbon (C) and nitrogen (N) metabolism causing a C/N shift; up-regulation of tetrapyrrole synthesis and down-regulation of chlorophyll synthesis and photosynthesis. Integrated results revealed that genes involved in lipid peroxidation (LPO) were up-regulated and a marker metabolite for LPO (azelaic acid) accumulated during HR. This coincided with a greater accumulation of GABA (neurotransmitter and an insect antifeedant) at the feeding site. Validation of microarray results by semi-quantitative RT-PCR revealed temporal variation in gene expression profiles.

CONCLUSIONS

The study revealed extensive reprogramming of the transcriptome and metabolome of RP upon GMB1 infestation leading to an HR that was induced by the generation and release of reactive oxygen species i.e. singlet oxygen and resulted in LPO-mediated cell death. RP thus used HR as a means to limit nutrient supply to the feeding maggots and simultaneously accumulated GABA, strategies that could have led to maggot mortality. The integrated results of transcript and metabolic profiling, for the first time, provided insights into an HR+ type of resistance in rice against gall midge.

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.

Development of interspecies hybrids to increase ginseng biomass and ginsenoside yield

Interspecific hybrids between Panax ginseng and P. quinquefolius results in hybrid vigor and higher ginsenoside contents. Ginseng is one of the most important herbs with valued pharmaceutical effects contributing mainly by the presence of bioactive ginsenosides in the roots. However, ginseng industry is impeded largely by its biological properties, because ginseng plants are slow-growing perennial herbs with lower yield. To increase the ginseng yield and amounts of ginsenosides, we developed an effective ginseng production system using the F(1) progenies obtained from the interspecific reciprocal cross between two Panax species: P. ginseng and P. quinquefolius. Although hybrid plants show reduced male fertility, F(1) hybrids with the maternal origin either from P. ginseng or P. quinquefolius displayed heterosis; they had larger roots and higher contents of ginsenosides as compared with non-hybrid parental lines. Remarkably, the F(1) hybrids with the maternal origin of P. quinquefolius had much higher ginsenoside contents, especially ginsenoside Re and Rb1, than those with the maternal origin of P. ginseng. Additionally, non-targeted metabolomic profiling revealed a clear increase of a large number of primary and secondary metabolites including fatty acids, amino acids and ginsenosides in hybrid plants. To effectively identify the F(1) hybrids for the large-scale cultivation, we successfully developed a molecular marker detection system for discriminating F(1) reciprocal hybrids. In summary, this work provided a practical system for reciprocal hybrid ginseng production, which would facilitate the ginseng production in the future.

Insect-induced effects on plants and possible effectors used by galling and leaf-mining insects to manipulate their host-plant

Gall-inducing insects are iconic examples in the manipulation and reprogramming of plant development, inducing spectacular morphological and physiological changes of host-plant tissues within which the insect feeds and grows. Despite decades of research, effectors involved in gall induction and basic mechanisms of gall formation remain unknown. Recent research suggests that some aspects of the plant manipulation shown by gall-inducers may be shared with other insect herbivorous life histories. Here, we illustrate similarities and contrasts by reviewing current knowledge of metabolic and morphological effects induced on plants by gall-inducing and leaf-mining insects, and ask whether leaf-miners can also be considered to be plant reprogrammers. We review key plant functions targeted by various plant reprogrammers, including plant-manipulating insects and nematodes, and functionally characterize insect herbivore-derived effectors to provide a broader understanding of possible mechanisms used in host-plant manipulation. Consequences of plant reprogramming in terms of ecology, coevolution and diversification of plant-manipulating insects are also discussed.

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

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

Micromorphology and development of the epicuticular structure on the epidermal cell of ginseng leaves

BACKGROUND

A leaf cuticle has different structures and functions as a barrier to water loss and as protection from various environmental stressors.

METHODS

Leaves of Panax ginseng were examined by scanning electron microscopy and transmission electron microscopy to investigate the characteristics and development of the epicuticular structure.

RESULTS

Along the epidermal wall surface, the uniformly protuberant fine structure was on the adaxial surface of the cuticle. This epicuticular structure was highly wrinkled and radially extended to the marginal region of epidermal cells. The cuticle at the protuberant positions maintained the same thickness. The density of the wall matrix under the structures was also similar to that of the other wall region. By contrast, none of this structure was distributed on the abaxial surface, except in the region of the stoma. During the early developmental phase of the epicuticular structure, small vesicles appeared on wall-cuticle interface in the peripheral wall of epidermal cells. Some electron-opaque vesicles adjacent to the cuticle were fused and formed the cuticle layer, whereas electron-translucent vesicles contacted each other and progressively increased in size within the epidermal wall.

CONCLUSION

The outwardly projected cuticle and epidermal cell wall (i.e., an epicuticular wrinkle) acts as a major barrier to block out sunlight in ginseng leaves. The small vesicles in the peripheral region of epidermal cells may suppress the cuticle and parts of epidermal wall, push it upward, and consequently contribute to the formation of the epicuticular structure.

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

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

Unbalanced activation of glutathione metabolic pathways suggests potential involvement in plant defense against the gall midge Mayetiola destructor in wheat

Glutathione, γ-glutamylcysteinylglycine, exists abundantly in nearly all organisms. Glutathione participates in various physiological processes involved in redox reactions by serving as an electron donor/acceptor. We found that the abundance of total glutathione increased up to 60% in resistant wheat plants within 72 hours following attack by the gall midge Mayetiola destructor, the Hessian fly. The increase in total glutathione abundance, however, is coupled with an unbalanced activation of glutathione metabolic pathways. The activity and transcript abundance of glutathione peroxidases, which convert reduced glutathione (GSH) to oxidized glutathione (GSSG), increased in infested resistant plants. However, the enzymatic activity and transcript abundance of glutathione reductases, which convert GSSG back to GSH, did not change. This unbalanced regulation of the glutathione oxidation/reduction cycle indicates the existence of an alternative pathway to regenerate GSH from GSSG to maintain a stable GSSG/GSH ratio. Our data suggest the possibility that GSSG is transported from cytosol to apoplast to serve as an oxidant for class III peroxidases to generate reactive oxygen species for plant defense against Hessian fly larvae. Our results provide a foundation for elucidating the molecular processes involved in glutathione-mediated plant resistance to Hessian fly and potentially other pests as well.