Caterpillar attack triggers accumulation of the toxic maize protein RIP2

Comparative proteome analysis of Spodoptera litura-infested Zea mays reveals a robust defense strategy targeting insect peritrophic membrane

Herbivorous insects such as Spodoptera litura, pose a constant threat to agricultural crops. The incompetence of contemporary pest management tools and techniques stipulates unravelling of molecular dogma, that drives pest-plant interaction. From our previous observations, we inferred that despite being a voracious polyphagous herbivore, S. litura growth and adaptability is severely hampered on maize foliage diet. In this investigation we explored further and demonstrated the impact of maize diet on the insect gut peritrophic membrane (PM, a crucial membrane involved in compartmentalizing digestive events and absorption of nutrients), its structural analysis using scanning electron microscopy (SEM) revealed damaged and perforated PM. Further, this study delves into the intricate resistance mechanism adapted by Z. mays against S. litura by conducting a comparative proteome analysis. We have detected 345 differentially abundant proteins (DAPs) at p < 0.05 and fold change ≥1. The DAPs were categorized as plant defense, secondary metabolite synthesis, redox homeostasis, cytoskeleton/cell wall biosynthesis, primary metabolism, transport and molecular processes. We remarkably report differential expression of proteolysis- and defense-related proteins that have potential to target insect gut, digestion and absorption of nutrients. Our findings contribute to a deeper understanding of the molecular dynamics governing maize resistance against S. litura. Understanding of such intricate molecular dialogues at these interfaces could provide valuable information on the arms race between plants and herbivores, it may pave the way for innovative pest management strategies.

The proteomic landscape of fall armyworm oral secretion reveals its role in plant adaptation

BACKGROUND

The fall armyworm (FAW, Spodoptera frugiperda (J.E. Smith)) is a polyphagous agricultural pest with rapidly evolving adaptations to host plants. We found the oral secretion (OS) of FAW from different plants influences plant defense response differentially, suggesting its role in adapting to host plants. However, the protein expression profile of FAW OS respond to different plants is largely unknown.

RESULTS

Here, from the mass spectrometry assay, we identified a total of 256 proteins in the OS of FAW fed on cotton (Gossypium hirsutum L.), tobacco (Nicotiana benthamiana Domin), maize (Zea mays L.) and artificial diet. The FAW OS primarily comprise of 60 proteases, 32 esterases and 92 non-enzymatic proteins. It displays high plasticity across different diets. We found that more than half of the esterases are lipases which have been reported as insect elicitors to enhance plant defense response. The lipase accumulation in cotton-fed larvae was the highest, followed by maize-fed larvae. In the presence of lipase inhibitors, the enhanced induction on defense genes in wounded leaves by OS was attenuated. However, the putative effectors were most highly accumulated in the OS from FAW larvae fed on maize compared to those fed on other diets. We identified that one of them (VRLP4) reduces the OS-mediated induction on defense genes in wounded leaves.

CONCLUSION

Together, our investigation presents the proteomic landscape of the OS of FAW influenced by different diets and reveals diet-mediated plasticity of OS is involved in FAW adaptation to host plants. © 2024 Society of Chemical Industry.

Evolution and natural selection of ribosome-inactivating proteins in bacteria, fungi, and plants

Ribosome-inactivating proteins (RIPs) are found in bacteria, fungi, and plants, with a wide range of biological resistances such as anti-fungal, anti-viral, anti-insect, and anti-tumor. They can be roughly divided into proactive defense bacterial or fungal types and passive defense plant types. We identified 1592 RIP genes in bacteria, fungi, and plants. Approximately 88 % of the 764 bacterial RIPs were Shiga or Shiga-like toxins which were exotoxins and could rapidly enter cells to possess strong biotoxicity, and about 98 % of fungal RIPs were predicted as secreted proteins. RIPs were not detected in non-seed plants such as algae, bryophytes, and ferns. However, we found RIPs in some flowering and non-flowering seed plants. The existence of plant RIPs might be related to the structure of seeds or fruits, which might be associated with whether seeds are easy to survive and spread. The evolutionary characteristics of RIPs were different between dicotyledons and monocotyledons. In addition, we also found that RIP2 genes might emerge very early and be plant-specific. Some plant RIP1 genes might evolve from RIP2 genes. This study provides new insights into the evolution of RIPs.

Nutrition vs association: plant defenses are altered by arbuscular mycorrhizal fungi association not by nutritional provisioning alone

BACKGROUND

While it is known that arbuscular mycorrhizal fungi (AMF) can improve nutrient acquisition and herbivore resistance in crops, the mechanisms by which AMF influence plant defense remain unknown. Plants respond to herbivory with a cascade of gene expression and phytochemical biosynthesis. Given that the production of defensive phytochemicals requires nutrients, a commonly invoked hypothesis is that the improvement to plant defense when grown with AMF is simply due to an increased availability of nutrients. An alternative hypothesis is that the AMF effect on herbivory is due to changes in plant defense gene expression that are not simply due to nutrient availability. In this study, we tested whether changes in plant defenses are regulated by nutritional provisioning alone or the response of plant to AMF associations. Maize plants grown with or without AMF and with one of three fertilizer treatments (standard, 2 × nitrogen, or 2 × phosphorous) were infested with fall armyworm (Spodoptera frugiperda; FAW) for 72 h. We measured general plant characteristics (e.g. height, number of leaves), relative gene expression (rtPCR) of three defensive genes (lox3, mpi, and pr5), total plant N and P nutrient content, and change in FAW mass per plant.

RESULTS

We found that AMF drove the defense response of maize by increasing the expression of mpi and pr5. Furthermore, while AMF increased the total phosphorous content of maize it had no impact on maize nitrogen. Fertilization alone did not alter upregulation of any of the 3 induced defense genes tested, suggesting the mechanism through which AMF upregulate defenses is not solely via increased N or P plant nutrition.

CONCLUSION

This work supports that maize defense may be optimized by AMF associations alone, reducing the need for artificial inputs when managing FAW.

Belowground and aboveground herbivory differentially affect the transcriptome in roots and shoots of maize

Plants recognize and respond to feeding by herbivorous insects by upregulating their local and systemic defenses. While defense induction by aboveground herbivores has been well studied, far less is known about local and systemic defense responses against attacks by belowground herbivores. Here, we investigated and compared the responses of the maize transcriptome to belowground and aboveground mechanical damage and infestation by two well-adapted herbivores: the soil-dwelling western corn rootworm Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) and the leaf-chewing fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae). In responses to both herbivores, maize plants were found to alter local transcription of genes involved in phytohormone signaling, primary and secondary metabolism. Induction by real herbivore damage was considerably stronger and modified the expression of more genes than mechanical damage. Feeding by the corn rootworm had a strong impact on the shoot transcriptome, including the activation of genes involved in defense and development. By contrast, feeding by the fall armyworm induced only few transcriptional changes in the roots. In conclusion, feeding by a leaf chewer and a root feeder differentially affects the local and systemic defense of maize plants. Besides revealing clear differences in how maize plants respond to feeding by these specialized herbivores, this study reveals several novel genes that may play key roles in plant-insect interactions and thus sets the stage for in depth research into the mechanism that can be exploited for improved crop protection.

SIGNIFICANCE STATEMENT

Extensive transcriptomic analyses revealed a clear distinction between the gene expression profiles in maize plants upon shoot and root attack, locally as well as distantly from the attacked tissue. This provides detailed insights into the specificity of orchestrated plant defense responses, and the dataset offers a molecular resource for further genetic studies on maize resistance to herbivores and paves the way for novel strategies to enhance maize resistance to pests.

Plant Nutrition Influences Resistant Maize Defense Responses to the Fall Armyworm (Spodoptera frugiperda)

Plants are often confronted by different groups of herbivores, which threaten their growth and reproduction. However, they are capable of mounting defenses against would-be attackers which may be heightened upon attack. Resistance to insects often varies among plant species, with different genotypes exhibiting unique patterns of chemical and physical defenses. Within this framework, plant access to nutrients may be critical for maximal functioning of resistance mechanisms and are likely to differ among plant genotypes. In this study, we aimed to test the hypothesis that access to nutrition would alter the expression of plant resistance to insects and alter insect performance in a manner consistent with fertilization regime. We used two maize (Zea mays) genotypes possessing different levels of resistance and the fall armyworm (Spodoptera frugiperda) as model systems. Plants were subjected to three fertilization regimes prior to assessing insect-mediated responses. Upon reaching V4 stage, maize plants were separated into two groups, one of which was infested with fall armyworm larvae to induce plant defenses. Plant tissue was collected and used in insect bioassays and to measure the expression of defense-related genes and proteins. Insect performance differed between the two plant genotypes substantially. For each genotype, fertilization altered larval performance, where lower fertilization rates hindered larval growth. Induction of plant defenses by prior herbivory substantially reduced naïve fall armyworm growth in both genotypes. The effects between fertilization and induced defenses were complex, with low fertilization reducing induced defenses in the resistant maize. Gene and protein expression patterns differed between the genotypes, with herbivory often increasing expression, but differing between fertilization levels. The soluble protein concentrations did not change across fertilization levels but was higher in the susceptible maize genotype. These results demonstrate the malleability of plant defenses and the cascading effects of plant nutrition on insect herbivory.

Soil chemistry determines whether defensive plant secondary metabolites promote or suppress herbivore growth

Plant secondary (or specialized) metabolites mediate important interactions in both the rhizosphere and the phyllosphere. If and how such compartmentalized functions interact to determine plant-environment interactions is not well understood. Here, we investigated how the dual role of maize benzoxazinoids as leaf defenses and root siderophores shapes the interaction between maize and a major global insect pest, the fall armyworm. We find that benzoxazinoids suppress fall armyworm growth when plants are grown in soils with very low available iron but enhance growth in soils with higher available iron. Manipulation experiments confirm that benzoxazinoids suppress herbivore growth under iron-deficient conditions and in the presence of chelated iron but enhance herbivore growth in the presence of free iron in the growth medium. This reversal of the protective effect of benzoxazinoids is not associated with major changes in plant primary metabolism. Plant defense activation is modulated by the interplay between soil iron and benzoxazinoids but does not explain fall armyworm performance. Instead, increased iron supply to the fall armyworm by benzoxazinoids in the presence of free iron enhances larval performance. This work identifies soil chemistry as a decisive factor for the impact of plant secondary metabolites on herbivore growth. It also demonstrates how the multifunctionality of plant secondary metabolites drives interactions between abiotic and biotic factors, with potential consequences for plant resistance in variable environments.

A sugarcane mosaic virus vector for rapid in planta screening of proteins that inhibit the growth of insect herbivores

Spodoptera frugiperda (fall armyworm) is a notorious pest that threatens maize production worldwide. Current control measures involve the use of chemical insecticides and transgenic maize expressing Bacillus thuringiensis (Bt) toxins. Although additional transgenes have confirmed insecticidal activity, limited research has been conducted in maize, at least partially due to the technical difficulty of maize transformation. Here, we describe implementation of a sugarcane mosaic virus (SCMV) vector for rapidly testing the efficacy of both endogenous maize genes and heterologous genes from other organisms for the control of S. frugiperda in maize. Four categories of proteins were tested using the SCMV vector: (i) maize defence signalling proteins: peptide elicitors (Pep1 and Pep3) and jasmonate acid conjugating enzymes (JAR1a and JAR1b); (ii) maize defensive proteins: the previously identified ribosome-inactivating protein (RIP2) and maize proteinase inhibitor (MPI), and two proteins with predicted but unconfirmed anti-insect activities, an antimicrobial peptide (AMP) and a lectin (JAC1); (iii) lectins from other plant species: Allium cepa agglutinin (ACA) and Galanthus nivalis agglutinin (GNA); and (iv) scorpion and spider toxins: peptides from Urodacus yaschenkoi (UyCT3 and UyCT5) and Hadronyche versuta (Hvt). In most cases, S. frugiperda larval growth was reduced by transient SCMV-mediated overexpression of genes encoding these proteins. Additionally, experiments with a subset of the SCMV-expressed genes showed effectiveness against two aphid species, Rhopalosiphum maidis (corn leaf aphid) and Myzus persicae (green peach aphid). Together, these results demonstrate that SCMV vectors are a rapid screening method for testing the efficacy and insecticidal activity of candidate genes in maize.

Maize Endochitinase Expression in Response to Fall Armyworm Herbivory

A large percentage of crop loss is due to insect damage, especially caterpillar damage. Plant chitinases are considered excellent candidates to combat these insects since they can degrade chitin in peritrophic matrix (PM), an important protective structure in caterpillar midgut. Compared to chemical insecticides, chitinases could improve host plant resistance and be both economically and environmentally advantageous. The focus of this research was to find chitinase candidates that could improve plant resistance by effectively limiting caterpillar damage. Five classes of endochitinase (I-V) genes were characterized in the maize genome, and we isolated and cloned four chitinase genes (chitinase A, chitinase B, chitinase I, and PRm3) present in two maize (Zea mays L.) inbred lines Mp708 and Tx601, with different levels of resistance to caterpillar pests. We also investigated the expression of these maize chitinases in response to fall armyworm (Spodoptera frugiperda, FAW) attack. The results indicated that both chitinase transcript abundance and enzymatic activity increased in response to FAW feeding and mechanical wounding. Furthermore, chitinases retained activity inside the caterpillar midgut and enzymatic activity was detected in the food bolus and frass. When examined under scanning electron microscopy, PMs from Tx601-fed caterpillars showed structural damage when compared to diet controls. Analysis of chitinase transcript abundance after caterpillar feeding and proteomic analysis of maize leaf trichomes in the two inbreds implicated chitinase PRm3 found in Tx601 as a potential insecticidal protein.

Evaluation of African Maize Cultivars for Resistance to Fall Armyworm Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) Larvae

The fall armyworm (FAW) has recently invaded and become an important pest of maize in Africa causing yield losses reaching up to a third of maize annual production. The present study evaluated different aspects of resistance of six maize cultivars, cropped by farmers in Kenya, to FAW larvae feeding under laboratory and field conditions. We assessed the arrestment and feeding of FAW neonate larvae in no-choice and choice experiments, development of larvae-pupae, food assimilation under laboratory conditions and plant damage in a field experiment. We did not find complete resistance to FAW feeding in the evaluated maize cultivars, but we detected differences in acceptance and preference when FAW larvae were given a choice between certain cultivars. Moreover, the smallest pupal weight and the lowest growth index were found on 'SC Duma 43' leaves, which suggests an effect of antibiosis of this maize hybrid against FAW larvae. In contrast, the highest growth index was recorded on 'Rachar' and the greatest pupal weight was found on 'Nyamula' and 'Rachar'. The density of trichomes on the leaves of these maize cultivars seems not to be directly related to the preference of neonates for feeding. Plant damage scores were not statistically different between cultivars in the field neither under natural nor artificial infestation. However, plant damage scores in 'Nyamula' and 'Jowi' tended to be lower in the two last samplings of the season compared to the two initial samplings under artificial infestation. Our study provides insight into FAW larval preferences and performance on some African maize cultivars, showing that there are differences between cultivars in these variables; but high levels of resistance to larvae feeding were not found.

Double infection of Wolbachia and Spiroplasma alters induced plant defense and spider mite fecundity

BACKGROUND

Herbivore-associated bacterial symbionts can change plant physiology and influence herbivore fitness. The spider mite Tetranychus truncatus is a notorious pest harboring various bacterial symbionts; however, the effect of bacterial symbionts on host plant physiology remains unclear. Here, we investigated whether infection with the endosymbionts Wolbachia and Spiroplasma altered spider mite performance on tomato plants and affected plant-induced defenses.

RESULTS

Wolbachia and Spiroplasma were mainly located in the gnathosoma and ovaries of their spider mite hosts. Wolbachia and Spiroplasma significantly improved spider mite reproductive performance in cultivated and wild-type tomato. However, in plants deficient in jasmonic acid (JA) and salicylic acid (SA), there were no significant differences in reproduction between spider mites infected with Wolbachia and Spiroplasma and uninfected mites. The results indicated that the reproduction benefits conferred by endosymbionts may relate to plant defenses. Both spider mites infected with Wolbachia and Spiroplasma and uninfected mites induced similar levels of JA and SA accumulation in tomato, whereas tomato plants damaged by spider mites infected with both Wolbachia and Spiroplasma showed lower expression levels of JA- and SA-responsive genes than those damaged by uninfected spider mites. In addition, mites infected with Wolbachia and Spiroplasma mites consumed more tomato amino acids compared to uninfected spider mites, which may have contributed to host fecundity.

CONCLUSIONS

Our results suggest that the reproduction benefits conferred by endosymbionts may be associated with changes in plant defense parameters and the concentrations of plant amino acids. The results highlight the importance of endosymbionts in interactions between spider mites and their host plants. © 2020 Society of Chemical Industry.

New ribosome-inactivating proteins and other proteins with protein synthesis-inhibiting activities

Ribosome-inactivating proteins (RIPs) consist of three varieties. Type 1 RIPs are single-chained and approximately 30-kDa in molecular weight. Type 2 RIPs are double-chained and composed of a type 1 RIP chain and a lectin chain. Type III RIPs, such as maize b-32 barley and JIP60 which are produced as single-domain proenzymes, possess an N-terminal domain corresponding to the A domain of RIPs and fused to a C-terminal domain. In addition to the aforementioned three types of RIPs originating from flowering plants, there are recently discovered proteins and peptides with ribosome-inactivating and protein synthesis inhibitory activities but which are endowed with characteristics such as molecular weights distinctive from those of the regular RIPs. These new/unusual RIPs discussed in the present review encompass metazoan RIPs from Anopheles and Culex mosquitos, antimicrobial peptides derived from RIP of the pokeweed Phytolacca dioica, maize RIP (a type III RIP derived from a precursor form), RIPs from the garden pea and the kelp. In addition, RIPs with a molecular weight smaller than those of regular type 1 RIPs are produced by plants in the Cucurbitaceae family including the bitter gourd, bottle gourd, sponge gourd, ridge gourd, wax gourd, hairy gourd, pumpkin, and Chinese cucumber. A small type II RIP from camphor tree (Cinnamomum camphora) seeds and a snake gourd type II RIP with its catalytic chain cleaved into two have been reported. RIPs produced from mushrooms including the golden needle mushroom, king tuber mushroom, straw mushroom, and puffball mushroom are also discussed in addition to a type II RIP from the mushroom Polyporus umbellatus. Bacterial (Spiroplasma) RIPs associated with the fruitfly, Shiga toxin, and Streptomyces coelicolor RIP are also dealt with. The aforementioned proteins display a diversity of molecular weights, amino acid sequences, and mechanisms of action. Some of them are endowed with exploitable antipathogenic activities.

The peritrophic membrane as a target of proteins that play important roles in plant defense and microbial attack

The peritrophic membrane (or peritrophic matrix: PM) is a thin membranous structure that lies along the midgut epithelium in the midgut lumen and consists of chitin and proteins. PM exists between ingested food material and midgut epithelium cells and it is on the frontline of insect-plant and insect-microbe interactions. Therefore, proteins that play major roles in plant defense against herbivorous insects and in microbial attack on insects should penetrate, destroy or modify the PM to accomplish their roles. Recently, it has become clear that some proteins crucial to plant defense or microbial attack have the PM as their primary target. In addition, several plant defense proteins have been reported to affect the PM, although it is still unclear whether the PM is their primary target. This review introduces several of these proteins: fusolin and enhancin, two proteins produced by insect viruses that greatly enhance infection of the viruses by disrupting the PM; the MLX56 family proteins found in mulberry latex as defense proteins against insect herbivores, which modify the PM to a thick structure that inhibits digestive processes; Mir1-CP, a defense cysteine protease from maize that inhibits the growth of insects at very low concentrations and degrades the PM structures; and chitinases and lectins. The importance, necessary characteristics, and modes of action of PM-targeting proteins are then discussed from a strategic point of view, by spotlighting the importance of selective permeability of the PM. Finally, the review discusses the possibility of applying PM-targeting proteins for the control of pest insects.

Integration of two herbivore-induced plant volatiles results in synergistic effects on plant defence and resistance

Plants can use induced volatiles to detect herbivore- and pathogen-attacked neighbors and prime their defenses. Several individual volatile priming cues have been identified, but whether plants are able to integrate multiple cues from stress-related volatile blends remains poorly understood. Here, we investigated how maize plants respond to two herbivore-induced volatile priming cues with complementary information content, the green leaf volatile (Z)-3-hexenyl acetate (HAC) and the aromatic volatile indole. In the absence of herbivory, HAC directly induced defence gene expression, whereas indole had no effect. Upon induction by simulated herbivory, both volatiles increased jasmonate signalling, defence gene expression, and defensive secondary metabolite production and increased plant resistance. Plant resistance to caterpillars was more strongly induced in dual volatile-exposed plants than plants exposed to single volatiles.. Induced defence levels in dual volatile-exposed plants were significantly higher than predicted from the added effects of the individual volatiles, with the exception of induced plant volatile production, which showed no increase upon dual-exposure relative to single exposure. Thus, plants can integrate different volatile cues into strong and specific responses that promote herbivore defence induction and resistance. Integrating multiple volatiles may be beneficial, as volatile blends are more reliable indicators of future stress than single cues.

Plant Defensive β-Glucosidases Resist Digestion and Sustain Activity in the Gut of a Lepidopteran Herbivore

Two-component activated chemical defenses are a major part of many plants' strategies to disrupt herbivory. The activation step is often the β-glucosidase-catalyzed removal of a glucose moiety from a pro-toxin, leading to an unstable and toxic aglycone. While some β-glucosidases have been well studied, several aspects of their roles in vivo, such as their precise sites of enzymatic activity during and after ingestion, and the importance of particular isoforms in plant defense are still not fully understood. Here, plant defensive β-glucosidases from maize, white mustard and almonds were shown to resist digestion by larvae of the generalist lepidopteran Spodoptera littoralis, and the majority of the ingested activities toward both general and plant pro-toxic substrates was recovered in the frass. Among other proteins potentially involved in defense, we identified specific plant β-glucosidases and a maize β-glucosidase aggregating factor in frass from plant-fed insects using proteomic methods. We therefore found that, while S. littoralis larvae efficiently degraded bulk food protein during digestion, β-glucosidases were among a small number of plant defensive proteins that resist insect digestive proteolysis. These enzymes remain intact in the gut lumen and frass and can therefore further catalyze the activation of plant defenses after ingestion, especially in pH-neutral regions of the digestive system. As most of the ingested enzymatic activity persists in the frass, and only particular β-glucosidases were detected via proteomic analyses, our data support the involvement of specific isoforms (maize ZmGlu1 and S. alba MA1 myrosinase) in defense in vivo.

Root exudate metabolites drive plant-soil feedbacks on growth and defense by shaping the rhizosphere microbiota

By changing soil properties, plants can modify their growth environment. Although the soil microbiota is known to play a key role in the resulting plant-soil feedbacks, the proximal mechanisms underlying this phenomenon remain unknown. We found that benzoxazinoids, a class of defensive secondary metabolites that are released by roots of cereals such as wheat and maize, alter root-associated fungal and bacterial communities, decrease plant growth, increase jasmonate signaling and plant defenses, and suppress herbivore performance in the next plant generation. Complementation experiments demonstrate that the benzoxazinoid breakdown product 6-methoxy-benzoxazolin-2-one (MBOA), which accumulates in the soil during the conditioning phase, is both sufficient and necessary to trigger the observed phenotypic changes. Sterilization, fungal and bacterial profiling and complementation experiments reveal that MBOA acts indirectly by altering root-associated microbiota. Our results reveal a mechanism by which plants determine the composition of rhizosphere microbiota, plant performance and plant-herbivore interactions of the next generation.

Plants can modify soil microbiota through root exudation, but how this process influences plant health in turn is often unclear. Here, Hu et al. show that maize benzoxazinoids released into the soil modify root-associated microbiota and thereby increase leaf defenses of the next plant generation.

Plant Bio-Wars: Maize Protein Networks Reveal Tissue-Specific Defense Strategies in Response to a Root Herbivore

In this study we examined global changes in protein expression in both roots and leaves of maize plants attacked by the root herbivore, Western corn rootworm (WCR, Diabrotica virgifera virgifera). The changes in protein expression Are indicative of metabolic changes during WCR feeding that enable the plant to defend itself. This is one of the first studies to look above- and below-ground at global protein expression patterns of maize plants grown in soil and infested with a root herbivore. We used advanced proteomic and network analyses to identify metabolic pathways that contribute to global defenses deployed by the insect resistant maize genotype, Mp708, infested with WCR. Using proteomic analysis, 4878 proteins in roots and leaves were detected and of these 863 showed significant changes of abundance during WCR infestation. Protein abundance patterns were analyzed using hierarchical clustering, protein correlation and protein-protein interaction networks. All three data analysis pipelines showed that proteins such as jasmonic acid biosynthetic enzymes, serine proteases, protease inhibitors, proteins involved in biosynthesis and signaling of ethylene, and enzymes producing reactive oxygen species and isopentenyl pyrophosphate, a precursor for volatile production, were upregulated in roots during WCR infestation. In leaves, highly abundant proteins were involved in signal perception suggesting activation of systemic signaling. We conclude that these protein networks contribute to the overall herbivore defense mechanisms in Mp708. Because the plants were grown in potting mix and not sterilized sand, we found that both microbial and insect defense-related proteins were present in the roots. The presence of the high constitutive levels of reduced ascorbate in roots and benzothiazole in the root volatile profiles suggest a tight tri-trophic interaction among the plant, soil microbiomes and WCR-infested roots suggesting that defenses against insects coexist with defenses against bacteria and fungi due to the interaction between roots and soil microbiota. In this study, which is one of the most complete descriptions of plant responses to root-feeding herbivore, we established an analysis pipeline for proteomics data that includes network biology that can be used with different types of "omics" data from a variety of organisms.

Altering Plant Defenses: Herbivore-Associated Molecular Patterns and Effector Arsenal of Chewing Herbivores

Chewing herbivores, such as caterpillars and beetles, while feeding on the host plant, cause extensive tissue damage and release a wide array of cues to alter plant defenses. Consequently, the cues can have both beneficial and detrimental impacts on the chewing herbivores. Herbivore-associated molecular patterns (HAMPs) are molecules produced by herbivorous insects that aid them to elicit plant defenses leading to impairment of insect growth, while effectors suppress plant defenses and contribute to increased susceptibility to subsequent feeding by chewing herbivores. Besides secretions that originate from glands (e.g., saliva) and fore- and midgut regions (e.g., oral secretions) of chewing herbivores, recent studies have shown that insect frass and herbivore-associated endosymbionts also play a critical role in modulating plant defenses. In this review, we provide an update on a growing body of literature that discusses the chewing insect HAMPs and effectors and the mechanisms by which they modulate host defenses. Novel "omic" approaches and availability of new tools will help researchers to move forward this discipline by identifying and characterizing novel insect HAMPs and effectors and how these herbivore-associated cues are perceived by host plant receptors.

A Maize Inbred Exhibits Resistance Against Western Corn Rootwoorm, Diabrotica virgifera virgifera

Insect resistance against root herbivores like the western corn rootworm (WCR, Diabrotica virgifera virgifera) is not well understood in non-transgenic maize. We studied the responses of two American maize inbreds, Mp708 and Tx601, to WCR infestation using biomechanical, molecular, biochemical analyses, and laser ablation tomography. Previous studies performed on several inbreds indicated that these two maize genotypes differed in resistance to pests including fall armyworm (Spodoptera frugiperda) and WCR. Our data confirmed that Mp708 shows resistance against WCR, and demonstrates that the resistance mechanism is based in a multi-trait phenotype that includes increased resistance to cutting in nodal roots, stable root growth during insect infestation, constitutive and induced expression of known herbivore-defense genes, including ribosomal inhibitor protein 2 (rip2), terpene synthase 23 (tps23) and maize insect resistance cysteine protease-1 (mir1), as well high constitutive levels of jasmonic acid and production of (E)-β-caryophyllene. In contrast, Tx601 is susceptible to WCR. These findings will facilitate the use of Mp708 as a model to explore the wide variety of mechanisms and traits involved in plant defense responses and resistance to herbivory by insects with several different feeding habits.

Transcriptome profiling revealed novel transcriptional regulators in maize responses to Ostrinia furnacalis and jasmonic acid

Chewing insects cause severe yield losses in crop production worldwide. Crop plants counteract chewing insects by transcriptionally promoting a repertoire of defense gene products that are either toxic to, or attractive to the natural enemies of, pest insects. However, the complexity of the transcriptional reprogramming in plant defense response against chewing insects is still not well understood. In this study, the genome-wide early responses in maize seedlings to Asian corn borer (ACB, Ostrinia furnacalis) and also to jasmonic acid(JA), the pivotal phytohormone controlling plant defense response against herbivory, were transcriptionally profiled by RNA-Seq. Clustering of differentially expressed genes (DEGs) along with functional enrichment analysis revealed important biological processes regulated in response to ACB infestation and/or jasmonic acid. Moreover, DEGs with distinct expression patterns were differentially enriched with diverse families of cis-elements on their promoters. Multiple inventories of differentially expressed transcription factors (DETFs) in each DEG group were also analyzed. A transient expression assay using transfected maize protoplastswas established to examine the potential roles of DETFs in maize defense response and JA signaling, and this was used to show that ZmNAC60, an ACB- and JA-inducible DETF, represented a novel positive regulator of JA and defense pathway genes. This study provided a comprehensive transcriptional picture for the early dynamics of maize defense responses and JA signaling, and the identification of DETFs offered potential targets for further functional genomics investigation of master regulators in maize defense responses against herbivory.

Helicoverpa zea gut-associated bacteria indirectly induce defenses in tomato by triggering a salivary elicitor(s)

Insect gut-associated microbes modulating plant defenses have been observed in beetles and piercing-sucking insects, but the role of caterpillar-associated bacteria in regulating plant induced defenses has not been adequately examined. We identified bacteria from the regurgitant of field-collected Helicoverpa zea larvae using 16S ribosomal RNA (rRNA) gene sequencing and matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry. A combination of biochemical, molecular, and confocal electron microscopy methods were used to determine the role of caterpillar-associated bacteria in mediating defenses in Solanum lycopersicum (tomato). Laboratory-reared H. zea inoculated with one of the bacteria identified in field-collected H. zea, Enterobacter ludwigii, induced expression of the tomato defense-related enzyme polyphenol oxidase and genes regulated by jasmonic acid (JA), whereas the salicylic acid (SA)-responsive pathogenesis-related gene was suppressed. Additionally, saliva and its main component glucose oxidase from inoculated caterpillars played an important role in elevating tomato anti-herbivore defenses. However, there were only low detectable amounts of regurgitant or bacteria on H. zea-damaged tomato leaves. Our results suggest that H. zea gut-associated bacteria indirectly mediate plant-insect interactions by triggering salivary elicitors. These findings provide a proof of concept that introducing gut bacteria to a herbivore may provide a novel approach to pest management through indirect induction of plant resistance.

Extensive Evolution of Cereal Ribosome-Inactivating Proteins Translates into Unique Structural Features, Activation Mechanisms, and Physiological Roles

Ribosome-inactivating proteins (RIPs) are a class of cytotoxic enzymes that can depurinate rRNAs thereby inhibiting protein translation. Although these proteins have also been detected in bacteria, fungi, and even some insects, they are especially prevalent in the plant kingdom. This review focuses on the RIPs from cereals. Studies on the taxonomical distribution and evolution of plant RIPs suggest that cereal RIPs have evolved at an enhanced rate giving rise to a large and heterogeneous RIP gene family. Furthermore, several cereal RIP genes are characterized by a unique domain architecture and the lack of a signal peptide. This advanced evolution of cereal RIPs translates into distinct structures, activation mechanisms, and physiological roles. Several cereal RIPs are characterized by activation mechanisms that include the proteolytic removal of internal peptides from the N-glycosidase domain, a feature not documented for non-cereal RIPs. Besides their role in defense against pathogenic fungi or herbivorous insects, cereal RIPs are also involved in endogenous functions such as adaptation to abiotic stress, storage, induction of senescence, and reprogramming of the translational machinery. The unique properties of cereal RIPs are discussed in this review paper.

A small RNA targets pokeweed antiviral protein transcript

Ribosome-inactivating proteins (RIPs) are a class of plant defense proteins with N-glycosidase activity (EC 3.2.2.22). Pokeweed antiviral protein (PAP) is a Type I RIP isolated from the pokeweed plant, Phytolacca americana, thought to confer broad-spectrum virus resistance in this plant. Through a combination of standard molecular techniques and RNA sequencing analysis, we report here that a small RNA binds and cleaves the open reading frame of PAP mRNA. Additionally, sRNA targeting of PAP is dependent on jasmonic acid (JA), a plant hormone important for defense against pathogen infection and herbivory. Levels of small RNA increased with JA treatment, as did levels of PAP mRNA and protein, suggesting that the small RNA functions to moderate the expression of PAP in response to this hormone. The association between JA and PAP expression, mediated by sRNA299, situates PAP within a signaling pathway initiated by biotic stress. The consensus sequence of sRNA299 was obtained through bioinformatic analysis of pokeweed small RNA sequencing. To our knowledge, this is the first account of a sRNA targeting a RIP gene.

The Pokeweed Leaf mRNA Transcriptome and Its Regulation by Jasmonic Acid

The American pokeweed plant, Phytolacca americana, is recognized for synthesizing pokeweed antiviral protein (PAP), a ribosome inactivating protein (RIP) that inhibits the replication of several plant and animal viruses. The plant is also a heavy metal accumulator with applications in soil remediation. However, little is known about pokeweed stress responses, as large-scale sequencing projects have not been performed for this species. Here, we sequenced the mRNA transcriptome of pokeweed in the presence and absence of jasmonic acid (JA), a hormone mediating plant defense. Trinity-based de novo assembly of mRNA from leaf tissue and BLASTx homology searches against public sequence databases resulted in the annotation of 59 096 transcripts. Differential expression analysis identified JA-responsive genes that may be involved in defense against pathogen infection and herbivory. We confirmed the existence of several PAP isoforms and cloned a potentially novel isoform of PAP. Expression analysis indicated that PAP isoforms are differentially responsive to JA, perhaps indicating specialized roles within the plant. Finally, we identified 52 305 natural antisense transcript pairs, four of which comprised PAP isoforms, suggesting a novel form of RIP gene regulation. This transcriptome-wide study of a Phytolaccaceae family member provides a source of new genes that may be involved in stress tolerance in this plant. The sequences generated in our study have been deposited in the SRA database under project # SRP069141.

Differential expression of the chemosensory transcriptome in two populations of the stemborer Sesamia nonagrioides

Among the proposed mechanisms of local adaptation to different ecological environments, transcriptional changes may play an important role. In this study, we investigated whether such variability occurred within the chemosensory organs of a herbivorous insect, for which chemosensation guides most of its host preferences. A European and an African population of the noctuid Sesamia nonagrioides that display significant differences in their ecological preferences were collected on Zea mays and Typha domingensis, respectively. RNAseq were used between the two populations for digital expression profiling of chemosensory organs from larval antennae and palps. Preliminary data on adult female antennae and ovipositors were also collected. We found 6,550 differentially expressed transcripts in larval antennae and palps. Gene ontology enrichment analyses suggested that transcriptional activity was overrepresented in the French population and that virus and defense activities were overrepresented in the Kenyan population. In addition, we found differential expression of a variety of cytochrome P450s, which may be linked to the different host-plant diets. Looking at olfactory genes, we observed differential expression of numerous candidate odorant-binding proteins, chemosensory proteins, and one olfactory receptor, suggesting that differences in olfactory sensitivity participate in insect adaptation.

"Omics" of maize stress response for sustainable food production: opportunities and challenges

Maize originated in the highlands of Mexico approximately 8700 years ago and is one of the most commonly grown cereal crops worldwide, followed by wheat and rice. Abiotic stresses (primarily drought, salinity, and high and low temperatures), together with biotic stresses (primarily fungi, viruses, and pests), negatively affect maize growth, development, and eventually production. To understand the response of maize to abiotic and biotic stresses and its mechanism of stress tolerance, high-throughput omics approaches have been used in maize stress studies. Integrated omics approaches are crucial for dissecting the temporal and spatial system-level changes that occur in maize under various stresses. In this comprehensive analysis, we review the primary types of stresses that threaten sustainable maize production; underscore the recent advances in maize stress omics, especially proteomics; and discuss the opportunities, challenges, and future directions of maize stress omics, with a view to sustainable food production. The knowledge gained from studying maize stress omics is instrumental for improving maize to cope with various stresses and to meet the food demands of the exponentially growing global population. Omics systems science offers actionable potential solutions for sustainable food production, and we present maize as a notable case study.

Interaction of plant defense compounds with the insect gut: new insights from genomic and molecular analyses

The co-evolutionary conflict between insect herbivores and their host plants is profoundly influenced by biochemical reactions associated with passage of toxin-laden plant material through the herbivore digestive canal. Insect herbivores provide excellent models in which to understand the mechanistic interplay between nutrition and detoxification, how plant defense compounds hijack these processes, and how insects adapt to host defense chemistry. Expanding genome sequence information and genetic approaches to manipulate gene function in both interacting partners are providing new insights into the genetic underpinnings of host preference and plasticity in gut physiology. Fundamental knowledge gained from these studies has practical application in understanding how insects evolve resistance to pesticides, and may also inform efforts to better understand how plant chemicals impact human health.

Accumulation of 5-hydroxynorvaline in maize (Zea mays) leaves is induced by insect feeding and abiotic stress

Plants produce a wide variety of defensive metabolites to protect themselves against herbivores and pathogens. Non-protein amino acids, which are present in many plant species, can have a defensive function through their mis-incorporation during protein synthesis and/or inhibition of biosynthetic pathways in primary metabolism. 5-Hydroxynorvaline was identified in a targeted search for previously unknown non-protein amino acids in the leaves of maize (Zea mays) inbred line B73. Accumulation of this compound increases during herbivory by aphids (Rhopalosiphum maidis, corn leaf aphid) and caterpillars (Spodoptera exigua, beet armyworm), as well as in response to treatment with the plant signalling molecules methyl jasmonate, salicylic acid and abscisic acid. In contrast, ethylene signalling reduced 5-hydroxynorvaline abundance. Drought stress induced 5-hydroxynorvaline accumulation to a higher level than insect feeding or treatment with defence signalling molecules. In field-grown plants, the 5-hydroxynorvaline concentration was highest in above-ground vegetative tissue, but it was also detectable in roots and dry seeds. When 5-hydroxynorvaline was added to aphid artificial diet at concentrations similar to those found in maize leaves and stems, R. maidis reproduction was reduced, indicating that this maize metabolite may have a defensive function. Among 27 tested maize inbred lines there was a greater than 10-fold range in the accumulation of foliar 5-hydroxynorvaline. Genetic mapping populations derived from a subset of these inbred lines were used to map quantitative trait loci for 5-hydroxynorvaline accumulation to maize chromosomes 5 and 7.

Additive effects of two quantitative trait loci that confer Rhopalosiphum maidis (corn leaf aphid) resistance in maize inbred line Mo17

Plants show considerable within-species variation in their resistance to insect herbivores. In the case of Zea mays (cultivated maize), Rhopalosiphum maidis (corn leaf aphids) produce approximately twenty times more progeny on inbred line B73 than on inbred line Mo17. Genetic mapping of this difference in maize aphid resistance identified quantitative trait loci (QTL) on chromosomes 4 and 6, with the Mo17 allele reducing aphid reproduction in each case. The chromosome 4 QTL mapping interval includes several genes involved in the biosynthesis of DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one), a maize defensive metabolite that also is required for callose accumulation in response to aphid feeding. Consistent with the known association of callose with plant defence against aphids, R. maidis reproduction on B73×Mo17 recombinant inbred lines was negatively correlated with both DIMBOA content and callose formation. Further genetic mapping, as well as experiments with near-isogenic lines, confirmed that the Mo17 allele causes increased DIMBOA accumulation relative to the B73 allele. The chromosome 6 aphid resistance QTL functions independently of DIMBOA accumulation and has an effect that is additive to that of the chromosome 4 QTL. Thus, at least two separate defence mechanisms account for the higher level of R. maidis resistance in Mo17 compared with B73.

Identification of a bioactive Bowman-Birk inhibitor from an insect-resistant early maize inbred

Breeding of maize, Zea mays, has improved insect resistance, but the genetic and biochemical basis of many of these improvements is unknown. Maize oligonucleotide microarrays were utilized to identify differentially expressed genes in leaves of three maize inbreds, parents Oh40B and W8 and progeny Oh43, developed in the 1940s. Oh43 had enhanced leaf resistance to corn earworm larvae, Helicoverpa zea, and fall armyworm larvae, Spodoptera frugiperda, compared to one or both parents. Among ca. 100 significantly differentially expressed genes, expression of a Bowman-Birk trypsin inhibitor (BBI) gene was at least ca. 8-fold higher in Oh43 than in either parent. The Oh43 BBI gene was expressed as a recombinant protein. Purified BBI inhibited trypsin and the growth of fall armyworm larvae when added to insect diet. These experiments indicate that comparative gene expression analysis combined with insect resistance measurements of early inbreds can identify previously unrecognized resistance genes.

Herbivore cues from the fall armyworm (Spodoptera frugiperda) larvae trigger direct defenses in maize

In addition to feeding damage, herbivores release cues that are recognized by plants to elicit defenses. Caterpillar oral secretions have been shown to trigger herbivore defense responses in several different plant species. In this study, the effects of two fall armyworm (Spodoptera frugiperda) oral secretions (saliva and regurgitant) on caterpillar defense responses in maize (Zea mays) were examined. Only minute amounts of regurgitant were deposited on the maize leaf during larval feeding bouts and its application to leaves failed to induce the expression of several herbivore defense genes. On the other hand, caterpillars consistently deposited saliva on leaves during feeding and the expression of several maize defense genes significantly increased in response to saliva application and larval feeding. However, feeding by ablated caterpillars with impaired salivation did not induce these defenses. Furthermore, bioassays indicated that feeding by unablated caterpillars significantly enhanced defenses when compared with that of ablated caterpillars. Another critical finding was that the maize genotype and stage of development affected the expression of defense genes in response to wounding and regurgitant treatments. These results demonstrate that fall armyworm saliva contains elicitors that trigger herbivore defenses in maize.