Potential role of lipoxygenases in defense against insect herbivory

Gall midge Baldratia salicorniae Kieffer (Diptera: Cecidomyiidae) infestation on Salicornia europaea L. induces the production of specialized metabolites with biotechnological potential

Saltmarsh plants have several defense mechanisms against threatening abiotic conditions, such as salinity, inundation, or exposure to intense radiation, less is known regarding response to insect pests attack. Salicornia europaea L. plant stands are produced as cash crops in Portuguese coastal areas. In 2017, these crops suffered significant attacks from a gall midge fly (Baldratia salicorniae Kieffer), reducing its economic value. To understand how this attack influenced S. europaea chemical composition, infested and non-infested branches were collected, and their extracts were analysed by GS-MS and UHPLC-MS. Results revealed that different degrees of infestations displayed different chemical composition. Several compounds were for the first time identified in S. europaea, such as, arachidic acid, alpha-tocopherol, henicos-1-ene, and squalene. Most evident results were the reduced amount of alkanes in the infested conditions, which seems to be a direct consequence of insect infestation. Several compounds identified in the infested branches are known to have negative effects on insect larvae by reducing larval growth (linoleic acid) or increasing insect mortality (oleic acid). Halophyte plants production is increasing and it is accompanied by the urge to develop early control strategies against potential pests. These strategies may include ecological friendly solutions such as endogenous production of specialized metabolites to retrieve plant self-defences. Further, our results showed that B. salicorniae herbivory also induced the production of higher number of specialized metabolites with important known biological activities. In years in which high infestations reduce organoleptic qualities for fresh consumption plants can be used in biorefinery industries for metabolite extraction.

Relevance of methionine sulfoxide reductase(s) (MSR) as candidate proteins in redox homeostasis-mediated resistance response to Helicoverpa armigera (Hübner) in the pigeonpea wild relative Cajanus platycarpus (Benth.) Maesen

Pod borer, Helicoverpa armigera, a polyphagus herbivore causes extensive economic losses to crops, including pigeonpea. Exploitation of pod borer resistance in wild relatives is pertinent due to the absence of resistance sources in cultivated pigeonpea and crossing-incompatibility with the resistant wild relatives. We present leads obtained in deeper understanding of pod borer resistance mechanism in Cajanus platycarpus, a pigeonpea wild relative. Surge in cellular ROS during herbivory leads to redox-PTMs (post translational modifications) of methionine-rich proteins including antioxidant enzymes, causing oxidative damage. Plants then officiate methionine sulfoxide reductases (MSRs), that maintain the redox status of methionine and hence homeostasis. We demonstrate functionality of MSRs (MSRA and MSRB) in the resistance response of the wild relative to pod borer. Among 5 MSRA and 3 MSRB genes, CpMSRA2 and CpMSRB1 were herbivore-responsive based on expression during herbivory. Clues about the stress-responsiveness were obtained upon analyses of cis-elements and co-expressing genes. Apparently, the wild relative followed a non-canonical mode of redox management, as divulged by antioxidant enzymes and the scavenging capacity. Differential lipid peroxidation as an early response provided evidences for an effective redox management in the wild relative. This is the first report signifying redox homeostasis in the resistance response towards herbivory.

Hemp Pest Spectrum and Potential Relationship between Helicoverpa zea Infestation and Hemp Production in the United States in the Face of Climate Change

Simple Summary

Cultivation of industrial hemp Cannabis sativa in the United States is now being expanded due to the recent legalization of the crop. Multiple insect pests attack the crop. One of the common pests is the corn earworm Helicoverpa zea that causes extensive damage to the marketable parts of hemp. Changing global climate may lead to expansion of the geographic range of insect pests. Thus, growers of this crop in the United States have to face new and intense pest problems now and in the years to come. Here, we assess the potential relationship between corn earworm infestation and hemp production in the US in the face of climate change. We also provide an update on the arthropods associated with hemp cultivation across the US. Climate change can affect aspects of interactions between hemp and corn earworm. Temperature and photoperiod affect the development and diapause process in H. zea. Drought leads to a reduction in hemp growth. Overall, our assessment suggests the selection of varieties resistant to stresses from climate and insects. Host plant diversity may prevent populations of corn earworm from reaching outbreak levels. Ongoing research on effective management of H. zea on hemp is critical.

Abstract

There has been a resurgence in the cultivation of industrial hemp, Cannabis sativa L., in the United States since its recent legalization. This may facilitate increased populations of arthropods associated with the plant. Hemp pests target highly marketable parts of the plant, such as flowers, stalks, and leaves, which ultimately results in a decline in the quality. Industrial hemp can be used for several purposes including production of fiber, grain, and cannabidiol. Thus, proper management of pests is essential to achieve a substantial yield of hemp in the face of climate change. In this review, we provide updates on various arthropods associated with industrial hemp in the United States and examine the potential impact of climate change on corn earworm (CEW) Helicoverpa zea Boddie, a major hemp pest. For example, temperature and photoperiod affect the development and diapause process in CEW. Additionally, drought can lead to a reduction in hemp growth. Host plant diversity of CEW may prevent populations of the pest from reaching outbreak levels. It is suggested that hemp varieties resistant to drought, high soil salinity, cold, heat, humidity, and common pests and diseases should be selected. Ongoing research on effective management of CEW in hemp is critical.

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.

Role of reactive oxygen species and isoflavonoids in soybean resistance to the attack of the southern green stink bug

Southern green stink bugs (Nezara viridula L.) are one of the major pests in many soybean producing areas. They cause a decrease in yield and affect seed quality by reducing viability and vigor. Alterations have been reported in the oxidative response and in the secondary metabolites in different plant species due to insect damage. However, there is little information available on soybean-stink bug interactions. In this study we compare the response of undamaged and damaged seeds by Nezara viridula in two soybean cultivars, IAC-100 (resistant) and Davis (susceptible), grown under greenhouse conditions. Pod hardness, H₂O₂ generation, enzyme activities in guaiacol peroxidase (GPOX), catalase (CAT) and superoxide dismutase (SOD) as well as lipoxygenase expression and isoflavonoid production were quantified. Our results showed a greater resistance of IAC-100 to pod penetration, a decrease in peroxide content after stink bug attack, and higher GPOX, CAT and SOD activities in seeds due to the genotype and to the genotype-interaction with the herbivory treatment. Induction of LOX expression in both cultivars and higher production of isoflavonoids in IAC-100 were also detected. It was then concluded that the herbivory stink bug induces pathways related to oxidative stress and to the secondary metabolites in developing seeds of soybean and that differences between cultivars hold promise for a plant breeding program.

Documenting Resistance and Physiological Changes in Soybean Challenged by Aphis glycines Matsumura (Hemiptera: Aphididae)

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is a limiting factor in soybean production in the North Central region of the USA. The objectives of this work were to identify sources of resistance to A. glycines in 14 soybean genotypes, and also document changes in total protein, peroxidase, and chlorophyll in response to aphid feeding. A reduced number of A. glycines was observed on the genotypes UX 2569-159-2-01 and UX 2570-171- 04, indicating the presence of antixenosis and/or antibiosis. UX 2569-159-2-01 expressed the highest level of resistance; whereas, UX 2570-171-04 had moderate levels of resistance to A. glycines. Chlorophyll content was relatively unaffected by A. glycines, except for a reduction in UX 2569-159-2-01 infested plants at 5 and 15 days after infestation (DAI). No changes were detected in total protein content between infested and control plants for the genotypes analyzed; however, peroxidase activity was higher in infested UX 2570-171-04 at both 5 and 10 DAI. This improvement in peroxidase content in infested UX 2570-171-04 may be playing multiple roles in the plant tolerance.

Insights using the molecular model of Lipoxygenase from Finger millet (Eleusine coracana (L.))

Lipoxygenase-1 (LOX-1) protein provides defense against pests and pathogens and its presence have been positively correlated with plant resistance against pathogens. Linoleate is a known substrate of lipoxygenase and it induces necrosis leading to the accumulation of isoflavonoid phytoalexins in plant leaves. Therefore, it is of interest to study the structural features of LOX-1 from Finger millet. However, the structure ofLOX-1 from Finger millet is not yet known. A homology model of LOX-1 from Finger millet is described. Domain architecture study suggested the presence of two domains namely PLAT (Phospho Lipid Acyl Transferase) and lipoxygenase. Molecular docking models of linoleate with lipoxygenase from finger millet, rice and sorghum are reported. The features of docked models showed that finger millet have higher pathogen resistance in comparison to other cereal crops. This data is useful for the molecular cloning of fulllength LOX-1 gene for validating its role in improving plant defense against pathogen infection and for various other biological processes.

Induced resistance by oxidative shifts in pigeonpea (Cajanus cajan L.) following Helicoverpa armigera (Hübner) herbivory

BACKGROUND

Oxidative responses in leaves, developing seeds and the pod wall of nine pigeonpea genotypes were investigated against Helicoverpa armigera feeding. Out of nine genotypes, four were moderately resistant, three were intermediate and two were moderately susceptible genotypes.

RESULTS

A significant shift in the oxidative status of pigeonpea following herbivory was depicted by the upregulation of diamine oxidase (DAO), polyamine oxidase (PAO) and lipoxygenase 2 (LOX 2) activities. Polyphenol oxidase (PPO) activity was significantly higher in the infested pod wall and leaves of moderately resistant genotypes than in those of moderately susceptible genotypes. H. armigera infestation markedly enhanced phenylalanine ammonia lyase (PAL) and tyrosine ammonia lyase (TAL) activities in wounded tissues. The decline in ascorbate peroxidase (APX) activity and ascorbate content was lower in moderately resistant genotypes than in moderately susceptible genotypes. A significant decrease in LOX 3 activity was also observed in the infested pod wall of moderately resistant and intermediate genotypes. A lower malondialdehyde (MDA) content and higher proline content of the infested pod wall and developing seeds was observed. Higher activities of PPO, PAL and proline content in leaves of uninfested moderately resistant genotypes could either be an unrelated observation or alternatively could help in identifying H. armigera-resistant genotypes.

CONCLUSION

The increase in activities of PPO, DAO, PAO, PAL and TAL and higher proline and lower MDA content upon herbivory suggested their integrated contribution in providing resistance to pigeonpea against H. armigera.

Induced resistance to Helicoverpa armigera through exogenous application of jasmonic acid and salicylic acid in groundnut, Arachis hypogaea

BACKGROUND

Induced resistance to Helicoverpa armigera through exogenous application of jasmonic acid (JA) and salicylic acid (SA) was studied in groundnut genotypes (ICGV 86699, ICGV 86031, ICG 2271 and ICG 1697) with different levels of resistance to insects and the susceptible check JL 24 under greenhouse conditions. Activities of oxidative enzymes and the amounts of secondary metabolites and proteins were quantified at 6 days after JA and SA application/insect infestation. Data were also recorded on plant damage and H. armigera larval weights and survival.

RESULTS

Higher levels of enzymatic activities and amounts of secondary metabolites were observed in the insect-resistant genotypes pretreated with JA and then infested with H. armigera than in JL 24. The insect-resistant genotypes suffered lower insect damage and resulted in poor survival and lower weights of H. armigera larvae than JL 24. In some cases, JA and SA showed similar effects.

CONCLUSION

JA and SA induced the activity of antioxidative enzymes in groundnut plants against H. armigera, and reduced its growth and development. However, induced response to application of JA was greater than to SA, and resulted in reduced plant damage, and larval weights and survival, suggesting that induced resistance can be used as a component of pest management in groundnut.

Caterpillar attack triggers accumulation of the toxic maize protein RIP2

Some plant-derived anti-herbivore defensive proteins are induced by insect feeding, resist digestion in the caterpillar gut and are eliminated in the frass. We have identified several maize proteins in fall armyworm (Spodoptera frugiperda) frass that potentially play a role in herbivore defense. Furthermore, the toxicity of one of these proteins, ribosome-inactivating protein 2 (RIP2), was assessed and factors regulating its accumulation were determined. To understand factors regulating RIP2 protein accumulation, maize (Zea mays) plants were infested with fall armyworm larvae or treated with exogenous hormones. The toxicity of recombinant RIP2 protein against fall armyworm was tested. The results show that RIP2 protein is synthesized as an inactive proenzyme that can be processed in the caterpillar gut. Also, caterpillar feeding, but not mechanical wounding, induced foliar RIP2 protein accumulation. Quantitative real-time PCR indicated that RIP2 transcripts were rapidly induced (1 h) and immunoblot analysis indicated that RIP2 protein accumulated soon after attack and was present in the leaf for up to 4 d after caterpillar removal. Several phytohormones, including methyl jasmonate, ethylene, and abscisic acid, regulated RIP2 protein expression. Furthermore, bioassays of purified recombinant RIP2 protein against fall armyworm significantly retarded caterpillar growth. We conclude that the toxic protein RIP2 is induced by caterpillar feeding and is one of a potential suite of proteins that defend maize against chewing herbivores.

Oxidative enzyme changes in sorghum infested by shoot fly

This research investigated the role of oxidative enzymes in the defense response of sorghum, Sorghum bicolor (L.) Moench (Poales: Poaceae), to the sorghum shoot fly, Atherigona soccata Rondani (Diptera: Muscidae). Changes in polyphenol oxidase and peroxidase activity and total protein content were observed in resistant and susceptible sorghum genotypes in response to A. soccata feeding. Resistant plants exhibited higher levels of peroxidase and polyphenol oxidase activities and total protein content compared with susceptible plants. Peroxidase and polyphenol oxidase activities and total protein content in the infested resistant and susceptible genotypes were higher when compared with their control plants, respectively. These findings suggest that resistant genotypes may be able to tolerate shoot fly feeding by increasing their peroxidase and polyphenol oxidase activities. Among the enzymes examined, differences in isozyme profiles for peroxidase and polyphenol oxidase were detected between control and infested IS 18551, M35-1, 296B, SSV 84, and DJ 6514 plants. Differences in protein profiles were observed between A. soccata infested and their respective uninfested controls of all the genotypes. In conclusion, this study revealed that these defense enzymes and proteins might attribute to the resistance mechanisms in sorghum plants against A. soccata infestation.

Foliar oxidative stress and insect herbivory: Primary compounds, secondary metabolites, and reactive oxygen species as components of induced resistance

Oxidative responses of plants to pathogens and other environmental stresses have received considerable recent attention. We propose that an oxidative response also occurs following attack by herbivores. Our data strongly indicate a shift in the oxidative status of soybean following herbivory by the insectHelicoverpa zea. Herbivory caused significant increases in lipid peroxidation and ·OH radical formation. The activity of several oxidative enzymes including lipoxygenases, peroxidase, diamine oxidase, ascorbate oxidase, and NADH oxidase I increased after herbivory on soybean. The enhanced production of phenolic compounds is indicated by an increase in the activity of phenylalanine ammonia lyase in wounded tissues. On the other hand, the level of soybean foliar antioxidants such as ascorbic acid, total carotenoids, nonprotein thiols, and catalase decreased significantly following herbivory. These results implicate primary compounds (e.g., ascorbic acid, proteins), secondary metabolites (e.g., phenolics), and reactive oxygen species (e.g., hydroxyl radical, hydrogen peroxide) as multiple components of induced resistance. The oxidative changes in the host plant correspond with increased oxidative damage in the midgut of insects feeding on previously wounded plants. Decreases in nonprotein thiols and reduced ascorbic acid occurred in midgut epithelial tissue from insects feeding on wounded plants compared to the insects on control plants. In contrast, midgut hydroperoxides and dehydroascorbic acid concentrations were greater in insects on wounded plants compared to their counterparts on control plants. We conclude that oxidative responses in soybean may have both positive and negative effects upon the host plant: a decrease in herbivory and an increase in oxidative damage to the plant. The salient benefit to the plant, in terms of insect resistance, is the relative balance between these opposing effects.

Plant as a plenteous reserve of lectin

Lectins are clusters of glycoproteins of nonimmune foundation that combine specifically and reversibly to carbohydrates, mainly the sugar moiety of glycoconjugates, resulting in cell agglutination and precipitation of glycoconjugates. They are universally distributed in nature, being established in plants, fungi, viruses, bacteria, crustacea, insects, and animals, but leguminacae plants are rich source of lectins. The present review reveals the structure, biological properties, and application of plant lectins.

Oxidative stress in pea seedling leaves in response to Acyrthosiphon pisum infestation

In this study we examined whether and to what extent oxidative stress is induced in seedling leaves of Pisum sativum L. cv. Cysterski in response to pea aphid (Acyrthosiphon pisum Harris) infestation. A. pisum caused oxidative stress conditions in pea leaves through enhanced production of the reactive oxygen species (ROS) hydrogen peroxide (H2O2) and superoxide anion radical (O2(·-)). Early, strong generation of H2O2 was observed at 24h in aphid-infested leaves. The highest level of H2O2 at this time point may be related to the functioning of H2O2 as a signaling molecule, triggering defense mechanisms in pea leaves against A. pisum. Additionally, the strong generation and continuous increase of O2(·-) production in aphid-infested leaves from 0 to 96 h enhanced the defense potential to protect against aphid herbivory. Also in the study cytochemical localization of H2O2 and O2(·-) in pea leaves after aphid infestation was determined using the confocal microscope. Relative release of H2O2 and O2(·-) was estimated by staining leaves with specific fluorochromes, i.e. dichlorodihydro-fluorescein diacetate (DCFH-DA) and dihydroethidium (DHE), respectively. DCFH-DA and DHE derived fluorescence was observed to cover a much larger tissue area in aphid-infested leaves, whereas little or no fluorescence was observed in the control leaves. Enhanced activity of the antioxidant enzymes superoxide dismutase (SOD, 1.15.1.1) and catalase (CAT, 1.11.1.6) is one of the most essential elements of defense responses in pea seedling leaves to oxidative stress. Additionally, generation of semiquinones, stable free radicals with g-values of 2.0020 and 2.0035, detected by electron paramagnetic resonance spectroscopy (EPR), was suggested as a protective action of pea that may contribute to build-up of a defensive barrier or activate other defense mechanisms. Concentrations of semiquinone radicals in aphid-infested seedling leaves not only were generally higher than in the control plants but also significantly increased with cultivation time. On the other hand, the small increase in content of thiobarbituric acid reactive substances (TBARS), a product of lipid peroxidation, and the percentage of injury (3-8%) indicated that the cellular damage was caused by oxidative stress. The induced changes in levels of H2O2, O2(·-) and semiquinone radicals as well as activities of antioxidant enzymes in the pea defense responses were proportional to the population size of A. pisum. These findings indicate that the defensive strategies against A. pisum infestation were stimulated in seedling leaves of P. sativum L. cv. Cysterski. Our observations of the enhanced defense responses of P. sativum to infestation by A. pisum reveal some aspects and contribute to current knowledge of regulatory mechanisms in plant-aphid interactions.

Towards uncovering the roles of switchgrass peroxidases in plant processes

Herbaceous perennial plants selected as potential biofuel feedstocks had been understudied at the genomic and functional genomic levels. Recent investments, primarily by the U.S. Department of Energy, have led to the development of a number of molecular resources for bioenergy grasses, such as the partially annotated genome for switchgrass (Panicum virgatum L.), and some related diploid species. In its current version, the switchgrass genome contains 65,878 gene models arising from the A and B genomes of this tetraploid grass. The availability of these gene sequences provides a framework to exploit transcriptomic data obtained from next-generation sequencing platforms to address questions of biological importance. One such question pertains to discovery of genes and proteins important for biotic and abiotic stress responses, and how these components might affect biomass quality and stress response in plants engineered for a specific end purpose. It can be expected that production of switchgrass on marginal lands will expose plants to diverse stresses, including herbivory by insects. Class III plant peroxidases have been implicated in many developmental responses such as lignification and in the adaptive responses of plants to insect feeding. Here, we have analyzed the class III peroxidases encoded by the switchgrass genome, and have mined available transcriptomic datasets to develop a first understanding of the expression profiles of the class III peroxidases in different plant tissues. Lastly, we have identified switchgrass peroxidases that appear to be orthologs of enzymes shown to play key roles in lignification and plant defense responses to hemipterans.

Fluctuations in peroxidase and catalase activities of resistant and susceptible black gram (Vigna mungo (L.) Hepper) genotypes elicited by Bemisia tabaci (Gennadius) feeding

Whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleryrodidae), is a serious pest of black gram, (Vigna mungo (L.) Hepper), an important legume pulse crop grown in north India. This research investigated the potential role of selected plant oxidative enzymes in resistance/susceptibility to whitefly in nine black gram genotypes. Oxidative enzyme activity was estimated spectrophotometrically from leaf samples collected at 30 and 50 d after sowing (DAS) from whitefly infested and uninfested plants. The enzymes showed different activity levels at different times after the infestation. The results indicated that in general, whitefly infestation increased the activities of peroxidase and decreased the catalase activity. Resistant genotypes NDU 5-7 and KU 99-20 recorded higher peroxidase and catalase activities at 30 and 50 DAS under whitefly-stress conditions as compared with non-stressed plants. The results suggest that the enhanced activities of the enzymes may contribute to bioprotection of black gram plants against B. tabaci infestation. The potential mechanisms to explain the correlation of resistance to whitefly in black gram genotypes with higher activities of oxidative enzymes are also discussed.

Role of trichomes in defense against herbivores: comparison of herbivore response to woolly and hairless trichome mutants in tomato (Solanum lycopersicum)

Trichomes contribute to plant resistance against herbivory by physical and chemical deterrents. To better understand their role in plant defense, we systemically studied trichome morphology, chemical composition and the response of the insect herbivores Helicoverpa zea and Leptinotarsa decemlineata (Colorado potato beetle = CPB) on the tomato hairless (hl), hairy (woolly) mutants and wild-type Rutgers (RU) and Alisa Craig (AC) plants. Hairless mutants showed reduced number of twisted glandular trichomes (types I, IV, VI and VII) on leaf and stem compared to wild-type Rutgers (RU), while woolly mutants showed high density of non-glandular trichomes (types II, III and V) but only on the leaf. In both mutants, trichome numbers were increased by methyl jasmonate (MeJA), but the types of trichomes present were not affected by MeJA treatment. Glandular trichomes contained high levels of monoterpenes and sesquiterpenes. A similar pattern of transcript accumulation was observed for monoterpene MTS1 (=TPS5) and sesquiterpene synthase SST1 (=TPS9) genes in trichomes. While high density of non-glandular trichome on leaves negatively influenced CPB feeding behavior and growth, it stimulated H. zea growth. High glandular trichome density impaired H. zea growth, but had no effect on CPB. Quantitative real-time polymerase chain reaction (qRT-PCR) showed that glandular trichomes highly express protein inhibitors (PIN2), polyphenol oxidase (PPOF) and hydroperoxide lyase (HPL) when compared to non-glandular trichomes. The SlCycB2 gene, which participates in woolly trichome formation, was highly expressed in the woolly mutant trichomes. PIN2 in trichomes was highly induced by insect feeding in both mutant and wild-type plants. Thus, both the densities of trichomes and the chemical defenses residing in the trichomes are inducible.

Soybean vegetative lipoxygenases are not vacuolar storage proteins

The paraveinal mesophyll (PVM) of soybean is a distinctive uniseriate layer of branched cells situated between the spongy and palisade chlorenchyma of leaves that contains an abundance of putative vegetative storage proteins, Vspα and Vspβ, in its vacuoles. Soybean vegetative lipoxygenases (five isozymes designated as Vlx(A-E)) have been reported to co-localise with Vsp in PVM vacuoles; however, conflicting results regarding the tissue-level and subcellular localisations of specific Vlx isozymes have been reported. We employed immuno-cytochemistry with affinity-purified, isozyme-specific antibodies to reinvestigate the subcellular locations of soybean Vlx isozymes during a sink limitation experiment. VlxB and VlxC were localised to the cytoplasm and nucleoplasm of PVM cells, whereas VlxD was present in the cytoplasm and nucleoplasm of mesophyll chlorenchyma (MC) cells. Label was not associated with storage vacuoles or any evident protein bodies, so our results cast doubt on the hypothesis that Vlx isozymes function as vegetative storage proteins.

Plant latex and other exudates as plant defense systems: roles of various defense chemicals and proteins contained therein

Plant latex and other exudates are saps that are exuded from the points of plant damage caused either mechanically or by insect herbivory. Although many (ca. 10%) of plant species exude latex or exudates, and although the defensive roles of plant latex against herbivorous insects have long been suggested by several studies, the detailed roles and functions of various latex ingredients, proteins and chemicals, in anti-herbivore plant defenses have not been well documented despite the wide occurrence of latex in the plant kingdom. Recently, however, substantial progress has been made. Several latex proteins, including cysteine proteases and chitin-related proteins, have been shown to play important defensive roles against insect herbivory. In the mulberry (Morus spp.)-silkworm (Bombyx mori) interaction, an old and well-known model system of plant-insect interaction, plant latex and its ingredients--sugar-mimic alkaloids and defense protein MLX56--are found to play key roles. Complicated molecular interactions between Apocynaceae species and its specialist herbivores, in which cardenolides and defense proteins in latex play key roles, are becoming more and more evident. Emerging observations suggested that plant latex, analogous to animal venom, is a treasury of useful defense proteins and chemicals that has evolved through interspecific interactions. On the other hand, specialist herbivores developed sophisticated adaptations, either molecular, physiological, or behavioral, against latex-borne defenses. The existence of various adaptations in specialist herbivores itself is evidence that latex and its ingredients function as defenses at least against generalists. Here, we review molecular and structural mechanisms, ecological roles, and evolutionary aspects of plant latex as a general defense against insect herbivory and we discuss, from recent studies, the unique characteristics of latex-borne defense systems as transport systems of defense substances are discussed based on recent studies.

Testing the importance of jasmonate signalling in induction of plant defences upon cabbage aphid (Brevicoryne brassicae) attack

Background

Phloem-feeding aphids deprive plants of assimilates, but mostly manage to avoid causing the mechanical tissue damage inflicted by chewing insects. Nevertheless, jasmonate signalling that is induced by infestation is important in mediating resistance to phloem feeders. Aphid attack induces the jasmonic acid signalling pathway, but very little is known about the specific impact jasmonates have on the expression of genes that respond to aphid attack.

Results

We have evaluated the function that jasmonates have in regulating Arabidopsis thaliana responses to cabbage aphid (Brevicoryne brassicae) by conducting a large-scale transcriptional analysis of two mutants: aos, which is defective in jasmonate production, and fou2, which constitutively induces jasmonic acid biosynthesis. This analysis enabled us to determine which genes' expression patterns depend on the jasmonic acid signalling pathway. We identified more than 200 genes whose expression in non-challenged plants depended on jasmonate levels and more than 800 genes that responded differently to infestation in aos and fou2 plants than in wt. Several aphid-induced changes were compromised in the aos mutant, particularly genes connected to regulation of transcription, defence responses and redox changes. Due to jasmonate-triggered pre-activation of fou2, its transcriptional profile in non-challenged plants mimicked the induction of defence responses in wt. Additional activation of fou2 upon aphid attack was therefore limited. Insect fitness experiments revealed that the physiological consequences of fou2 mutation contributed to more effective protection against B. brassicae. However, the observed resistance of the fou2 mutant was based on antibiotic rather than feeding deterrent properties of the mutant as indicated by an analysis of aphid feeding behaviour.

Conclusions

Analysis of transcriptional profiles of wt, aos and fou2 plants revealed that the expression of more than 200 genes is dependent on jasmonate status, regardless of external stimuli. Moreover, the aphid-induced response of more than 800 transcripts is regulated by jasmonate signalling. Thus, in plants lacking jasmonates many of the defence-related responses induced by infestation in wt plants are impaired. Constant up-regulation of jasmonate signalling as evident in the fou2 mutant causes reduction in aphid population growth, likely as a result of antibiotic properties of fou2 plants. However, aos mutation does not seem to affect aphid performance when the density of B. brassicae populations on plants is low and aphids are free to move around.

Survey of a salivary effector in caterpillars: glucose oxidase variation and correlation with host range

Salivary glucose oxidase (GOX) has been reported in a few insect species where it plays a role in protection against infectious disease. Our recent research has focused on the role of this salivary enzyme in the noctuid Helicoverpa zea, where it functions as an effector to suppress the induced defenses of the host plant Nicotiana tabacum. In this study, we examined the labial gland GOX activities in 23 families of Lepidoptera (85 species) and two families of plant-feeding Hymenoptera (three species). We analyzed the relationship between host breadth and GOX activities, and we found a significant relationship, where highly polyphagous species were more likely to possess relatively high levels of GOX compared to species with more limited host range. We also examined the effect of diet on GOX activity and found that the host plant had a significant effect on enzyme activity. The significance of these findings is discussed in relation to caterpillar host breadth.

Plant immunity to insect herbivores

Herbivorous insects use diverse feeding strategies to obtain nutrients from their host plants. Rather than acting as passive victims in these interactions, plants respond to herbivory with the production of toxins and defensive proteins that target physiological processes in the insect. Herbivore-challenged plants also emit volatiles that attract insect predators and bolster resistance to future threats. This highly dynamic form of immunity is initiated by the recognition of insect oral secretions and signals from injured plant cells. These initial cues are transmitted within the plant by signal transduction pathways that include calcium ion fluxes, phosphorylation cascades, and, in particular, the jasmonate pathway, which plays a central and conserved role in promoting resistance to a broad spectrum of insects. A detailed understanding of plant immunity to arthropod herbivores will provide new insights into basic mechanisms of chemical communication and plant-animal coevolution and may also facilitate new approaches to crop protection and improvement.

Stability of plant defense proteins in the gut of insect herbivores

Plant defense against insect herbivores is mediated in part by enzymes that impair digestive processes in the insect gut. Little is known about the evolutionary origins of these enzymes, their distribution in the plant kingdom, or the mechanisms by which they act in the protease-rich environment of the animal digestive tract. One example of such an enzyme is threonine (Thr) deaminase (TD), which in tomato (Solanum lycopersicum) serves a dual role in isoleucine (Ile) biosynthesis in planta and Thr degradation in the insect midgut. Here, we report that tomato uses different TD isozymes to perform these functions. Whereas the constitutively expressed TD1 has a housekeeping role in Ile biosynthesis, expression of TD2 in leaves is activated by the jasmonate signaling pathway in response to herbivore attack. Ingestion of tomato foliage by specialist (Manduca sexta) and generalist (Trichoplusia ni) insect herbivores triggered proteolytic removal of TD2's C-terminal regulatory domain, resulting in an enzyme that degrades Thr without being inhibited through feedback by Ile. This processed form (pTD2) of TD2 accumulated to high levels in the insect midgut and feces (frass). Purified pTD2 exhibited biochemical properties that are consistent with a postingestive role in defense. Shotgun proteomic analysis of frass from tomato-reared M. sexta identified pTD2 as one of the most abundant proteins in the excrement. Among the other tomato proteins identified were several jasmonate-inducible proteins that have a known or proposed role in anti-insect defense. Subtilisin-like proteases and other pathogenesis-related proteins, as well as proteins of unknown function, were also cataloged. We conclude that proteomic analysis of frass from insect herbivores provides a robust experimental approach to identify hyperstable plant proteins that serve important roles in defense.

Ultraweak photon emission from herbivory-injured maize plants

Following perception of herbivory or infection, plants exhibit a wide range of inducible responses. In this study, we found ultraweak photon emissions from maize leaves damaged by Helicoverpa armigera (Noctuidae). Interestingly, mechanically damaged maize leaves treated with caterpillar regurgitants emitted the same intensity and pattern of photon emissions as those from maize leaves damaged by caterpillars. Furthermore, two-dimensional imaging of the leaf section treated with the oral secretions clearly shows that photon emissions were observed specifically at the lip of the wound exposed to the secretions. These results suggest that the direct interaction between maize leaf cells and chemicals contained in caterpillar regurgitants triggers these photon emissions.

Characterization of oxidative enzyme changes in buffalograsses challenged by Blissus occiduus

This research investigated the role of oxidative enzymes in the defense response of buffalograss, Buchloë dactyloides (Nuttall) Engelmann, to Blissus occiduus Barber. Changes in catalase and peroxidase activity were observed in both resistant and susceptible buffalograsses in response to chinch bug feeding. Susceptible plants were shown to have a lower level of catalase activity compared with their respective control plants. By contrast, catalase activities of resistant plants were similar between infested and control buffalograsses throughout the study. Resistant plants had higher levels of peroxidase activity compared with their control plants, whereas peroxidase activities for control and infested susceptible plants remained at similar levels or were slightly lower for infested plants. These findings suggest that chinch bug feeding leads to a loss in catalase activity in susceptible buffalograsses. In contrast, resistant buffalograsses may be able to tolerate chinch bug feeding by increasing their peroxidase activity. Polyphenol oxidase activities were similar between control and infested plants for the buffalograsses evaluated. Among the enzymes examined, no differences in isozyme profiles for peroxidase and polyphenol oxidase were detected between control and infested 378, NE91-118, Cody, and Tatanka plants. Gels stained for catalase identified differences in the isozyme profiles of infested and uninfested 378 plants; however, infested and control NE91-118, Tatanka, and Cody plants has similar isozyme profiles. No differences in protein profiles were observed between chinch buginfested 378, NE91-118, Cody, and Tatanka plants and their respective uninfested controls.

Sequestration of host plant carotenoids in the larval tissues of Helicoverpa zea

To determine the cause of the unique yellow coloration in mandibular glands of soybean-fed Helicoverpa zea larvae, the accumulation of carotenoids in various tissues of last instar larvae fed soybean, cotton and tomato foliage was quantified. Five carotenoids were detected in the foliage of all host plants but at significantly different concentrations. Xanthophylls rather than carotenes were most likely to accumulate in larval tissues. Carotenoids accumulated at different rates and some were significantly affected by larval diet. Highest levels of carotenoid accumulation, notably lutein, were detected in the testes, followed by midgut epithelium, fat body and integument. The midgut epithelium contained the greatest and the testes the least diversity of carotenoid types. Low levels of lutein were detected in both labial and mandibular glands. Tomato foliage had the highest carotenoid content and caterpillar tissues fed these leaves often had the highest amounts of carotenoid. However, the accumulation of carotenoids did not protect larvae from antibiotic effects of tomato foliage because these caterpillars had the highest mortality and slowest growth rates of all the three host plants. Transport and absorption of lipid and oxidative stress may be some reasons for differential carotenoid accumulation.

Oxidative responses of resistant and susceptible cereal leaves to symptomatic and nonsymptomatic cereal aphid (Hemiptera: Aphididae) feeding

The impact of the leaf-chlorosis-eliciting Russian wheat aphid, Diuraphis noxia (Mordvilko), and the nonchlorosis-eliciting bird cherry-oat aphid, Rhopalosiphum padi (L.), feeding on D. noxia-susceptible and -resistant cereals was examined during the period (i.e., 3, 6, and 9 d after aphid infestation) that leaf chlorosis developed. After aphid number, leaf rolling and chlorosis ratings, and fresh leaf weight were recorded on each sampling date, total protein content, peroxidase, catalase, and polyphenol oxidase activities of each plant sample were determined spectrophotometrically. Although R. padi and D. noxia feeding caused significant increase of total protein content in comparison with the control cereal leaves, the difference in total protein content between R. padi and D. noxia-infested leaves was not significant. Although R. padi-feeding did not elicit any changes of peroxidase specific activity in any of the four cereals in comparison with the control leaves, D. noxia feeding elicited greater increases of peroxidase specific activity only on resistant 'Halt' wheat (Triticum aestivum L.) and susceptible 'Morex' barley (Hordeum vulgare L.), but not on susceptible 'Arapahoe' and resistant 'Border' oat (Avena sativa L.). D. noxia-feeding elicited a ninefold increase in peroxidase specific activity on Morex barley and a threefold on Halt wheat 9 d after the initial infestation in comparison with control leaves. Furthermore, D. noxia feeding did not elicit any differential changes of catalase and polyphenol oxidase activities in comparison with either R. padi feeding or control leaves. The findings suggest that D. noxia feeding probably results in oxidative stress in plants. Moderate increase of peroxidase activity (approximately threefold) in resistant Halt compared with susceptible Arapahoe wheat might have contributed to its resistance to D. noxia, whereas the ninefold peroxidase activity increase may have possibly contributed to barley's susceptibility. Different enzymatic responses in wheat, barley, and oat to D. noxia and R. padi feeding indicate the cereals have different mechanisms of aphid resistance.

The mid-pericarp cell layer in soybean pod walls is a multicellular compartment enriched in specific lipoxygenase isoforms

Specific lipoxygenase isoforms immunolocalize to the cytosol of a single cell layer in the soybean (Glycine max L.) pod wall. The cells of this layer, termed the mid-pericarp layer (MPL), are larger than adjacent cells and are highly branched. The entire MPL appears to form an elaborate interdigitated network within the pod wall. A particularly striking feature of the MPL is the presence of extensive regions of very thin, approximately 30 nm, cell wall, which connect the cells of the MPL. It was demonstrated that after mechanical wounding of the pod wall, 40-kD fluorescein-dextran was able to move throughout the MPL. In addition, when pod walls are cut, an exudate flows from the MPL that is highly enriched in lipoxygenase isoforms (approximately 40% of the total protein). The MPL of soybean pod walls may represent a novel multicellular compartment involved in defense of leguminous plants.

Effects of total dietary nitrogen and nitrogen form on the development of xylophagous leafhoppers

The influence of primary nutrients on the development of the leafhopper Homalodisca coagulata (Say) was examined using Glycine max (L.) Merrill subjected to treatments using different forms of nitrogen fertilization (urea and nitrate) and inoculation with Rhizobium bacteria. Concentrations of amino acids, organic acids, carbohydrates, and ureides in xylem fluid varied with treatment; differences were most pronounced in young plants (6 weeks after planting), and subsided as plants aged. Plants receiving urea fertilization had lower concentrations of organic nitrogen and carbon, but more balanced profiles of amino acids. Leafhoppers on urea-fertilized plants had much higher success in development (62 to 87% survival to adult stage) than H. coagulata developing on nitrate-fertilized plants (31-42%). The sex ratio of successfully developed adults varied greatly with treatments, indicating that female survival was much higher on urea-fertilized plants. Males and females utlilized diets differently as indicated by the carbon and nitrogen content of adult carcasses. Consumption rates and nitrogen assimilation efficiencies were higher for young (third instar) leafhoppers on urea-fertilized plants. Instar duration and total time of development were not affected by treatment. Highest rates of success in development on diets providing the lowest (yet most balanced) concentrations of total nitrogen, amino acids, and organic compounds underscored the importance of nutrient balance for leafhopper development. Arch. Copyright 1999 Wiley-Liss, Inc.

Impact of dietary allelochemicals on gypsy moth (Lymantria dispar) caterpillars: importance of midgut alkalinity

Midgut pH of gypsy moth larvae was depressed artificially with buffered diet to examine the impact of alkalinity on the caterpillars' ability to tolerate a dietary polyphenol and a quinone. A 2x3 factorial design was used, with 2 levels of succinate buffer and 3 dietary amendments (tannic acid, juglone, or control). Development was monitored during the third and fourth instars, with consumption, food passage rates, midgut pH, and midgut redox potential (Eh) measured in the fourth instar. Diet buffering successfully depressed midgut pH to hypothetically suboptimal acidic levels without reductions in survivorship, but it did reduce larval growth and impede development. Buffering dramatically reduced survivorship of fourth instar larvae eating diets containing tannic acid or juglone. Growth increased on unbuffered diet amended with tannic acid, but not with juglone. Caterpillars passed food through the gut more slowly when feeding on buffered tannic acid diet or on unbuffered juglone diet. These results indicate that maintenance of midgut alkalinity is critical to tolerance of dietary tannic acid and juglone, and that these allelochemicals have very different activities in the caterpillar gut.