Symbiont Digestive Range Reflects Host Plant Breadth in Herbivorous Beetles

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虫害诱导植物合成防御性次生代谢产物的研究进展

昆虫对植物的取食活动可以激活植物的防御反应,诱导植物通过调控自身的代谢网络合成防御性次生代谢产物,抵御外界不良刺激。虫害诱导植物合成防御性次生代谢产物及其机制研究已成为近年来的研究热点之一。现对虫害诱导的植物防御性次生代谢产物、昆虫危害产生的各类激发子、植物对激发子的识别、虫害应答相关的信号转导通路及其对次生代谢物质积累的调控进行了综述,可为虫害诱导植物合成防御性次生代谢产物的机制研究提供参考,为植物虫害防治研究、植物次生代谢物质的生产和利用提供理论依据。     

Defense Reactions in Host and Nonhost Plants Against the Soybean Rust Fungus (Phakopsora pachyrhizi Syd.)

Growth and development of two races of the soybean rust fungus (Phakopsora pachyrhizi Syd.) were compared on host and nonhost plants. Both groups had several lines of defense, each of which could stop a part of attacking uredospores. Germ tubes and appressoria were produced equally well on hosts and nonhosts. A reduced formation of penetration hyphae contributed to the resistance of nonhosts and resistant host genotypes. In the epidermal cells of wheat and barley leaves, lower frequencies of penetration hyphae coincided with the production of papillae-like structures which were not penetrated. The last line of defense of all nonhosts was localized in the epidermal cell where the growth of the penetration hyphae was arrested definitively. The colony development in these species was suppressed completely. In highly resistant host genotypes the last defense reaction occurred later as a hypersensitive cell collapse which interrupted the growth of the rust colony.     

A Sequential Approach to Risk Assessment of Transgenic Plants Expressing Protease Inhibitors: Effects on Nontarget Herbivorous Insects

Protease inhibitors expressed in transgenic plants can provide enhanced levels of resistance to important pest species. A sequential approach for testing the effects of protease inhibitor-expressing crops on nontarget herbivorous insects has been developed. The approach consists of five tiers. The first two tiers comprise the selection phase. In tier one, field surveys are used to characterise the nontarget invertebrate fauna of a crop. In tier 2, histochemical assays are used to identify the subset of herbivores with a particular class of digestive proteolytic enzymes. In the assessment phase a combination of laboratory worst-case scenario studies (tier 3) and controlled environment or small-scale field trials (tier 4) are used to evaluate the impact of the protease inhibitor-expressing plants on the selected nontarget species. In the final tier, field trials are used to compare the relative effect of transgenic plants and current management practices, such as pesticide use, on selected species. The first four tiers of the approach are described using potatoes expressing cystatins, a family of cysteine proteinase inhibitors, as an example. Although the plants have enhanced levels of resistance to potato cyst nematodes (PCN), Globodera pallida and Globodera rostochiensis, the results establish that they have negligible impact on the nontarget herbivorous insect, Eupteryx aurata.     

Regulation of a Chemical Defense against Herbivory Produced by Symbiotic Fungi in Grass Plants

Neotyphodium uncinatum and Neotyphodium siegelii are fungal symbionts (endophytes) of meadow fescue (MF; Lolium pratense), which they protect from insects by producing loline alkaloids. High levels of lolines are produced following insect damage or mock herbivory (clipping). Although loline alkaloid levels were greatly elevated in regrowth after clipping, loline-alkaloid biosynthesis (LOL) gene expression in regrowth and basal tissues was similar to unclipped controls. The dramatic increase of lolines in regrowth reflected the much higher concentrations in young (center) versus older (outer) leaf blades, so LOL gene expression was compared in these tissues. In MF-N. siegelii, LOL gene expression was similar in younger and older leaf blades, whereas expression of N. uncinatum LOL genes and some associated biosynthesis genes was higher in younger than older leaf blades. Because lolines are derived from amino acids that are mobilized to new growth, we tested the amino acid levels in center and outer leaf blades. Younger leaf blades of aposymbiotic plants (no endophyte present) had significantly higher levels of asparagine and sometimes glutamine compared to older leaf blades. The amino acid levels were much lower in MF-N. siegelii and MF-N. uncinatum compared to aposymbiotic plants and MF with Epichloë festucae (a closely related symbiont), which lacked lolines. We conclude that loline alkaloid production in young tissue depleted these amino acid pools and was apparently regulated by availability of the amino acid substrates. As a result, lolines maximally protect young host tissues in a fashion similar to endogenous plant metabolites that conform to optimal defense theory.Loline alkaloids (LAs; Hofmeister, 1892; Siegel et al., 1990; TePaske et al., 1993; Blankenship et al., 2001) are protective secondary metabolites produced by some Epichloë and Neotyphodium spp. (epichloae), fungi that live as systemic symbionts in many cool season grasses (Poaceae subfamily Pooideae). The lolines are active against a broad spectrum of insects (Schardl et al., 2007) and are derived from l-Pro (Pro) and l-homoserine (Hse; Blankenship et al., 2005). Mock herbivory (clipping plants) is reported to induce higher levels of lolines in several grass-epichloë symbiota (Craven et al., 2001; Bultman et al., 2004; Gonthier et al., 2008), suggesting that the epichloae have evolved to regulate their metabolism in a manner appropriate for defense of their hosts. However, little is known of the regulation of LA synthesis in symbio and whether these symbionts follow prevailing models for how plants deploy chemical defenses against herbivores (McKey, 1979; Rhoades, 1979; Barto and Cipollini, 2005).The loline-alkaloid biosynthesis (LOL) gene cluster contains nine genes likely to direct LA production (Spiering et al., 2005). Neotyphodium uncinatum contains two highly similar LOL clusters (LOL1 and LOL2), and a single LOL cluster has been found in each of the LA-producing species, Neotyphodium coenophialum, Neotyphodium siegelii, and some strains of Epichloë festucae, among others (Spiering et al., 2005; Kutil et al., 2007). Fermentation cultures of N. uncinatum produce lolines, and studies involving application of labeled precursors and intermediates have almost completely elucidated the LA biosynthetic pathway (Blankenship et al., 2005; Spiering et al., 2005; Faulkner et al., 2006; Schardl et al., 2007). Putative roles of the LOL gene products—based on sequence relationships to known enzyme classes—fit well with the pathway. Furthermore, an RNA interference knockdown of lolC reduces LA levels, and a lolP knockout prevents conversion of N-methylloline to N-formylloline (Spiering et al., 2005, 2008). Expression kinetics of the LOL genes are tightly correlated with each other and with the LA production phase in N. uncinatum cultures (Zhang et al., 2009). This finding raises the question whether and how LOL gene expression in symbio relates to changes in LA levels in response to development and stresses in host plants.LA production in symbio may be influenced by physiological differences among plant tissues and developmental stages, as well as differences in nutritional status and environmental stresses (Kennedy and Bush, 1983; Belesky et al., 1987; Justus et al., 1997; Tong et al., 2006). Given the anti-insect activity of lolines, effects of plant damage on LA levels are of particular interest. Mock herbivory (clipping of leaves) leads to apparent increases in LA concentrations in regrowth tissues of tall fescue (TF; Lolium arundinaceum) symbiotic with N. coenophialum (Bultman et al., 2004; Sullivan et al., 2007) and of meadow fescue (MF; Lolium pratense) symbiotic with N. uncinatum or N. siegelii (Craven et al., 2001). Despite the higher LA levels, however, clipping or damage of TF-N. coenophialum by the herbivore Spodoptera frugiperda (fall armyworm) was reported to elicit only minor, marginally significant (P = 0.052) effects on expression of lolC (Sullivan et al., 2007). A study of the Glyceria striata-Epichloë glyceriae symbiotum demonstrated significantly higher expression of lolC and higher LA production when the grass was artificially damaged, whereas the effect of damage by S. frugiperda on LA concentrations and lolC expression was not significant (Gonthier et al., 2008).Prevailing concepts about how plants deploy chemical defenses include the optimal defense theory (ODT; McKey, 1979; Rhoades, 1979) and the growth differentiation balance hypothesis (GDBH; Barto and Cipollini, 2005). The ODT addresses the distribution of chemical defenses in the plant, predicting that such defenses will be concentrated in tissues that have relatively little means to physically inhibit herbivory (e.g. in young tissues) and are important in the fitness of the plant. The GBDH addresses the location of biosynthesis and predicts that mature tissues are more likely to produce secondary metabolites than are actively growing tissues, which instead need to use resources for biomass production. It is intriguing to consider whether the epichloae obey the predictions of ODT and GDBH, considering that many epichloae protect their hosts by synthesizing insecticidal alkaloids, but they are also evolutionarily derived from plant-pathogenic fungi (Moon et al., 2004) and do not always enhance host fitness (Faeth et al., 2004). In order to address these questions, it is necessary to understand how secondary metabolism of the epichloae is regulated in symbio. The production of lolines in MF-N. uncinatum and MF-N. siegelii is an ideal test case because the lolines accumulate to very high levels—up to 1.9% dry weight—in regrowth of clipped plants (Craven et al., 2001). Here, we test the hypotheses that LOL gene expression and substrate availability correlate with LA levels in younger versus older leaf tissues and in response to clipping in MF-N. uncinatum and MF-N. siegelii symbiota.     

Phytophagous Insects on Native and Non-Native Host Plants: Combining the Community Approach and the Biogeographical Approach

During the past centuries, humans have introduced many plant species in areas where they do not naturally occur. Some of these species establish populations and in some cases become invasive, causing economic and ecological damage. Which factors determine the success of non-native plants is still incompletely understood, but the absence of natural enemies in the invaded area (Enemy Release Hypothesis; ERH) is one of the most popular explanations. One of the predictions of the ERH, a reduced herbivore load on non-native plants compared with native ones, has been repeatedly tested. However, many studies have either used a community approach (sampling from native and non-native species in the same community) or a biogeographical approach (sampling from the same plant species in areas where it is native and where it is non-native). Either method can sometimes lead to inconclusive results. To resolve this, we here add to the small number of studies that combine both approaches. We do so in a single study of insect herbivory on 47 woody plant species (trees, shrubs, and vines) in the Netherlands and Japan. We find higher herbivore diversity, higher herbivore load and more herbivory on native plants than on non-native plants, generating support for the enemy release hypothesis.     

Permeability of the Peritrophic Envelopes of Herbivorous Insects to Dextran Sulfate: a Test of the Polyanion Exclusion Hypothesis

We tested the hypothesis that the permeability of the peritrophic envelope in herbivorous insects is greatly reduced for polyanions as a result of an extensive network of anionic sites in the proteoglycans of the matrix. 14C-Dextran sulfate (polyanionic, 8000 M(w)) and fluorescein isothiocyanate-labeled (FITC) dextran (monoanionic, 9400 M(w)) were introduced together into the endoperitrophic space of the midguts of Orgyia leucostigma (Lepidoptera) larvae and Melanoplus sanguinipes (Orthoptera) adults. In all cases more of the 14C-dextran sulfate permeated the peritrophic envelope than the FITC-dextran, the opposite of the result predicted by the polyanion exclusion hypothesis. We conclude that polyanion exclusion is not a mechanism that contributes significantly to the permeability properties of the peritrophic envelopes of these two species, or that explains the failure of tannic acid to cross the peritrophic envelopes of lepidopteran larvae. Copyright 1997 Elsevier Science Ltd. All rights reserved     

Activation of Host Defense Mechanisms by Elevated Production of H2O2 in Transgenic Plants

Active oxygen species have been postulated to perform multiple functions in plant defense, but their exact role in plant resistance to diseases is not fully understood. We have recently demonstrated H2O2-mediated disease resistance in transgenic potato (Solanum tuberosum) plants expressing a foreign gene encoding glucose oxidase. In this study we provide further evidence that the H2O2-mediated disease resistance in potato is effective against a broad range of plant pathogens. We have investigated mechanisms underlying the H2O2-mediated disease resistance in transgenic potato plants. The constitutively elevated levels of H2O2 induced the accumulation of total salicylic acid severalfold in the leaf tissue of transgenic plants, although no significant change was detected in the level of free salicylic acid. The mRNAs of two defense-related genes encoding the anionic peroxidase and acidic chitinase were also induced. In addition, an increased accumulation of several isoforms of extracellular peroxidase, including a newly induced one, was observed. This was accompanied by a significant increase in the lignin content of stem and root tissues of the transgenic plants. The results suggest that constitutively elevated sublethal levels of H2O2 are sufficient to activate an array of host defense mechanisms, and these defense mechanisms may be a major contributing factor to the H2O2-mediated disease resistance in transgenic plants.     

Fluorimetry Method for Assaying Cellobiohydrolase Activity in Digestive Tracts of Small Herbivorous Mammals

Biology Bulletin - The fluorimetry method for assaying cellobiohydrolase activity (CBHA), originally developed for pure cultures of microorganims, has been modified for studying samples of the...     

Adaptive Radiation in Insects and Plants: Time and Opportunity

SYNOPSIS. Insects and their hostplants represent the major partof terrestrial diversity, yet we are just beginning to understandwhy there are so very many species. By far the most influentialmodel of insect/plant diversification has been Ehrlich and Raven's(1964) hypothesis of insect/plant coevolution. While the coevolutionmodel was based on macroevolutionary patterns in plant defensesand hostplant affiliations, most of the subsequent work hasbeen on its possible ecological and genetic mechanisms, withrelatively little systematic scrutiny of the evolutionary patternsEhrlich and Raven described. We explore the possible roles insect/plantinteractions may play in the long-term evolution of insect andplant lineages, and review some of the evidence on whether ornot insects and plants have exerted reciprocal influences oneach other's diversification. Insects and plants have diversified over roughly the same timeintervals, and many insect host/affiliations are evolutionarilyconserved, thus reflecting long/term, phylogenetic history.Rather than accumulating herbivores at a rate proportional totheir geographic area of distribution or biomass, some plantgroups pose apparent chemical barriers to potential herbivorecolonists, and seem accessible to relatively few insect lineages,possibly preadapted by use of chemically similar or relatedhostplants. Evolutionary innovations in plant defenses and ininsect feeding habits seem to have spurred their respectiveadaptive radiations, thus ecological opportunity may influencelong-term evolutionary success. The greater diversity of insectsand plants in the tropics, compared to the temperate zone, probablyreflects the greater age of tropical habitats as well as climaticbarriers that limit successful invasion of the temperate zoneto just those primitively tropical groups able to evolve strategiesfor both overwintering and use of temperate resources. Thoughevidence is still sparse, successful invasion of the temperatezone may promote subsequent radiations of both insects and plants. We conclude that much of the available evidence from systematicsis consistent with Ehrlich and Raven's suggestion that muchof insect and plant diversification has been spurred by a seriesof ecological opportunities over evolutionary time.     

The Polar Flagellin of Azospirillum brasilense REC3 Induces a Defense Response in Strawberry Plants Against the Fungus Macrophomina phaseolina

Microbe-associated molecular patterns (MAMPs) are conserved molecules able to trigger plant resistance. The aim of this work was to evaluate the capacity of Azospirillum brasilense REC3 polar flagellin AzFlap as a MAMP, eliciting biochemical, histological, and molecular defense responses that can provide strawberry plants protection against the pathogenic fungus Macrophomina phaseolina. Strawberry plants were treated with AzFlap on leaves or with the isolate REC3 on leaves or roots. Salicylic acid content, biofilm formation, callose and lignin depositions, stomatal closure, ROS, and the expression of defense-related genes such as FaPR1, FaCAT, FaRBOH-D, FaRBOH-F, FaCHI23, FaCHI2-2, and FaGSL5 were evaluated. Phytopathogenic assays in plants treated with AzFlap or REC3 and infected with M. phaseolina were also performed. Results showed that plants leaf treated with AzFlap or root treated with REC3 caused the accumulation of ROS, salicylic acid, callose, lignin, the increase of biofilm formation on leaves, and stomatal closure. The evaluation of the expression of genes associated to defense response indicated the activation of the innate immunity of strawberry plants. The level of gene expression was strongly time and treatment dependent, suggesting a complex regulation of defense signaling. Root inoculations with REC3 or foliar treatment with AzFlap were able to reduce plant mortality, showing the effectiveness of both treatments to control M. phaseolina. These results indicate that flagellin AzFlap from A. brasilense REC3 behaves as a MAMP that activates a defense response against M. phaseolina in strawberry plants.

     

Insects Can Count: Sensory Basis of Host Discrimination in Parasitoid Wasps Revealed

The solitary parasitoid Leptopilina heterotoma is one of the best studied organisms concerning the ecology, behaviour and physiology of host discrimination. Behavioural evidence shows that L. heterotoma uses its ovipositor to discriminate not only between parasitized and unparasitized Drosophila melanogaster larvae, but also to discriminate between hosts with different numbers of parasitoid eggs. The existing knowledge about how and when the parasitoid marks the host motivated us to unravel the chemosensory basis of host discrimination by L. heterotoma that allows it to choose the “best” host available. In this paper we report on electrophysiological recordings of multi-neural responses from the single taste sensillum on the tip of the unpaired ovipositor valve. We stimulated this sensillum with haemolymph of unparasitized, one-time-parasitized and two-times-parasitized Drosophila larvae. We demonstrate for the first time that quantitative characteristics of the neural responses to these haemolymph samples differed significantly, implying that host discrimination is encoded by taste receptor neurons in the multi-neuron coeloconic ovipositor sensillum. The activity of three of the six neurons present in the sensillum suffices for host discrimination and support the hypothesis that L. heterotoma females employ an ensemble code of parasitization status of the host.     

Consequences of Fragmentation of Tropical Moist Forest for Birds and Their Role in Predation of Herbivorous Insects

The consequences of habitat alteration on the role of understory insectivorous birds as predators of herbivorous insects in tropical forests are poorly understood. To examine whether fragmentation may affect the top–down controls of herbivory, we compared the number of species, individuals, and the community structure of insectivorous birds between fragments and continuous tropical moist forest in Mexico. We also registered insect herbivore abundances and conducted a larvae predation experiment to evaluate the potential role of insectivorous birds as predators of herbivorous insects. We recorded 63 bird species from 22 families, 43 percent of which were insectivorous birds. Species richness, abundance, and diversity of the avian community were higher in continuous forest compared with forest fragments. For insectivorous birds in particular, there was low similarity in avian insectivore communities between forest types, and forest fragments had more heavily dominated communities of avian insectivores. During the dry season, forest fragments presented significantly higher predation rates on artificial caterpillars, and lower abundance of herbivorous Lepidoptera larvae, compared with continuous forest. Furthermore, there was a significant negative correlation between artificial caterpillar predation rate and larval Lepidoptera abundance, with higher rates of predation in sample sites of low Lepidoptera abundance. Hence, the potentially greater light in the dry season combined with a more dominated avian insectivore community in forest fragments may facilitate increased predation by avian insectivores, resulting in a decline in abundance of larval Lepidoptera, with implications for the process of insect‐driven herbivory in forest fragments.     

Microbial Symbionts in Insects Influence Down-Regulation of Defense Genes in Maize

Diabrotica virgifera virgifera larvae are root-feeding insects and significant pests to maize in North America and Europe. Little is known regarding how plants respond to insect attack of roots, thus complicating the selection for plant defense targets. Diabrotica virgifera virgifera is the most successful species in its genus and is the only Diabrotica beetle harboring an almost species-wide Wolbachia infection. Diabrotica virgifera virgifera are infected with Wolbachia and the typical gut flora found in soil-living, phytophagous insects. Diabrotica virgifera virgifera larvae cannot be reared aseptically and thus, it is not possible to observe the response of maize to effects of insect gut flora or other transient microbes. Because Wolbachia are heritable, it is possible to investigate whether Wolbachia infection affects the regulation of maize defenses. To answer if the success of Diabrotica virgifera virgifera is the result of microbial infection, Diabrotica virgifera virgifera were treated with antibiotics to eliminate Wolbachia and a microarray experiment was performed. Direct comparisons made between the response of maize root tissue to the feeding of antibiotic treated and untreated Diabrotica virgifera virgifera show down-regulation of plant defenses in the untreated insects compared to the antibiotic treated and control treatments. Results were confirmed via QRT-PCR. Biological and behavioral assays indicate that microbes have integrated into Diabrotica virgifera virgifera physiology without inducing negative effects and that antibiotic treatment did not affect the behavior or biology of the insect. The expression data and suggest that the pressure of microbes, which are most likely Wolbachia, mediate the down-regulation of many maize defenses via their insect hosts. This is the first report of a potential link between a microbial symbiont of an insect and a silencing effect in the insect host plant. This is also the first expression profile for a plant attacked by a root-feeding insect.     

Utilization of Marine Seaweeds as a Promising Defense Against COVID-19: a Mini-review

Marine Biotechnology - COVID-19 is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) which mainly affects the respiratory system. It has been declared as...     

Liposomes in Host Defense Mechanism: A Key Role for Macrophages

Abstract

Liposomes can be used as carriers for antigens, immunomodulators and cytotoxic drugs. Such liposomes may serve as a tool to manipulate immune and non-immune host defense mechanisms. In most cases their effects are mediated by macrophages. Macrophages seem to be involved in humoral (antibody) responses and in cytotoxic T-lymphocyte responses. They are also important in non-immune defense mechanisms against foreign invaders and altered self. Which macrophages can be influenced by the liposome encapsulated molecules depends on the administration route of the liposomes. The macrophages ingest the liposomes. Once within the cell, lysosomal phospholipases disrupt the phospholipid bilayers. In this way, encapsulated molecules are released in the cell. Such liposome delivered molecules can be processed (antigens), activate the macrophage (immunomodulators) or disturb the metabolism of the cells (cytotoxic drugs). That the latter inhibition of macrophage functions may result in immunopotentiation is explained by the fact that certain macrophages are regulating immune functions by suppression.