Defense Compounds Research Articles

Can Moisture-Indicating Understory Plants Be Used to Predict Survivorship of Large Lodgepole Pine Trees During Severe Outbreaks of Mountain Pine Beetle?

Abstract Why do some mature lodgepole pines survive mountain pine beetle outbreaks while most are killed? Here we test the hypothesis that mature trees growing in sites with vascular plant indicators of high relative soil moisture are more likely to survive mountain pine beetle outbreaks than mature trees associated with indicators of lower relative soil moisture. Working in the Clearwater Valley of south central British Columbia, we inventoried understory plants growing near large-diameter and small-diameter survivors and nonsurvivors of a mountain pine beetle outbreak in the mid-2000s. When key understory species were ranked according to their accepted soil moisture indicator value, a significant positive correlation was found between survivorship in large-diameter pine and inferred relative high soil moisture status—a finding consistent with the well-documented importance of soil moisture in the mobilization of defense compounds in lodgepole pine. We suggest that indicators of soil moisture may be useful in predicting the survival of large pine trees in future pine beetle outbreaks. Study Implications: A recent outbreak of the mountain pine beetle resulted in unprecedented levels of lodgepole pine mortality across southern inland British Columbia. Here, we use moisture-dependent understory plants to show that large lodgepole pine trees growing in sites with high relative moisture are more likely than similar trees in drier sites to survive severe outbreaks of mountain pine beetle—a finding that may be related to a superior ability to mobilize chemical defense compounds compared with drought-stressed trees.

Conifer cell cultures as a tool for studying the induction of terpenoid defense compounds

ABSTRACT Conifer trees often increase their production of terpene resins after attack by herbivores or pathogens, a defense response that may be critical in their survival. Yet it has proved difficult to study such terpene induction under laboratory conditions because of the need for mature trees and specific organs and cells at the proper stage of development. Here we explore the potential of Norway spruce (Picea abies) cell cultures to be a convenient system for studying terpene induction in this species. We demonstrated that P. abies cell suspension cultures accumulated a mixture of monoterpenes (but not of sesquiterpenes or diterpenes), similar to that found in intact trees, but at level of only 0.5–2.5% those in intact trees. However, treatment with the hormone methyl jasmonate and the fungal elicitor chitosan increased monoterpene accumulation 2–4-fold. Based on in vitro assays of terpene synthase activity, this increase was driven by an increased rate of biosynthesis, and was likely not due to pH changes caused by hormone or fungal elicitor treatment. Since these defense responses of P. abies cell cultures are typical of the response of intact spruce needles and bark, such cultures may have promise as a laboratory system for studying the induction of terpene resin defenses.

Generalist dietary responses to individual versus combined plant toxin odors: an African elephant study

Abstract The detoxification limitation hypothesis posits that, unless plant defense compounds interact synergistically or additively to increase their harmful effects, generalist herbivores will prefer consuming combinations of these compounds more than singular compounds. Monoterpenes are odoriferous defense compounds that may be toxic to mammalian herbivores when ingested in sufficient quantities. Previous research has shown that the addition of individual monoterpenes to food sources reduces consumption by generalist mammalian herbivores. By using African elephants as a case study, we aimed to determine whether odors from monoterpene combinations (i.e., two or more monoterpenes) also deter generalist mammalian herbivory, and whether generalist herbivores prefer the odors of monoterpene combinations more than individual monoterpenes. First, we tested whether the odor of monoterpene combinations that resemble the monoterpene profiles of a high-acceptability, intermediate-acceptability, and low-acceptability plant deter herbivory. We found that elephants preferred plants without the added odors of the monoterpene combinations. Second, we explored how elephants responded to individual monoterpenes found within the combinations compared with the combinations at the same set concentration, and found that the elephants did not always prefer the combinations more than the individual monoterpenes. Moreover, the more diverse the combination, the less frequently it was preferred when compared with the individual monoterpene odors. Our results imply that generalist herbivores do not necessarily prefer combinations of plant chemical defenses at comparatively lower concentrations and that, consequently, the composition and diversity of monoterpene profiles in plants likely determine the efficacy of these compounds as an olfactory defense against mammalian herbivory.

Identification of key amino acid residues in AtUMAMIT29 for transport of glucosinolates.

Glucosinolates are key defense compounds of plants in Brassicales order, and their accumulation in seeds is essential for the protection of the next generation. Recently, members of the Usually Multiple Amino acids Move In and Out Transporter (UMAMIT) family were shown to be essential for facilitating transport of seed-bound glucosinolates from site of synthesis within the reproductive organ to seeds. Here, we set out to identify amino acid residues responsible for glucosinolate transport activity of the main seed glucosinolate exporter UMAMIT29 in Arabidopsis thaliana. Based on a predicted model of UMAMIT29, we propose that the substrate transporting cavity consists of 51 residues, of which four are highly conserved residues across all the analyzed homologs of UMAMIT29. A comparison of the putative substrate binding site of homologs within the brassicaceous-specific, glucosinolate-transporting clade with the non-brassicaceous-specific, non-glucosinolate-transporting UMAMIT32 clade identified 11 differentially conserved sites. When each of the 11 residues of UMAMIT29 was individually mutated into the corresponding residue in UMAMIT32, five mutant variants (UMAMIT29#V27F, UMAMIT29#M86V, UMAMIT29#L109V, UMAMIT29#Q263S, and UMAMIT29#T267Y) reduced glucosinolate transport activity over 75% compared to wild-type UMAMIT29. This suggests that these residues are key for UMAMIT29-mediated glucosinolate transport activity and thus potential targets for blocking the transport of glucosinolates to the seeds.

Host adaptation and specialization in Tetranychidae mites.

Composite generalist herbivores are comprised of host-adapted populations that retain the ability to shift hosts. The degree and overlap of mechanisms used by host-adapted generalist and specialist herbivores to overcome the same host plant defenses are largely unknown. Tetranychidae mites are exceptionally suited to address the relationship between host adaptation and specialization in herbivores as this group harbors closely related species with remarkably different host ranges-an extreme generalist the two-spotted spider mite (Tetranychus urticae Koch [Tu]) and the Solanaceous specialist Tetranychus evansi (Te). Here, we used tomato-adapted two-spotted spider mite (Tu-A) and Te populations to compare mechanisms underlying their host adaptation and specialization. We show that both mites attenuate induced tomato defenses, including protease inhibitors (PIs) that target mite cathepsin L digestive proteases. While Te solely relies on transcriptional attenuation of PI induction, Tu and Tu-A have elevated constitutive activity of cathepsin L proteases, making them less susceptible to plant anti-digestive proteins. Tu-A and Te also rely on detoxification of tomato constitutive defenses. Te uses esterase and P450 activities, while Tu-A depends on the activity of all major detoxification enzymatic classes to disarm tomato defensive compounds to a lesser extent. Thus, even though both Tu-A and Te use similar mechanisms to counteract tomato defenses, Te can better cope with them. This finding is congruent with the ecological and evolutionary times required to establish mite adaptation and specialization states, respectively.

Condensed tannins mediate the effect of long-term nitrogen addition on soil nematodes in a boreal spruce forest

Fertilization and combustion of fossil fuels have increased nitrogen (N) deposition over the last decades. Most boreal forests are N-limited; thus, increased N availability may have considerable implications for boreal forest food web structure and functioning. Soil biota are important drivers of ecosystem processes through their effect on carbon (C) and nutrient cycling. Among these, nematodes are ubiquitous in soil and respond quickly to environmental changes. They are thus useful in determining changes in ecosystem status. Boreal forest plants produce large quantities of condensed tannins (hereafter tannins), and these may decrease following N addition. As defence compounds, tannins may alter soil community structure by reducing microbial activity and deter microfauna. Using nematodes as indicators, we here investigated the linkages between N-induced changes in soil food web and soil tannin concentration. We utilized an experimental site where N has been added since 2003 at a rate of ca. 37 kg N ha−1 year−1. This have resulted in a shift in the understory, and consequently a 23 % lower soil tannin concentration. Fertilization led to a less structured nematode community dominated by bacterivores. Channel Index was 35 % lower in fertilized plots, indicating an increasing decomposition dominance by bacteria. Furthermore, the dominance of bacteria appears to have been inhibited by soil tannin concentrations in the fertilized plots. Overall, our study demonstrate that soil community structure strongly changes upon increased N availability, and lower soil tannin concentrations further facilitate the dominance of bacteria.

Larvicidal constituents from Poncirus trifoliata root extracts.

In the search for effective and environmentally friendly mosquito control agents, we have examined natural sources, such as microbes and plants, and the synthetic analogs of natural products. These plants and microbes have evolved in their ecological niches to produce defensive compounds against other competing organisms in their surroundings such as microbes, plants, and insects as a means to enhance their survival. Thus, some of these plants and microbes have bioactive compounds with insecticidal, fungicidal, and phytotoxic activities. In our previous research, we successfully isolated bioactive constituents from natural sources. We have carried out synthetic modifications and total synthesis of marginally active isolated compounds to achieve significantly higher active compounds. We have focused on plants in the Rutaceae family as the members of this family are known to possess bioactive compounds with algicidal, antifungal, insecticidal, and fungicidal activities. In this article, we report the isolation and structure elucidation of mosquito larvicidal constituents from Poncirus trifoliata (Rutaceae) root extract.

Diversification of JAZ-MYC signaling function in immune metabolism.

Jasmonate (JA) re-programs metabolism to confer resistance to diverse environmental threats. Jasmonate stimulates the degradation of JASMONATE ZIM-DOMAIN (JAZ) proteins that repress the activity of MYC transcription factors. In Arabidopsis thaliana, MYC and JAZ are encoded by 4 and 13 genes, respectively. The extent to which expansion of the MYC and JAZ families has contributed to functional diversification of JA responses is not well understood. Here, we investigated the role of MYC and JAZ paralogs in controlling the production of defense compounds derived from aromatic amino acids (AAAs). Analysis of loss-of-function and dominant myc mutations identified MYC3 and MYC4 as the major regulators of JA-induced tryptophan metabolism. We developed a JAZ family-based, forward genetics approach to screen randomized jaz polymutants for allelic combinations that enhance tryptophan biosynthetic capacity. We found that mutants defective in all members (JAZ1/2/5/6) of JAZ group I over-accumulate AAA-derived defense compounds, constitutively express marker genes for the JA-ethylene branch of immunity and are more resistant to necrotrophic pathogens but not insect herbivores. In defining JAZ and MYC paralogs that regulate the production of amino-acid-derived defense compounds, our results provide insight into the specificity of JA signaling in immunity.

Altered methionine metabolism impacts phenylpropanoid production and plant development in Arabidopsis thaliana.

Phenylpropanoids are specialized metabolites derived from phenylalanine. Glucosinolates are defense compounds derived mainly from methionine and tryptophan in Arabidopsis. It was previously shown that the phenylpropanoid pathway and glucosinolate production are metabolically linked. The accumulation of indole-3-acetaldoxime (IAOx), the precursor of tryptophan-derived glucosinolates, represses phenylpropanoid biosynthesis through accelerated degradation of phenylalanine ammonia lyase (PAL). As PAL functions at the entry point of the phenylpropanoid pathway, which produces indispensable specialized metabolites such as lignin, aldoxime-mediated phenylpropanoid repression is detrimental to plant survival. Although methionine-derived glucosinolates in Arabidopsis are abundant, any impact of aliphatic aldoximes (AAOx) derived from aliphatic amino acids such as methionine on phenylpropanoid production remains unclear. Here, we investigate the impact of AAOx accumulation on phenylpropanoid production using Arabidopsis aldoxime mutants, ref2 and ref5. REF2 and REF5 metabolize aldoximes to respective nitrile oxides redundantly, but with different substrate specificities. ref2 and ref5 mutants have decreased phenylpropanoid contents due to the accumulation of aldoximes. As REF2 and REF5 have high substrate specificity toward AAOx and IAOx, respectively, it was assumed that ref2 accumulates AAOx, not IAOx. Our study indicates that ref2 accumulates both AAOx and IAOx. Removing IAOx partially restored phenylpropanoid content in ref2, but not to the wild-type level. However, when AAOx biosynthesis was silenced, phenylpropanoid production and PAL activity in ref2 were completely restored, suggesting an inhibitory effect of AAOx on phenylpropanoid production. Further feeding studies revealed that the abnormal growth phenotype commonly observed in Arabidopsis mutants lacking AAOx production is a consequence of methionine accumulation.

Exploring the canthariphilous species of the Tolfa Mountains (Latium, Central Italy): taxonomic diversity, phenology, and putative sources of cantharidin

Canthariphilous species are those arthropods attracted to cantharidin (CTD), a defensive compound produced by two beetle families (Meloidae and Oedemeridae). Although several species are known to be attracted to CTD, canthariphily was recently discovered in new species, suggesting that the list of canthariphilous species is still far from being complete. A systematic sampling focused to detect canthariphilous species has never been performed in Italy. The present research provides a list of seven canthariphilous species (Diptera: Ceratopogonidae, Anthomyiidae; Coleoptera: Anthicidae) from the Tolfa Mountains (Latium, Central Italy) resulting from a one-year sampling with CTD-baited and control traps. New species (Atrichopogon atriscapulus and A. tolfensis) were found to be attracted to CTD, and other species, already known as canthariphilous, were recorded for the first time in the Italian fauna (A. atriscapulus and A. meloesugans). A new scenario about the ecological significance of CTD in the sexual selection of canthariphilous species was speculated in A. meloesugans. Finally, a list of CTD-producing species occurring in the sample area was provided to suggest putative natural CTD sources.

Plasticity in plant defense and the role of phytochemical dissimilarity in limiting specialist herbivory

Phytochemical diversity is an effective plant defensive attribute, but much more research has focused on genetic and environmental controls of specific defensive compounds than phytochemical diversity per se. Documenting plasticity in phytochemical richness and plant chemical composition as opposed to individual compounds is important for understanding plant defense. This study outlines a multi-site transplant experiment in Cerrado gallery forests in central Brazil, utilizing Piper arboreum (Piperaceae), a prevalent and widespread neotropical shrub. Clones from four distinct populations were planted either at their origin site or in a different forest. Secondary metabolite composition varied between populations initially and then changed after transplanting. Interestingly, clones with chemical profiles that were distinct from the populations where they were introduced experienced reduced specialist chrysomelid herbivory compared to clones that were more chemically similar to the existing P. arboreum populations where they were planted. Specialist Lepidoptera herbivory also declined in clones transplanted to a new forest, but this change could not be ascribed to chemical profiles. In contrast, generalist herbivory was unaffected by chemical dissimilarity and transplanting. This research adds to the expanding body of evidence suggesting that phytochemical diversity is a dynamic trait exerting unique effects on different herbivore guilds.

Caterpillars Detoxify Diterpenoid from Nepeta stewartiana by the Molting Hormone Gene CYP306A1.

Herbivorous insects are well known for detoxifying a broad range of the defense compounds produced by the plants that they feed on, but knowledge of the mechanisms of detoxification is still very limited. Here, we describe a system in which two species of lepidopteran caterpillars metabolize an abietane diterpene from the plants of Nepeta stewartiana Diels to an oxygenated derivative that is less active biologically. We found that this transformation could be catalyzed by a cytochrome P450 enzyme in caterpillars, which are associated with molting. Most interestingly, abietane diterpene targets the molting-associated gene CYP306A1 to alter the content of molting hormones in the insect at specific developmental stages and competitively inhibit molting hormone metabolism. These findings identify the mechanism by which caterpillars are able to detoxify abietane diterpenoid through hydroxylation at the C-19 position, which may be opening up exciting research questions into the mechanisms of interaction between plants and insects.

Isolation and characterization of phenanthrene-degrading bacteria from urban soil

Phenanthrene (PHE) is one of the most abundant Polycyclic Aromatic Hydrocarbons (PAHs) found in urban soil. So far, majority of microbe-mediated degradation approaches have relied on microbes isolated from polluted or mined soils. These microbes, if introduced into non-native soil, may not able to establish themselves, or alter the native microbiome of soil, making their commercial use a challenge. Therefore, this study was conducted to screen and isolate bacteria, from urban soil that degrade PHE. Degradation of 50 mg/L PHE by day 4 was highest in Providencia rettgeri VMP5 (98.63%) followed by Bacillus tropicus VMP4 (89.9%), Bacillus sp. VMP2 (82.63%), P. stuartii VMP3 (39.72%) and least Dellaglioa algida VMP1 (3.32%). Besides, VMP5, showed the presence of several fatty acid ethyl esters that act as biosurfactants and anti-microbial agent, which may play a crucial role in increasing its adaptability and degradation potential. VMP4 and VMP2 were also found to produce more defense compounds which may be crucial for their adaptability to PAH rich environment. Thus, VMP5, VMP4 and VMP2 showed promising remediation potential that can be further explored to develop commercially viable consortia for PHE mitigation in urban soil.

Gut Bacteria of Wood-Boring Streltzoviella insularis and Their Involvement in Pinoresinol Degradation of Ash Tree (Fraxinus sp.) Host.

Insect gut microbiota have been widely reported to help the insects to overcome host tree defense. Streltzoviella insularis (Lepidoptera: Cossidae) is one of the most common wood borers in China, attacking various hosts, including ash trees (Fraxinus sp.), but little is known about its gut microbial associates and their involvement in host tree defense. We isolated gut bacteria of S. insularis larvae, analyzed their ability to degrade pinoresinol (a defense compound of ash trees) and cellulose, and identified pinoresinol degradation products. Larval mortality increased with increasing pinoresinol concentration (reflecting natural variation observed in the host trees). All the five detected gut bacteria isolates were able to degrade pinoresinol, two of which were also capable of cellulose degradation. Furthermore, gut bacteria were also shown to degrade pinoresinol via the gluconeogenesis pathway. These results suggest that S. insularis-associated microorganisms help to overcome host pinoresinol defense and possibly contribute to insects or gut microbial nutrition via carbohydrate synthesis.

Reductive Conversion Leads to Detoxification of Salicortin-like Chemical Defenses (Salicortinoids) in Lepidopteran Specialist Herbivores (Notodontidae).

Lepidopteran specialist herbivores of the Notodontidae family have adapted to thrive on poplar and willow species (Salicaceae). Previous research showed that Cerura vinula, a member of the Notodontidae family occurring throughout Europe and Asia, uses a unique mechanism to transform salicortinoids, the host plant's defense compounds, into quinic acid-salicylate conjugates. However, how the production of this conjugates relates to the detoxification of salicortinoids and how this transformation proceeds mechanistically have remained unknown. To find the mechanisms, we conducted gut homogenate incubation experiments with C. vinula and re-examined its metabolism by analyzing the constituents of its frass. To estimate the contribution of spontaneous degradation, we examined the chemical stability of salicortinoids and found that salicortinoids were degraded very quickly by midgut homogenates and that spontaneous degradation plays only a marginal role in the metabolism. We learned how salicortinoids are transformed into salicylate after we discovered reductively transformed derivatives, which were revealed to play key roles in the metabolism. Unless they have undergone the process of reduction, salicortinoids produce toxic catechol. We also studied constituents in the frass of the Notodontidae species Cerura erminea, Clostera anachoreta, Furcula furcula, Notodonta ziczac, and Pheosia tremula, and found the same metabolites as those described for C. vinula. We conclude that the process whereby salicortinoids are reductively transformed represents an important adaption of the Notodontidae to their Salicaceae host species.

Daytime of leaf wounding determines plant biomass and affect the interplay between growth and defense in Brassica crops.

Recent discoveries pointed out the importance of the mutual correlation between timing of environmental stress and plant fitness. However, the internal reshaping of plant growth under daily stress sensing, and their metabolic coordination remain to be investigated. Thus, we aim to study the connection between daytime, growth and defense to understand how plant fitness is affected by diurnal stress inputs. To accomplish that, we examined if simulated herbivory (leaf wounding) in the morning, mid-day or evening differentially influenced plant defensive states vs growth in three different crop species of the Brassica genus, broccoli (Brassica oleraceae), turnip greens (B. rapa) and rapeseed (B. napus). The data presented here revealed that plant's ability to tolerate wounding stress is diurnally regulated in Brassica crops. Trade-offs between plant biomass and investment in glucosinolates (GSL) and phenolics were affected by daytime of leaf stress. Negative correlations between plant biomass and induction of defensive compounds were found for morning and evening treated plants. However, these correlations were positives for mid-day treatment. Interestingly, we revealed new connection between plant growth and changes of aliphatic GSLs and flavonoids in response to wounding. These data suggest that metabolic stress-dependent circadian oscillations of leaves defenses could be one mechanism conferring competitive advantage to plants able to anticipate daily environmental variations by synchronize growth with them. Moreover, this work provides first insights of how secondary metabolites are linked to growth response in a timely manner.

The Fall Armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), Influences Nilaparvata lugens Population Growth Directly, by Preying on Its Eggs, and Indirectly, by Inducing Defenses in Rice.

The fall armyworm (FAW), Spodoptera frugiperda, has become one of the most important pests on corn in China since it invaded in 2019. Although FAW has not been reported to cause widespread damage to rice plants in China, it has been sporadically found feeding in the field. If FAW infests rice in China, the fitness of other insect pests on rice may be influenced. However, how FAW and other insect pests on rice interact remains unknown. In this study, we found that the infestation of FAW larvae on rice plants prolonged the developmental duration of the brown planthopper (BPH, Nilaparvata lugens (Stål)) eggs and plants damaged by gravid BPH females did not induce defenses that influenced the growth of FAW larvae. Moreover, co-infestation by FAW larvae on rice plants did not influence the attractiveness of volatiles emitted from BPH-infested plants to Anagrus nilaparvatae, an egg parasitoid of rice planthoppers. FAW larvae were able to prey on BPH eggs laid on rice plants and grew faster compared to those larvae that lacked available eggs. Studies revealed that the delay in the development of BPH eggs on FAW-infested plants was probably related to the increase in levels of jasmonoyl-isoleucine, abscisic acid and the defensive compounds in the rice leaf sheaths on which BPH eggs were laid. These findings indicate that, if FAW invades rice plants in China, the population density of BPH may be decreased by intraguild predation and induced plant defenses, whereas the population density of FAW may be increased.

ZmDRR206 Regulates Nutrient Accumulation in Endosperm through Its Role in Cell Wall Biogenesis during Maize Kernel Development.

Dirigent proteins (DIRs) contribute to plant fitness by dynamically reorganizing the cell wall and/or by generating defense compounds during plant growth, development, and interactions with environmental stresses. ZmDRR206 is a maize DIR, it plays a role in maintaining cell wall integrity during seedling growth and defense response in maize, but its role in regulating maize kernel development is unclear. Association analysis of candidate genes indicated that the natural variations of ZmDRR206 were significantly associated with maize hundred-kernel weight (HKW). ZmDRR206 plays a dominant role in storage nutrient accumulation in endosperm during maize kernel development, ZmDRR206 overexpression resulted in small and shrunken maize kernel with significantly reduced starch content and significantly decreased HKW. Cytological characterization of the developing maize kernels revealed that ZmDRR206 overexpression induced dysfunctional basal endosperm transfer layer (BETL) cells, which were shorter with less wall ingrowth, and defense response was constitutively activated in developing maize kernel at 15 and 18 DAP by ZmDRR206 overexpression. The BETL-development-related genes and auxin signal-related genes were down-regulated, while cell wall biogenesis-related genes were up-regulated in developing BETL of the ZmDRR206-overexpressing kernel. Moreover, the developing ZmDRR206-overexpressing kernel had significantly reduced contents of the cell wall components such as cellulose and acid soluble lignin. These results suggest that ZmDRR206 may play a regulatory role in coordinating cell development, storage nutrient metabolism, and stress responses during maize kernel development through its role in cell wall biogenesis and defense response, and provides new insights into understanding the mechanisms of kernel development in maize.

Actinobacillus pleuropneumoniae FliY and YdjN are involved in cysteine/cystine utilization, oxidative resistance, and biofilm formation but are not determinants of virulence.

Actinobacillus pleuropneumoniae (A. pleuropneumoniae) is a member of Actinobacillus in family Pasteurellaceae. It is the causative agent of porcine pleuropneumonia, which has caused huge economic losses to pig industry over the world. Cysteine is a precursor of many important biomolecules and defense compounds in the cell. However, molecular mechanisms of cysteine transport in A. pleuropneumoniae are unclear. In this study, gene-deleted mutants were generated and investigated, to reveal the roles of potential cysteine/cystine transport proteins FliY and YdjN of A. pleuropneumoniae. Our results indicated that the growth of A. pleuropneumoniae was not affected after fliY or ydjN single gene deletion, but absence of both FliY and YdjN decreased the growth ability significantly, when cultured in the chemically defined medium (CDM) supplemented with cysteine or cystine as the only sulfur source. A. pleuropneumoniae double deletion mutant ΔfliYΔydjN showed increased sensitivity to oxidative stress. Besides, trans-complementation of YdjN into ΔfliYΔydjN and wild type leads to increased biofilm formation in CDM. However, the virulence of ΔfliYΔydjN was not attenuated in mice or pigs. These findings suggest that A. pleuropneumoniae FliY and YdjN are involved in the cysteine/cystine acquisition, oxidative tolerance, and biofilm formation, but not contribute to the pathogenicity of A. pleuropneumoniae.

Two-way plant-mediated interactions between a plant parasitic nematode and a foliar herbivore arthropod

Interactions between belowground and aboveground heterotrophic communities with no direct physical contact can be connected by the plant as a mediator. Plants respond to the attack of herbivores by producing a suite of defensive compounds that can affect the choice and performance of other herbivores. The aim of this study was to determine the impact of one herbivore's activity on the acceptability of that plant to another species of herbivore. Two herbivores were tested; root-knot nematode, Meloidogyne incognita (RKN), a belowground plant-parasitic nematode, and the two-spotted spider mite, Tetranychus urticae (TSSM), an aboveground folivore. We conducted herbivore preference and performance tests on Lima bean (LB) (Phaseolus lunatus cv. Henderson) as an optimal host for TSSM, and tomato (Solanum lycopersicum cv. Rutgers) which is a sub-optimal host for TSSM but optimal host for RKN. We used two-choice glass olfactometers to measure the response of RKN to plants that were exposed to TSSM versus a clean LB plant. RKN infected the clean plants at a significantly greater rate than the TSSM exposed plants. TSSM preference was measured, using leaf discs and two-choice olfactometers containing a RKN infected plant versus a clean plant at different days post-inoculation (DPI) of the RKN. TSSM preferred the clean plants to those with 25-day old RKN infections on LB, but preferred RKN infected tomato plants at 1 DPI. We also tested the effect of the inoculation (1 DPI) of the entomopathogenic nematode (EPN) Steinernema carpocapsae on tomato plants and TSSM preferred the EPN inoculated plants. We carried out a non-choice performance test for TSSM on both LB and tomato on plants inoculated with RKN versus clean plants and observed no effect of RKN exposure on TSSM performance. This research shows that plants can mediate interactions between below and aboveground herbivores that share the same plant.