Why Do Herbivorous Mites Suppress Plant Defenses?

Temporal transcriptome and metabolome study revealed molecular mechanisms underlying rose responses to red spider mite infestation and predatory mite antagonism

Introduction

Red spider mite (Tetranychus urticae) infestation (SMI) is a detrimental factor for roses grown indoors. Although predatory mite (Neoseiulus californicus) antagonism (PMA) is often utilized to alleviate SMI damage, little is known about the defensive response of greenhouse-grown roses to SMI and the molecular mechanism by which PMA protects roses.

Methods

To determine the transcriptome and metabolome responses of roses to SMI and PMA, the leaves of a rose cultivar ("Fairy Zixia/Nightingale") were infested with T. urticae, followed by the introduction of predator mite. Leaf samples were collected at various time points and subjected to transcriptome and metabolome analyses.

Results

We found that 24 h of SMI exerted the most changes in the expression of defense-related genes and metabolites in rose leaves. KEGG pathway analysis of differentially expressed genes (DEGs) and metabolites revealed that rose responses to SMI and PMA were primarily enriched in pathways such as sesquiterpenoid and triterpenoid biosynthesis, benzoxazinoid biosynthesis, stilbenoid, diarylheptanoid and gingerol biosynthesis, phytosterol biosynthesis, MAPK signaling pathway, phenylpropanoid biosynthesis, and other pathways associated with resistance to biotic stress. Rose reacted to SMI and PMA by increasing the expression of structural genes and metabolite levels in phytosterol biosynthesis, mevalonate (MVA) pathway, benzoxazinoid biosynthesis, and stilbenoid biosynthesis. In addition, PMA caused a progressive recover from SMI, allowing rose to revert to its normal growth state. PMA restored the expression of 190 essential genes damaged by SMI in rose leaves, including transcription factors DRE1C, BH035, MYB14, EF110, WRKY24, NAC71, and MY108. However, after 144 h of PMA treatment, rose responsiveness to stimulation was diminished, and after 192 h, the metabolic levels of organic acids and lipids were recovered in large measure.

Conclusion

In conclusion, our results offered insights on how roses coordinate their transcriptome and metabolome to react to SMI and PMA, therefore shedding light on how roses, T. urticae, and N. californicus interact.

A new spider mite elicitor triggers plant defence and promotes resistance to herbivores

Herbivore-associated elicitors (HAEs) are active molecules produced by herbivorous insects. Recognition of HAEs by plants induces defence that resist herbivore attacks. We previously demonstrated that the tomato red spider mite Tetranychus evansi triggered defence in Nicotiana benthamiana. However, our knowledge of HAEs from T. evansi remains limited. Here, we characterize a novel HAE, Te16, from T. evansi and dissect its function in mite-plant interactions. We investigate the effects of Te16 on spider mites and plants by heterologous expression, virus-induced gene silencing assay, and RNA interference. Te16 induces cell death, reactive oxygen species (ROS) accumulation, callose deposition, and jasmonate (JA)-related responses in N. benthamiana leaves. Te16-mediated cell death requires a calcium signalling pathway, cytoplasmic localization, the plant co-receptor BAK1, and the signalling components SGT1 and HSP90. The active region of Te16-induced cell death is located at amino acids 114-293. Moreover, silencing Te16 gene in T. evansi reduces spider mite survival and hatchability, but expressing Te16 in N. benthamiana leaves enhances plant resistance to herbivores. Finally, Te16 gene is specific to Tetranychidae species and is highly conserved in activating plant immunity. Our findings reveal a novel salivary protein produced by spider mites that elicits plant defence and resistance to insects, providing valuable clues for pest management.

The effect of spermine on Tetranychus urticae-Cucumis sativus interaction

BACKGROUND

Two spotted spider mite, Tetranychus urticae (Acari: Tetranychidae) is one of the most important plant pests in the world. Due to increased resistance of mites to acaricides, it is necessary to use other methods such as inducing resistance in plants by natural compounds for pests' management. Polyamins such as spermine are effective in increasing plant resistance to biotic and abiotic stressors. In this research, the effect of spermine treatments in cucumber plants on life table parameters of T. urticae was investigated. Also, top-down effect of spermine and T. urticae on cucumber biochemical parameters was measured. In the experiments, 1, 2 and 3 mM spermine concentrations were used.

RESULTS

Amongst the spermine treatments, those mites that fed on cucumbers which received 1 mM spermine showed the shortest protonymphal period and higher ovipositon period, fecundity, gross and net reproductive rates and life expectancy compare to control. Treatment with 2 mM spermine lead to the longest teleochrysalis period and shortest range of age-stage-specific fecundity period. In addition, 2 mM spermine lowered intrinsic and finite rate of population increase in T. urticae. The longest larval period of T. urticae was observed in 3 mM spermine. Feeding of T. urticae from cucumber plants increased hydrogen peroxide (H2O2), malondialdehyde (MDA) content, electrolyte leakage (EL) level and ascorbate peroxidase (APX) activity but inhibited catalase (CAT) activity in this plant. Infested cucumber plants treated with 2 mM spermine showed lower H2O2 and MDA content and highest activity of APX and CAT on day 1 and 3 compare to the others. The 3 mM spermine increased H2O2 content in infested plants during the whole experiment as well as non-infested plants in day 5 and 9 only. This treatment induced the highest MDA content and lowest catalase activity on day1, 3 and 5 of experiment in infested plants.

CONCLUSION

This study showed that 2 mM spermine was the only effective concentration that reduce cucumber sensitivity to T. urticae. The trend of changes in biochemical parameters, especially H2O2, in 3 mM spermine was abnormal, and this concentration could be considered toxic.

Horizontally Transferred Salivary Protein Promotes Insect Feeding by Suppressing Ferredoxin-Mediated Plant Defenses

Herbivorous insects such as whiteflies, planthoppers, and aphids secrete abundant orphan proteins to facilitate feeding. Yet, how these genes are recruited and evolve to mediate plant-insect interaction remains unknown. In this study, we report a horizontal gene transfer (HGT) event from fungi to an ancestor of Aleyrodidae insects approximately 42 to 190 million years ago. BtFTSP1 is a salivary protein that is secreted into host plants during Bemisia tabaci feeding. It targets a defensive ferredoxin 1 in Nicotiana tabacum (NtFD1) and disrupts the NtFD1-NtFD1 interaction in plant cytosol, leading to the degradation of NtFD1 in a ubiquitin-dependent manner. Silencing BtFTSP1 has negative effects on B. tabaci feeding while overexpressing BtFTSP1 in N. tabacum benefits insects and rescues the adverse effect caused by NtFD1 overexpression. The association between BtFTSP1 and NtFD1 is newly evolved after HGT, with the homologous FTSP in its fungal donor failing to interact and destabilize NtFD1. Our study illustrates the important roles of horizontally transferred genes in plant-insect interactions and suggests the potential origin of orphan salivary genes.

GARP: A family of glycine and alanine-rich proteins that helps spider mites feed on plants

Spider mites (Tetranychidae) are destructive agricultural pests which have evolved strategies to overcome plant defenses, such as the ability to puncture the leaves of their hosts to feed. The expression of many genes with unknown functions is altered during feeding, but little is known about the role of these genes in plant-mite interactions. Here, we identified 3 novel gene families through analysis of genomic and transcriptomic data from 3 spider mite species. These GARP family genes encode glycine and alanine-rich proteins; they are present in mites (Acariformes) but absent in ticks (Parasitiformes) in the subclass Acari, indicating that these genes have undergone a significant expansion in spider mites and thus play important adaptive roles. Transcriptomic analysis revealed that the expression of GARP genes is strongly correlated with feeding and the transfer to new hosts. We used RNA interference to silence GARP1d in the two-spotted spider mite Tetranychus urticae, which inhibited feeding and egg laying and significantly increased mortality when the mites were transferred to soybean shoots; a similar effect was observed after TuVATPase was silenced. However, no changes in mite mortality were observed after TuGARP1d-silenced mites were placed on an artificial diet, which was different from the effect of TuVATPase silencing. Our results indicate that GARP family members play important roles in mite-plant interactions. Additional studies are needed to clarify the mechanisms underlying these interactions.

Intraspecific variation for host immune activation by the spider mite Tetranychus evansi

Many parasites can interfere with their host's defences to maximize their fitness. Here, we investigated if there is heritable variation in the spider mite Tetranychus evansi for traits associated with how they interact with their host plant. We also determined if this variation correlates with mite fecundity. Tetranychus evansi can interfere with jasmonate (JA) defences which are the main determinant of anti-herbivore immunity in plants. We investigated (i) variation in fecundity in the presence and absence of JA defences, making use of a wild-type tomato cultivar and a JA-deficient mutant (defenseless-1), and (ii) variation in the induction of JA defences, in four T. evansi field populations and 59 inbred lines created from an outbred population originating from controlled crosses of the four field populations. We observed a strong positive genetic correlation between fecundity in the presence (on wild-type) and the absence of JA defences (on defenseless-1). However, fecundity did not correlate with the magnitude of induced JA defences in wild-type plants. Our results suggest that the performance of the specialist T. evansi is not related to their ability to manipulate plant defences, either because all lines can adequately reduce levels of defences, or because they are resistant to them.

Transplastomic tomatoes expressing double-stranded RNA against a conserved gene are efficiently protected from multiple spider mites

Spider mites are serious pests and have evolved significant resistance to many chemical pesticides, thus making their control challenging. Several insect pests can be combated by plastid-mediated RNA interference (PM-RNAi), but whether PM-RNAi can be utilized to control noninsect pests is unknown. Here, we show that three species of spider mites (Tetranychus evansi, Tetranychus truncatus, and Tetranychus cinnabarinus) take up plastid RNA upon feeding. We generated transplastomic tomato plants expressing double-stranded RNA (dsRNA) targeted against a conserved region of the spider mite β-Actin mRNA. Transplastomic plants exhibited high levels of resistance to all three spider mite species, as evidenced by increased mortality and suppression of target gene expression. Notably, transplastomic plants induced a more robust RNAi response, caused higher mortality, and were overall better protected from spider mites than dsRNA-expressing nuclear transgenic plants. Our data demonstrate the potential of PM-RNAi as an efficient pest control measure for spider mites and extend the application range of the technology to noninsect pests.

Transcriptome analysis revealed detoxification gene expression changes in Tetranychus cinnabarinus challenged with ethyl oleate

Natural acaricides are potential biorational mite control alternatives to conventional chemical acaricides. However, little is known about the molecular mechanism of defense response to natural acaricides in mites. We previously reported significant acaricidal properties of ethyl oleate (EO) against Tetranychus cinnabarinus (here referred to as a sibling species of two-spotted spider mite, Tetranychus urticae), a highly polyphagous pest devastating crops in fields and greenhouses worldwide. In this study, we explored the molecular responses of T. cinnabarinus exposed to EO using RNA-Seq and differentially expressed gene (DEG) analysis. A total of 131, 185, and 154 DEGs were identified in T. cinnabarinus after 1, 6, and 24 h of EO treatment. In addition, 36 putative detoxification-related DEGs, including 10 cytochrome P450s (P450s), three glutathione S-transferases (GSTs), nine UDP-glycosyltransferases (UGTs), eight esterases (ESTs), and six ATP-binding cassette transporters (ABC transporters), were identified. Interestingly, the upregulation of these detoxification-related genes might be the main defense response of T. cinnabarinus exposed to EO. A quantitative real-time PCR analysis indicated that the expression profiles of 19 random DEGs were consistent with the RNA-Seq results. These findings serve as valuable information for a better understanding of the acaricide-mite interaction and molecular mechanisms involved in the defense response of T. cinnabarinus against EO.

Impact of Future Elevated Carbon Dioxide on C3 Plant Resistance to Biotic Stresses

Before the end of the century, atmospheric carbon dioxide levels are predicted to increase to approximately 900 ppm. This will dramatically affect plant physiology and influence environmental interactions and, in particular, plant resistance to biotic stresses. This review is a broad survey of the current research on the effects of elevated CO₂ (eCO₂) on phytohormone-mediated resistance of C₃ agricultural crops and related model species to pathogens and insect herbivores. In general, while plants grown in eCO₂ often have increased constitutive and induced salicylic acid levels and suppressed induced jasmonate levels, there are exceptions that implicate other environmental factors, such as light and nitrogen fertilization in modulating these responses. Therefore, this review sets the stage for future studies to delve into understanding the mechanistic basis behind how eCO₂ will affect plant defensive phytohormone signaling pathways under future predicted environmental conditions that could threaten global food security to inform the best agricultural management practices.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Interactions With Plant Defences Isolate Sympatric Populations of an Herbivorous Mite

Host plant specialisation can promote evolutionary divergence between herbivore populations associated with different plant species. While the mechanisms by which specialist species exploit their hosts have been studied widely across taxa, less is known about the mechanisms that allow intraspecific variants to arise and to be maintained across spatial and temporal scales. To understand whether adaptations to plant defences against herbivory contribute to the co-existence of genetically distinct populations of an herbivore, we investigate the interaction between honeysuckle (Lonicera periclymenum) and sympatric specialist and generalist populations of the spider mite Tetranychus urticae. We found that mite folivory induces the production of sticky droplets on honeysuckle, which have a defensive role: they increase mite mortality directly, and potentially indirectly by increasing the arrestment of a predator. We show that droplet induction and the preference to feed on honeysuckle depend on mite genotype, where the generalist avoids this host and the specialist suppresses droplet production. These traits are heritable and dominant in F1 hybrids between generalists and specialists. Selection pressure from honeysuckle and differences in host preference likely reduce the opportunity of mating encounters on this host. We propose that the interplay between selection from host plant defences and ecological barriers to hybridisation contribute to the persistence of genetically distinct populations of a single species in sympatry.

Mycorrhizal Symbiosis Triggers Local Resistance in Citrus Plants Against Spider Mites

Citrus plants are a highly mycotrophic species with high levels of fungal colonization. Citrus aurantium rootstocks typically show abundant root colonization by Rhizophagus irregularis three weeks after inoculation. Mycorrhizal symbiosis protects plants against multiple biotic stressors, however, such protection against spider mites remains controversial. We examined mycorrhiza-induced resistance (MIR) in citrus against the two-spotted spider mite Tetranychus urticae. Mycorrhized C. aurantium displayed reduced levels of damage in leaves and lower mite oviposition rates, compared to non-mycorrhized controls. Mycorrhization did not affect host choice of mites in Y-tube assays; of note, C. aurantium has innate strong antixenotic resistance against this mite. Analysis of metabolism pathways in mycorrhized citrus plants showed upregulated expression of the oxylipin-related genes LOX-2 and PR-3 early after infestation. Accordingly, jasmonic acid (JA), 12-oxo phytodienoic acid (OPDA), and JA-Ile concentrations were increased by mycorrhization. Non-targeted metabolomic analysis revealed the amino acid, oxocarboxylic acid, and phenylpropanoid metabolism as the three major pathways with more hits at 24 h post infection (hpi) in mycorrhized plants. Interestingly, there was a transition to a priming profile of these pathways at 48 hpi following infestation. Three flavonoids (i.e., malic acid, coumaric acid, and diconiferyl alcohol) were among the priming compounds. A mixture containing all these compounds provided efficient protection against the mite. Unexpectedly, systemic resistance did not improve after 72 h of primary infestation, probably due to the innate strong systemic resistance of C. aurantium. This is the first study to show that MIR is functional against T. urticae in locally infested citrus leaves, which is mediated by a complex pool of secondary metabolites and is likely coordinated by priming of JA-dependent responses.

Rapid specialization of counter defenses enables two-spotted spider mite to adapt to novel plant hosts

Genetic adaptation, occurring over a long evolutionary time, enables host-specialized herbivores to develop novel resistance traits and to efficiently counteract the defenses of a narrow range of host plants. In contrast, physiological acclimation, leading to the suppression and/or detoxification of host defenses, is hypothesized to enable broad generalists to shift between plant hosts. However, the host adaptation mechanisms used by generalists composed of host-adapted populations are not known. Two-spotted spider mite (TSSM; Tetranychus urticae) is an extreme generalist herbivore whose individual populations perform well only on a subset of potential hosts. We combined experimental evolution, Arabidopsis thaliana genetics, mite reverse genetics, and pharmacological approaches to examine mite host adaptation upon the shift of a bean (Phaseolus vulgaris)-adapted population to Arabidopsis. We showed that cytochrome P450 monooxygenases are required for mite adaptation to Arabidopsis. We identified activities of two tiers of P450s: general xenobiotic-responsive P450s that have a limited contribution to mite adaptation to Arabidopsis and adaptation-associated P450s that efficiently counteract Arabidopsis defenses. In approximately 25 generations of mite selection on Arabidopsis plants, mites evolved highly efficient detoxification-based adaptation, characteristic of specialist herbivores. This demonstrates that specialization to plant resistance traits can occur within the ecological timescale, enabling the TSSM to shift to novel plant hosts.

Cooperative Behaviors in Group-Living Spider Mites

Cooperative behaviors are evolutionary stable if the direct and/or indirect fitness benefits exceed the costs of helping. Here we discuss cooperation and behaviors akin to cooperation in subsocial group-living species of two genera of herbivorous spider mites (Tetranychidae), i.e., the largely polyphagous Tetranychus spp. and the nest-building Stigmaeopsis spp., which are specialized on grasses, such as bamboo. These spider mites are distributed in patches on various spatial scales, that is, within and among leaves of individual host plants and among individual hosts of single or multiple plant species. Group-living of spider mites is brought about by plant-colonizing foundresses ovipositing at local feeding sites and natal site fidelity, and by multiple individuals aggregating in the same site in response to direct and/or indirect cues, many of which are associated with webbing. In the case of the former, emerging patches are often composed of genetically closely related individuals, while in the case of the latter, local patches may consist of kin of various degrees and/or non-kin and even heterospecific spider mites. We describe and discuss ultimate and proximate aspects of cooperation by spider mites in host plant colonization and exploitation, dispersal, anti-predator behavior, and nesting-associated behaviors and conclude with theoretical and practical considerations of future research on cooperation in these highly rewarding model animals.

What Could Arrest an Eriophyoid Mite on a Plant? The Case of Aculops allotrichus from the Black Locust Tree

Simple Summary

For a phytophagous arthropod, plant cues and the presence of conspecifics or heterospecifics on a plant are important sources of information about food availability and predation risk. In eriophyoid mites, which are a group of economically important plant parasites, this issue is still poorly understood. In the previous study, females of the eriophyoid Aculops allotrichus prolonged their stay on unprofitable, old black locust leaves with injured conspecifics, suggesting their specific attraction to pierced individuals. The aim of this study was to shed light on this phenomenon. The series of tests using intact and artificially injured conspecifics, heterospecifics, pollen and sand grains revealed the high specificity of cues associated with mite injury. Only the presence of conspecifics triggered female arrestment on unprofitable old leaves, and this effect was more pronounced on leaf patches with injured than with intact eriophyoids. To our best knowledge, A. allotrichus is the first described herbivore in which injured conspecifics, instead of causing an alarm to be raised, keep the foraging individuals within a risky patch. Our tests using three non-host plants also suggest that A. allotrichus presumably needs more time to identify and evaluate non-host plants than host plants.

Abstract

Aculops allotrichus is a vagrant eriophyoid that lives gregariously on the leaves of the black locust tree. This study demonstrated that conspecifics can have a significant impact on A. allotrichus females on unprofitable, old black locust leaves and can arrest them on those leaves. The effect was more pronounced in females that were exposed to artificially injured individuals than to intact ones. They not only prolonged their sojourn on leaf discs with pierced conspecifics, but also preferred the leaf disc halves with damaged individuals to clean ones. Aculops allotrichus is the first described herbivore in which artificially injured conspecifics, instead of causing alarm, keep the foraging individuals within a risky patch. Other objects, such as artificially injured or intact heterospecifics, pollen or sand, were irrelevant to the eriophyoid females on old leaf patches. In tests with old leaves of maple, magnolia and hard kiwi vine, the females postponed their movement from non-host leaf discs, which suggests that they may need more time to recognise and evaluate unfamiliar plants than familiar ones.

Soybean (Glycine max L Merr) host-plant defenses and resistance to the two-spotted spider mite (Tetranychus urticae Koch)

In southern Ontario, Canada, the two-spotted spider mite (Tetranychus urticae) is an emerging pest of soybean (Glycine max) due to the increasing incidence of warmer, drier weather conditions. One key strategy to manage soybean pests is breeding resistant cultivars. Resistance to pathogens and herbivores in soybean has been associated with isoflavonoid phytoalexins, a group of specialized metabolites commonly associated with root, leaf and seed tissues. A survey of 18 Ontario soybean cultivars for spider mite resistance included evaluations of antibiosis and tolerance in relation to isoflavonoid and other metabolites detected in the leaves. Ten-day and 4-week trials beginning with early growth stage plants were used to compare survival, growth, fecundity as well as damage to leaves. Two-spotted spider mite (TSSM) counts were correlated with HPLC measurements of isoflavonoid concentration in the leaves and global metabolite profiling by high resolution LC-MS to identify other metabolites unique to the most resistant (R) and susceptible (S) cultivars. Within 10 days, no significant difference (P>0.05) in resistance to TSSM was determined between cultivars, but after 4 weeks, one cultivar, OAC Avatar, was revealed to have the lowest number of adult TSSMs and their eggs. Other cultivars showing partial resistance included OAC Wallace and OAC Lakeview, while Pagoda was the most tolerant to TSSM feeding. A low, positive correlation between isoflavonoid concentrations and TSSM counts and feeding damage indicated these compounds alone do not explain the range of resistance or tolerance observed. In contrast, other metabolite features were significantly different (P<0.05) in R versus S cultivars. In the presence of TSSM, the R cultivars had significantly greater (P<0.05) concentrations of the free amino acids Trp, Val, Thr, Glu, Asp and His relative to S cultivars. Furthermore, the R cultivar metabolites detected are viable targets for more in-depth analysis of their potential roles in TSSM defense.

Gall- and erineum-forming Eriophyes mites alter photosynthesis and volatile emissions in an infection severity-dependent manner in broad-leaved trees Alnus glutinosa and Tilia cordata

Highly host-specific eriophyoid gall- and erineum-forming mites infest a limited range of broadleaf species, with the mites from the genus Eriophyes particularly widespread on Alnus spp. and Tilia spp. Once infected, the infections can be massive, covering a large part of leaf area and spreading through the plant canopy, but the effects of Eriophyes mite gall formation on the performance of host leaves are poorly understood. We studied the influence of three frequent Eriophyes infections, E. inangulis gall-forming mites on Alnus glutinosa, and E. tiliae gall-forming and E. exilis erineum-forming mites on Tilia cordata, on foliage morphology, chemistry, photosynthetic characteristics, and constitutive and induced volatile emissions. For all types of infections, leaf dry mass per unit area, net assimilation rate per area and stomatal conductance strongly decreased with increasing severity of infection. Mite infections resulted in enhancement or elicitation of emissions of fatty acid-derived volatiles, isoprene, benzenoids and carotenoid breakdown products in an infection severity-dependent manner for all different infections. Monoterpene emissions were strongly elicited in T. cordata mite infections, but these emissions were suppressed in E. inangulis-infected A. glutinosa. Although the overall level of mite-induced emissions was surprisingly low, these results highlight the uniqueness of the volatile profiles and offer opportunities for using volatile fingerprints and overall emission rates to diagnose infections by Eriophyes gall- and erineum-forming mites on temperate trees and assess their impact on the physiology of the affected trees.

Genome streamlining in a minute herbivore that manipulates its host plant

The tomato russet mite, Aculops lycopersici, is among the smallest animals on earth. It is a worldwide pest on tomato and can potently suppress the host's natural resistance. We sequenced its genome, the first of an eriophyoid, and explored whether there are genomic features associated with the mite's minute size and lifestyle. At only 32.5 Mb, the genome is the smallest yet reported for any arthropod and, reminiscent of microbial eukaryotes, exceptionally streamlined. It has few transposable elements, tiny intergenic regions, and is remarkably intron-poor, as more than 80% of coding genes are intronless. Furthermore, in accordance with ecological specialization theory, this defense-suppressing herbivore has extremely reduced environmental response gene families such as those involved in chemoreception and detoxification. Other losses associate with this species' highly derived body plan. Our findings accelerate the understanding of evolutionary forces underpinning metazoan life at the limits of small physical and genome size.

Early Molecular Responses of Tomato to Combined Moderate Water Stress and Tomato Red Spider Mite Tetranychus evansi Attack

Interaction between plants and their environment is changing as a consequence of the climate change and global warming, increasing the performance and dispersal of some pest species which become invasive species. Tetranychus evansi also known as the tomato red spider mite, is an invasive species which has been reported to increase its performance when feeding in the tomato cultivar Moneymaker (MM) under water deficit conditions. In order to clarify the underlying molecular events involved, we examined early plant molecular changes occurring on MM during T. evansi infestation alone or in combination with moderate drought stress. Hormonal profiling of MM plants showed an increase in abscisic acid (ABA) levels in drought-stressed plants while salicylic acid (SA) levels were higher in drought-stressed plants infested with T. evansi, indicating that SA is involved in the regulation of plant responses to this stress combination. Changes in the expression of ABA-dependent DREB2, NCED1, and RAB18 genes confirmed the presence of drought-dependent molecular responses in tomato plants and indicated that these responses could be modulated by the tomato red spider mite. Tomato metabolic profiling identified 42 differentially altered compounds produced by T. evansi attack, moderate drought stress, and/or their combination, reinforcing the idea of putative manipulation of tomato plant responses by tomato red spider mite. Altogether, these results indicate that the tomato red spider mite acts modulating plant responses to moderate drought stress by interfering with the ABA and SA hormonal responses, providing new insights into the early events occurring on plant biotic and abiotic stress interaction.

Juvenile Spider Mites Induce Salicylate Defenses, but Not Jasmonate Defenses, Unlike Adults

When plants detect herbivores they strengthen their defenses. As a consequence, some herbivores evolved the means to suppress these defenses. Research on induction and suppression of plant defenses usually makes use of particular life stages of herbivores. Yet many herbivorous arthropods go through development cycles in which their successive stages have different characteristics and lifestyles. Here we investigated the interaction between tomato defenses and different herbivore developmental stages using two herbivorous spider mites, i.e., Tetranychus urticae of which the adult females induce defenses and T. evansi of which the adult females suppress defenses in Solanum lycopersicum (tomato). First, we monitored egg-to-adult developmental time on tomato wild type (WT) and the mutant defenseless-1 (def-1, unable to produce jasmonate-(JA)-defenses). Then we assessed expression of salivary effector genes (effector 28, 84, SHOT2b, and SHOT3b) in the consecutive spider mite life stages as well as adult males and females. Finally, we assessed the extent to which tomato plants upregulate JA- and salicylate-(SA)-defenses in response to the consecutive mite developmental stages and to the two sexes. The consecutive juvenile mite stages did not induce JA defenses and, accordingly, egg-to-adult development on WT and def-1 did not differ for either mite species. Their eggs however appeared to suppress the SA-response. In contrast, all the consecutive feeding stages upregulated SA-defenses with the strongest induction by T. urticae larvae. Expression of effector genes was higher in the later developmental stages. Comparing expression in adult males and females revealed a striking pattern: while expression of effector 84 and SHOT3b was higher in T. urticae females than in males, this was the opposite for T. evansi. We also observed T. urticae females to upregulate tomato defenses, while T. evansi females did not. In addition, of both species also the males did not upregulate defenses. Hence, we argue that mite ontogenetic niche shifts and stage-specific composition of salivary secreted proteins probably together determine the course and efficiency of induced tomato defenses.

Targeted mutagenesis using CRISPR-Cas9 in the chelicerate herbivore Tetranychus urticae

The use of CRISPR-Cas9 has revolutionized functional genetic work in many organisms, including more and more insect species. However, successful gene editing or genetic transformation has not yet been reported for chelicerates, the second largest group of terrestrial animals. Within this group, some mite and tick species are economically very important for agriculture and human health, and the availability of a gene-editing tool would be a significant advancement for the field. Here, we report on the use of CRISPR-Cas9 in the spider mite Tetranychus urticae. The ovary of virgin adult females was injected with a mix of Cas9 and sgRNAs targeting the phytoene desaturase gene. Natural mutants of this laterally transferred gene have previously shown an easy-to-score albino phenotype. Albino sons of injected virgin females were mated with wild-type females, and two independent transformed lines where created and further characterized. Albinism inherited as a recessive monogenic trait. Sequencing of the complete target-gene of both lines revealed two different lesions at expected locations near the PAM site in the target-gene. Both lines did not genetically complement each other in dedicated crosses, nor when crossed to a reference albino strain with a known genetic defect in the same gene. In conclusion, two independent mutagenesis events were induced in the spider mite T. urticae using CRISPR-Cas9, hereby providing proof-of-concept that CRISPR-Cas9 can be used to create gene knockouts in mites.

Tetranychus evansi spider mite populations suppress tomato defenses to varying degrees

Plant defense suppression is an offensive strategy of herbivores, in which they manipulate plant physiological processes to increase their performance. Paradoxically, defense suppression does not always benefit the defense-suppressing herbivores, because lowered plant defenses can also enhance the performance of competing herbivores and can expose herbivores to increased predation. Suppression of plant defense may therefore entail considerable ecological costs depending on the presence of competitors and natural enemies in a community. Hence, we hypothesize that the optimal magnitude of suppression differs among locations. To investigate this, we studied defense suppression across populations of Tetranychus evansi spider mites, a herbivore from South America that is an invasive pest of solanaceous plants including cultivated tomato, Solanum lycopersicum, in other parts of the world. We measured the level of expression of defense marker genes in tomato plants after infestation with mites from eleven different T. evansi populations. These populations were chosen across a range of native (South American) and non-native (other continents) environments and from different host plant species. We found significant variation at three out of four defense marker genes, demonstrating that T. evansi populations suppress jasmonic acid- and salicylic acid-dependent plant signaling pathways to varying degrees. While we found no indication that this variation in defense suppression was explained by differences in host plant species, invasive populations tended to suppress plant defense to a smaller extent than native populations. This may reflect either the genetic lineage of T. evansi-as all invasive populations we studied belong to one linage and both native populations to another-or the absence of specialized natural enemies in invasive T. evansi populations.

Plant Defenses Against Tetranychus urticae: Mind the Gaps

The molecular interactions between a pest and its host plant are the consequence of an evolutionary arms race based on the perception of the phytophagous arthropod by the plant and the different strategies adopted by the pest to overcome plant triggered defenses. The complexity and the different levels of these interactions make it difficult to get a wide knowledge of the whole process. Extensive research in model species is an accurate way to progressively move forward in this direction. The two-spotted spider mite, Tetranychus urticae Koch has become a model species for phytophagous mites due to the development of a great number of genetic tools and a high-quality genome sequence. This review is an update of the current state of the art in the molecular interactions between the generalist pest T. urticae and its host plants. The knowledge of the physical and chemical constitutive defenses of the plant and the mechanisms involved in the induction of plant defenses are summarized. The molecular events produced from plant perception to the synthesis of defense compounds are detailed, with a special focus on the key steps that are little or totally uncovered by previous research.

Food decisions of an omnivorous thrips are independent from the indirect effects of jasmonate-inducible plant defences on prey quality

Plant defensive substances can affect the quality of herbivores as prey for predators either directly or indirectly. Directly when the prey has become toxic since it ingested toxic plant material and indirectly when these defences have affected the size and/or nutritional value (both quality parameters) of prey or their abundance. To disentangle direct and indirect effects of JA-defences on prey quality for predators, we used larvae of the omnivorous thrips Frankliniella occidentalis because these are not directly affected by the jasmonate-(JA)-regulated defences of tomato. We offered these thrips larvae the eggs of spider mites (Tetranychus urticae or T. evansi) that had been feeding from either normal tomato plants, JA-impaired plants, or plants treated with JA to artificially boost defences and assessed their performance. Thrips development and survival was reduced on the diet of T. evansi eggs relative to the diet of T. urticae eggs yet these effects were independent from the absence/presence of JA-defences. This indicates that the detrimental effects of tomato JA-defences on herbivores not necessarily also affects their quality as prey.

Transcriptomic Plasticity in the Arthropod Generalist Tetranychus urticae Upon Long-Term Acclimation to Different Host Plants

The two-spotted spider mite Tetranychus urticae is an important pest with an exceptionally broad host plant range. This generalist rapidly acclimatizes and adapts to a new host, hereby overcoming nutritional challenges and a novel pallet of constitutive and induced plant defenses. Although recent studies reveal that a broad transcriptomic response upon host plant transfer is associated with a generalist life style in arthropod herbivores, it remains uncertain to what extent these transcriptional changes are general stress responses or host-specific. In the present study, we analyzed and compared the transcriptomic changes that occur in a single T. urticae population upon long-term transfer from Phaseolus vulgaris to a similar, but chemically defended, host (cyanogenic Phaseolus lunatus) and to multiple economically important crops (Glycine max, Gossypium hirsutum, Solanum lycopersicum and Zea mays). These long-term host plant transfers were associated with distinct transcriptomic responses with only a limited overlap in both specificity and directionality, suggestive of a fine-tuned transcriptional plasticity. Nonetheless, analysis at the gene family level uncovered overlapping functional processes, recruiting genes from both well-known and newly discovered detoxification families. Of note, our analyses highlighted a possible detoxification role for Tetranychus-specific short-chain dehydrogenases and single PLAT domain proteins, and manual genome annotation showed that both families are expanded in T. urticae Our results shed new light on the molecular mechanisms underlying the remarkable adaptive potential for host plant use of generalist arthropods and set the stage for functional validation of important players in T. urticae detoxification of plant secondary metabolites.

Distinct Signatures of Host Defense Suppression by Plant-Feeding Mites

Tomato plants are attacked by diverse herbivorous arthropods, including by cell-content-feeding mites, such as the extreme generalist Tetranychus urticae and specialists like Tetranychus evansi and Aculops lycopersici. Mite feeding induces plant defense responses that reduce mite performance. However, T. evansi and A. lycopersici suppress plant defenses via poorly understood mechanisms and, consequently, maintain a high performance on tomato. On a shared host, T. urticae can be facilitated by either of the specialist mites, likely due to the suppression of plant defenses. To better understand defense suppression and indirect plant-mediated interactions between herbivorous mites, we used gene-expression microarrays to analyze the transcriptomic changes in tomato after attack by either a single mite species (T. urticae, T. evansi, A. lycopersici) or two species simultaneously (T. urticae plus T. evansi or T. urticae plus A. lycopersici). Additionally, we assessed mite-induced changes in defense-associated phytohormones using LC-MS/MS. Compared to non-infested controls, jasmonates (JAs) and salicylate (SA) accumulated to higher amounts upon all mite-infestation treatments, but the response was attenuated after single infestations with defense-suppressors. Strikingly, whereas 8 to 10% of tomato genes were differentially expressed upon single infestations with T. urticae or A. lycopersici, respectively, only 0.1% was altered in T. evansi-infested plants. Transcriptome analysis of dual-infested leaves revealed that A. lycopersici primarily suppressed T. urticae-induced JA defenses, while T. evansi dampened T. urticae-triggered host responses on a transcriptome-wide scale. The latter suggests that T. evansi not solely down-regulates plant gene expression, but rather directs it back towards housekeeping levels. Our results provide valuable new insights into the mechanisms underlying host defense suppression and the plant-mediated facilitation of competing herbivores.

Unraveling Rice Tolerance Mechanisms Against Schizotetranychus oryzae Mite Infestation

Rice is the staple food for over half of the world's population. Infestation of Schizotetranychus oryzae (Acari: Tetranychidae) causes great losses in rice productivity. To search for rice genotypes that could better tolerate S. oryzae infestation, we evaluated morphological and production parameters in Brazilian cultivars, and identified two cultivars with contrasting responses. Leaf damage during infestation was similar for all cultivars. However, infestation in Puitá INTA-CL resulted in reduction in the number of seeds per plant, percentage of full seeds, weight of 1,000 seeds, and seed length, whereas infestation in IRGA 423 increased weight of 1,000 seeds and seed length. Reduction in seed weight per plant caused by infestation was clearly higher in Puitá INTA-CL (62%) compared to IRGA 423 (no reduction detected), thus Puitá INTA-CL was established as susceptible, and IRGA 423 as tolerant to S. oryzae infestation. Photosynthetic parameters were less affected by infestation in IRGA 423 than in Puitá INTA-CL, evidencing higher efficiency of energy absorption and use. S. oryzae infestation also caused accumulation of H2O2, decreased cell membrane integrity (indicative of cell death), and accelerated senescence in leaves of Puitá INTA-CL, while leaves of IRGA 423 presented higher levels of total phenolics compounds. We performed proteomics analysis of Puitá INTA-CL and IRGA 423 leaves after 7 days of infestation, and identified 60 differentially abundant proteins (28 more abundant in leaves of Puitá INTA-CL and 32 in IRGA 423). Proteins related to plant defense, such as jasmonate synthesis, and related to other mechanisms of tolerance such as oxidative stress, photosynthesis, and DNA structure maintenance, together with energy production and general metabolic processes, were more abundant in IRGA 423. We also detected higher levels of silicon (as amorphous silica cells) in leaves of infested IRGA 423 plants compared to Puitá INTA-CL, an element previously linked to plant defense, indicating that it could be involved in tolerance mechanisms. Taken together, our data show that IRGA 423 presents tolerance to S. oryzae infestation, and that multiple mechanisms might be employed by this cultivar. These findings could be used in biotechnological approaches aiming to increase rice tolerance to mite infestation.

Unraveling Rice Tolerance Mechanisms Against Schizotetranychus oryzae Mite Infestation

Rice is the staple food for over half of the world's population. Infestation of Schizotetranychus oryzae (Acari: Tetranychidae) causes great losses in rice productivity. To search for rice genotypes that could better tolerate S. oryzae infestation, we evaluated morphological and production parameters in Brazilian cultivars, and identified two cultivars with contrasting responses. Leaf damage during infestation was similar for all cultivars. However, infestation in Puitá INTA-CL resulted in reduction in the number of seeds per plant, percentage of full seeds, weight of 1,000 seeds, and seed length, whereas infestation in IRGA 423 increased weight of 1,000 seeds and seed length. Reduction in seed weight per plant caused by infestation was clearly higher in Puitá INTA-CL (62%) compared to IRGA 423 (no reduction detected), thus Puitá INTA-CL was established as susceptible, and IRGA 423 as tolerant to S. oryzae infestation. Photosynthetic parameters were less affected by infestation in IRGA 423 than in Puitá INTA-CL, evidencing higher efficiency of energy absorption and use. S. oryzae infestation also caused accumulation of H₂O₂, decreased cell membrane integrity (indicative of cell death), and accelerated senescence in leaves of Puitá INTA-CL, while leaves of IRGA 423 presented higher levels of total phenolics compounds. We performed proteomics analysis of Puitá INTA-CL and IRGA 423 leaves after 7 days of infestation, and identified 60 differentially abundant proteins (28 more abundant in leaves of Puitá INTA-CL and 32 in IRGA 423). Proteins related to plant defense, such as jasmonate synthesis, and related to other mechanisms of tolerance such as oxidative stress, photosynthesis, and DNA structure maintenance, together with energy production and general metabolic processes, were more abundant in IRGA 423. We also detected higher levels of silicon (as amorphous silica cells) in leaves of infested IRGA 423 plants compared to Puitá INTA-CL, an element previously linked to plant defense, indicating that it could be involved in tolerance mechanisms. Taken together, our data show that IRGA 423 presents tolerance to S. oryzae infestation, and that multiple mechanisms might be employed by this cultivar. These findings could be used in biotechnological approaches aiming to increase rice tolerance to mite infestation.