Can Plant Defence Mechanisms Provide New Approaches for the Sustainable Control of the Two-Spotted Spider Mite Tetranychus urticae?

Mycorrhiza-induced resistance in citrus against Tetranychus urticae is plant species dependent and inversely correlated to basal immunity

BACKGROUND

Mycorrhizal plants show enhanced resistance to biotic stresses, but few studies have addressed mycorrhiza-induced resistance (MIR) against biotic challenges in woody plants, particularly citrus. Here we present a comparative study of two citrus species, Citrus aurantium, which is resistant to Tetranychus urticae, and Citrus reshni, which is highly susceptible to T. urticae. Although both mycorrhizal species are protected in locally infested leaves, they show very distinct responses to MIR.

RESULTS

Previous studies have indicated that C. aurantium is insensitive to MIR in systemic tissues and MIR-triggered antixenosis. Conversely, C. reshni is highly responsive to MIR which triggers local, systemic and indirect defense, and antixenosis against the pest. Transcriptional, hormonal and inhibition assays in C. reshni indicated the regulation of jasmonic acid (JA)- and abscisic acid-dependent responses in MIR. The phytohormone jasmonic acid isoleucine (JA-Ile) and the JA biosynthesis gene LOX2 are primed at early timepoints. Evidence indicates a metabolic flux from phenylpropanoids to specific flavones that are primed at 24 h post infestation (hpi). MIR also triggers the priming of naringenin in mycorrhizal C. reshni, which shows a strong correlation with several flavones and JA-Ile that over-accumulate in mycorrhizal plants. Treatment with an inhibitor of phenylpropanoid biosynthesis C4H enzyme impaired resistance and reduced the symbiosis, demonstrating that phenylpropanoids and derivatives mediate MIR in C. reshni.

CONCLUSION

MIR's effectiveness is inversely correlated to basal immunity in different citrus species, and provides multifaceted protection against T. urticae in susceptible C. reshni, activating rapid local and systemic defenses that are mainly regulated by the accumulation of specific flavones and priming of JA-dependent responses. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

QTL analysis on a lemon population provides novel insights on the genetic regulation of the tolerance to the two-spotted spider mite attack

BACKGROUND

Among the Citrus species, lemon (Citrus limon Burm f.) is one of the most affected by the two-spotted spider mite (Tetranychus urticae Koch). Moreover, chemical control is hampered by the mite's ability to develop genetic resistance against acaricides. In this context, the identification of the genetic basis of the host resistance could represent a sustainable strategy for spider mite control. In the present study, a marker-trait association analysis was performed on a lemon population employing an association mapping approach. An inter-specific full-sib population composed of 109 accessions was phenotyped through a detached-leaf assays performed in modified Huffaker cells. Those individuals, complemented with two inter-specific segregating populations, were genotyped using a target-sequencing approach called SPET (Single Primer Enrichment Technology), the resulting SNPs were employed for the generation of an integrated genetic map.

RESULTS

The percentage of damaged area in the full-sib population showed a quantitative distribution with values ranging from 0.36 to 9.67%. A total of 47,298 SNPs were selected for an association mapping study and a significant marker linked with resistance to spider mite was detected on linkage group 5. In silico gene annotation of the QTL interval enabled the detection of 13 genes involved in immune response to biotic and abiotic stress. Gene expression analysis showed an over expression of the gene encoding for the ethylene-responsive transcription factor ERF098-like, already characterized in Arabidopsis and in rice for its involvement in defense response.

CONCLUSION

The identification of a molecular marker linked to the resistance to spider mite attack can pave the way for the development of marker-assisted breeding plan for the development of novel selection coupling favorable agronomical traits (e.g. fruit quality, yield) with a higher resistance toward the mite.

Management potentiality of emulsions based on essential oil of Piper macedoi and Piper arboreum for spider mite: a possible alternative for agroecological control

Tetranychus urticae, popularly known as spider mite, is a pest that causes several economic losses to crops. Thus, this work evaluated the effect of essential oils from the leaves of Piper macedoi and Piper arboreum on managing T. urticae. The chemical compounds present in essential oils were identified by gas chromatography. Tests were carried out to evaluate the acaricidal activity by fumigation effect and direct contact with T. urticae. The results showed that LC50 values for the essential oils of P. macedoi and P. arboreum in the fumigation effect were 16.15 and 50.53 µL L-1 air, respectively. Using the contact application route, the LC50 values for the essential oil of P. macedoi was 17.16 µL mL-1, and for P arboreum, it was 15.17 µL mL-1. So, this work showed that essential oils of Piper macedoi and Piper arboreum could be used as possible alternative to managing T. urticae.

Attraction of the two-spotted spider mite, Tetranychus urticae (Acari: Tetranychidae), to healthy and damaged strawberry plants mediated by volatile cues

Tetranychus urticae Koch (Acari: Tetranychidae), the two-spotted spider mite, is a pest that limits strawberry production in Mexico. Little is known about the interactions that occur between T. urticae and healthy strawberry plants or strawberry plants infested by conspecific spider mites. Therefore, in this study we evaluated the attraction of T. urticae to healthy strawberry plants mediated by volatile organic compounds (VOCs), and to plants damaged by conspecifics mediated by herbivore-induce plant volatiles (HIPVs). First, we conducted dual-choice tests using a Y-tube olfactometer with plants and extracts obtained through dynamic aeration. The volatile composition of the extracts was identified using gas chromatography-mass spectrometry (GC-MS). Once the compounds were identified, we also conducted dual-choice tests with selected synthetic compounds. Tetranychus urticae exhibited greater attraction to both healthy and damaged plants compared to the control (clean air). However, when healthy and damaged plants were offered simultaneously, there was no significant preference observed. Bioassays with extracts obtained by dynamic aeration yielded similar results. The identified compounds were terpenes and aromatic hydrocarbons. We found qualitative and quantitative changes between the VOCs emitted by the healthy plant and the HIPVs from mite-damaged plants. The individual compounds α-pinene (10 ng), pseudocumene (10 ng), and limonene (1 ng) and 10 ng of the blend made of α-pinene + pseudocumene + mesitylene + limonene (5:34:57:4) attracted more T. urticae than the control. However, the binary blend of pseudocumene + limonene (91:9) was more attractive than the other binary or three-compound blends evaluated. These results may contribute to developing strategies for the management of this pest.

Chemical characterization and evaluation of acaricidal potential of rosemary essential oil and its main compound α-pinene on the two-spotted spider mite, Tetranychus urticae

Two-spotted spider mite causes significant damage to various crops, often requiring synthetic acaricides for their control. However, the frequent use of these products causes several environmental problems. Thus, this work aimed to evaluate the acaricidal action, using the application by fumigation on female adults of Tetranychus urticae of essential oil from the leaves of Rosmarinus officinalis L., and its major compound, α-pinene. In addition, the essential oil was characterized by gas chromatography and mass spectrometry. α-pinene was the compound with the highest relative area in the oil (29.2%). In fumigation tests, α-pinene showed more significant toxicity than rosemary essential oil on two-spotted spider mite females with LC50 and LC90 values of 1.58 and 49.61 µL/L air, showing the impact of the chemical composition of the essential oil on the biological activity.

Evaluation of the Stability of a 1,8-Cineole Nanoemulsion and Its Fumigant Toxicity Effect against the Pests Tetranychus urticae, Rhopalosiphum maidis and Bemisia tabaci

Pest control is a main concern in agriculture. Indiscriminate application of synthetic pesticides has caused negative impacts leading to the rapid development of resistance in arthropod pests. Plant secondary metabolites have been proposed as a safer alternative to conventional pesticides. Monoterpenoids have reported bioactivities against important pests; however, due to their high volatility, low water solubility and chemical instability, the application of these compounds has been limited. Nanosystems represent a potential vehicle for the broad application of monoterpenoids. In this study, an 1,8-cineole nanoemulsion was prepared by the low energy method of phase inversion, characterization of droplet size distribution and polydispersity index (PDI) was carried out by dynamic light scattering and stability was evaluated by centrifugation and Turbiscan analysis. Fumigant bioactivity was evaluated against Tetranychus urticae, Rhopalosiphum maidis and Bemisia tabaci. A nanoemulsion with oil:surfactant:water ratio of 0.5:1:8.5 had a droplet size of 14.7 nm and PDI of 0.178. Formulation was stable after centrifugation and the Turbiscan analysis showed no particle migration and a delta backscattering of ±1%. Nanoemulsion exhibited around 50% more bioactivity as a fumigant on arthropods when compared to free monoterpenoid. These results suggest that nanoformulations can provide volatile compounds of protection against volatilization, improving their bioactivity.

A comprehensive meta-analysis reveals the key variables and scope of seed defense priming

Background

When encountered with pathogens or herbivores, the activation of plant defense results in a penalty in plant fitness. Even though plant priming has the potential of enhancing resistance without fitness cost, hurdles such as mode of application of the priming agent or even detrimental effects in plant fitness have yet to be overcome. Here, we review and propose seed defense priming as an efficient and reliable approach for pathogen protection and pest management.

Methods

Gathering all available experimental data to date, we evaluated the magnitude of the effect depending on plant host, antagonist class, arthropod feeding guild and type of priming agent, as well as the influence of parameter selection in measuring seed defense priming effect on plant and antagonist performance.

Results

Seed defense priming enhances plant resistance while hindering antagonist performance and without a penalty in plant fitness. Specifically, it has a positive effect on crops and cereals, while negatively affecting fungi, bacteria and arthropods. Plant natural compounds and biological isolates have a stronger influence in plant and antagonist performance than synthetic chemicals and volatiles.

Discussion

This is the first meta-analysis conducted evaluating the effect of seed defense priming against biotic stresses studying both plant and pest/pathogen performance. Here, we proved its efficacy in enhancing both, plant resistance and plant fitness, and its wide range of application. In addition, we offered insight into the selection of the most suitable priming agent and directed the focus of interest for novel research.

Effects of tomato inoculation with the entomopathogenic fungus Metarhizium brunneum on spider mite resistance and the rhizosphere microbial community

Entomopathogenic fungi have been well exploited as biocontrol agents that can kill insects through direct contact. However, recent research has shown that they can also play an important role as plant endophytes, stimulating plant growth, and indirectly suppressing pest populations. In this study, we examined the indirect, plant-mediated, effects of a strain of entomopathogenic fungus, Metarhizium brunneum on plant growth and population growth of two-spotted spider mites (Tetranychus urticae) in tomato, using different inoculation methods (seed treatment, soil drenching and a combination of both). Furthermore, we investigated changes in tomato leaf metabolites (sugars and phenolics), and rhizosphere microbial communities in response to M. brunneum inoculation and spider mite feeding. A significant reduction in spider mite population growth was observed in response to M. brunneum inoculation. The reduction was strongest when the inoculum was supplied both as seed treatment and soil drench. This combination treatment also yielded the highest shoot and root biomass in both spider mite-infested and non-infested plants, while spider mite infestation increased shoot but reduced root biomass. Fungal treatments did not consistently affect leaf chlorogenic acid and rutin concentrations, but M. brunneum inoculation via a combination of seed treatment and soil drenching reinforced chlorogenic acid (CGA) induction in response to spider mites and under these conditions the strongest spider mite resistance was observed. However, it is unclear whether the M. brunneum-induced increase in CGA contributed to the observed spider mite resistance, as no general association between CGA levels and spider mite resistance was observed. Spider mite infestation resulted in up to two-fold increase in leaf sucrose concentrations and a three to five-fold increase in glucose and fructose concentrations, but these concentrations were not affected by fungal inoculation. Metarhizium, especially when applied as soil drench, impacted the fungal community composition but not the bacterial community composition which was only affected by the presence of spider mites. Our results suggest that in addition to directly killing spider mites, M. brunneum can indirectly suppress spider mite populations on tomato, although the underlying mechanism has not yet been resolved, and can also affect the composition of the soil microbial community.

On the Future Perspectives of Some Medicinal Plants within Lamiaceae Botanic Family Regarding Their Comprehensive Properties and Resistance against Biotic and Abiotic Stresses

Lamiaceae is one of the largest botanical families, encompassing over 6000 species that include a variety of aromatic and medicinal spices. The current study is focused on three plants within this botanical family: basil (Ocimum basilicum L.), thyme (Thymus vulgaris L.), and summer savory (Satureja hortensis L.). These three species contain primary and secondary metabolites such as phenolic and flavonoid compounds, fatty acids, antioxidants, and essential oils and have traditionally been used for flavoring, food preservation, and medicinal purposes. The goal of this study is to provide an overview of the nutraceutical, therapeutic, antioxidant, and antibacterial key features of these three aromatics to explore new breeding challenges and opportunities for varietal development. In this context, a literature search has been performed to describe the phytochemical profile of both primary and secondary metabolites and their pharmacological uses, as well as to further explore accession availability in the medicine industry and also to emphasize their bioactive roles in plant ecology and biotic and abiotic stress adaptability. The aim of this review is to explore future perspectives on the development of new, highly valuable basil, summer savory, and thyme cultivars. The findings of the current review emphasize the importance of identifying the key compounds and genes involved in stress resistance that can also provide valuable insights for further improvement of these important medicinal plants.

Detection of adaptive genetic diversity and chemical composition in date palm cultivars and their implications in controlling red palm weevil, Rhynchophorus ferrugineus Oliver

This study, about RPW and date palms, is under the scope of date palm bioecology and nutrition (nutritional ecology) which includes the integration of several areas of research such as date palm biochemistry, genetics, and RPW infestation behavior through various date palm cultivars. Date palm (Phoenix dactylifera L.; Arecaceae) production is under threat from the red palm weevil (RPW), Rhynchophorus ferrugineus Oliver. A better understanding of genetic diversity within date palm cultivars can be useful for its implementation within the insect IPM program in the future. Three indices, namely simple-sequence repeats (SSR) markers to elucidate genetic diversity, chemical components, and a natural infestation index of RPW, were used to evaluate the resistant or susceptible date palm cultivars in Qassim. Based on a field survey of RPW infestation within 79 date palm farms involving 11 cultivars at Qassim, the sensitivity and resistance cultivars were determined. The resistant date palm cultivars were Nabtat Ali, Shakrah, red Sukary, and um Kobar which had the lowest degree of RPW abundance %. Values of the essential minerals, nitrogen, phosphorus, potassium, and calcium within the date palm cultivars were also estimated. RPW abundance % was negatively correlated with the calcium content of date palm cultivars. The principal component analysis (PCA) revealed that the calcium content and RPW abundance % were highly affected by the cultivars. SSR markers of the date palm cluster tree divided genotypes into two main groups at similarity coefficients between 0.56 and 0.91. The 1st group included; Nabtet Ali, Red Sukary, Um Kobar, and Shakrah with similarity coefficients between 0.56, this group was the most resistant cultivars. Therefore, SSR markers were able to characterize and resolve genetic diversity in date palm cultivars for RPW resistance. When SSR markers coupled with higher calcium (Ca) content can efficiently replace indices in characterizing resistant date-palm genotypes with a high confidence level. Integration between date palm genetic diversity, chemical structures, and RPW infestations rates promoted the understanding of the interplay between the diversity of RPW management (short-time scale), and the resistance genes, plant nutrition, and dynamics of the diversity of RPW through domestication and diversification (long-timescale). Therefore, our results may lead to a change in RPW control strategies by switching to using safe alternative pesticide control methods (Resistant cultivars of date palm), which are underestimated and may reveal the impact of low-cost, but highly effective agricultural practices in the field of date production in the world. Understanding the genetic structure and calcium content of date palm cultivars mechanisms could help to predict date palm resistance against RPW populations in the new IPM strategy in RPW control.

Frequencies and mechanisms of pesticide resistance in Tetranychus urticae field populations in China

The two-spotted spider mite Tetranychus urticate is an important agricultural pest worldwide. It is extremely polyphagous and has developed resistance to many pesticides. Here, we assessed the pesticide resistance of seven field populations of T. urticae in China, their target site mutations and the activities of their detoxification enzymes. The results showed that abamectin and the traditional pesticides pyridaben, profenofos and bifenthrin had higher resistance or lower toxicity than more recently developed pesticides including chlorfenapyr, spinetoram, cyflumetofen, cyenopyrafen, bifenazate and B-azolemiteacrylic. The frequency of point mutations related to abamectin resistance, G314D in the glutamate-gated chloride channel 1 (GluCl1) and G326E in GluCl3, ranged 47%-70% and 0%-97%, respectively. The frequency of point mutations in A1215D and F1538I of the voltage-gated sodium channel gene (VGSC), which may increase resistance to pyrethroids, ranged 88%-100% and 10%-100%, respectively. For target sites related to organophosphate resistance, mutation frequencies ranged 25%-92% for G119S and 0%-23% for A201S in the acetycholinesterase gene (Ace). Mutation G126S in the bifenazate resistance-related cytochrome b gene (Cytb) was observed in three of the seven T. urticae populations. Higher activities of detoxification enzymes (P450, GST, CarEs and UGTs) were observed in two T. urticae populations, with significant difference in the XY-SX population. These results provide useful information on the status of pesticide resistance of T. urticae in China and suggest that T. urticae field populations may have multiple resistance mechanisms.

The Resistance of Seven Host Plants to Tetranychus merganser Boudreaux (Acari: Tetranychidae)

Simple Summary

The red spider mite, Tetranychus merganser is one of the most economically important pests in papaya and prickle pear cactus cultivars, causing major damage to fruit and defoliation. In recent years, T. merganser has increased the number of its host plants. The mechanisms of resistance of a plant to herbivorous arthropod include antixenosis and antibiosis. Antixenosis refers to the plant mechanism to affect feeding and oviposition of arthropods; antibiosis refers to the plant capacity to affect the biology of the arthropod. The aim of this research is to assess antibiosis and antixenosis as resistance mechanisms in seven host plants (Thevetia ahouai, Carica papaya, Phaseolus vulgaris, Moringa oleifera, Pittosporum tobira, Helietta parvifolia, Capsicum annuum var. glabriusculum) to red spider mites. Oviposition and damage by feeding of T. merganser were greater on C. papaya than on the other host plants. The population growth of the spider mite was lower in P. tobira and T. ahouai than in the other host plants. Results based on the analysis of demographic parameters, food intake, survival and oviposition of T. merganser females suggest that P. tobira and T. ahouai were the most resistant to red spider mites, whereas C. papaya was the most susceptible of the seven host plants. The resistant plants can be studied as alternatives in the management of red spider mites.

Abstract

Red spider mites, Tetranychus merganser Boudreaux (Acari: Tetranychidae), is an agricultural pest that causes economic losses in papaya and nopal crops in Mexico. The aim of this research was to assess antibiosis and antixenosis as resistance mechanisms in seven host plants (Thevetia ahouai, Carica papaya, Phaseolus vulgaris, Moringa oleifera, Pittosporum tobira, Helietta parvifolia, Capsicum annuum var. glabriusculum) to red spider mites. Antixenosis was evaluated by non-preference for oviposition and feeding, antibiosis by infinitesimal rate of increase, finite rate of increase and doubling time, and the percentage of spider mites mortality. Oviposition and damage by feeding of T. merganser were significantly greater on C. papaya than on the other host plants. The growth rate of the spider mite was significantly lower in P. tobira and T. ahouai than in the other host plants. The percentage of hatched eggs of T. merganser was significantly higher in P. vulgaris than in the other plant species. Based on the demographic parameters, survival, food intake, and oviposition, these results indicated that compared with C. papaya, P. tobira and T. ahouai were more resistant. These results may be due to the fact that they were plants species of different families. The resistant plants can be studied as alternatives in the management of T. merganser.

Overexpression of leucoanthocyanidin reductase or anthocyanidin reductase elevates tannins content and confers cassava resistance to two-spotted spider mite

The two-spotted spider mite (TSSM) is a destructive cassava pest. Intensive demonstration of resistance mechanism greatly facilitates the creation of TSSM-resistant cassava germplasm. Gene to metabolite network plays a crucial role in modulating plant resistance, but little is known about the genes and related metabolites which are responsible for cassava resistance to TSSM. Here, a highly resistant (HR) and a highly susceptible (HS) cassava cultivar were used, integrative and comparative transcriptomic and metabolomic analyses between these two cultivars after TSSM infestation revealed that several genes and metabolites were closely related and significantly different in abundance. In particular, the expression of leucoanthocyanidin reductase (LAR) and anthocyanidin reductase (ANR) genes showed a high positive correlation with most of the metabolites in the tannin biosynthesis pathway. Furthermore, transgenic cassava lines overexpressing either of the genes elevated tannin concentrations and conferred cassava resistance to TSSM. Additionally, different forms of tannins possessed distinct bioactivity on TSSM, of which total condensed tannins (LC₅₀ = 375.68 mg/l) showed maximum lethal effects followed by procyanidin B1 (LC₅₀ = 3537.10 mg/l). This study accurately targets LAR, ANR and specific tannin compounds as critical genes and metabolites in shaping cassava resistance to TSSM, which could be considered as biomarkers for evaluation and creation of pest-resistant cassava germplasm.

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.

Salicylic Acid Induced Resistance in Drought-Stressed Pistachio Seedlings Influences Physiological Performance of Agonoscena pistaciae (Hemiptera: Aphalaridae)

Induced host plant resistance is a potential approach to insect and disease management. Salicylic acid (SA) acts as a signal molecule to induce resistance in plants against sap-sucking insects. The effects of salicylic acid-induced resistance against common pistachio psylla, Agonoscena pistaciae Burckhardt and Lauterer, were investigated in well-watered and drought-stressed pistachio, Pistacia vera L. cv. Akbari, seedlings. Agonoscena pistaciae exhibited a significant preference for plants treated with SA as compared with untreated controls or those subjected to drought stress. Plants subjected to both drought stress and SA treatment were equivalently colonized as compared with control plants but were more attractive than those subjected to drought stress alone. Psyllid mortality increased on plants subjected to simultaneous drought stress and SA treatment as compared with controls. Salicylic acid treatment mediated production of defensive enzymes in plants, including superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), glutathione reductase (GR), guaiacol peroxidase (GPX) and polyphenol oxidase (PPO), as well as that of other metabolites such as phenol, malondialdehyde (MDA), H2O2, free amino acids, and pigments via phenylpropanoid pathways under conditions of drought. Despite increases in activity of detoxification (glutathione S transferase, carboxylesterase) and antioxidative (SOD, CAT, APX, phenoloxidase, GPX) enzymes in psyllids, reduced survival of A. pistaciae on drought stressed and SA-treated plants was likely caused by excessive H2O2 and high phenolic content in treated plants. Based on our results, we postulate that salicylic acid-induced defense against A. pistaciae under drought conditions could be manipulated to enhance antibiosis against this key pest in pistachio orchards.

Changes in plant responses induced by an arthropod influence the colonization behavior of a subsequent herbivore

BACKGROUND

Plants in nature can be sequentially attacked by different arthropod herbivores. Feeding by one arthropod species may induce plant-defense responses that might affect the performance of a later-arriving herbivorous species. Understanding these interactions can help in developing pest-management strategies. In tomato, the sweet-potato whitefly Bemisia tabaci and the two-spotted spider mite Tetranychus urticae are key pests that frequently cohabit on the same plant. We studied whether colonization by one species can either facilitate or impede later colonization of tomato plants by conspecific or heterospecific individuals.

RESULTS

B. tabaci females showed a strong preference for and increased oviposition on plants previously colonized by conspecifics. In contrast, plants infested with T. urticae repelled B. tabaci females and reduced their oviposition rate by 86%. Although females of T. urticae showed no preference between conspecific-infested or uninfested plants, we observed a 50% reduction in the number of eggs laid on conspecific-infested plants. Both herbivorous arthropods up-regulated the expression of genes involving the jasmonic acid and abscisic acid pathways, increasing emissions of fatty-acid derivatives, but only B. tabaci increased the expression of genes related to the salicylic acid pathway and the total amount of phenylpropanoids released. Terpenoids were the most abundant compounds in the volatile blends; many terpenoids were emitted at different rates, which might have influenced the arthropods' host selection.

CONCLUSION

Our results indicate that B. tabaci infestation facilitated subsequent infestations by conspecifics and mites, while T. urticae infestation promoted herbivore-induced resistance. Based on both the molecular and behavioral findings, a novel sustainable pest-management strategy is discussed.

Overexpression of a plasma membrane protein generated broad-spectrum immunity in soybean

Plants fight-off pathogens and pests by manifesting an array of defence responses using their innate immunity mechanisms. Here we report the identification of a novel soybean gene encoding a plasma membrane protein, transcription of which is suppressed following infection with the fungal pathogen, Fusarium virguliforme. Overexpression of the protein led to enhanced resistance against not only against F. virguliforme, but also against spider mites (Tetranychus urticae, Koch), soybean aphids (Aphis glycines, Matsumura) and soybean cyst nematode (Heterodera glycines). We, therefore, name this protein as Glycine max disease resistance 1 (GmDR1; Glyma.10g094800). The homologues of GmDR1 have been detected only in legumes, cocoa, jute and cotton. The deduced GmDR1 protein contains 73 amino acids. GmDR1 is predicted to contain an ecto- and two transmembrane domains. Transient expression of the green fluorescent protein fused GmDR1 protein in soybean leaves showed that it is a plasma membrane protein. We investigated if chitin, a pathogen-associated molecular pattern (PAMP), common to all pathogen and pests considered in this study, can significantly enhance defence pathways among the GmDR1-overexpressed transgenic soybean lines. Chitin induces marker genes of the salicylic- and jasmonic acid-mediated defence pathways, but suppresses the defence pathway regulated by ethylene. Chitin induced SA- and JA-regulated defence pathways may be one of the mechanisms involved in generating broad-spectrum resistance among the GmDR1-overexpressed transgenic soybean lines against two serious pathogens and two pests including spider mites, against which no known resistance genes have been identified in soybean and among the most other crop species.

Classification of Plant Electrophysiology Signals for Detection of Spider Mites Infestation in Tomatoes

Herbivorous arthropods, such as spider mites, are one of the major causes of annual crop losses. They are usually hard to spot before a severe infestation takes place. When feeding, these insects cause external perturbation that triggers changes in the underlying physiological process of a plant, which are expressed by a generation of distinct variations of electrical potential. Therefore, plant electrophysiology data portray information of the plant state. Analyses involving machine learning techniques applied to plant electrical response triggered by spider mite infestation have not been previously reported. This study investigates plant electrophysiological signals recorded from 12 commercial tomatoes plants contaminated with spider mites and proposes a workflow based on Gradient Boosted Tree algorithm for an automated differentiation of the plant’s normal state from the stressed state caused by infestation. The classification model built using the signal samples recorded during daylight and employing a reduced feature subset performs with an accuracy of 80% in identifying the plant’s stressed state. Furthermore, the Hjorth complexity encloses the most relevant information for discrimination of the plant status. The obtained findings open novel access towards automated detection of insect infestation in greenhouse crops and, consequently, more optimal prevention and treatment approaches.

Overexpression of chalcone isomerase in apple reduces phloridzin accumulation and increases susceptibility to herbivory by two-spotted mites

Apples (Malus spp.) accumulate significant quantities of the dihydrochalcone glycoside, phloridzin, whilst pears (Pyrus spp.) do not. To explain this difference, we hypothesized that a metabolic bottleneck in the phenylpropanoid pathway might exist in apple. Expression analysis indicated that transcript levels of early phenylpropanoid pathway genes in apple and pear leaves were similar, except for chalcone isomerase (CHI), which was much lower in apple. Apples also showed very low CHI activity compared with pear. To relieve the bottleneck at CHI, transgenic apple plants overexpressing the Arabidopsis AtCHI gene were produced. Unlike other transgenic apples where phenylpropanoid flux was manipulated, AtCHI overexpression (CHIox) plants were phenotypically indistinguishable from wild-type, except for an increase in red pigmentation in expanding leaves. CHIox plants accumulated slightly increased levels of flavanols and flavan-3-ols in the leaves, but the major change was a 2.8- to 19-fold drop in phloridzin concentrations compared with wild-type. The impact of these phytochemical changes on insect preference was studied using a two-choice leaf assay with the polyphagous apple pest, the two-spotted spider mite (Tetranychus urticae Koch). Transgenic CHIox leaves were more susceptible to herbivory, an effect that could be reversed (complemented) by application of phloridzin to transgenic leaves. Taken together, these findings shed new light on phenylpropanoid biosynthesis in apple and suggest a new physiological role for phloridzin as an antifeedant in leaves.

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.

Analysis of the Physicochemical Properties of Acaricides Based on Lipinski's Rule of Five

This study aimed to explore the similarity between the physical and chemical properties of different acaricides, determine whether Lipinski's Rule of Five (RO5) used in the design of drug molecules is suitable for screening acaricides, and provide methods for selection of new acaricides. We evaluated and predicted the molecular properties of >180 acaricides using Molinspiration. We calculated physicochemical property parameters, such as log p, molecular weight (MW), and number of hydrogen bond donors (HBDs), hydrogen bond acceptors (HBAs), and rotatable bonds (Rot B). We then conducted qualitative and quantitative analyses of the physicochemical properties of acaricides. The MW of all acaricides ranged from 141 to 663, with an average value of 337.8. The number of HBDs ranged from 0 to 5, with an average value of 0.46. The number of HBAs ranged from 0 to 9, with an average value of 4.07. The log p ranged from -0.79 to 8.74, with an average value of 4.61. The number of Rot B ranged from 0 to 14, with an average value of 5.62. Except for the microbial and plant-derived acaricides, the molecular properties of the remaining acaricides are in accordance with the Lipinski's RO5. Therefore, the Lipinski's RO5 can provide a basis for screening new acaricide drugs.

Comparative transcriptome provides molecular insight into defense-associated mechanisms against spider mite in resistant and susceptible common bean cultivars

Common bean (Phaseolus vulgaris L.) is a major source of proteins and one of the most important edible foods for more than three hundred million people in the world. The common bean plants are frequently attacked by spider mite (Tetranychus urticae Koch), leading to a significant decrease in plant growth and economic performance. The use of resistant cultivars and the identification of the genes involved in plant-mite resistance are practical solutions to this problem. Hence, a comprehensive study of the molecular interactions between resistant and susceptible common bean cultivars and spider mite can shed light into the understanding of mechanisms and biological pathways of resistance. In this study, one resistant (Naz) and one susceptible (Akhtar) cultivars were selected for a transcriptome comparison at different time points (0, 1 and 5 days) after spider mite feeding. The comparison of cultivars in different time points revealed several key genes, which showed a change increase in transcript abundance via spider mite infestation. These included genes involved in flavonoid biosynthesis process; a conserved MYB-bHLH-WD40 (MBW) regulatory complex; transcription factors (TFs) TT2, TT8, TCP, Cys2/His2-type and C2H2-type zinc finger proteins; the ethylene response factors (ERFs) ERF1 and ERF9; genes related to metabolism of auxin and jasmonic acid (JA); pathogenesis-related (PR) proteins and heat shock proteins.

An Arabidopsis TIR-Lectin Two-Domain Protein Confers Defense Properties against Tetranychus urticae

Plant immunity depends on fast and specific transcriptional reprogramming triggered by the perception of biotic stresses. Numerous studies have been conducted to better understand the response of plants to the generalist herbivore two-spotted spider mite (Tetranychus urticae). However, how plants perceive mites and how this perception is translated into changes in gene expression are largely unknown. In this work, we identified a gene induced in Arabidopsis (Arabidopsis thaliana) upon spider mite attack that encodes a two-domain protein containing predicted lectin and Toll/Interleukin-1 receptor domains. The gene, previously named PP2-A5, belongs to the Phloem Protein2 family. Biotic assays showed that PP2-A5 confers tolerance to T. urticae Overexpression or knockout of PP2-A5 leads to transcriptional reprogramming that alters the balance of hormone accumulation and corresponding signaling pathways. The nucleocytoplasmic location of this protein supports a direct interaction with regulators of gene transcription, suggesting that the combination of two putative signaling domains in a single protein may provide a novel mechanism for regulating gene expression. Together, our results suggest that PP2-A5 improves the ability to defend against T. urticae by participating in the tight regulation of hormonal cross talk upon mite feeding. Further research is needed to determine the mechanism by which this two-domain protein functions and to clarify its molecular role in signaling following a spider mite attack.

Physiological Response of Basil Plants to Twospotted Spider Mite (Acari: Tetranychidae) Infestation

The induction of plant resistance against pests is considered a potential method of controlling mite infestation as it restricts the use of chemical pesticides in herbal crops. Our goal was to investigate whether plant physiological response to mite feeding varied depending on basil cultivar and/or duration of mite infestation. The effect of plant acceptance, mite mortality rate, and changes in physiological parameters: malondialdehyde content (MDA), hydrogen peroxide (H2O2) concentration, and antioxidant enzyme activities, including guaiacol peroxidase (GPX) and catalase (CAT) were examined in this study. Tetranychus urticae Koch (Acari: Tetranychidae) infestation induced oxidative stress in three Ocimum basilicum L. cultivars: 'Sweet basil,' 'Purpurascens,' and 'Fino Verde.' The analysis of mite behavior and alteration in metabolic plant profiles showed different sensitivities of basil cultivars to biotic stress that were dependent on the cultivar and duration of infestation. All basil plants were suitable as host plants for T. urticae, but they varied in the level of susceptibility to mite feeding. O. basilicum 'Fino Verde' was the most suitable host for the twospotted spider mite. In turn, O. basilicum 'Purpurascens' was characterized by the lowest level of susceptibility to T. urticae feeding. The lowest acceptance, the highest mortality of twospotted spider mite individuals as well as decreased levels of H2O2 and MDA, significantly increased GPX activity and low level of CAT activity were recorded in O. basilicum 'Purpurascens' leaves. Research on plant responses to biotic stress can inform breeding cultivars resistant to arthropod attack.

Making a Better Home: Modulation of Plant Defensive Response by Brevipalpus Mites

False-spider mites of the genus Brevipalpus are highly polyphagous pests that attack hundreds of plant species of distinct families worldwide. Besides causing direct damage, these mites may also act as vectors of many plant viruses that threaten high-value ornamental plants like orchids and economically important crops such as citrus and coffee. To better understand the molecular mechanisms behind plant-mite interaction we used an RNA-Seq approach to assess the global response of Arabidopsis thaliana (Arabidopsis) plants along the course of the infestation with Brevipalpus yothersi, the main vector species within the genus. Mite infestation triggered a drastic transcriptome reprogramming soon at the beginning of the interaction and throughout the time course, deregulating 1755, 3069 and 2680 genes at 6 hours after infestation (hai), 2 days after infestation (dai), and 6 dai, respectively. Gene set enrichment analysis revealed a clear modulation of processes related to the plant immune system. Co-expressed genes correlated with specific classes of transcription factors regulating defense pathways and developmental processes. Up-regulation of defensive responses correlated with the down-regulation of growth-related processes, suggesting the triggering of the growth-defense crosstalk to optimize plant fitness. Biological processes (BPs) enriched at all time points were markedly related to defense against herbivores and other biotic stresses involving the defense hormones salicylic acid (SA) and jasmonic acid (JA). Levels of both hormones were higher in plants challenged with mites than in the non-infested ones, supporting the simultaneous induction of genes from both pathways. To further clarify the functional relevance of the plant hormonal pathways on the interaction, we evaluated the mite performance on Arabidopsis mutants impaired in SA- or JA-mediated response. Mite oviposition was lower on mutants defective in SA biosynthesis (sid2) and signaling (npr1), showing a function for SA pathway in improving the mite reproduction, an unusual mechanism compared to closely-related spider mites. Here we provide the first report on the global and dynamic plant transcriptome triggered by Brevipalpus feeding, extending our knowledge on plant-mite interaction. Furthermore, our results suggest that Brevipalpus mites manipulate the plant defensive response to render the plant more susceptible to their colonization by inducing the SA-mediated pathway.