Insect-Borne Plant Pathogens and Their Vectors: Ecology, Evolution, and Complex Interactions

The role of host plants in driving pathogen susceptibility in insects through chemicals, immune responses and microbiota

In this comprehensive exploration, we delve into the pivotal role of host plants in shaping the intricate interactions between herbivorous insects and their pathogens. Recent decades have seen a surge in studies that demonstrate that host plants are crucial drivers of the interactions between insects and pathogens, providing novel insights into the direct and indirect interactions that shape tri-trophic interactions. These studies have built on a wide range of pathogens, from viruses to bacteria, and from protozoans to fungi. We summarise these studies, and discuss the mechanisms of plant-mediated insect resistance to infection, ranging from the toxicity of plant chemicals to pathogens to enhancement of anti-pathogen immune responses, and modulation of the insect's microbiome. Although we provide evidence for the roles of all these mechanisms, we also point out that the majority of existing studies are phenomenological, describing patterns without addressing the underlying mechanisms. To further our understanding of these tri-trophic interactions, we therefore urge researchers to design their studies to enable them specifically to distinguish the mechanisms by which plants affect insect susceptibility to pathogens.

Cotton leaf curl Multan virus activates autophagy in the whitefly AsiaII7, weakening its vectorial capacity for transmission

BACKGROUND

Autophagy plays an important role against pathogen infections in both insects and plants. Insect vectors employ autophagy as an intrinsic antiviral defense mechanism against viral infections, whereas viruses can exploit autophagy to enhance their transmission via insect vectors. The Cotton leaf curl Multan virus (CLCuMuV) is transmitted by the AsiaII7 cryptic species of Bemisia tabaci, however, the role of autophagy is involved in regulating the transmission of this virus remains unclear.

RESULT

In this study, it was observed that CLCuMuV infection induced autophagy in AsiaII7 whitefly, as evidenced by an elevated in the level of ATG8-II and the upregulation of Atg3, Atg8, Atg9 and Atg12. Both the administration of the autophagy inhibitor bafilomycin A1 and the silencing of Atg9 expression increased the viral load and enhanced CLCuMuV transmission. Conversely, the activation of autophagy via rapamycin feeding significantly reduced the amount of CLCuMuV and inhibited the efficiency of virus transmission.

CONCLUSION

CLCuMuV infection can activate the autophagy pathway in whiteflies. The activation of autophagy leads to the subsequent degradation of the virus and suppresses CLCuMuV transmission efficiency, whereas suppression of autophagy promotes virus transmission. Our research provides insight into the potential role of autophagy in antiviral defense mechanisms. © 2025 Society of Chemical Industry.

Neuropeptide Ecdysis-Triggering Hormone and Its Receptor Mediate the Fecundity Improvement of 'Candidatus Liberibacter Asiaticus'-Infected Diaphorina citri Females and CLas Proliferation

The severe Asiatic form of huanglongbing (HLB), caused by "Candidatus Liberibacter asiaticus" (CLas), threatens global citrus production via the citrus psyllid, Diaphorina citri. Culturing challenges of CLas necessitate reducing D. citri populations for disease management. CLas boosts the fecundity of CLas-positive (CLas+) D. citri and fosters its own proliferation by modulating the insect host's juvenile hormone (JH), but the intricate endocrine regulatory mechanisms remain elusive. Here, it is reported that the D. citri ecdysis-triggering hormone (DcETH) and its receptor DcETHR play pivotal roles in the reciprocal benefits between CLas and D. citri within the ovaries, influencing energy metabolism and reproductive development in host insects; miR-210, negatively regulates DcETHR expression, contributing to this symbiotic interaction. CLas infection reduces 20-hydroxyecdysone (20E) levels and stimulates DcETH release, elevating JH production via DcETHR, enhancing fecundity and CLas proliferation. Furthermore, circulating JH levels suppress 20E production in CLas+ ovaries. Collectively, the orchestrated functional interplay involving 20E, ETH, and JH increases energy metabolism and promotes the fecundity of CLas+ D. citri and CLas proliferation. These insights not only broaden the knowledge of how plant pathogens manipulate the reproductive behavior of insect hosts but also offer novel targets and strategies for combatting HLB and D. citri.

The Insect Vector Diaphorina citri Exhibits a Lower Level of Fatty Acids upon Infection with the Phytopathogenic Bacterium 'Candidatus Liberibacter asiaticus'

Diaphorina citri (Hemiptera: Liviidae) is the main vector for the bacterium 'Candidatus Liberibacter asiaticus', which is associated with citrus greening, also known as Huanglongbing. D. citri transmits 'Ca. L. asiaticus' during its feeding on citrus phloem sap. Transmission occurs in a circulative, propagative, and persistent manner. 'Ca. L. asiaticus' has a small genome (1.2 Mb). Therefore, it acquires most of its nutrients and energetic nucleotides from its hosts. The objective of this study was to assess the effect of 'Ca. L. asiaticus' infection on the level of the free fatty acids in its vector. The fatty acids were extracted from adult D. citri using ethyl acetate, derivatized with boron trifluoride-methanol, and analyzed using gas chromatography-mass spectrometry. Nine fatty acids were identified in the D. citri extracts. Oleic acid was the most predominant fatty acid, followed by stearic and palmitic acid, whereas the rest of the fatty acids were present in low amounts. In general, the levels of the detected fatty acids in 'Ca. L. asiaticus'-infected D. citri were lower than those found in healthy psyllids. Our findings showed that the reduction of fatty acids in 'Ca. L. asiaticus'-infected psyllids resulted from the higher activity of β-oxidation to generate acetyl-coenzyme A, which causes more production of ATP. Our results indicated that 'Ca. L. asiaticus' may enhance the β-oxidation of fatty acids in its vector insect to fulfill its nutrient and energetic nucleotide requirements.

Targeting the Xylella fastidiosa spittlebug vector Neophilaenus campestris in the olive cover crops with the entomopathogenic fungus Metarhizium brunneum

Introduction

Neophilaenus campestris (Fallén) (Hemiptera: Aphrophoridae) is among the most abundant, highly dispersible, and widely distributed Xylella fastidiosa Wells (Xanthomonadales: Xanthomonadaceae) vectors to olive tree in Europe, with emphasis in Andalucía. The development of efficient and environmentally friendly vector management strategies is greatly needed. Entomopathogenic ascomycetes are among the few alternatives for the microbial control of pierce-sucking spittlebugs due to their unique contact mode and ability to endophytically colonize crops. These characteristics allow for several strategic uses aimed at reducing vector populations and/or their disease transmission potential. This study included a two-year field experiment to evaluate the Metarhizium brunneum Petch. (Ascomycota: Hypocreales) strain EAMa 01/58-Su sprayed onto N. campestris population naturally present in the olive grove cover in Cordoba (Spain).

Methods

Experiments were conducted in early spring, and efficacy was evaluated using the Henderson-Tilton formula, as well as by analyzing changes in the relative population density of both nymphs and adults.

Results and discussion

The fungus was detected in the soil and endophytically in the natural cover throughout the 8 days monitoring period, in which the fungal treatment significantly reduced both the nymph and the adult populations. Notably, the efficacy of the fungal treatment was 100.0% and 85.0% for foams and adults in 2023, and 62.5% and 72.0% for foams and adults in 2024, respectively. Results indicate a significant reduction in the population density of both vector developmental stages, highlighting the potential of this fungal strain for managing X. fastidiosa vectors in olive cover crops.

A phloem-limited RNA phytovirus infection does not positively modulate vector preference and fitness in primary and alternate hosts

Colonizing aphids play an important role in the transmission of RNA phytoviruses in the family Solemoviridae. According to "host manipulation hypothesis," phloem limited and persistently transmitted Solemoviridae viruses modulate host physiology that positively affects vector behavior and fitness and facilitates virus spread. However, it is unclear if virus-modulated host effects on vectors across pathosystems involving Solemoviridae members are always positive. Cotton leafroll dwarf virus (CLRDV) is a recently introduced Solemoviridae member in the United States, and it is transmitted by the cotton aphid (Aphis gossypii). Effects of CLRDV infection on vector behavior and fitness were evaluated on its primary host plant, cotton (Gossypium hirsutum), and an alternate host plant, hibiscus (Hibiscus acetosella). In this study, changes to viruliferous and non-viruliferous aphid preference and aphid fitness on virus-infected and non-infected hosts were examined. In contrast to the hypothesized preference of non-viruliferous aphids for infected plants and vice-versa, both viruliferous and non-viruliferous A. gossypii preferred non-infected cotton and hibiscus plants over CLRDV-infected plants. This suggested that the preference of non-viruliferous vectors to non-infected plants might negatively impact virus acquisition, whereas the preference of viruliferous vectors toward non-infected plants could positively facilitate virus inoculation. The total fecundity and intrinsic rate of increase of aphids were higher on non-infected plants compared with CLRDV-infected plants. The lack of enhanced fitness benefits on CLRDV-infected hosts also could negatively impact virus spread. Overall, this study suggested that "host manipulation hypothesis" favoring vector attraction and enhanced fitness on infected plants does not apply to all pathosystems involving Solemoviridae members.

The diversity, pathogenic spectrum, and ecological significance of arthropod viruses

Research on arthropod viruses initially focused on those associated with diseases in vertebrates, particularly humans, as well as in plants of economic importance. However, the more recent deployment of metatranscriptomic sequencing of diverse arthropod species has facilitated the discovery of a multitude of novel arthropod viruses, in turn revealing that pathogenic viruses represent only a small component of the arthropod virome. In addition, arthropods may play a pivotal role in viral evolution and ecological dynamics, and have the potential to act as reservoirs for pathogens affecting vertebrates or plants. Due to active interactions between arthropod populations and diverse organisms - including fungi, plants, vertebrates, and even other arthropods in both aquatic and terrestrial ecosystems - there is an increased risk of the spillover of arthropod viruses to other organisms, including mammals. Herein, we review our current understanding of the diversity and ecology of arthropod viruses. We outline what is known about pathogenic arthropod viruses in diverse host types and emphasize the unique niche of arthropods as the source of emerging viral infectious diseases. Finally, we describe the evolutionary interactions between arthropod viruses and their hosts in ecosystems, at the same time highlighting their ecological significance with respect to regulating host populations.

Invasion of the four kingdoms: the parasite journey across plant and non-plant hosts

Parasites have a rich and long natural history among biological entities, and it has been suggested that parasites are one of the most significant factors in the evolution of their hosts. However, it has been emphasized less frequently how co-evolution has undoubtedly also shaped the paths of parasites. It may seem safe to assume that specific differences among the array of potential hosts for particular parasites have restricted and diversified their evolutionary pathways and strategies for survival. Nevertheless, if one looks closely enough at host and parasite, one finds commonalities, both in terms of host defences and parasite strategies to out-manoeuvre them. While such analyses have been the source of numerous reviews, they are generally limited to interactions between, at most, one kingdom of parasite with two kingdoms of host (e.g. similarities in animal and plant host responses against fungi). With the aim of extending this view, we herein critically evaluate the similarities and differences across all four eukaryotic host kingdoms (plants, animals, fungi, and protists) and their parasites. In doing so, we show that hosts tend to share common strategies for defence, including both physical and behavioural barriers, and highly evolved immune responses, in particular innate immunity. Parasites have, similarly, evolved convergent strategies to counter these defences, including mechanisms of active penetration, and evading the host's innate and/or adaptive immune responses. Moreover, just as hosts have evolved behaviours to avoid parasites, many parasites have adaptations to manipulate host phenotype, physiologically, reproductively, and in terms of behaviour. Many of these strategies overlap in the host and parasite, even across wide phylogenetic expanses. That said, specific differences in host physiology and immune responses often necessitate different adaptations for parasites exploiting fundamentally different hosts. Taken together, this review facilitates hypothesis-driven investigations of parasite-host interactions that transcend the traditional kingdom-based research fields.

Enhanced association of whitefly-begomovirus competence with plant-mediated mutualism

BACKGROUND

Vector-borne viruses often manipulate plant defenses against insect vectors, thereby impacting vector population dynamics and in turn virus spread. However, the factors regulating the outcome of insect vector-virus-plant tripartite interactions, such as the feature of virus-vector combinations, are understudied.

RESULTS

Using eight whitefly (Bemisia tabaci)-begomovirus combinations exhibiting different degrees of competence, namely virus transmission efficiency, we examined the association between whitefly-begomovirus competence and plant-mediated mutualism. We found that three begomoviruses, tomato yellow leaf curl virus (TYLCV), cotton leaf curl Multan virus (CLCuMuV) and Sri Lankan cassava mosaic virus (SLCMV), can effectively infect but cause distinct symptoms in tobacco (Nicotiana tabacum) plants. Although the efficient vectors Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) whiteflies performed significantly better on TYLCV-infected plants than on control plants, the less-efficient vector Asia II 1 performed similarly on TYLCV-infected and uninfected plants. CLCuMuV infection of plants significantly enhanced the performance of the efficient vector Asia II 1, whereas the performance of the inefficient vector MEAM1 was unaffected by the virus infection status of the plants. SLCMV infection of plants significantly increased the survival and fecundity of the efficient vector Asia II 1, but did not affect the performance of the poorer vectors MEAM1 and MED.

CONCLUSION

Combined analysis of our data and case studies from the literature indicates that plant-mediated mutualism between whiteflies and the begomoviruses they transmit is more likely to occur in competent combinations. Our findings shed novel light on the ecological principles governing the variations in insect vector-virus-plant tripartite interactions. © 2024 Society of Chemical Industry.

Rice stripe mosaic virus M protein antagonizes G-protein-induced antiviral autophagy in insect vectors

In the field, 80% of plant viruses are transmitted by insect vectors. When ingested by a sap-sucking insect such as Recilia dorsalis, persistently transmitted viruses such as rice stripe mosaic virus (RSMV) infect the gut epithelium and eventually pass to the salivary glands where they will be transmitted to the next rice (Oryza sativa) plant. To efficiently exploit insect vectors for transmission, plant viruses must overcome various immune mechanisms within the vectors, including autophagy. However, understanding how plant viruses overcome insect autophagic defenses remains limited. In this study, we provide evidence that infection with RSMV triggers an autophagic antiviral response in leafhopper cells. In this response, the G protein of RSMV binds to a leafhopper AMP-activated protein kinase (AMPK), leading to enhanced phosphorylation of Beclin-1 (BECN1), thereby inducing autophagy. Knockdown of AMPK and genes encoding members of the phosphoinositide 3-kinase (PI3K) complex composed of the autophagy-related protein 14 (ATG14), BECN1, and vacuolar protein sorting 34 (VPS34) facilitated viral infection in leafhoppers. To suppress leafhopper-induced autophagy, RSMV M protein specifically interacts with ATG14, resulting in the disintegration of PI3K complexes. This leads to reduced phosphatidylinositol-3-phosphate content and thus inhibits the G-protein- induced autophagy. Our study sheds light on the mechanism by which this rice virus evades insect autophagy antiviral defenses.

The Turnip Yellows Virus Capsid Protein Promotes Access of Its Main Aphid Vector Myzus persicae to Phloem Tissues

Many plant viruses modify the phenotype of their hosts, which may influence the behaviour of their vectors and facilitate transmission. Among them is the turnip yellows virus (TuYV), which can modify the orientation, feeding, and performance of its main aphid vector, Myzus persicae. However, the virus factors driving these mechanisms have not been elucidated. In this study, we compared the feeding behaviour and fecundity of aphids on TuYV-infected and transgenic Arabidopsis thaliana expressing individual TuYV proteins (CP, RT and P0) to define the role of these proteins in aphid-plant interactions. Aphids on TuYV-infected plants had shorter pathway phases and ingested phloem sap for longer times, which is expected to promote the acquisition of the phloem-limited TuYV. No change in aphid fecundity was observed on TuYV-infected plants. The transmission-conducive feeding behaviour changes could be fully reproduced by phloem-specific expression of the capsid protein (CP) in transgenic plants, whereas expression of P0 had minor and RT had no effects on aphid feeding behaviour. We then carried out a metabolomic analysis to determine plant compounds that could be involved in the modification of the aphid behaviour. A few metabolites were specific for TuYV-infected or CP-transgenic A. thaliana, and are good candidates for inducing behavioural changes.

Detecting novel plant pathogen threats to food system security by integrating the Plant Reactome and remote sensing

Plant diseases constantly threaten crops and food systems, while global connectivity further increases the risks of spreading existing and exotic pathogens. Here, we first explore how an integrative approach involving plant pathway knowledgegraphs, differential gene expression data, and biochemical data informing Raman spectroscopy could be used to detect plant pathways responding to pathogen attacks. The Plant Reactome (https://plantreactome.gramene.org) demonstrates the potential to synthesize knowledgegraphs depicting plant-pathogen interactions, leveraging availability of publicly available OMIC data sets related to major diseases of rice and maize. Plant pathway signatures may then guide the development of drone and satellite remote-sensing methods for early monitoring of disease outbreaks across farms and landscapes. A review of current proximal- and remote-sensing technology demonstrates the potential for actionable early pathogen detection. We furthermore identify knowledge gaps that need to be addressed for developing these tools as components of effective strategies for safeguarding global food security against current and emerging pathogens.

The plant growth-promoting rhizobacterium Azospirillum brasilense reduces symptoms and aphid population growth on wheat plants infected with barley yellow dwarf virus

There is increasing interest in the potential of plant growth-promoting rhizobacteria (PGPR) in agriculture to improve plant quality and control pests and diseases. Emerging evidence indicates that some PGPR can influence interactions between plants and their pathogens, while less work has explored whether PGPR may also influence interactions between plants and arthropod vectors. We address this issue in a major agricultural pathosystem involving wheat infection by barley yellow dwarf virus (BYDV), the most economically important aphid-transmitted viral disease of cereal crops. We found that plant association with the PGPR Azospirillum brasilense mitigated both viral effects on plant growth and population growth of the BYDV aphid vector, Rhopalosiphum padi. Although effects varied across A. brasilense strains, PGPR treatments that attenuated virus effects were also associated with reduced induction of salicylic acid in response to infection, suggesting PGPR inoculation may induce systemic resistance against BYDV. These findings suggest that PGPR may have significant capacity for application in the sustainable management of crop growth. However, further investigation of the complex interactions among PGPR, plants, pathogens and their vectors is needed to better understand this potential.

An insect virus differentially alters gene expression among life stages of an insect vector and enhances bacterial phytopathogen transmission

Diaphorina citri transmits Candidatus Liberibacter asiaticus (CLas) between citrus plants which causes the expression of huanglongbing disease in citrus. D. citri flavi-like virus (DcFLV) co-occurs intracellularly with CLas in D. citri populations in the field. However, the impact(s) of DcFLV presence on the insect vector and its interaction with the CLas phytopathogen remain unclear. We compared CLas acquisition and transmission efficiencies as well as transcriptomic expression between viruliferous and non-viruliferous psyllids at multiple life stages. Viruliferous nymphs acquired higher titers of CLas than non-viruliferous nymphs, whereas viruliferous adults acquired less CLas than those without virus. The presence of DcFLV increased the transmission of CLas by both nymphs and adults. Furthermore, RNA-seq and functional gene expression analyses revealed that endoplasmic reticulum stress-, autophagy-, and defense-related genes were significantly upregulated in viruliferous adult psyllids, whereas most of these genes were downregulated in viruliferous nymphs. Our work demonstrates that DcFLV differentially modulates various cellular and physiological functions in D. citri in a life stage-dependent manner and promotes the acquisition of CLas at the nymphal stage and transmission of the pathogen at the adult stage of the vector. Collectively, our results suggest that D. citri vectors with DcFLV exhibit greater pathogen transmission efficiency than those without virus.

IMPORTANCE

Huanglongbing (HLB), caused by fastidious bacteria from three Candidatus Liberibacter species, is the most damaging disease impacting the citrus industry worldwide. Spread by the Asian citrus psyllid (Diaphorina citri) in Asia and the Americas, HLB causes substantial financial losses, and has reduced citrus production in Florida by more than 90%. Although there are ongoing efforts to limit spread of the disease, effective HLB management remains elusive. Suppressing vector populations and decreasing CLas transmission are the two strategies that need to be urgently improved. Recently, a D. citri flavi-like virus (DcFLV) was characterized within its D. citri host, and it co-occurs intracellularly with CLas in psyllid populations. Here, we show that viruliferous nymphs exhibit higher CLas acquisition than non-viruliferous nymphs. Furthermore, both viruliferous adults and nymphs exhibit increased CLas transmission efficiency. We suggest the possibility of manipulating DcFLV in D. citri populations to reduce CLas transmission for HLB disease management.

A phytoplasma effector suppresses insect melanization immune response to promote pathogen persistent transmission

Insect melanization triggered by the conversion of prophenoloxidase to active phenoloxidase via serine proteases (SPs) is an important immediate immune response. However, how phytoplasmas evade this immune response to promote their propagation in insect vectors remains unknown. Here, we demonstrate that infection of leafhopper vectors with rice orange leaf phytoplasma (ROLP) activates the mild melanization response in hemolymph. ROLP-encoded effector protein SRP1 is highly expressed in leafhopper hemolymph, where it competitively binds to SP2, thereby inhibiting SP2-mediated cleavage of prophenoloxidase into active phenoloxidase. Consequently, microinjection of SRP1 effectively suppresses the melanization response and enhances ROLP propagation. The histidine residue at position 23 of SRP1 is essential for SRP1-SP2 interaction, and the mutation of this position abolishes its ability to inhibit such SP2-meidated cleavage, ultimately promoting melanization response and inhibiting ROLP propagation. Our findings provide insights into how phytoplasmas antagonize insect melanization response to facilitate their persistent transmission.

Endosymbionts as hidden players in tripartite pathosystem of interactions and potential candidates for sustainable viral disease management

The convoluted relationships between plants, viruses, and arthropod vectors housing bacterial endosymbionts are pivotal in the spread of harmful plant viral diseases. Endosymbionts play key roles in: manipulating host responses, influencing insect resistance to pesticides, shaping insect evolution, and bolstering virus acquisition, retention, and transmission. This interplay presents an innovative approach for developing sustainable strategies to manage plant diseases. Recent progress in targeting specific endosymbionts through genetic modifications, biotechnological advancements, and RNA interference shows potential for curbing viral spread and disease progression. Additionally, employing synthetic biology techniques like CRISPR/Cas9 to engineer endosymbionts and disrupt crucial interactions necessary for viral transmission in arthropod vectors holds promise for effective control measures. In this review, these obligate and facultative bacterial cruxes have been discussed to elaborate on their mechanistic involvement in the regulation and/or inhibition of tripartite pathways of interactions. Furthermore, we provide an in-depth understanding of endosymbionts' synergistic and antagonistic effects on: insect biology, plant immunity, and virus acquisition and transmission. Finally, we point out open questions for future research and provide research directions concerning the deployment of genetically engineered symbionts to affect plant-virus-vector interactions for sustainable disease management. By addressing existing knowledge gaps and charting future research paths, a deeper comprehension of the role of endosymbionts in plant-virus-vector interactions can pave the way for innovative and successful disease management strategies. The exploration of antiviral therapies, paratransgenesis, and pathogen-blocking tactics using engineered endosymbionts introduces pioneering solutions for lessening the impact of plant viral diseases and green pest management.

Citrus Yellow Vein Clearing Virus Infection in Lemon Influences Host Preference of the Citrus Whitefly by Affecting the Host Metabolite Composition

Plant viruses have been known to alter host metabolites that influence the attraction of insect vectors. Our study investigated whether Citrus yellow vein clearing virus (CYVCV) infection influences vector attractiveness, focusing on the citrus whitefly, Dialeurodes citri (Ashmead). Free choice assays showed that citrus whiteflies exhibited a preference for settling on CYVCV-infected lemon plants versus healthy control plants. Using chromatography techniques, we found that the levels of sugars were similar in leaves and stems of both plant groups, while the contents of several amino acids in leaf or stem samples and non-volatile phenolic compounds in the leaf samples of CYVCV-infected and healthy plants differ drastically. In addition, volatile terpenes/terpenoids decreased significantly in virus-infected plants compared to healthy controls. Several of the identified volatile compounds such as α-phellandrene, α-terpinolene, p-cymene, linalool, and citral are known for their whitefly repellent properties. Further Y-tube olfactometer bioassays revealed that emissions of volatile organic compounds (VOCs) from infected plants attracted more citrus whiteflies, but not alate spirea aphids, Aphis spiraecola Patch, than those from healthy plants, suggesting that the VOCs released from CYVCV-infected lemon plants may specifically affect citrus whiteflies. Therefore, we suggest that, in addition to the visual cue of yellow vein symptoms, the preference of citrus whiteflies that settled on CYVCV-infected lemon plants was attributed to a reduction in the levels of repellent volatile compounds.

Manipulation of juvenile hormone signaling by the fire blight pathogen Erwinia amylovora mediates fecundity enhancement of pear psylla

BACKGROUND

In nature, plant pathogens often rely on insect vectors for transmission. Through long-term evolution, plant pathogens and insect vectors have established a mutually beneficial symbiotic relationship. Fire blight, caused by the Gram-negative bacterium Erwinia amylovora (Eam), poses a significant global threat to apple and pear production due to its rapid dissemination among host plants of the Rosaceae family. Despite evidence of E. amylovora transmission by various insects, the association between this pathogen and the pear psylla Cacopsylla chinensis, a common vector insect in pear orchards, remains unclear.

RESULTS

Sampling investigations and qRT-PCR results revealed that C. chinensis, from 11 pear orchards severely affected by fire blight disease in Xinjiang of China, harbored varying levels of this pathogen. Eam-positive females exhibited significantly higher fecundity compared to Eam-negative individuals, displaying accelerated ovarian development and a notable increase in egg production. Further RNAi results revealed that juvenile hormone (JH) receptor methoprene-tolerant (CcMet) and a crucial downstream gene Krüppel-homologue 1 (CcKr-h1) mediated the fecundity improvement of C. chinensis induced by Eam. Additionally, miR-2b, which targets CcKr-h1, was identified as being involved in Eam-induced fecundity enhancement in C. chinensis.

CONCLUSION

This study unveils, for the first time, that Eam colonize and amplify the fecundity of C. chinensis females. Host miR-2b targets CcKr-h1 of the JH signaling pathway to regulate the heightened fecundity of C. chinensis induced by Eam. These findings not only broaden our understanding of the interaction between plant pathogens and insect vectors, but also provide novel strategies for managing fire blight and pear psylla. © 2024 Society of Chemical Industry.

Enhancing aphid resistance in horticultural crops: a breeding prospective

Increasing agricultural losses caused by insect infestations are a significant problem, so it is important to generate pest-resistant crop varieties to address this issue. Several reviews have examined aphid-plant interactions from an entomological perspective. However, few have specifically focused on plant resistance mechanisms to aphids and their applications in breeding for aphid resistance. In this review, we first outline the types of resistance to aphids in plants, namely antixenosis, tolerance (cell wall lignification, resistance proteins), and antibiosis, and we discuss strategies based on each of these resistance mechanisms to generate plant varieties with improved resistance. We then outline research on the complex interactions amongst plants, viruses, and aphids, and discuss how aspects of these interactions can be exploited to improve aphid resistance. A deeper understanding of the epigenetic mechanisms related to induced resistance, i.e. the phenomenon where plants become more resistant to a stress they have encountered previously, may allow for its exploitation in breeding for aphid resistance. Wild relatives of crop plants serve as important sources of resistance traits. Genes related to these traits can be introduced into cultivated crop varieties by breeding or genetic modification, and de novo domestication of wild varieties can be used to exploit multiple excellent characteristics, including aphid resistance. Finally, we discuss the use of molecular design breeding, genomic data, and gene editing to generate new aphid-resistant, high-quality crop varieties.

The conserved role of miR-2 and novel miR-109 in the increase in fecundity of Diaphorina citri induced by symbiotic bacteria and pathogenic fungi

Infection with pathogens can increase the fecundity and other fitness-related traits of insect vectors for their own advantage. Our previous research has reported the pivotal role of DcKr-h1 in the fecundity improvement of Diaphorina citri induced by the bacterium, "Candidatus Liberibacter asiaticus" (CLas), and the fungus, Cordyceps fumosorosea (Cf). However, the posttranscriptional regulation of this process remains poorly understood. Given the significance of miRNAs in gene regulation, we delved into their roles in shaping phenotypes and their underlying molecular mechanisms. Our results indicated that two miRNAs, miR-2 and novel-miR-109, jointly inhibited DcKr-h1 expression by binding to its 3' untranslated region (UTR). In the D. citri-CLas interaction, the expression levels of miR-2 and novel-miR-109 in the ovaries of CLas-positive psyllids were lower compared to CLas-negative individuals. Overexpression of miR-2 or novel-miR-109 significantly decreased fecundity and CLas titer in ovaries and caused reproductive defects reminiscent of DcKr-h1 knockdown. Similarly, in the D. citri-Cf interaction, the levels of miR-2 and novel-miR-109 markedly decreased in the ovaries. Upregulation of miR-2 or novel-miR-109 also resulted in reduced fecundity and ovary defects similar to those caused by DcKr-h1 silencing. Moreover, feeding antagomir-2 or antagomir-109 partially rescued the defective phenotypes caused by DcKr-h1 silencing in both model systems, and miR-2 and novel-miR-109 were repressed by juvenile hormone (JH) and regulated the genes associated with egg development. This study shows a conserved regulatory mechanism, whereby JH suppresses the expression of miR-2 and novel-miR-109 which, together with JH-induced transcription of DcKr-h1, increases female fecundity induced by both symbiotic bacteria and pathogenic fungi.

IMPORTANCE

Infection with pathogens can increase the fecundity and other fitness-related traits of insect vectors for their own advantage. Our previous research has reported that DcKr-h1 plays a critical role in the increase in fecundity of Diaphorina citri induced by the bacterium, "Candidatus Liberibacter asiaticus" (CLas) and the fungus, Cordyceps fumosorosea (Cf). However, the posttranscriptional regulation of this process remains poorly understood. Given the significance of miRNAs in gene regulation, we delved into their roles in shaping phenotypes and their underlying molecular mechanisms. Our results indicated that two miRNAs, miR-2 and novel-miR-109, jointly inhibited DcKr-h1 expression by binding to its 3' untranslated region (UTR). In both D. citri-CLas and D. citri-Cf interactions, the increased juvenile hormone (JH) titer and reduced abundance of miR-2 and novel-miR-109 ensure high levels of DcKr-h1 expression, consequently stimulating ovarian development and enhancing fecundity. These observations provide evidence that miR-2 and miR-109 are crucial players in the JH-dependent increase in fecundity in psyllids induced by infection with different pathogens.

Viral proteins resolve the virus-vector conundrum during hemipteran-mediated transmission by subverting salicylic acid signaling pathway

Hemipteran insects transmit viruses when infesting plants, during which vectors activate salicylic acid (SA)-regulated antiviral defenses. How vector-borne plant viruses circumvent these antiviral defenses is largely unexplored. During co-infections of begomoviruses and betasatellites in plants, betasatellite-encoded βC1 proteins interfere with SA signaling and reduce the activation of antiviral resistance. βC1 inhibits SA-induced degradation of NbNPR3 (Nicotiana benthamiana nonexpressor of pathogenesis-related genes 3), a negative regulator of SA signaling. βC1 does not bind directly to NbNPR3, but regulates NbNPR3 degradation via heat shock protein 90s (NbHSP90s). NbHSP90s bind to both NbNPR3 and βC1 and suppress SA signaling. This viral success strategy appears to be conserved as it is also documented for viral proteins encoded by two aphid-borne viruses. Our findings reveal an exquisite mechanism that facilitates the persistence of vector-borne plant viruses and provide important insights into the intricacies of the virus life cycle.

Herbivore-induced volatiles reduce the susceptibility of neighboring tomato plants to transmission of a whitefly-borne begomovirus

Plant viruses exist in a broader ecological community that includes non-vector herbivores that can impact vector abundance, behavior, and virus transmission within shared host plants. However, little is known about the effects of non-vector herbivore infestation on virus transmission by vector insects on neighboring plants through inter-plant airborne chemicals. In this study, we investigated how volatiles emitted from tomato plants infested with the two-spotted spider mite (Tetranychus urticae) affect the infection of neighboring plants by tomato yellow leaf curl virus (TYLCV) transmitted by whitefly (Bemisia tabaci). Exposure of neighboring tomato plants to volatiles released from T. urticae-infested tomato plants reduced subsequent herbivory as well as TYLCV transmission and infection, and the jasmonic acid signaling pathway was essential for generation of the inter-plant defense signals. We also demonstrated that (E)-β-ocimene and methyl salicylic acid were two volatiles induced by T. urticae that synergistically attenuated TYLCV transmission and infection in tomato. Thus, our findings suggest that plant-plant communication via volatiles likely represents a widespread defensive mechanism that substantially contributes to plant fitness. Understanding such phenomena may help us to predict the occurrence and epidemics of multiple herbivores and viruses in agroecosystems, and ultimately to manage pest and virus outbreaks.

Leafhopper salivary carboxylesterase suppresses JA-Ile synthesis to facilitate initial arbovirus transmission in rice phloem

Plant jasmonoyl-L-isoleucine (JA-Ile) is a major defense signal against insect feeding, but whether or how insect salivary effectors suppress JA-Ile synthesis and thus facilitate viral transmission in the plant phloem remains elusive. Insect carboxylesterases (CarEs) are the third major family of detoxification enzymes. Here, we identify a new leafhopper CarE, CarE10, that is specifically expressed in salivary glands and is secreted into the rice phloem as a saliva component. Leafhopper CarE10 directly binds to rice jasmonate resistant 1 (JAR1) and promotes its degradation by the proteasome system. Moreover, the direct association of CarE10 with JAR1 clearly impairs JAR1 enzyme activity for conversion of JA to JA-Ile in an in vitro JA-Ile synthesis system. A devastating rice reovirus activates and promotes the co-secretion of virions and CarE10 via virus-induced vesicles into the saliva-storing salivary cavities of the leafhopper vector and ultimately into the rice phloem to establish initial infection. Furthermore, a virus-mediated increase in CarE10 secretion or overexpression of CarE10 in transgenic rice plants causes reduced levels of JAR1 and thus suppresses JA-Ile synthesis, promoting host attractiveness to insect vectors and facilitating initial viral transmission. Our findings provide insight into how the insect salivary protein CarE10 suppresses host JA-Ile synthesis to promote initial virus transmission in the rice phloem.

MCMV-infected maize attracts its insect vector Frankliniella occidentalis by inducing β-myrcene

Maize lethal necrosis is attributed to the accumulation of maize chlorotic mottle virus (MCMV), an invasive virus transmitted by insect vectors. The western flower thrips (WFT) can shift host to maize, thus promoting the spread of MCMV. However, our understanding of the characteristics and interactions involved in the transmission of MCMV is still limited. This study finds that non-viruliferous WFTs showed a 57.56% higher preference for MCMV-infected maize plants compared to healthy maize plants, while viruliferous WFTs showed a 53.70% higher preference for healthy maize plants compared to MCMV-infected maize plants. We also show for the first time that both adults and larvae of WFT could successfully acquire MCMV after 1 min of acquisition access period (AAP), and after 48 h of AAP, WFT could transmit MCMV in an inoculation access period of 1 h without a latent period. Both adults and larvae of WFT can transmit MCMV for up to 2 days. Furthermore, the decreasing number of viruliferous WFTs and transmission rates as time progressed, together with the transcriptomic evidence, collectively suggest that WFTs transmit MCMV in a semi-persistent method, a mode of transmission requiring minutes to several hours for acquisition access and having a retention time of several hours to a few days. Additionally, β-myrcene can attract WFTs significantly and is detected in Nicotiana benthamiana plants transiently expressing MCMV CP (coat protein), which is consistent with results in MCMV-infected maize plants through the metabolomic profiling and the preference analyses of WFT. Therefore, this study demonstrates the indirect interaction between MCMV and WFT by inducing maize to synthesize β-myrcene to attract insect vectors. The exploration of specific interactions between MCMV and WFT could help to expand the mechanism studies of virus-vector-host plant interaction and put forward a new insight for the combined control of MCMV and WFT through the manipulation of plant volatiles and key insect genes.

Tomato Spotted Wilt Virus Suppresses the Antiviral Response of the Insect Vector, Frankliniella occidentalis, by Elevating an Immunosuppressive C18 Oxylipin Level Using Its Virulent Factor, NSs

Orthotospovirus tomatomaculae (tomato spotted wilt virus, TSWV) is transmitted by the western flower thrips, Frankliniella occidentalis. Epoxyoctadecamonoenoic acids (EpOMEs) function as immune-suppressive factors, particularly in insects infected by viral pathogens. These oxylipins are produced by cytochrome P450 monooxygenases (CYPs) and are degraded by soluble epoxide hydrolase (sEH). In this study, we tested the hypothesis that TSWV modulates the EpOME level in the thrips to suppress antiviral responses and enhance its replication. TSWV infection significantly elevated both 9,10-EpOME and 12,13-EpOME levels. Following TSWV infection, the larvae displayed apoptosis in the midgut along with the upregulated expression of four caspase genes. However, the addition of EpOME to the viral treatment notably reduced apoptosis and downregulated caspase gene expressions, which led to a marked increase in TSWV titers. The CYP and sEH genes of F. occidentalis were identified, and their expression manipulation using RNA interference (RNAi) treatments led to significant alternations in the insect's immune responses and TSWV viral titers. To ascertain which viral factor influences the host EpOME levels, specialized RNAi treatments targeting genes encoded by TSWV were administered to larvae infected with TSWV. These treatments demonstrated that NSS expression is pivotal in manipulating the genes involved in EpOME metabolism. These results indicate that NSs of TSWV are crucially linked with the elevation of host insect EpOME levels and play a key role in suppressing the antiviral responses of F. occidentalis.

A plant virus differentially alters DNA methylation in two cryptic species of a hemipteran vector

Epigenetic patterns including DNA methylation are known to vary between distantly related species, but it is not clear how these patterns differ at an intraspecific level. The sweetpotato whitefly, Bemisia tabaci (Gennadius) (Aleyrodidae; Hemiptera), encompasses several cryptic species. These cryptic species possess highly similar genomes but exhibit substantial biological and physiological differences. B. tabaci cryptic species are invasive, highly polyphagous, and transmit an array of plant infecting single stranded DNA viruses (ssDNA) -begomoviruses. In this study, DNA methylation patterns around genes and genomic features of two prominent B. tabaci cryptic species were investigated following acquisition of a monopartite ssDNA virus -tomato yellow curl virus. The cryptic species investigated included: B (also known as Middle East Asia Minor 1) and Q (also known as Mediterranean). Genomic features, such as promoters, gene bodies, and transposable elements were assessed for methylation levels in both B and Q cryptic species. Despite overall similar trends, both cryptic species showed differences in methylation levels between these genomic features. Virus induced differentially methylated regions were associated with predominantly distinct genes in B and Q cryptic species. All differentially methylated regions were assessed for differential gene expression and alternative splicing events with and without virus acquisition. DNA methylation levels were found to have a negative correlation with differential gene expression in both B and Q cryptic species. The differentially expressed genes were further grouped into hyper- and hypomethylated clusters. These clusters included genes with implications for virus-vector interactions including immune functions and xenobiotics' detoxification. The observed DNA methylation pattern differences within each cryptic species could, in part, explain some of the biological and physiological differences between them.

Effects of climate change and human activities on vector-borne diseases

Vector-borne diseases are transmitted by haematophagous arthropods (for example, mosquitoes, ticks and sandflies) to humans and wild and domestic animals, with the largest burden on global public health disproportionately affecting people in tropical and subtropical areas. Because vectors are ectothermic, climate and weather alterations (for example, temperature, rainfall and humidity) can affect their reproduction, survival, geographic distribution and, consequently, ability to transmit pathogens. However, the effects of climate change on vector-borne diseases can be multifaceted and complex, sometimes with ambiguous consequences. In this Review, we discuss the potential effects of climate change, weather and other anthropogenic factors, including land use, human mobility and behaviour, as possible contributors to the redistribution of vectors and spread of vector-borne diseases worldwide.

Effector enrichment by Candidatus Liberibacter promotes Diaphorina citri feeding via Jasmonic acid pathway suppression

BACKGROUND

Citrus huanglongbing (HLB) is a devastating disease caused by Candidatus Liberibacter asiaticus (CLas) that affects the citrus industry. In nature, CLas relies primarily on Diaphorina citri Kuwayama as its vector for dissemination. After D. citri ingests CLas-infected citrus, the pathogen infiltrates the insect's body, where it thrives, reproduces, and exerts regulatory control over the growth and metabolism of D. citri. Previous studies have shown that CLas alters the composition of proteins in the saliva of D. citri, but the functions of these proteins remain largely unknown.

RESULTS

In this study, we detected two proteins (DcitSGP1 and DcitSGP3) with high expression levels in CLas-infected D. citri. Quantitative PCR and Western blotting analysis showed that the two proteins were highly expressed in the salivary glands and delivered into the host plant during feeding. Silencing the two genes significantly decreased the survival rate for D. citri, reduced phloem nutrition sucking and promoted jasmonic acid (JA) defenses in citrus. By contrast, after overexpressing the two genes in citrus, the expression levels of JA pathway-associated genes decreased.

CONCLUSION

Our results suggest that CLas can indirectly suppress the defenses of citrus and support feeding by D. citri via increasing the levels of effectors in the insect's saliva. This discovery facilitates further research into the interaction between insect vectors and pathogens. © 2024 Society of Chemical Industry.

Tomato yellow leaf curl virus manipulates Bemisia tabaci, MEAM1 both directly and indirectly through changes in visual and volatile cues

The sweetpotato whitefly, Bemisia tabaci MEAM1, is one of the most devastating pests of row-crop vegetables worldwide, damaging crops directly through feeding and indirectly through the transmission of many different viruses, including the geminivirus Tomato yellow leaf curl virus (TYLCV). Y-tube olfactometer tests were conducted at different stages of TYLCV infection in tomatoes to understand how TYLCV affects B. tabaci behavior. We also recorded changes in tomato hosts' color and volatile profiles using color spectrophotometry and gas chromatography-mass spectrometry (GC-MS). We found that the infection status of B. tabaci and the infection stage of TYLCV influenced host selection, with uninfected whiteflies showing a preference for TYLCV-infected hosts, especially during the late stages of infection. Viruliferous B. tabaci attraction to visual targets significantly differed from non-viruliferous B. tabaci. Late-stage infected hosts had larger surface areas reflecting yellow-green wavelengths and higher emissions of methyl salicylate in their volatile profiles. These findings shed new light on several critical mechanisms involved in the viral manipulation of an insect vector and its economically important host.

Plant growth-promoting rhizobacteria modulate induced corn defense against Spodoptera litura (Lepidoptera: Noctuidae)

Common cutworm, Spodoptera litura is an important pest of corn causing significant crop yield loss. Synthetic insecticides have mostly been used to combat this pest, raising human and environmental health concerns. Plant growth-promoting rhizobacteria (PGPR) could compensate for or augment the harmful effects of agrochemicals. Herein, we aimed to assess whether PGPR-induced defenses in corn plants impact the host-plant selection behavior of S. litura. Headspace volatile organic compounds were analyzed using gas chromatography-mass spectrometry. Larvae fed inoculated corn exhibited lower weights and relative growth rate than noninoculated plants. Under choice experiments, PGPR-treated plants significantly reduced percentage leaf damage area and oviposition rate compared to untreated plants. Volatile organic compound ratio emission varied significantly between control and PGPR treatments, which, in part, explains feeding and oviposition deterrence in PGPR-treated plants. The results demonstrate that PGPR inoculation can enhance corn resistance to S. litura, making it a promising candidate for crop protection strategies.

Land use around influences the entomological community in lettuce horticultural systems

The complexity of the agroecosystem can also be assessed by the different land uses in the system and the surroundings, being a relevant way to assess the heterogeneity of the landscape and the effects on the community of interest, in this case, entomofauna. Thus, the objective of this work is to verify how the use of soil in the surroundings of Chilean lettuce horticultural systems, in the Coquimbo Region, alters the entomological community of the crop. Insect sampling was conducted (February 2021 to March 2022) using yellow pan traps. Two sites will be sampled on each of the seven studied lettuce crops. Land use and land cover classes were defined: Forests, water bodies, shrub vegetation, grasslands, barren lands, impermeable surfaces, and urban areas. After land use and land cover classification, buffers of 500 to 5,000 m were created around each data collection point. For data analysis, the percentages of land use of different classes were compared with the ecological attributes: Abundance of insects, abundance of insect pests, richness of entomological families and types of oral apparatus (licker-sucker, mandible, picker-sucker, and sucker). Land uses at different distances from horticultural systems affected the entomological community.

Adipokinetic hormone signaling mediates the enhanced fecundity of Diaphorina citri infected by 'Candidatus Liberibacter asiaticus'

Diaphorina citri serves as the primary vector for 'Candidatus Liberibacter asiaticus (CLas),' the bacterium associated with the severe Asian form of huanglongbing. CLas-positive D. citri are more fecund than their CLas-negative counterparts and require extra energy expenditure. Therefore, understanding the molecular mechanisms linking metabolism and reproduction is of particular importance. In this study, we found adipokinetic hormone (DcAKH) and its receptor (DcAKHR) were essential for increasing lipid metabolism and fecundity in response to CLas infection in D. citri. Knockdown of DcAKH and DcAKHR not only resulted in the accumulation of triacylglycerol and a decline of glycogen, but also significantly decreased fecundity and CLas titer in ovaries. Combined in vivo and in vitro experiments showed that miR-34 suppresses DcAKHR expression by binding to its 3' untranslated region, whilst overexpression of miR-34 resulted in a decline of DcAKHR expression and CLas titer in ovaries and caused defects that mimicked DcAKHR knockdown phenotypes. Additionally, knockdown of DcAKH and DcAKHR significantly reduced juvenile hormone (JH) titer and JH signaling pathway genes in fat bodies and ovaries, including the JH receptor, methoprene-tolerant (DcMet), and the transcription factor, Krüppel homolog 1 (DcKr-h1), that acts downstream of it, as well as the egg development related genes vitellogenin 1-like (DcVg-1-like), vitellogenin A1-like (DcVg-A1-like) and the vitellogenin receptor (DcVgR). As a result, CLas hijacks AKH/AKHR-miR-34-JH signaling to improve D. citri lipid metabolism and fecundity, while simultaneously increasing the replication of CLas, suggesting a mutualistic interaction between CLas and D. citri ovaries.

Beet curly top virus affects vector biology: the first transcriptome analysis of the beet leafhopper

Curly top disease, caused by beet curly top virus (BCTV), is among the most serious viral diseases affecting sugar beets in western USA. The virus is exclusively transmitted by the beet leafhopper (BLH, Circulifer tenellus) in a circulative and non-propagative manner. Despite the growing knowledge on virus-vector interactions, our understanding of the molecular interactions between BCTV and BLH is hampered by limited information regarding the virus impact on the vector and the lack of genomic and transcriptomic resources for BLH. This study unveils the significant impact of BCTV on both the performance and transcriptome response of BLHs. Viruliferous BLHs had higher fecundity than non-viruliferous counterparts, which was evident by upregulation of differentially expressed transcripts (DETs) associated with development, viability and fertility of germline and embryos in viruliferous insects. Conversely, most DETs associated with muscle movement and locomotor activities were downregulated in viruliferous insects, implying potential behavioural modifications by BCTV. Additionally, a great proportion of DETs related to innate immunity and detoxification were upregulated in viruliferous insects. Viral infection also induced notable alterations in primary metabolisms, including energy metabolism, namely glucosidases, lipid digestion and transport, and protein degradation, along with other cellular functions, particularly in chromatin remodelling and DNA repair. This study represents the first comprehensive transcriptome analysis for BLH. The presented findings provide new insights into the multifaceted effects of viral infection on various biological processes in BLH, offering a foundation for future investigations into the complex virus-vector relationship and potential management strategies for curly top disease.

Novel insights into hotspots of insect vectors of GLRaV-3: Dynamics and global distribution

Grapevine leafroll-associated virus 3 (GLRaV-3) is the most prevalent and economically damaging virus in grapevines and is found on nearly all continents, except Antarctica. Ten mealybugs act as vector insects transmitting the GLRaV-3. Understanding the potential distribution range of vector insects under climate change is crucial for preventing and managing vector insects and controlling and delaying the spread of GLRaV-3. This study investigated the potential geographical range of insect vectors of GLRaV-3 worldwide using MaxEnt (maximum entropy) based on occurrence data under environmental variables. The potential distributions of these insects were projected for the 2030s, 2050s, 2070s, and 2090s under the three climate change scenarios. The results showed that the potential distribution range of most vector insects is concentrated in Southeastern North America, Europe, Asia, and Southeast Australia. Most vector insects contract their potential distribution ranges under climate-change conditions. The stacked model suggested that potential distribution hotspots of vector insects were present in Southeastern North America, Europe, Southeast Asia, and Southeast Australia. The potential distribution range of hotspots would shrink with climate change. These results provide important information for governmental decision-makers and farmers in developing control and management strategies against vector insects of GLRaV-3. They can also serve as references for studies on other insect vectors.

Reduced cold tolerance of viral-infected leafhoppers attenuates viral persistent epidemics

Most arthropod-borne viruses produce intermittent epidemics in infected plants. However, the underlying mechanisms of these epidemics are unclear. Here, we demonstrated that rice stripe mosaic virus (RSMV), a viral pathogen, significantly increases the mortality of its overwintering vector, the leafhopper species Recilia dorsalis. Cold-stress assays indicated that RSMV reduces the cold tolerance of leafhoppers, a process associated with the downregulation of leafhopper cuticular protein genes. An RSMV-derived small RNA (vsiR-t00355379) was found to facilitate the downregulation of a leafhopper endocuticle gene that is mainly expressed in the abdomen (named RdABD-5) and is conserved across dipteran species. The downregulation of RdABD-5 expression in R. dorsalis resulted in fewer and thinner endocuticle lamellae, leading to decreased cold tolerance. This effect was correlated with a reduced incidence rate of RSMV in early-planted rice plants. These findings contribute to our understanding of the mechanism by which viral pathogens reduce cold tolerance in arthropod vectors and suggest an approach to managing the fluctuating prevalence of arboviruses.

IMPORTANCE

Increasing arthropod vector dispersal rates have increased the susceptibility of crop to epidemic viral diseases. However, the incidence of some viral diseases fluctuates annually. In this study, we demonstrated that a rice virus reduces the cold tolerance of its leafhopper vector, Recilia dorsalis. This effect is linked to the virus-derived small RNA-mediated downregulation of a gene encoding a leafhopper abdominal endocuticle protein. Consequently, the altered structural composition of the abdominal endocuticle reduces the overwinter survival of leafhoppers, resulting in a lower incidence of RSMV infection in early-planted rice plants. Our findings illustrate the important roles of RNA interference in virus-vector insect-environment interactions and help explain the annual fluctuations of viral disease epidemics in rice fields.

All-you-can-eat buffet: A spider-specialized bat species (Myotis emarginatus) turns into a pest fly eater around cattle

Determining the dietary spectrum of European insectivorous bats over time is the cornerstone of their conservation, as it will aid our understanding of foraging behavior plasticity in response to plummeting insect populations. Despite the global decline in insects, a restricted number of arthropod pest species thrive. Yet past research has overlooked the potential of European bats to suppress pests harmful to woodlands or livestock, in spite of their economic relevance. Here we investigated the diet composition, its breeding season variations and pest consumption of an insectivorous bat species (Myotis emarginatus), at the northern edge of its range (Wallonia, Belgium). We also explored the prey ecology to gain insight into the hunting strategies and foraging habitats of this bat species. We used DNA metabarcoding to amplify two COI markers within 195 bat droppings collected in June, July and August, thereby identifying 512 prey taxa predominated by Diptera, Araneae and Lepidoptera. Overall, in 97% of the samples we detected at least one of the 58 potential pest taxa, 41 of which targeting trees. The June samples were marked by a diet rich in orb-weaver spiders, in accordance with the archetypal diet of M. emarginatus bats. However, during the highly energy demanding July-August parturition and lactation period, roughly 55% of the dropping samples contained two cattle fly pests (Stomoxys calcitrans and Musca domestica). Moreover, among the 88 Diptera species preyed upon by M. emarginatus in July and August, these flies accounted for around 50% of the taxa occurrences. This plasticity-the switch from a spider-rich to a fly-rich diet-seems providential considering the dramatic ongoing drop in insect populations but this involves ensuring bat-friendly cattle farming. Our results revealed that bats widely consume pest entomofauna, thereby highlighting their potential role as allies of forest managers and farmers.

Brevipalpus yothersi Baker (Tenuipalpidae) development in sweet orange plants is influenced by previous mite infestation and the presence of shelters

Citrus leprosis is the most important viral disease affecting citrus. The disease is caused predominantly by CiLV-C and is transmitted by Brevipalpus yothersi Baker mites. This study brings some insight into the colonization of B. yothersi in citrus [(Citrus × sinensis (L.) Osbeck (Rutaceae)] previously infested by viruliferous or non-viruliferous B. yothersi. It also assesses the putative role of shelters on the behavior of B. yothersi. Expression of PR1 and PR4 genes, markers of plant defense mechanisms, were evaluated by RT-qPCR to correlate the role of the plant hormonal changes during the tri-trophic virus-mite-plant interplay. A previous infestation with either non-viruliferous and viruliferous mites positively influenced oviposition and the number of adult individuals in the resulting populations. Mite populations were higher on branches that had received a previous mite infestation than branches that did not. There was an increase in the expression of PR4, a marker gene in the jasmonic acid (JA) pathway, in the treatment with non-viruliferous mites, indicating a response from the plant to their feeding. Conversely, an induced expression of PR1, a marker gene in the salicylic acid (SA) pathway, was observed mainly in the treatment with viruliferous mites, which suggests the activation of a plant response against the pathogen. The earlier mite infestation, as well as the presence of leprosis lesions and a gypsum mixture as artificial shelters, all fostered the growth of the B. yothersi populations after the second infestation, regardless of the presence or absence of CiLV-C. Furthermore, it is suggested that B. yothersi feeding actually induces the JA pathway in plants. At the same time, the CiLV-C represses the JA pathway and induces the SA pathway, which benefits the mite vector.

Balancing metabolism and reproduction

The bacterium responsible for a disease that infects citrus plants across Asia facilitates its own proliferation by increasing the fecundity of its host insect.

An optical system to detect, surveil, and kill flying insect vectors of human and crop pathogens

Sustainable and effective means to control flying insect vectors are critically needed, especially with widespread insecticide resistance and global climate change. Understanding and controlling vectors requires accurate information about their movement and activity, which is often lacking. The Photonic Fence (PF) is an optical system that uses machine vision, infrared light, and lasers to identify, track, and interdict vectors in flight. The PF examines an insect's outline, flight speed, and other flight parameters and if these match those of a targeted vector species, then a low-power, retina-safe laser kills it. We report on proof-of-concept tests of a large, field-sized PF (30 mL × 3 mH) conducted with Aedes aegypti, a mosquito that transmits dangerous arboviruses, and Diaphorina citri, a psyllid which transmits the fatal huanglongbing disease of citrus. In tests with the laser engaged, < 1% and 3% of A. aegypti and D. citri, respectfully, were recovered versus a 38% and 19% recovery when the lacer was silenced. The PF tracked, but did not intercept the orchid bee, Euglossa dilemma. The system effectively intercepted flying vectors, but not bees, at a distance of 30 m, heralding the use of photonic energy, rather than chemicals, to control flying vectors.

A near-complete genome assembly of Monochamus alternatus a major vector beetle of pinewood nematode

The Japanese sawyer beetle, Monochamus alternatus, is not only one of the most important wood boring pest itself, but also a major vector of the invasive pinewood nematode (PWN), which is the causal agent of the devastative pine wilt disease (PWD) and threats the global pine forest. Here, we present a near-complete genome of M. alternatus at the chromosome level. The assembled genome was 792.05 Mb with contig N50 length of 55.99 Mb, which is the largest N50 size among the sequenced Coleoptera insects currently. 99.57% of sequence was anchored onto ten pseudochromosomes (one X-chromosome and nine autosomes), and the final genome harbored only 13 gaps. BUSCO evaluation revealed the presence of 99.0% of complete core genes. Thus, our genome assembly represented the highest-contiguity genome assembly as well as high completeness in insects so far. We identified 20,471 protein-coding genes, of which 20,070 (98.04%) were functionally annotated. The genome assembly of M. alternatus provides a valuable resource for exploring the evolution of the symbiosis between PWN and the vector insects.

Virus-induced changes in root volatiles attract soil nematode vectors to infected plants

Plant-derived volatiles mediate interactions among plants, pathogenic viruses, and viral vectors. These volatile-dependent mechanisms have not been previously demonstrated belowground, despite their likely significant role in soil ecology and agricultural pest impacts. We investigated how the plant virus, tobacco rattle virus (TRV), attracts soil nematode vectors to infected plants. We infected Nicotiana benthamiana with TRV and compared root growth relative to that of uninfected plants. We tested whether TRV-infected N. benthamiana was more attractive to nematodes 7 d post infection and identified a compound critical to attraction. We also infected N. benthamiana with mutated TRV strains to identify virus genes involved in vector nematode attraction. Virus titre and associated impacts on root morphology were greatest 7 d post infection. Tobacco rattle virus infection enhanced 2-ethyl-1-hexanol production. Nematode chemotaxis and 2-ethyl-1-hexanol production correlated strongly with viral load. Uninfected plants were more attractive to nematodes after the addition of 2-ethyl-1-hexanol than were untreated plants. Mutation of TRV RNA2-encoded genes reduced the production of 2-ethyl-1-hexanol and nematode attraction. For the first time, this demonstrates that virus-driven alterations in root volatile emissions lead to increased chemotaxis of the virus's nematode vector, a finding with implications for sustainable management of both nematodes and viral pathogens in agricultural systems.

N6-methyladenosine modification of the mRNA for a key gene in purine nucleotide metabolism regulates virus proliferation in an insect vector

The titer of viruses that persist and propagate in their insect vector must be high enough for transmission yet not harm the insect, but the mechanism of this dynamic balance is unclear. Here, expression of inosine monophosphate dehydrogenase (LsIMPDH), a rate-limiting enzyme for guanosine triphosphate (GTP) synthesis, is shown to be downregulated by increased levels of N6-methyladenosine (m6A) on LsIMPDH mRNA in rice stripe virus (RSV)-infected small brown planthoppers (SBPHs; Laodelphax striatellus), the RSV vector, which decreases GTP content, thus limiting viral proliferation. Moreover, planthopper methyltransferase-like protein 3 (LsMETTL3) and m6A reader protein LsYTHDF3 are found to catalyze and recognize the m6A on LsIMPDH mRNA, respectively, and cooperate in destabilizing LsIMPDH transcripts. Co-silencing assays show that negative regulation of viral proliferation by both LsMETTL3 and LsYTHDF3 is partially dependent on LsIMPDH. This distinct mechanism limits virus replication in an insect vector, providing a potential gene target to block viral transmission.

Preference of Pentalonia nigronervosa for infected banana plants tends to reverse after Banana bunchy top virus acquisition

Pentalonia nigronervosa Coquerel (Hemiptera: Aphididae) is the vector of the Banana Bunchy Top Virus (BBTV), the most serious viral disease of banana (Musa spp.) in the world. Before acquiring the virus, the vector is more attracted to infected banana plants in response to the increased emissions of volatile organic compounds (VOCs). Here, we test the hypothesis that BBTV acquisition directly modifies the preference of P. nigronervosa for infected banana plants, and that the change in preference results from the alteration of the organs linked to the VOC detection or to the behaviour of the vector. We found that the preference of P. nigronervosa for infected banana plants reverses after virus acquisition in dessert banana, while it remains similar between healthy and infected banana plants before and after the acquisition of BBTV. At the same time, aphids reared on infected bananas had smaller forewing areas and hind tibia length than aphids reared on healthy bananas, although the number of secondary rhinaria on the antennae was lower on dessert banana-reared aphids than plantain-reared aphids, this was not affected by the infection status of the aphid. These results support the "vector manipulation hypothesis-VMH" of pathogens to promote their spread. They have implications for the BBTV management.

Rice volatile compound (E)-β-caryophyllene induced by rice dwarf virus (RDV) attracts the natural enemy Cyrtorhinus lividipennis to prey on RDV insect vectors

BACKGROUND

Rice dwarf virus (RDV)-induced rice plant volatiles (E)-β-caryophyllene and 2-heptanol modulate the olfactory behavior of RDV insect vectors that promote viral acquisition and transmission. However, it remains elusive whether these two volatiles could influence the behaviors of the natural enemies of RDV insect vectors. Herein, we determined the effects of these two volatiles on the olfactory and predatory behaviors of Cyrtorhinus lividipennis (Hemiptera: Miridae), an important predator of RDV insect vectors in rice paddies.

RESULTS

The results showed that C. lividipennis preferred RDV-infected rice plant odors over RDV-free rice plant odors. C. lividipennis was attracted by (E)-β-caryophyllene, but showed no behavioral responses to 2-heptanol. The attraction of (E)-β-caryophyllene towards C. lividipennis was further confirmed using oscas1 rice plants, which do not release (E)-β-caryophyllene in response to RDV infection, through a series of complementary assays. The oviposition preference of the RDV vector insect Nephotettix cincticeps (Hemiptera: Cicadellidae) showed no significant difference between RDV-infected and RDV-free wild-type plants, nor between oscas1-RDV and oscas1 plants. However, the predation rate of C. lividipennis for N. cincticeps eggs on RDV-infected plants was higher than that on RDV-free plants, whereas there was no significant difference between oscas1-RDV and oscas1 plants.

CONCLUSION

(E)-β-caryophyllene induced by RDV attracted more C. lividipennis to prey on N. cincticeps eggs and played a crucial role in plant-virus-vector-enemy interactions. These novel findings will promote the design of new strategies for disease control by controlling the populations of insect vectors, for example recruiting more natural enemies by virus-induced plant volatiles. © 2023 Society of Chemical Industry.

Pleiotropic effects of N-acylhomoserine lactone synthase ExpI on virulence, competition, and transmission in Pectobacterium carotovorum subsp. carotovorum Pcc21

BACKGROUND

Pectobacterium species are necrotrophic phytopathogenic bacteria that cause soft rot disease in economically important crops. The successful infection of host plants relies on interactions among virulence factors, competition, and transmission within hosts. Pectobacteria primarily produce and secrete plant cell-wall degrading enzymes (PCWDEs) for virulence. The regulation of PCWDEs is controlled by quorum sensing (QS). Thus, the QS system is crucial for disease development in pectobacteria through PCWDEs.

RESULTS

In this study, we identified a Tn-insertion mutant, M2, in the expI gene from a transposon mutant library of P. carotovorum subsp. carotovorum Pcc21 (hereafter Pcc21). The mutant exhibited reduced production and secretion of PCWDEs, impaired flagellar motility, and increased sensitivity to hydrogen peroxide, resulting in attenuated soft rot symptoms in cabbage and potato tubers. Transcriptomic analysis revealed the down-regulation of genes involved in the production and secretion in the mutant, consistent with the observed phenotype. Furthermore, the Pcc21 wild-type transiently colonized in the gut of Drosophila melanogaster within 12 h after feeding, while the mutant compromised colonization phenotype. Interestingly, Pcc21 produces a bacteriocin, carocin D, to compete with other bacteria. The mutant exhibited up-regulation of carocin D-encoding genes (caroDK) and inhibited the growth of a closely related bacterium, P. wasabiae.

CONCLUSION

Our results demonstrated the significance of ExpI in the overall pathogenic lifestyle of Pcc21, including virulence, competition, and colonization in plant and insect hosts. These findings suggest that disease outcome is a result of complex interactions mediated by ExpI across multiple steps. © 2023 Society of Chemical Industry.

Diffuse Associations Between Drosophila suzukii and Filamentous Fungal Microbes May Alter Caneberry Disease Dynamics

Interactions between microorganisms and frugivorous insects can modulate fruit rot disease epidemiology. Insect feeding and/or oviposition wounds may create opportunities for fungal infection. Passive and active dispersal of fungal inoculums by adult insects also increases disease incidence. In fall-bearing raspberries and blackberries, such vectoring interactions could increase crop damage from the invasive pestiferous vinegar fly Drosophila suzukii (spotted-wing drosophila). Periods of peak D. suzukii activity are known to overlap with several species of primary fruit rot pathogen, particularly Botrytis cinerea and Cladosporium cladosporioides, and previous work indicates that larvae co-occur with and feed on various filamentous fungi at low rates. To further our understanding of the epidemiological consequences that may emerge from these associations, we surveyed the filamentous fungal community associated with adult D. suzukii, isolating and molecularly identifying fungi externally and internally (indicating feeding) from field-collected adults over 3 years. We isolated and identified 37 unique genera of fungi in total, including known raspberry pathogens. Most fungi were detected infrequently, and flies acquired and carried fungi externally at higher richness, frequency, and density relative to internally. In a worst-case scenario laboratory vectoring assay, D. suzukii adults were able to transfer B. cinerea and C. cladosporioides to sterile media at 0, 24, 48, and 72 h after exposure to sporulating cultures in Petri dishes. These results collectively suggest an adventitious vectoring association between D. suzukii and fruit rot fungi that has the potential to alter caneberry disease dynamics.

Infection with 'Candidatus Liberibacter asiaticus' improves the fecundity of Diaphorina citri aiding its proliferation: A win-win strategy

The evolution of insect vector-pathogen relationships has long been of interest in the field of molecular ecology. One system of special relevance, due to its economic impacts, is that between Diaphorina citri and 'Candidatus Liberibacter asiaticus' (CLas), the cause of the severe Asian form of huanglongbing. CLas-positive D. citri are more fecund than their CLas-negative counterparts, boosting opportunities for pathogens to acquire new vector hosts. The molecular mechanism behind this life-history shift remains unclear. Here, we found that CLas promoted ovarian development and increased the expression of the vitellogenin receptor (DcVgR) in ovaries. DcVgR RNAi significantly decreased fecundity and CLas titer in ovaries, extended the preoviposition period, shortened the oviposition period and blocked ovarian development. Given their importance in gene regulation, we explored the role of miRNAs in shaping these phenotypes and their molecular triggers. Our results showed that one miRNA, miR-275, suppressed DcVgR expression by binding to its 3' UTR. Overexpression of miR-275 knocked down DcVgR expression and CLas titer in ovaries, causing reproductive defects that mimicked DcVgR knockdown phenotypes. We focused, further, on roles of the Juvenile Hormone (JH) pathway in shaping the observed fecundity phenotype, given its known impacts on ovarian development. After CLas infection, this pathway was upregulated, thereby increasing DcVgR expression. From these combined results, we conclude that CLas hijacks the JH signalling pathway and miR-275, thereby targeting DcVgR to increase D. citri fecundity. These changes simultaneously increase CLas replication, suggesting a pathogen-vector host mutualism, or a seemingly helpful, but cryptically costly life-history manipulation.

Virulence factors of the genus Fusarium with targets in plants

Fusarium spp. comprise various species of filamentous fungi that cause severe diseases in plant crops of both agricultural and forestry interest. These plant pathogens produce a wide range of molecules with diverse chemical structures and biological activities. Genetic functional analyses of some of these compounds have shown their role as virulence factors (VF). However, their mode of action and contributions to the infection process for many of these molecules are still unknown. This review aims to analyze the state of the art in Fusarium VF, emphasizing their biological targets on the plant hosts. It also addresses the current experimental approaches to improve our understanding of their role in virulence and suggests relevant research questions that remain to be answered with a greater focus on species of agroeconomic importance. In this review, a total of 37 confirmed VF are described, including 22 proteinaceous and 15 non-proteinaceous molecules, mainly from Fusarium oxysporum and Fusarium graminearum and, to a lesser extent, in Fusarium verticillioides and Fusarium solani.

Plant and microbial features governing an endophytic lifestyle

Beneficial microorganisms colonizing internal plant tissues, the endophytes, support their host through plant growth promotion, pathogen protection, and abiotic stress alleviation. Their efficient application in agriculture requires the understanding of the molecular mechanisms and environmental conditions that facilitate in planta accommodation. Accumulating evidence reveals that commensal microorganisms employ similar colonization strategies as their pathogenic counterparts. Fine-tuning of immune response, motility, and metabolic crosstalk accounts for their differentiation. For a holistic perspective, in planta experiments with microbial collections and comprehensive genome data exploration are crucial. This review describes the most recent findings on factors involved in endophytic colonization processes, focusing on bacteria and fungi, and discusses required methodological approaches to unravel their relevance within a community context.

Endosymbionts modulate virus effects on aphid-plant interactions

Vector-borne pathogens frequently modify traits of their primary hosts and vectors in ways that influence disease transmission. Such effects can themselves be altered by the presence of other microbial symbionts, yet we currently have limited understanding of these interactions. Here we show that effects of pea enation mosaic virus (PEMV) on interactions between host plants and aphid vectors are modulated by the presence of different aphid endosymbionts. In a series of laboratory assays, we found strong interactive effects of virus infection and endosymbionts on aphid metabolomic profiles, population growth, behavior, and virus transmission during aphid feeding. Furthermore, the strongest effects-and those predicted to favor virus transmission-were most apparent in aphid lines harboring particular endosymbionts. These findings show that virus effects on host-vector interactions can be strongly influenced by other microbial symbionts and suggest a potentially important role for such interactions in disease ecology and evolution.