A review of the mechanisms and components that determine the transmission efficiency of Tomato yellow leaf curl virus (Geminiviridae; Begomovirus) by its whitefly vector

Begomoviruses associated with okra yellow vein mosaic disease (OYVMD): diversity, transmission mechanism, and management strategies

Okra yellow vein mosaic disease (OYVMD) is a major constraint to okra production globally. It is caused by several distinct begomoviruses, including okra yellow vein mosaic virus (OYVMV), that are transmitted by the whitefly. This study synthesizes current knowledge on the complex interactions between whiteflies, begomoviruses, and okra plants that enable viral spread and cause OYVMD. The acquisition and transmission cycle involves specific processes including virion ingestion during phloem-feeding, endocytosis and passage across insect tissues, secretion in saliva, and inoculation into plants. Molecular compatibilities between vector coat proteins, midgut proteins, and plant factors modulate virus replication and movement through barrier tissues. Abiotic stresses and host traits also impact whitefly behavior and virus epidemiology. Begomoviruses such as OYVMV have spread globally wherever whitefly vectors and susceptible okra varieties occur. Integrated management of the tripartite pathosystem that incorporates host resistance, cultural tactics, and biological control is required to mitigate the transmission of begomoviruses and OYVMD impact. Finally, resolving vector-virus interactions and developing interference strategies will help contribute to strengthening okra germplasm resistance which can support sustainable food production.

Tomato yellow leaf curl virus: Characteristics, influence, and regulation mechanism

Tomato yellow leaf curl virus (TYLCV), a DNA virus belonging to the genus Begomovirus, significantly impedes the growth and development of numerous host plants, including tomatoes and peppers. Due to its rapid mutation rate and frequent recombination events, achieving complete control of TYLCV proves exceptionally challenging. Consequently, identifying resistance mechanisms become crucial for safeguarding host plants from TYLCV-induced damage. This review article delves into the global distribution, dispersal patterns, and defining characteristics of TYLCV. Moreover, the intricate interplay between TYLCV and various influencing factors, such as insect vectors, susceptible host plants, and abiotic stresses, plays a pivotal role in plant-TYLCV interactions. The review offers an updated perspective on recent investigations focused on plant response mechanisms to TYLCV infection, including the intricate relationship between TYLCV, whiteflies, and regulatory factors. This comprehensive analysis aims to establish a foundation for future research endeavors exploring the molecular mechanisms underlying TYLCV infection and the development of plant resistance through breeding programs.

Temporal changes in the levels of virus and betasatellite DNA in B. tabaci feeding on CLCuD affected cotton during the growing season

Cotton, a key source of income for Pakistan, has suffered significantly by cotton leaf curl disease (CLCuD) since 1990. This disease is caused by a complex of phylogenetically-related begomovirus (genus Begomovirus, family Geminiviridae) species and a specific betasatellite (genus Betasatellite, family Tolecusatellitidae), cotton leaf curl Multan betasatellite. Additionally, another DNA satellite called alphasatellite (family Alphasatellitidae), is also frequently associated. All these virus components are vectored by a single species of whitefly (Bemisia tabaci). While many factors affect cotton productivity, including cotton variety, sowing time, and environmental cues such as temperature, humidity, and rainfall, CLCuD is a major biotic constraint. Although the understanding of begomoviruses transmission by whiteflies has advanced significantly over the past three decades, however, the in-field seasonal dynamics of the viruses in the insect vector remained an enigma. This study aimed to assess the levels of virus and betasatellite in whiteflies collected from cotton plants throughout the cotton growing season from 2014 to 2016. Notably, begomovirus levels showed no consistent pattern, with minimal variations, ranging from 0.0017 to 0.0074 ng.μg-1 of the genomic DNA in 2014, 0.0356 to 0.113 ng.μg-1 of the genomic DNA in 2015, and 0.0517 to 0.0791 ng.μg-1 of the genomic DNA in 2016. However, betasatellite levels exhibited a distinct pattern. During 2014 and 2015, it steadily increased throughout the sampling period (May to September). While 2016 showed a similar trend from the start of sampling (July) to September but a decline in October (end of sampling). Such a study has not been conducted previously, and could potentially provide valuable insights about the epidemiology of the virus complex causing CLCuD and possible means of controlling losses due to it.

A new pest suction machine to control Bemisia tabaci (Hemiptera: Aleyrodidae) in tomato greenhouses

The sweetpotato whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is challenging to control using chemical pesticides owing to its resistance to many insecticides. Thus, there has been an increasing demand for alternative control measures. Thus, this study evaluated the efficacy of a newly designed pest suction machine to manage whiteflies on tomato plants (Solanum lycopersicum L.) (Solanales: Solanaceae) in greenhouses over 2 seasons. The suction machine comprised a battery-powered cart with a mounted suction unit, an ultrasonic device, and green lights. Ultrasonic irradiation provided non-contact vibration, facilitating the movement of adult whiteflies away from the plants, and green lights attracted them to the suction device. This combination effectively captured whitefly adults, even with a weak suction force, saving electricity consumption. The efficacy of suction machine was further evaluated by measuring the number of whitefly adults caught by the machine and the number of adults and nymphs remaining on the tomato leaves. The whitefly population was considerably lower in the treated blocks than in the non-treated blocks in the autumn trial. The machine reduced the density of whitefly adults without using chemical pesticides. Although a lot of optimizations would be required, suction control is an additional and alternative strategy that may be incorporated in the integrated pest management of whiteflies on greenhouse tomato plants.

An Insight into Emerging Begomoviruses and their Satellite Complex causing Papaya Leaf Curl Disease

Papaya leaf curl disease (PaLCD) was primarily detected in India and causes major economic damage to agriculture crops grown globally, seriously threatening food security. Begomoviruses are communicated by the vector Bemisia tabaci, and their transmission efficiency and persistence in the vector are the highest, exhibiting the widest host range due to adaptation and evolution. Symptoms induced during PaLCD include leaf curl, leaf yellowing, interveinal chlorosis, and reduced fruit quality and yield. Consequently, plants have evolved several multi-layered defense mechanisms to resist Begomovirus infection and distribution. Subsequently, Begomovirus genomes organise circular ssDNA of size ~2.5-2.7 kb of overlapping viral transcripts and carry six-seven ORFs encoding multifunctional proteins, which are precisely evolved by the viruses to maintain the genome-constraint and develop complex but integrated interactions with a variety of host components to expand and facilitate successful infection cycles, i.e., suppression of host defense strategies. Geographical distribution is continuing to increase due to the advent and evolution of new Begomoviruses, and sweep to new regions is a future scenario. This review summarizes the current information on the biological functions of papaya-infecting Begomoviruses and their encoded proteins in transmission through vectors and modulating host-mediated responses, which may improve our understanding of how to challenge these significant plant viruses by revealing new information on the development of antiviral approaches against Begomoviruses associated with PaLCD.

Current Status and Complexity of Three Begomovirus Species in Pepper Plants in Lowlands and Highlands in Java Island, Indonesia

Three primary species from the Begomovirus genus, Pepper yellow leaf curl Indonesia virus (PepYLCIV), Tomato yellow leaf curl Kanchanaburi virus (TYLCKaV), and Tomato leaf curl New Delhi virus (ToLCNDV), are suspected of spreading throughout pepper production centers, and plants are infected by a single species or a combination of two or three species. This study was conducted to provide complete information about the symptoms, incidence and severity, whitefly biotypes, as well as the dominance status of the three Begomovirus species in pepper-producing areas in Java. A DNA analysis was carried out on leaf samples to identify Begomovirus species and biotypes of B. tabaci collected from 18 areas (16 districts) in lowlands (<400 m asl) and highlands (>700 m asl). The DNA analysis showed that B. tabaci biotype B was the most commonly detected in all locations compared to the A, AN, and Q biotypes. The incidence of begomovirus infection was at a high level, 93% and 88.78% in the lowlands and highlands, respectively. However, the severity of begomovirus infection was significantly higher in the lowlands (54.50%) than in the highlands (38.11%). A single infection of PepYLCIV was most dominant in all locations sampled and caused severe infection, followed by a mixed infection with TYLCKaV. Therefore, the current status of begomovirus infection, especially PepYLCIV, can provide advice to farmers using more tolerant and resistant varieties as well as a breeding strategy for resistant pepper varieties.

Genetic recombination among tomato yellow leaf curl virus isolates in commercial tomato crops in Kuwait drives emergence of virus diversity: a comparative genomic analysis

OBJECTIVE

Whitefly-transmitted tomato yellow leaf curl virus (TYLCV) continues to be a major constraint to tomato production in Kuwait. However, very limited information is available about the population structure and genetic diversity of TYLCV infecting tomato in Kuwait.

RESULTS

Whole genome sequences of 31 isolates of TYLCV, collected from commercial tomato crops grown in northern (Abdally) and southern (Al Wafra) parts of Kuwait, were deciphered. Eighteen isolates of TYLCV are identified as potential genetic recombinants. The isolates Abdally 6A and Abdally 3B reported in this study were identified to be potential recombinants. Compared to the 15 isolates from the Abdally area, and the three previously reported KISR isolates of Kuwait, six out of sixteen Al Wafra isolates showed an insertion of 19 extra nucleotides near the 5'-end. There are also four nucleotide variations before the 19-extra-nucleotides. The additional 19 nucleotides observed in nine isolates indicate that these isolates might have resulted from a single gene recombination/insertion event. Molecular phylogeny based on complete genome sequences of TYLCV isolates suggests transboundary movement of virus isolates due to geographic proximity. The information presented herein is quite useful for the comprehension of TYLCV biology, epidemiology and would aid in the management of disease in the long run.

Heat shock protein 60 in parasitic helminths: A role in immune responses and therapeutic applications

Heat shock protein 60 (HSP60) is an unique member of the heat shock protein family, being involved in parasite infections. To cope with harsh environments where parasites live, HSP60s are indispensable and involved in a variety of biological processes. HSP60s have relative low similarity among parasites, but their ATPase /Mg²⁺ active sites are highly conserved. The interactions of HSP60s with signaling pathway regulators in immune cells suggest a crucial role in immune responses, rendering them a potential therapeutic target. This paper reviews the current understandings of HSP60s in parasitic helminths in aspects of molecular characteristics, immunoregulatory responses and HSP60-based therapeutics.

Unraveling different begomoviruses, DNA satellites and cryptic species of Bemisia tabaci and their endosymbionts in vegetable ecosystem

Bemisia tabaci species complex contains more than 46 cryptic species. It has emerged as an important pest causing significant yield loss in many cultivated crops. This pest is also a vector for more than 100 species of begomoviruses, that are a major threat for the cultivation of many crops in different regions of the world. The relation between cryptic species of the B. tabaci species complex and associated begomoviruses that infect different crops remains unclear. In the present study, four cryptic species (Asia I, China 3, Asia II 5 and Asia II-1) of B. tabaci and four associated endosymbionts (Arsenophonus, Cardinium, Rickettsia and Wolbachia) were identified in different vegetable crops. The vector-based PCR detection revealed five different begomoviruses such as okra enation leaf curl virus (OELCuV), tomato leaf curl Palampur virus (ToLCPalV), squash leaf curl China virus (SLCCNV), chilli leaf curl virus (ChiLCuV), and tomato leaf curl New Delhi virus (ToLCNDV). Of these begomoviruses, the maximum infection rate was observed (9.1%) for OELCuV, followed by 7.3% for ToLCNDV. The infection rate of the other three viruses (SLCCNV, ChiLCuV, ToLCPalV) ranged from 0.9 to 2.7% in cryptic species of B. tabaci. Further, each cryptic species was infected with multiple virus species and the virus infection rate of Asia I, Asia II-5, China 3 and Asia II-1 was 21.2%, 15.1%, 15.1% and 0.6% respectively. Similarly, in case of betasatellites the highest infection rate was 12% for ToLCBDB, followed by 6% for OLCuB and PaLCB. With regard to alphasatellites, the highest infection rate was 18.2% for AEV and 3% for CLCuMuA. This study demonstrates the distribution of cryptic species of whitefly and their endosymbionts, and associated begomoviruses and DNA satellites in vegetable ecosystem. We believe that the information generated here is useful for evolving an effective pest management strategies for vegetable production.

F-actin dynamics in midgut cells enables virus persistence in vector insects

Hemipteran insects that transmit plant viruses in a persistent circulative manner acquire, retain and transmit viruses for their entire life. The mechanism enabling this persistence has remained unclear for many years. Here, we determined how wheat dwarf virus (WDV) persists in its leafhopper vector Psammotettix alienus. We found that WDV caused the up-regulation of actin-depolymerizing factor (ADF) at the mRNA and protein levels in the midgut cells of leafhoppers after experiencing a WDV acquisition access period (AAP) of 6, 12 or 24 h. Experimental inhibition of F-actin depolymerization by jasplakinolide and dsRNA injection led to lower virus accumulation levels and transmission efficiencies, suggesting that depolymerization of F-actin regulated by ADF is essential for WDV invasion of midgut cells. Exogenous viral capsid protein (CP) inhibited ADF depolymerization of actin filaments in vitro and in Spodoptera frugiperda 9 (Sf9) cells because the CP competed with actin to bind ADF and then blocked actin filament disassembly. Interestingly, virions colocalized with ADF after a 24-h AAP, just as actin polymerization occurred, indicating that the binding of CP with ADF affects the ability of ADF to depolymerize F-actin, inhibiting WDV entry. Similarly, the luteovirus barley yellow dwarf virus also induced F-actin depolymerization and then polymerization in the gut cells of its vector Schizaphis graminum. Thus, F-actin dynamics are altered by nonpropagative viruses in midgut cells to enable virus persistence in vector insects.

Pervasive Endosymbiont Arsenophonus Plays a Key Role in the Transmission of Cotton Leaf Curl Virus Vectored by Asia II-1 Genetic Group of Bemisia tabaci

Insects often coevolved with their mutualistic partners such as gut endosymbionts, which play a key in the physiology of host. Studies on such interactions between Bemisia tabaci and its primary and secondary endosymbionts have gained importance due to their indispensable roles in the biology of this insect. Present study reports the predominance of two secondary endosymbionts, Arsenophonus and Cardinium in the Asia II-1 genetic group of whitefly and elucidates their role in the transmission of its vectored Cotton leaf curl virus. Selective elimination of endosymbionts was optimized using serial concentration of ampicillin, chloramphenicol, kanamycin, tetracycline, and rifampicin administered to viruliferous whiteflies through sucrose diet. Primary endosymbiont, Portiera was unresponsive to all the antibiotics, however, rifampicin and tetracycline at 90 μg/ml selectively eliminated Arsenophonus from the whitefly. Elimination of Arsenophonus resulted in significant decrease in virus titer from viruliferous whitefly, further the CLCuV transmission efficiency of these whiteflies was significantly reduced compared to the control flies. Secondary endosymbiont, Cardinium could not be eliminated completely even with higher concentrations of antibiotics. Based on the findings, Arsenophonus plays a key role in the retention and transmission of CLCuV in the Asia II-1 genetic group of B. tabaci, while the role of Cardinium could not be established due to its unresponsiveness to antibiotics.

Effects of heat stress on virus transmission and virus-mediated apoptosis in whitefly Bemisia tabaci

Tomato yellow leaf curl virus (TYLCV), a plant DNA virus of the genus Begomovirus, is transmitted by whiteflies of the Bemisia tabaci species complex in a persistent manner. Our previous study indicated that activation of the apoptosis pathway in whiteflies could facilitate TYLCV accumulation and transmission. Considering that temperature change can influence the spread of insect-borne plant viruses, we focused on plant virus induced-apoptosis to investigate the underlying mechanism of temperature regulation on plant virus transmission via an insect vector. We found that heat stress (40°C) on whiteflies could facilitate TYLCV accumulation and increase transmission to tomato plants. Despite upregulation of caspase-1 and caspase-3 gene expression, heat stress failed to induce an increase in the activation of cleaved caspase-3 and DNA fragmentation in TYLCV-infected whiteflies. However, our data failed to determine the role of heat stress in apoptosis modulation of insect-plant virus interplay while still providing clues to understand insect vectors and their transmitted plant viruses.

Whitefly-Mediated Transmission and Subsequent Acquisition of Highly Similar and Naturally Occurring Tomato Yellow Leaf Curl Virus Variants

Begomoviruses are whitefly-transmitted viruses that infect many agricultural crops. Numerous reports exist on individual host plants harboring two or more begomoviruses. Mixed infection allows recombination events to occur among begomoviruses. However, very few studies have examined mixed infection of different isolates/variants/strains of a Begomovirus species in hosts. In this study, the frequency of mixed infection of tomato yellow leaf curl virus (TYLCV) variants in field-grown tomato was evaluated. At least 60% of symptomatic field samples were infected with more than one TYLCV variant. These variants differed by a few nucleotides and amino acids, resembling a quasispecies. Subsequently, in the greenhouse, single and mixed infection of two TYLCV variants (variant #2 and variant #4) that shared 99.5% nucleotide identity and differed by a few amino acids was examined. Plant-virus variant-whitefly interactions including transmission of one and/or two variants, variants' concentrations, competition between variants in inoculated tomato plants, and whitefly acquisition of one and/or two variants were assessed. Whiteflies transmitted both variants to tomato plants at similar frequencies; however, the accumulation of variant #4 was greater than that of variant #2 in tomato plants. Despite differences in variants' accumulation in inoculated tomato plants, whiteflies acquired variant #2 and variant #4 at similar frequencies. Also, whiteflies acquired greater amounts of TYLCV from singly infected plants than from mixed-infected plants. These results demonstrated that even highly similar TYLCV variants could differentially influence component (whitefly-variant-plant) interactions.

Differential expression of gut protein genes and population density of Arsenophonus contributes to sex-biased transmission of Bemisia tabaci vectored Cotton leaf curl virus

Whitefly, Bemisia tabaci (Gennadius) is an important pest of cotton causing direct damage as sap feeder and vector of Cotton leaf curl virus (CLCuV). Previous few studies suggest that female whiteflies are more efficient vector of begomovirusthan males, however the sex-biased transmission efficiency is still not clearly understood. Present studies with B. tabaci AsiaII-1 haplotype showed higher virus transmission efficiency of females compared to males. This variable begomovirus transmission efficiency has been related to previously identifiedkey factors associated with B. tabaci. The higher density of endosymbiont Arsenophonus and variable expression of some midgut proteins genes i.e. Cyclophilin, Knottin, Hsp40, Hsp70 may be possibly imparting higher vector competency to the females compared to males. The present studies suggest low abundance of Arsenophonus spp. as well as lower expressionof Cyclophilin genein males as compared to females. This is further supplemented by overexpression of Knottin, Hsp40, and Hsp70 genes in males compared to females and thus collectively all these factors might be playing a key role in low virus transmission efficiency of males. The relative density of Arsenophonus spp. and expression of midgut proteins genes in male and female whitefly first time enriches our understanding about sex-biased transmission efficiency of begomovirus.

Factors Determining Transmission of Persistent Viruses by Bemisia tabaci and Emergence of New Virus-Vector Relationships

Many plant viruses depend on insect vectors for their transmission and dissemination. The whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is one of the most important virus vectors, transmitting more than four hundred virus species, the majority belonging to begomoviruses (Geminiviridae), with their ssDNA genomes. Begomoviruses are transmitted by B. tabaci in a persistent, circulative manner, during which the virus breaches barriers in the digestive, hemolymph, and salivary systems, and interacts with insect proteins along the transmission pathway. These interactions and the tissue tropism in the vector body determine the efficiency and specificity of the transmission. This review describes the mechanisms involved in circulative begomovirus transmission by B. tabaci, focusing on the most studied virus in this regard, namely the tomato yellow leaf curl virus (TYLCV) and its closely related isolates. Additionally, the review aims at drawing attention to the recent knowhow of unorthodox virus—B. tabaci interactions. The recent knowledge of whitefly-mediated transmission of two recombinant poleroviruses (Luteoviridae), a virus group with an ssRNA genome and known to be strictly transmitted with aphids, is discussed with its broader context in the emergence of new whitefly-driven virus diseases.

The Global Dimension of Tomato Yellow Leaf Curl Disease: Current Status and Breeding Perspectives

Tomato yellow leaf curl disease (TYLCD) caused by tomato yellow leaf curl virus (TYLCV) and a group of related begomoviruses is an important disease which in recent years has caused serious economic problems in tomato (Solanum lycopersicum) production worldwide. Spreading of the vectors, whiteflies of the Bemisia tabaci complex, has been responsible for many TYLCD outbreaks. In this review, we summarize the current knowledge of TYLCV and TYLV-like begomoviruses and the driving forces of the increasing global significance through rapid evolution of begomovirus variants, mixed infection in the field, association with betasatellites and host range expansion. Breeding for host plant resistance is considered as one of the most promising and sustainable methods in controlling TYLCD. Resistance to TYLCD was found in several wild relatives of tomato from which six TYLCV resistance genes (Ty-1 to Ty-6) have been identified. Currently, Ty-1 and Ty-3 are the primary resistance genes widely used in tomato breeding programs. Ty-2 is also exploited commercially either alone or in combination with other Ty-genes (i.e., Ty-1, Ty-3 or ty-5). Additionally, screening of a large collection of wild tomato species has resulted in the identification of novel TYLCD resistance sources. In this review, we focus on genetic resources used to date in breeding for TYLCVD resistance. For future breeding strategies, we discuss several leads in order to make full use of the naturally occurring and engineered resistance to mount a broad-spectrum and sustainable begomovirus resistance.

Vector Transmission of Tomato Yellow Leaf Curl Thailand Virus by the Whitefly Bemisia tabaci: Circulative or Propagative?

Viruses that cause tomato yellow leaf curl disease are part of a group of viruses of the genus Begomovirus, family Geminiviridae. Tomato-infecting begomoviruses cause epidemics in tomato crops in tropical, subtropical, and Mediterranean climates, and they are exclusively transmitted by Bemisia tabaci in the field. The objective of the present study was to examine the transmission biology of the tomato yellow leaf curl Thailand virus (TYLCTHV) by B. tabaci, including virus-infected tissues, virus translocation, virus replication, and transovarial transmission. The results demonstrated that the virus translocates from the alimentary gut to the salivary glands via the hemolymph, without apparent replication when acquired by B. tabaci. Furthermore, the virus was detected in 10% of the first-generation progeny of viruliferous females, but the progeny was unable to cause the viral infection of host plants. There was no evidence of transovarial transmission of TYLCTHV in B. tabaci. When combined with the current literature, our results suggest that B. tabaci transmits TYLCTHV in a persistent-circulative mode. The present study enhances our understanding of virus-vector interaction and the transmission biology of TYLCTHV in B. tabaci.

Aphid endosymbiont facilitates virus transmission by modulating the volatile profile of host plants

BACKGROUND

Most plant viruses rely on vectors for their transmission and spread. One of the outstanding biological questions concerning the vector-pathogen-symbiont multi-trophic interactions is the potential involvement of vector symbionts in the virus transmission process. Here, we used a multi-factorial system containing a non-persistent plant virus, cucumber mosaic virus (CMV), its primary vector, green peach aphid, Myzus persicae, and the obligate endosymbiont, Buchnera aphidicola to explore this uncharted territory.

RESULTS

Based on our preliminary research, we hypothesized that aphid endosymbiont B. aphidicola can facilitate CMV transmission by modulating plant volatile profiles. Gene expression analyses demonstrated that CMV infection reduced B. aphidicola abundance in M. persicae, in which lower abundance of B. aphidicola was associated with a preference shift in aphids from infected to healthy plants. Volatile profile analyses confirmed that feeding by aphids with lower B. aphidicola titers reduced the production of attractants, while increased the emission of deterrents. As a result, M. persicae changed their feeding preference from infected to healthy plants.

CONCLUSIONS

We conclude that CMV infection reduces the B. aphidicola abundance in M. persicae. When viruliferous aphids feed on host plants, dynamic changes in obligate symbionts lead to a shift in plant volatiles from attraction to avoidance, thereby switching insect vector's feeding preference from infected to healthy plants.

The Actin Cytoskeleton Mediates Transmission of "Candidatus Liberibacter solanacearum" by the Carrot Psyllid

Several vector-borne plant pathogens have evolved mechanisms to exploit and to hijack vector host cellular, molecular, and defense mechanisms for their transmission. In the past few years, Liberibacter species, which are transmitted by several psyllid vectors, have become an economically important group of pathogens that have devastated the citrus industry and caused tremendous losses to many other important crops worldwide. The molecular mechanisms underlying the interactions of Liberibacter species with their psyllid vectors are poorly studied. "Candidatus Liberibacter solanacearum," which is associated with important vegetable diseases, is transmitted by the carrot psyllid Bactericera trigonica in a persistent manner. Here, we elucidated the role of the B. trigonica Arp2/3 protein complex, which plays a major role in regulation of the actin cytoskeleton, in the transmission of "Ca Liberibacter solanacearum." "Ca Liberibacter solanacearum" colocalized with ArpC2, a key protein in this complex, and this colocalization was strongly associated with actin filaments. Silencing of the psyllid ArpC2 disrupted the colocalization and the dynamics of F-actin. Silencing of RhoGAP21 and Cdc42, which act in the signaling cascade leading to upregulation of Arp2/3 and F-actin bundling, showed similar results. On the other hand, silencing of ArpC5, another component of the complex, did not induce any significant effects on F-actin formation. Finally, ArpC2 silencing caused a 73.4% reduction in "Ca Liberibacter solanacearum" transmission by psyllids, strongly suggesting that transmission of "Ca Liberibacter solanacearum" by B. trigonica is cytoskeleton dependent and "Ca Liberibacter solanacearum" interacts with ArpC2 to exploit the intracellular actin nucleation process for transmission. Targeting this unique interaction could lead to the development of a novel strategy for the management of Liberibacter-associated diseases.IMPORTANCE Plant diseases caused by vector-borne pathogens are responsible for tremendous losses and threaten some of the most important agricultural crops. A good example is the citrus greening disease, which is caused by bacteria of the genus Liberibacter and is transmitted by psyllids; it has devastated the citrus industry in the United States, China, and Brazil. Here, we show that psyllid-transmitted "Candidatus Liberibacter solanacearum" employs the actin cytoskeleton of psyllid gut cells, specifically the ArpC2 protein in the Arp2/3 complex of this system, for movement and transmission in the vector. Silencing of ArpC2 dramatically influenced the interaction of "Ca Liberibacter solanacearum" with the cytoskeleton and decreased the bacterial transmission to plants. This system could be targeted to develop a novel approach for the control of Liberibacter-associated diseases.

Red-light is an environmental effector for mutualism between begomovirus and its vector whitefly

Environments such as light condition influence the spread of infectious diseases by affecting insect vector behavior. However, whether and how light affects the host defense which further affects insect preference and performance, remains unclear, nor has been demonstrated how pathogens co-adapt light condition to facilitate vector transmission. We previously showed that begomoviral βC1 inhibits MYC2-mediated jasmonate signaling to establish plant-dependent mutualism with its insect vector. Here we show red-light as an environmental catalyzer to promote mutualism of whitefly-begomovirus by stabilizing βC1, which interacts with PHYTOCHROME-INTERACTING FACTORS (PIFs) transcription factors. PIFs positively control plant defenses against whitefly by directly binding to the promoter of terpene synthase genes and promoting their transcription. Moreover, PIFs interact with MYC2 to integrate light and jasmonate signaling and regulate the transcription of terpene synthase genes. However, begomovirus encoded βC1 inhibits PIFs' and MYC2' transcriptional activity via disturbing their dimerization, thereby impairing plant defenses against whitefly-transmitted begomoviruses. Our results thus describe how a viral pathogen hijacks host external and internal signaling to enhance the mutualistic relationship with its insect vector.

Diversity of tomato-infecting begomoviruses and spatiotemporal dynamics of an endemic viral species of the Brazilian Atlantic rain forest biome

Yield losses induced by a complex of begomoviruses are observed across all major tomato-producing areas in Brazil. Tomato severe rugose virus (ToSRV) is the most widespread begomovirus in the country. Conversely, tomato common mosaic virus (ToCmMV) displays a more restricted geographical distribution to areas associated with the Atlantic Rain Forest (ARF) biome, encompassing the States of Espírito Santo-ES, Minas Gerais-MG, and Rio de Janeiro-RJ. Here, we characterized 277 tomato-infecting isolates collected in fields located within the ARF biome from 2006 to 2018. ToSRV displayed the highest prevalence (n = 157), followed by ToCmMV (n = 95) and tomato interveinal chlorosis virus (n = 14). Four other begomoviruses were also detected, but with very low incidences. ToCmMV was the predominant begomovirus in the ARF biome up to 2014-2015 with very low ToSRV incidence. Subsequently, ToSRV became the most prevalent species in ES and RJ, but ToCmMV was still predominating in the "Zona da Mata" meso-region in MG. Due to the remarkable endemic distribution of ToCmMV, we carried out phylogeographical studies of this virus using information from all 28 available isolates with complete DNA-A sequences. The closest common ancestor of ToCmMV was more likely originated around Coimbra-MG area ≈ 25 years before the formal report of this viral species. So far, all surveys indicated tomatoes as the only natural hosts of ToCmMV with outbreaks occurring mainly (but not exclusively) in highland areas. ToSRV shows a more widespread incidence across both highland and lowland areas of the ARF biome.

Apoptosis in a Whitefly Vector Activated by a Begomovirus Enhances Viral Transmission

Of the approximately 1,100 known plant viruses, about one-third are DNA viruses that are vectored by insects. Plant virus infections often induce cellular and molecular responses in their insect vectors, which can, in many cases, affect the spread of viruses. However, the mechanisms underlying vector responses that affect virus accumulation and transmission are poorly understood. Here, we examined the role of virus-induced apoptosis in the transmission of begomoviruses, a group of single-stranded plant DNA viruses that are transmitted by whiteflies and cause extensive damage to many crops worldwide. We demonstrated that virus infection can induce apoptosis in the insect vector conferring protection to the virions from degradation, leading to enhanced viral accumulation and transmission to host plants. Our findings provide valuable clues for designing new strategies to block the transmission of insect-vectored plant viruses, particularly plant DNA viruses.

Apoptosis is generally considered the first line of defense against viral infection. However, the role of apoptosis in the interactions between plant viruses and their insect vectors has rarely been investigated. By studying plant DNA viruses of the genus Begomovirus within the family Geminiviridae, which are transmitted by whiteflies of the Bemisia tabaci species complex in a persistent manner, we revealed that virus-induced apoptosis in insect vectors can facilitate viral accumulation and transmission. We found that infection with tomato yellow leaf curl virus activated the apoptosis pathway in B. tabaci. Suppressing apoptosis by inhibitors or silencing caspase-3 significantly reduced viral accumulation, while the activation of apoptosis increased viral accumulation in vivo. Moreover, the positive effect of whitefly apoptosis on virus accumulation and transmission was not due to its cross talk with the autophagy pathway that suppresses begomovirus infection in whiteflies. We further showed that viral replication, rather than the viral coat protein, is likely the critical factor in the activation of apoptosis by the virus. These novel findings indicate that similarly to many animal and a few plant RNA viruses, plant DNA viruses may activate apoptosis in their insect vectors leading to enhanced viral accumulation and transmission.

IMPORTANCE Of the approximately 1,100 known plant viruses, about one-third are DNA viruses that are vectored by insects. Plant virus infections often induce cellular and molecular responses in their insect vectors, which can, in many cases, affect the spread of viruses. However, the mechanisms underlying vector responses that affect virus accumulation and transmission are poorly understood. Here, we examined the role of virus-induced apoptosis in the transmission of begomoviruses, a group of single-stranded plant DNA viruses that are transmitted by whiteflies and cause extensive damage to many crops worldwide. We demonstrated that virus infection can induce apoptosis in the insect vector conferring protection to the virions from degradation, leading to enhanced viral accumulation and transmission to host plants. Our findings provide valuable clues for designing new strategies to block the transmission of insect-vectored plant viruses, particularly plant DNA viruses.

The threat of seed-transmissible pepper yellow leaf curl Indonesia virus in chili pepper

Recently, chili pepper (Capsicum annuum) plants in Indonesia have been devastated by a notorious bipartite begomovirus infection named Pepper yellow leaf curl Indonesia virus (PepYLCIV), which causes a distinct decrease in chili pepper production. Pepper yellow diseases have been known since early 2000; however, the spread of this virus thus far is distressing. These diseases can reduce chili yields by 20-100% in Indonesia. As previously known, begomovirus can be transmitted through whitefly to several host plants from the families Solanaceae, Compositae, and Leguminosae. In the field, a single plant was observed with severe symptoms of pepper yellow leaf curl disease, while other plants in the same field were asymptomatic and healthy. The observation leads to the possibility that the virus can be transmitted from previously infected chili pepper plants through seeds, as begomovirus transmission through seeds has been reported before. This study was conducted using seeds from chili peppers infected with viruses from different places in Indonesia. Whole seeds, embryos, and seedlings from PepYLCIV infected seeds were investigated in this study by performing viral genome DNA extraction, uracil DNA glycosylase-PCR, and sequencing analysis. Results revealed that both DNA-A and DNA-B of PepYLCIV in seeds and embryos of infected chili pepper plants were detected. The results also showed that 25-67% of PepYLCIV DNA-A and 50-100% of DNA-B were detected from seedlings grown from infected chili pepper seed collected from different location, thus confirming PepYLCIV as a seed-transmissible virus in chili pepper plants.

Identification of begomoviruses from different cryptic species of Bemisia tabaci in Bangladesh

Bemisia tabaci is a global species complex consisting of at least 40 cryptic species. It is also a vector for at least 100 species of begomovirus, many of which cause severe crop damage. The relationship between begomoviruses and cryptic species of the B. tabaci species complex, however, remains unclear. Our previous study [13] was identified four cryptic species (Asia I, Asia II 1, Asia II 5, and Asia II 10) of B. tabaci from Bangladesh. Using those 110 whitefly samples, vector-based PCR analysis identified 8 different begomovirus species: BYVMV, BGYVV, OELCV, SLCCV, SLCV, TbCSV, ToLCBV, and ToLCNDV. The overall rate of virus infection was 26.4%, and BYVMV and ToLCNDV were the most frequently detected in the B. tabaci vector. Virus infection rates for Asia I, Asia II 1, Asia II 5, and Asia II 10 were 22.4% (15/67), 35% (7/20), 27.3% (6/22), and 100% (1/1), respectively. Each cryptic species infected multiple virus species, but SLCCV, TbCSV, and BGYVV were each only detected in, Asia I, Asia II 1, and Asia II 5, respectively. This study demonstrates the geographic distribution of various begomoviruses in Bangladesh and their relationships with cryptic species of B. tabaci.

Engineering Plant Resistance to Tomato Yellow Leaf Curl Thailand Virus Using a Phloem-Specific Promoter Expressing Hairpin RNA

Transgenic approaches employing RNA interference (RNAi) strategies have been successfully applied to generate desired traits in plants; however, variations between RNAi transgenic siblings and the ability to quickly apply RNAi resistance to diverse cultivars remain challenging. In this study, we assessed the promoter activity of a cauliflower mosaic virus 35S promoter (35S) and a phloem-specific promoter derived from rice tungro bacilliform virus (RTBV) and their efficacy to drive RNAi against the endogenous glutamate-1-semialdehyde aminotransferase gene (GSA) that acts as a RNAi marker, through chlorophyll synthesis inhibition, and against tomato yellow leaf curl Thailand virus (TYLCTHV), a begomovirus (family Geminiviridae) reported to be the prevalent cause of tomato yellow leaf curl disease (TYLCD) in Taiwan. Transgenic Nicotiana benthamiana expressing hairpin RNA of GSA driven by either the 35S or RTBV promoter revealed that RTBV::hpGSA induced stronger silencing along the vein and more uniformed silencing phenotype among its siblings than 35S::hpGSA. Analysis of transgenic N. benthamiana, 35S::hpTYLCTHV, and RTBV::hpTYLCTHV revealed that, although 35S::hpTYLCTHV generated a higher abundance of small RNA than RTBV::hpTYLCTHV, RTBV::hpTYLCTHV transgenic plants conferred better TYLCTHV resistance than 35S::hpTYLCTHV. Grafting of wild-type (WT) scions to TYLCTHV RNAi rootstocks allowed transferable TYLCTHV resistance to the scion. A TYLCTHV-inoculation assay showed that noninfected WT scions were only observed when grafted to RTBV::hpTYLCTHV rootstocks but not 35S::hpTYLCTHV nor WT rootstocks. Together, our findings demonstrate an approach that may be widely applied to efficiently confer TYLCD resistance.

The Combined Effect of Elevated O3 Levels and TYLCV Infection Increases the Fitness of Bemisia tabaci Mediterranean on Tomato Plants

Global change and biotic stress, such as tropospheric contamination and virus infection, can individually modify the quality of host plants, thereby altering the palatability of the plant for herbivorous insects. The bottom-up effects of elevated O3 and tomato yellow leaf curl virus (TYLCV) infection on tomato plants and the associated performance of Bemisia tabaci Mediterranean (MED) were determined in open-top chambers. Elevated O3 decreased eight amino acid levels and increased the salicylic acid (SA) and jasmonic acid (JA) content and the gene expression of pathogenesis-related protein (PR1) and proteinase inhibitor (PI1) in both wild-type (CM) and JA defense-deficient tomato genotype (spr2). TYLCV infection and the combination of elevated O3 and TYLCV infection increased eight amino acids levels, SA content and PR1 expression, and decreased JA content and PI1 expression in both tomato genotypes. In uninfected tomato, elevated O3 increased developmental time and decreased fecundity by 6.1 and 18.8% in the CM, respectively, and by 6.8 and 18.9% in the spr2, respectively. In TYLCV-infected tomato, elevated O3 decreased developmental time and increased fecundity by 4.6 and 14.2%, respectively, in the CM and by 4.3 and 16.8%, respectively, in the spr2. These results showed that the interactive effects of elevated O3 and TYLCV infection partially increased the amino acid content and weakened the JA-dependent defense, resulting in increased population fitness of MED on tomato plants. This study suggests that whiteflies would be more successful at TYLCV-infected plants than at uninfected plants in elevated O3 levels.

Lettuce Chlorosis Virus Disease: A New Threat to Cannabis Production

In a survey conducted in Cannabis sativa L. (cannabis) authorized farms in Israel, plants showed disease symptoms characteristic of nutrition deprivation. Interveinal chlorosis, brittleness, and occasional necrosis were observed in older leaves. Next generation sequencing analysis of RNA extracted from symptomatic leaves revealed the presence of lettuce chlorosis virus (LCV), a crinivirus that belongs to the Closteroviridae family. The complete viral genome sequence was obtained using RT-PCR and Rapid Amplification of cDNA Ends (RACE) PCR followed by Sanger sequencing. The two LCV RNA genome segments shared 85-99% nucleotide sequence identity with LCV isolates from GenBank database. The whitefly Bemisia tabaci Middle Eastern Asia Minor1 (MEAM1) biotype transmitted the disease from symptomatic cannabis plants to un-infected 'healthy' cannabis, Lactuca sativa, and Catharanthus roseus plants. Shoots from symptomatic cannabis plants, used for plant propagation, constituted a primary inoculum of the disease. To the best of our knowledge, this is the first report of cannabis plant disease caused by LCV.

Complete genome sequence of a new bipartite begomovirus infecting tomato in Brazil

A novel bipartite begomovirus infecting begomovirus-resistant tomato plants was detected via Illumina sequencing analysis, and its genome sequence was confirmed by Sanger sequencing. The DNA-A (2627 nt) and DNA-B (2587 nt) have a genome organization that is typical of New World bipartite begomoviruses, sharing 82.5% identity with tomato golden leaf distortion virus and 75.1% identity with sida chlorotic vein virus. Based on the current classification criteria for begomoviruses, this isolate should be considered a member of a new species, and the name "tomato interveinal chlorosis virus-2" (ToICV2) is proposed for this virus.

Differences in gene expression in whitefly associated with CYSDV-infected and virus-free melon, and comparison with expression in whiteflies fed on ToCV- and TYLCV-infected tomato

Background

Cucurbit yellow stunting disorder virus (CYSDV; genus Crinivirus, Closteroviridae) is transmitted in a semipersistent manner by the whitefly, Bemisia tabaci, and is efficiently transmitted by the widely prevalent B. tabaci cryptic species, MEAM1. In this study, we compared transcriptome profiles of B. tabaci MEAM1, after 24 h, 72 h and 7 days of acquisition feeding on melon plants infected with CYSDV (CYSDV-whiteflies) with those fed on virus-free melon, using RNA-Seq technology. We also compared transcriptome profiles with whiteflies fed on tomato plants separately infected with Tomato chlorosis virus (ToCV), a crinivirus closely related to CYSDV, and Tomato yellow leaf curl virus (TYLCV), a member of the genus Begomovirus, which has a distinctly different mode of transmission and their respective virus-free controls, to find common gene expression changes among viruliferous whiteflies feeding on different host plants infected with distinct (TYLCV) and related (CYSDV and ToCV) viruses.

Results

A total of 275 differentially expressed genes (DEGs) were identified in CYSDV-whiteflies, with 3 DEGs at 24 h, 221 DEGs at 72 h, and 51 DEGs at 7 days of virus acquisition. Changes in genes encoding orphan genes (54 genes), phosphatidylethanolamine-binding proteins (PEBP) (20 genes), and AAA-ATPase domain containing proteins (10 genes) were associated with the 72 h time point. Several more orphan genes (20 genes) were differentially expressed at 7 days. A total of 59 common DEGs were found between CYSDV-whiteflies and ToCV-whiteflies, which included 20 orphan genes and 6 lysosomal genes. A comparison of DEGs across the three different virus-host systems revealed 14 common DEGs, among which, eight showed similar and significant up-regulation in CYSDV-whiteflies at 72 h and TYLCV-whiteflies at 24 h, while down-regulation of the same genes was observed in ToCV-whiteflies at 72 h.

Conclusions

Dynamic gene expression changes occurred in CYSDV-whiteflies after 72 h feeding, with decreased gene expression changes associated with 7 days of CYSDV acquisition. Similarities in gene expression changes among CYSDV-whiteflies, ToCV-whiteflies and TYLCV-whiteflies suggest the possible involvement of common genes or pathways for virus acquisition and transmission by whiteflies, even for viruses with distinctly different modes of transmission.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5999-0) contains supplementary material, which is available to authorized users.

Higher Bacterial Diversity of Gut Microbiota in Different Natural Populations of Leafhopper Vector Does Not Influence WDV Transmission

The bacterial communities in the gut of an insect have important ecological and functional effects on the insect. However, the community composition and diversity of the gut microbiota in insects that vector plant viruses are poorly understood. As an important insect vector, Psammotettix alienus transmits various viruses including wheat dwarf virus (WDV). Here, we used the combination of leafhopper and WDV as model to survey the influence of gut microbiota on virus transmission characteristic of insect vector and vice versa. We have characterized 22 phyla and 249 genera of all gut bacterial communities in the leafhopper populations collected from six geographic regions in China. Community composition and diversity varied across different geographic populations. However, WDV transmission efficiencies of these six field populations were all greater than 80% with no significant difference. Interestingly, the transmission efficiency of WDV by laboratory reared insects with decreased gut bacterial diversity was similar to that of field populations. Furthermore, we found that the composition of the leafhopper gut bacteria was dynamic and could reversibly respond to WDV acquisition. Higher bacterial diversity and abundance of gut microbiota in different leafhopper populations did not influence their WDV transmission efficiency, while the acquisition of WDV changes gut microbiota by a dynamic and reversible manner. This report provides insight into the complex relationship between the gut microbiota, insect vector and virus.

Plant-Mediated Silencing of the Whitefly Bemisia tabaci Cyclophilin B and Heat Shock Protein 70 Impairs Insect Development and Virus Transmission

The whitefly B. tabaci is a global pest and transmits extremely important plant viruses especially begomoviruses, that cause substantial crop losses. B. tabaci is one of the top invasive species worldwide and have developed resistance to all major pesticide classes. One of the promising alternative ways for controlling this pest is studying its genetic makeup for identifying specific target proteins which are critical for its development and ability to transmit viruses. Tomato yellow leaf curl virus (TYLCV) is the most economically important and well-studied begomovirus transmitted by B. tabaci, in a persistent-circulative manner. Recently, we reported that B. tabaci Cyclophilin B (CypB) and heat shock protein 70 proteins (hsp70) interact and co-localize with TYLCV in the whitefly midgut, on the virus transmission pathway, and that both proteins have a significant role in virus transmission. Here, we extended the previous work and used the Tobacco rattle virus (TRV) plant-mediated RNA silencing system for knocking down both genes and testing the effect of their silencing on whitefly viability and virus transmission. Portions of these two genes were cloned into TRV constructs and tomato plants were infected and used for whitefly feeding and transmission experiments. Following whitefly feeding on TRV-plants, the expression levels of cypB and hsp70 in adult B. tabaci significantly decreased over 72 h feeding period. The knockdown in the expression of both genes was further shown in the first generation of silenced whiteflies, where phenotypic abnormalities in the adult, wing, nymph and bacteriosomes development and structure were observed. Additionally, high mortality rates that reached more than 80% among nymphs and adults were obtained. Finally, silenced whitefly adults with both genes showed decreased ability to transmit TYLCV under lab conditions. Our results suggest that plant-mediated silencing of both cypB and hsp70 have profound effects on whitefly development and its ability to transmit TYLCV.

Global genetic diversity and geographical distribution of Bemisia tabaci and its bacterial endosymbionts

Bemisia tabaci is one of the most threatening pests in agriculture, causing significant losses to many important crops on a global scale. The dramatic increase and availability of sequence data for B. tabaci species complex and its bacterial endosymbionts is critical for developing emerging sustainable pest management strategies which are based on pinpointing the global diversity of this important pest and its bacterial endosymbionts. To unravel the global genetic diversity of B. tabaci species complex focusing on its associated endosymbionts, along with Israeli whitefly populations collected in this study, we combined available sequences in databases, resulting in a total of 4,253 mitochondrial cytochrome oxidase I (mtCOI) sequences from 82 countries and 1,226 16S/23S rRNA endosymbiont sequences from 32 countries that were analyzed. Using Bayesian phylogenetic analysis, we identified two new B. tabaci groups within the species complex and described the global distribution of endosymbionts within this complex. Our analyses revealed complex divergence of the different endosymbiont sequences within the species complex, with overall one Hamiltonella, two Porteria (P1 and P2), two Arsenophonus (A1 and A2), two Wolbachia (super-groups O and B), four Cardinium (C1-C4) and three Rickettsia (R1-R3) groups were identified. Our comprehensive analysis provides an updated important resource for this globally important pest and its secondary symbionts, which have been a major subject for research in last three decades.

Combined infection with Tomato yellow leaf curl virus and Rickettsia influences fecundity, attraction to infected plants and expression of immunity-related genes in the whitefly Bemisia tabaci

We have recently shown that Rickettsia, a secondary facultative bacterial symbiont that infects the whitefly B. tabaci is implicated in the transmission of Tomato yellow leaf curl virus (TYLCV). Infection with Rickettsia improved the acquisition and transmission of the virus by B. tabaci adults. Here we performed a transcriptomic analysis with Rickettsia-infected and uninfected B. tabaci adults before and after TYLCV acquisition. The results show a dramatic and specific activation of the immune system in the presence of Rickettsia before TYLCV acquisition. However, when TYLCV was acquired, it induced massive activation of gene expression in the Rickettsia uninfected population, whereas in the Rickettsia-infected population the virus induced massive down-regulation of gene expression. Fitness and choice experiments revealed that while Rickettsia-infected whiteflies are always more attracted to TYLCV-infected plants, this attraction is not always beneficiary for their offspring. These studies further confirm the role of Rickettsia in many aspects of B. tabaci interactions with TYLCV, and possibly serves as an important factor in the dissemination of the virus.

Differential responses between a vector species Bemisia tabaci and a nonvector species Trialeurodes vaporariorum following ingestion of tomato yellow leaf curl virus

In transmitting plant viruses, insect vectors undergo physiological and behavioral alterations. The whitefly Bemisia tabaci is a vector of tomato yellow leaf curl virus (TYLCV), causing severe damages to various horticultural crop plants. To determine whether whitefly alteration is specific to vector species, the responses to TYLCV ingestion were compared between B. tabaci and Trialeurodes vaporariorum, a nonvector for TYLCV. The two species were reared on TYLCV-infected and noninfected tomato, a host of TYLCV, and their longevity and fecundity were determined while rearing in either tomato or eggplant, a nonhost of TYLCV. TYLCV-ingested B. tabaci increased their developmental rates but reduced fecundity when they were reared in either tomato or eggplant compared with those of TYLCV-free ones. In contrast, TYLCV-ingested T. vaporariorum did not show any of the aforementioned changes when reared on both plant species. In addition, TYLCV-ingested B. tabaci increased their levels of three heat shock protein genes ( hsp20, hsp70, and hsp90) against thermal stress, whereas TYLCV-ingested T. vaporariorum did not. The presence of TYLCV virions was identified in two colonies of both species via polymerase chain reaction analysis. TYLCV was detected in the whole body, saliva, and eggs of B. tabaci, while TYLCV was detected only in the whole body but not in the saliva and eggs of T. vaporariorum. The present results strongly indicated that TYLCV specifically manipulate physiological processes of the vector species, B. tabaci.

Whitefly-transmitted viruses threatening cassava production in Africa

Emerging plant viruses are one of the greatest problems facing crop production worldwide, and have severe consequences in the developing world where subsistence farming is a major source of food production, and knowledge and resources for management are limited. In Africa, evolution of two viral disease complexes, cassava mosaic begomoviruses (CMBs) (Geminiviridae) and cassava brown streak viruses (CBSVs) (Potyviridae), have resulted in severe pandemics that continue to spread and threaten cassava production. Identification of genetically diverse and rapidly evolving CMBs and CBSVs, extensive genetic variation in the vector, Bemisia tabaci (Hemiptera: Aleyrodidae), and numerous secondary endosymbiont profiles that influence vector phenotypes suggest that complex local and regional vector-virus-plant-environment interactions may be driving the evolution and epidemiology of these viruses.

World Management of Geminiviruses

Management of geminiviruses is a worldwide challenge because of the widespread distribution of economically important diseases caused by these viruses. Regardless of the type of agriculture, management is most effective with an integrated pest management (IPM) approach that involves measures before, during, and after the growing season. This includes starting with resistant cultivars and virus- and vector-free transplants and propagative plants. For high value vegetables, protected culture (e.g., greenhouses and screenhouses) allows for effective management but is limited owing to high cost. Protection of young plants in open fields is provided by row covers, but other measures are typically required. Measures that are used for crops in open fields include roguing infected plants and insect vector management. Application of insecticide to manage vectors (whiteflies and leafhoppers) is the most widely used measure but can cause undesirable environmental and human health issues. For annual crops, these measures can be more effective when combined with host-free periods of two to three months. Finally, given the great diversity of the viruses, their insect vectors, and the crops affected, IPM approaches need to be based on the biology and ecology of the virus and vector and the crop production system. Here, we present the general measures that can be used in an IPM program for geminivirus diseases, specific case studies, and future challenges.

Nanovirus DNA-N encodes a protein mandatory for aphid transmission

Nanoviruses possess a multipartite single-stranded DNA genome and are naturally transmitted to plants by various aphid species in a circulative non-propagative manner. Using the cloned genomic DNAs of faba bean necrotic stunt virus (FBNSV) for reconstituting nanovirus infections we analyzed the necessity of different virus components for infection and transmission by aphids. We found that in the absence of DNA-U1 and DNA-U2 symptom severity decreased, and in the absence of DNA-U1 the transmission efficiency decreased. Most significantly, we demonstrated that the protein encoded by DNA-N (NSP) is mandatory for aphid transmission. Moreover, we showed that the NSP of FBNSV could substitute for that of a distantly related nanovirus, pea necrotic yellow dwarf virus. Altering the FBNSV NSP by adding 13 amino acids to its carboxy-terminus resulted in an infectious but non-transmissible virus. We demonstrate that the NSP acts as a nanovirus transmission factor, the existence of which had been hypothesized earlier.

Bemisia tabaci-mediated facilitation in diversity of begomoviruses: Evidence from recent molecular studies

Begomoviruses are considered as one of the most notorious plant viruses worldwide, which cause substantial economic losses to various field crops. Management of begomoviruses has become a challenge due to the continuous evolution and the emergence of new strains. Bemisia tabaci is globally known to be the key vector of begomoviruses, having relatively high reproductivity, fast dispersal ability, high survival rate due to its polyphagous nature and high resistance to various groups of insecticides. Continuous transmission of begomoviruses by the vector has led to the development and spread of epidemics of various diseases worldwide. In this review, we have critically analyzed the various dynamics which facilitate the diversity of begomoviruses through their vector. The interaction of begomovirus-whitefly leads to continuous research activities regarding management of both virus and its vector, thus opening exciting new horizons to formulate potential control strategies to ensure a disease-free cropping environment.

A nation-wide genetic survey revealed a complex population structure of Bemisia tabaci in Pakistan

The whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a cryptic species complex distributed worldwide. In Pakistan, B. tabaci poses a serious threat to agriculture production. To understand its diversity in Pakistan, a large-scale sampling was conducted from various locations of all four provinces of the country and Mitochondrial cytochrome oxidase I (mtCOI) gene sequencing was used to determine the whiteflies genetically. The study revealed the presence of five different cryptic species in Pakistan namely Asia II-1, Asia II-5, Asia II-7, Asia II-8 and MEAM-1, respectively. Among them, Asia II-1, which was previously reported from a few areas in the country, had been found now to be prevalent all over the country covering 88.7% of all the sequenced samples. Based on the mtCOI sequences and genetic distance analyses, the diversity of Asia II-1 was much greater than all other cryptic species, which exist only in small patches.

Genetic diversity of begomoviruses in Pakistan captured through a vector based survey

Begomoviruses (Geminiviridea), transmitted by whiteflies, constitute one of the most dangerous groups of plant viruses posing a severe threat to economically important crops in tropical and sub-tropical areas. In this study, whiteflies were collected from various locations all over Pakistan. The begomoviruses carried by these whiteflies were detected by PCR with the degenerative primers pair AV94/Dep3. Analysis of the 177 sequences obtained in our study, revealed 14 distinct begomovirus species, including five which were not previously reported in this country. Putative novel strains of Corchorus yellow vein virus (CoYVV) and Chilli leaf curl virus (ChiLCV) showing less than 90% identity with the previously available taxa were also identified. The greatest number of begomoviruses per single site was detected in Sindh province, where up to five different begomovirus species were identified from the same cropping field. Moreover, Cotton leaf curl Multan virus - Rajasthan (CLCuMuV-Ra) was found prevalent in all the cotton growing areas. The data reported here may be useful in the development of control measures against begomoviruses.

Transcriptome profiling of whitefly guts in response to Tomato yellow leaf curl virus infection

BACKGROUND

Plant viruses in agricultural crops are of great concern worldwide, and over 75% of them are transmitted from infected to healthy plants by insect vectors. Tomato yellow leaf curl virus (TYLCV) is a begomovirus, which is the largest and most economically important group of plant viruses, transmitted by the whitefly Bemisia tabaci. The circulation of TYLCV in the insect involves complex insect-virus interactions, whereas the molecular mechanisms of these interactions remain ambiguous. The insect gut as a barrier for viral entry and dissemination is thought to regulate the vector specificity. However, due to its tiny size, information for the responses of whitefly gut to virus infection is limited.

METHODS

We investigated the transcriptional response of the gut of B. tabaci Middle East-Asia Minor 1 species to TYLCV infection using Illumina sequencing.

RESULTS

A total of 5207 differentially expressed genes (DEGs) between viruliferous and non-viruliferous whitefly guts were identified. Enrichment analyses showed that cargo receptor and ATP-binding cassette (ABC) transporters were enriched in DEGs, and might help the virus to cross gut barrier. TYLCV could perturb cell cycle and DNA repair as a possible result of its replication in the whitefly. Our data also demonstrated that TYLCV can activate whitefly defense responses, such as antimicrobial peptides. Meanwhile, a number of genes involved in intracellular signaling were activated by TYLCV infection.

CONCLUSIONS

Our results reveal the complex insect-virus relationship in whitefly gut and provide substantial molecular information for the role of insect midguts in virus transmission.

Integration of Omics Approaches toward Understanding Whitefly Transmission of Viruses

Viruses transmitted by whiteflies are predominantly classified as having either persistent circulative or semipersistent transmission, and the majority of studies have addressed transmission of viruses in the genera Begomovirus (family Geminiviridae) and Crinivirus (family Closteroviridae), respectively. Early studies on vector transmission primarily addressed individual aspects of transmission; however, with the breadth of new technology now available, an increasingly greater number of studies involve coordinated research that is beginning to assemble a more complete picture of how whiteflies and viruses have coevolved to facilitate transmission. In particular the integration of gene expression and metabolomic studies into broader research topics is providing knowledge of changes within the whitefly vector in response to the presence of viruses that would have been impossible to identify previously. Examples include comparative studies on the response of Bemisia tabaci to begomovirus and crinivirus infection of common host plants, evolution of whitefly endosymbiont relationships, and opportunities to evaluate responses to specific transmission-related events. Integration of metabolomics, as well as the application of electrical penetration graphing, can lead to an ability to monitor the changes that occur in vector insects associated with specific aspects of virus transmission. Through gaining more complete knowledge of the mechanisms behind whitefly transmission of viruses new control strategies will undoubtedly emerge for control of whiteflies and the viruses they transmit.

Comparative transcriptome analysis reveals networks of genes activated in the whitefly, Bemisia tabaci when fed on tomato plants infected with Tomato yellow leaf curl virus

The whitefly Bemisia tabaci can transmit hundreds of viruses to numerous agricultural crops in the world. Five genera of viruses, including Begomovirus and Crinivirus, are transmitted by B. tabaci. There is little knowledge about the genes involved in virus acquisition and transmission by whiteflies. Using a comparative transcriptomics approach, we evaluated the gene expression profiles of whiteflies (B. tabaci MEAM1) after feeding on tomato infected by a begomovirus, Tomato yellow leaf curl virus (TYLCV), in comparison to a recent study, in which whiteflies were fed on tomato infected by the crinivirus, Tomato chlorosis virus (ToCV). The data revealed similar temporal trends in gene expression, but large differences in the number of whitefly genes when fed on TYLCV or ToCV-infected tomato. Transcription factors, cathepsins, receptors, and a hemocyanin gene, which is implicated in mediating antiviral immune responses in other insects and possibly virus transmission, were some of the genes identified.

Flupyradifurone effectively manages whitefly Bemisia tabaci MED (Hemiptera: Aleyrodidae) and tomato yellow leaf curl virus in tomato

BACKGROUND

The cotton whitefly Bemisia tabaci (Gennadius) is among the most important pests of numerous crops and a vector of more than 100 plant viruses, causing significant crop losses worldwide. Managing this pest as well as inhibiting the transmission of major viruses such as tomato yellow leaf curl virus (TYLCV) are of utmost importance for sustainable yields. The efficacy against both whitefly and virus transmission of the novel systemic butenolide insecticide flupyradifurone was investigated in this study.

RESULTS

The inhibition of TYLCV transmission by flupyradifurone was compared to that by thiamethoxam, a neonicotinoid insecticide reported to inhibit virus transmission. The experiment was performed under high virus pressure conditions (10 viruliferous insects per plant for 48 h) using a fully characterized field strain of B. tabaci. The insecticides were foliarly applied at recommended label rates under greenhouse conditions. Flupyradifurone suppressed virus transmission by 85% while levels of suppression after thiamethoxam treatments were just 25% and significantly lower. In untreated control plots, 100% of plants were infected by TYLCV. The observed difference in the potential to suppress virus transmission is linked to a strong knockdown effect as well as prolonged feeding inhibition in flupyradifurone treatments.

CONCLUSION

Flupyradifurone is shown to be an extremely useful, fast-acting, new chemical tool in integrated crop management offering simultaneous control of whiteflies and strong suppression of viral infections via its rapid knockdown action and good residual activity. © 2017 Society of Chemical Industry.

Effects of different temperature regimes on survival of Diaphorina citri and its endosymbiotic bacterial communities

The Asian citrus psyllid, Diaphorina citri, is a major pest of citrus and vector of citrus greening (huanglongbing) in Asian. In our field-collected psyllid samples, we discovered that Fuzhou (China) and Faisalabad (Pakistan), populations harbored an obligate primary endosymbiont Candidatus Carsonella (gen. nov.) with a single species, Candidatus Carsonella ruddii (sp. nov.) and a secondary endosymbiont, Wolbachia surface proteins (WSP) which are intracellular endosymbionts residing in the bacteriomes. Responses of these symbionts to different temperatures were examined and their host survival assessed. Diagnostic PCR assays showed that the endosymbionts infection rates were not significantly reduced in both D. citri populations after 24 h exposure to cold or heat treatments. Although quantitative PCR assays showed significant reduction of WSP relative densities at 40°C for 24 h, a substantial decrease occurred as the exposure duration increased beyond 3 days. Under the same temperature regimes, Ca. C. ruddii density was initially less affected during the first exposure day, but rapidly reduced at 3-5 days compared to WSP. However, the mortality of the psyllids increased rapidly as exposure time to heat treatment increased. The responses of the two symbionts to unfavorable temperature regimes highlight the complex host-symbionts interactions between D. citri and its associated endosymbionts.

Genetics and Genomics of Cotton Leaf Curl Disease, Its Viral Causal Agents and Whitefly Vector: A Way Forward to Sustain Cotton Fiber Security

Cotton leaf curl disease (CLCuD) after its first epidemic in 1912 in Nigeria, has spread to different cotton growing countries including United States, Pakistan, India, and China. The disease is of viral origin-transmitted by the whitefly Bemisia tabaci, which is difficult to control because of the prevalence of multiple virulent viral strains or related species. The problem is further complicated as the CLCuD causing virus complex has a higher recombination rate. The availability of alternate host crops like tomato, okra, etc., and practicing mixed type farming system have further exaggerated the situation by adding synergy to the evolution of new viral strains and vectors. Efforts to control this disease using host plant resistance remained successful using two gene based-resistance that was broken by the evolution of new resistance breaking strain called Burewala virus. Development of transgenic cotton using both pathogen and non-pathogenic derived approaches are in progress. In future, screening for new forms of host resistance, use of DNA markers for the rapid incorporation of resistance into adapted cultivars overlaid with transgenics and using genome editing by CRISPR/Cas system would be instrumental in adding multiple layers of defense to control the disease-thus cotton fiber production will be sustained.

Clathrin-mediated endocytosis is involved in Tomato yellow leaf curl virus transport across the midgut barrier of its whitefly vector

Tomato yellow leaf curl virus (TYLCV) is a begomovirus transmitted by the whitefly Bemisia tabaci. The circulative translocation of the virus in the insect is known in its broad line. However, transit of TYLCV from the digestive tract into the haemolymph is poorly understood. We studied the involvement of clathrin in this process by disrupting the clathrin-mediated endocytosis and the endosome network using inhibitor feeding, antibody blocking and dsRNA silencing. We monitored the quantities of TYLCV in the whitefly and virus transmission efficiency. Following endocytosis and endosome network disruption, the quantity of virus was higher in the midgut relative to that of the whole insect body, and the quantity of virus in the haemolymph was reduced. The transmission efficiency of TYLCV by the treated insects was also reduced. These findings indicate that clathrin-mediated endocytosis and endosomes play an important role in the transport of TYLCV across the whitefly midgut.

The draft genome of whitefly Bemisia tabaci MEAM1, a global crop pest, provides novel insights into virus transmission, host adaptation, and insecticide resistance

BACKGROUND

The whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is among the 100 worst invasive species in the world. As one of the most important crop pests and virus vectors, B. tabaci causes substantial crop losses and poses a serious threat to global food security.

RESULTS

We report the 615-Mb high-quality genome sequence of B. tabaci Middle East-Asia Minor 1 (MEAM1), the first genome sequence in the Aleyrodidae family, which contains 15,664 protein-coding genes. The B. tabaci genome is highly divergent from other sequenced hemipteran genomes, sharing no detectable synteny. A number of known detoxification gene families, including cytochrome P450s and UDP-glucuronosyltransferases, are significantly expanded in B. tabaci. Other expanded gene families, including cathepsins, large clusters of tandemly duplicated B. tabaci-specific genes, and phosphatidylethanolamine-binding proteins (PEBPs), were found to be associated with virus acquisition and transmission and/or insecticide resistance, likely contributing to the global invasiveness and efficient virus transmission capacity of B. tabaci. The presence of 142 horizontally transferred genes from bacteria or fungi in the B. tabaci genome, including genes encoding hopanoid/sterol synthesis and xenobiotic detoxification enzymes that are not present in other insects, offers novel insights into the unique biological adaptations of this insect such as polyphagy and insecticide resistance. Interestingly, two adjacent bacterial pantothenate biosynthesis genes, panB and panC, have been co-transferred into B. tabaci and fused into a single gene that has acquired introns during its evolution.

CONCLUSIONS

The B. tabaci genome contains numerous genetic novelties, including expansions in gene families associated with insecticide resistance, detoxification and virus transmission, as well as numerous horizontally transferred genes from bacteria and fungi. We believe these novelties likely have shaped B. tabaci as a highly invasive polyphagous crop pest and efficient vector of plant viruses. The genome serves as a reference for resolving the B. tabaci cryptic species complex, understanding fundamental biological novelties, and providing valuable genetic information to assist the development of novel strategies for controlling whiteflies and the viruses they transmit.

The draft genome of whitefly Bemisia tabaci MEAM1, a global crop pest, provides novel insights into virus transmission, host adaptation, and insecticide resistance

Background

The whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is among the 100 worst invasive species in the world. As one of the most important crop pests and virus vectors, B. tabaci causes substantial crop losses and poses a serious threat to global food security.

Results

We report the 615-Mb high-quality genome sequence of B. tabaci Middle East-Asia Minor 1 (MEAM1), the first genome sequence in the Aleyrodidae family, which contains 15,664 protein-coding genes. The B. tabaci genome is highly divergent from other sequenced hemipteran genomes, sharing no detectable synteny. A number of known detoxification gene families, including cytochrome P450s and UDP-glucuronosyltransferases, are significantly expanded in B. tabaci. Other expanded gene families, including cathepsins, large clusters of tandemly duplicated B. tabaci-specific genes, and phosphatidylethanolamine-binding proteins (PEBPs), were found to be associated with virus acquisition and transmission and/or insecticide resistance, likely contributing to the global invasiveness and efficient virus transmission capacity of B. tabaci. The presence of 142 horizontally transferred genes from bacteria or fungi in the B. tabaci genome, including genes encoding hopanoid/sterol synthesis and xenobiotic detoxification enzymes that are not present in other insects, offers novel insights into the unique biological adaptations of this insect such as polyphagy and insecticide resistance. Interestingly, two adjacent bacterial pantothenate biosynthesis genes, panB and panC, have been co-transferred into B. tabaci and fused into a single gene that has acquired introns during its evolution.

Conclusions

The B. tabaci genome contains numerous genetic novelties, including expansions in gene families associated with insecticide resistance, detoxification and virus transmission, as well as numerous horizontally transferred genes from bacteria and fungi. We believe these novelties likely have shaped B. tabaci as a highly invasive polyphagous crop pest and efficient vector of plant viruses. The genome serves as a reference for resolving the B. tabaci cryptic species complex, understanding fundamental biological novelties, and providing valuable genetic information to assist the development of novel strategies for controlling whiteflies and the viruses they transmit.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0321-y) contains supplementary material, which is available to authorized users.