Virion stability is important for the circulative transmission of tomato yellow leaf curl sardinia virus by Bemisia tabaci, but virion access to salivary glands does not guarantee transmissibility

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.

Emerging evidence of seed transmission of begomoviruses: implications in global circulation and disease outbreak

Begomoviruses (family Geminiviridae) are known for causing devastating diseases in fruit, fibre, pulse, and vegetable crops throughout the world. Begomoviruses are transmitted in the field exclusively through insect vector whitefly (Bemisia tabaci), and the frequent outbreaks of begomoviruses are attributed largely due to the abundance of whitefly in the agri-ecosystem. Begomoviruses being phloem-borne were known not be transmitted through seeds of the infected plants. The recent findings of seed transmission of begomoviruses brought out a new dimension of begomovirus perpetuation and dissemination. The first convincing evidence of seed transmission of begomoviruses was known in 2015 for sweet potato leaf curl virus followed by several begomoviruses, like bhendi yellow vein mosaic virus, bitter gourd yellow mosaic virus, dolichos yellow mosaic virus, mungbean yellow mosaic virus, mungbean yellow mosaic India virus, pepper yellow leaf curl Indonesia virus, tomato leaf curl New Delhi virus, tomato yellow leaf curl virus, tomato yellow leaf curl Sardinia virus, and okra yellow mosaic Mexico virus. These studies brought out two perspectives of seed-borne nature of begomoviruses: (i) the presence of begomovirus in the seed tissues derived from the infected plants but no expression of disease symptoms in the progeny seedlings and (ii) the seed infection successfully transmitted the virus to cause disease to the progeny seedlings. It seems that the seed transmission of begomovirus is a feature of a specific combination of host-genotype and virus strain, rather than a universal phenomenon. This review comprehensively describes the seed transmitted begomoviruses reported in the last 9 years and the possible mechanism of seed transmission. An emphasis is placed on the experimental results that proved the seed transmission of various begomoviruses, factors affecting seed transmission and impact of begomovirus seed transmission on virus circulation, outbreak of the disease, and management strategies.

The essential role of clathrin-mediated endocytosis and early endosomes in the trafficking of begomoviruses through the primary salivary glands of their whitefly vectors

IMPORTANCE

Many plant viruses are transmitted by insect vectors in a circulative manner. For efficient transmission, the entry of the virus from vector hemolymph into the primary salivary gland (PSG) is a step of paramount importance. Yet, vector components mediating virus entry into PSG remain barely characterized. Here, we demonstrate the role of clathrin-mediated endocytosis and early endosomes in begomovirus entry into whitefly PSG. Our findings unravel the key components involved in begomovirus transport within the whitefly body and transmission by their whitefly vectors and provide novel clues for blocking begomovirus transmission.

Current status, breeding strategies and future prospects for managing chilli leaf curl virus disease and associated begomoviruses in Chilli (Capsicum spp.)

Chilli leaf curl virus disease caused by begomoviruses, has emerged as a major threat to global chilli production, causing severe yield losses and economic harm. Begomoviruses are a highly successful and emerging group of plant viruses that are primarily transmitted by whiteflies belonging to the Bemisia tabaci complex. The most effective method for mitigating chilli leaf curl virus disease losses is breeding for host resistance to Begomovirus. This review highlights the current situation of chilli leaf curl virus disease and associated begomoviruses in chilli production, stressing the significant issues that breeders and growers confront. In addition, the various breeding methods used to generate begomovirus resistant chilli cultivars, and also the complicated connections between the host plant, vector and the virus are discussed. This review highlights the importance of resistance breeding, emphasising the importance of multidisciplinary approaches that combine the best of traditional breeding with cutting-edge genomic technologies. subsequently, the article highlights the challenges that must be overcome in order to effectively deploy begomovirus resistant chilli varieties across diverse agroecological zones and farming systems, as well as understanding the pathogen thus providing the opportunities for improving the sustainability and profitability of chilli production.

Differential Transmission of Old and New World Begomoviruses by Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) Cryptic Species of Bemisia tabaci

Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) are two of the most invasive members of the sweetpotato whitefly, Bemisia tabaci, cryptic species complexes and are efficient vectors of begomoviruses. Bemisia tabaci MEAM1 is the predominant vector of begomoviruses in open-field vegetable crops in the southeastern United States. However, recently B. tabaci MED also has been detected in the landscape outside of greenhouses in Florida and Georgia. This study compared the transmission efficiency of one Old-World (OW) and two New-World (NW) begomoviruses prevalent in the southeastern United States, viz.., tomato yellow leaf curl virus (TYLCV), cucurbit leaf crumple virus (CuLCrV), and sida golden mosaic virus (SiGMV) between B. tabaci MEAM1 and B. tabaci MED. Bemisia tabaci MEAM1 efficiently transmitted TYLCV, CuLCrV, or SiGMV, whereas B. tabaci MED only transmitted TYLCV. Percent acquisition and retention of OW TYLCV following a 72 h acquisition access period was significantly higher for B. tabaci MED than B. tabaci MEAM1. In contrast, B. tabaci MEAM1 acquired and retained significantly more NW bipartite begomoviruses, CuLCrV or SiGMV, than B. tabaci MED. Quantitative analysis (qPCR) of virus DNA in whitefly internal tissues revealed reduced accumulation of CuLCrV or SiGMV in B. tabaci MED than in B. tabaci MEAM1. Fluorescent in situ hybridization (FISH) showed localization of CuLCrV or SiGMV in the midgut of B. tabaci MED and B. tabaci MEAM1. However, localization of CuLCrV or SiGMV was only observed in the primary salivary glands of B. tabaci MEAM1 and not B. tabaci MED. TYLCV localization was observed in all internal tissues of B. tabaci MEAM1 and B. tabaci MED. Overall, results demonstrate that both B. tabaci MEAM1 and B. tabaci MED are efficient vectors of OW TYLCV. However, for the NW begomoviruses, CuLCrV and SiGMV, B. tabaci MEAM1 seems to a better vector.

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.

Insights into the multifunctional roles of geminivirus-encoded proteins in pathogenesis

Geminiviruses are a major threat to agriculture in tropical and subtropical regions of the world. Geminiviruses have small genome with limited coding capacity. Despite this limitation, these viruses have mastered hijacking the host cellular metabolism for their survival. To compensate for the small size of their genome, geminiviruses encode multifunctional proteins. In addition, geminiviruses associate themselves with satellite DNA molecules which also encode proteins that support the virus in establishing successful infection. Geminiviral proteins recruit multiple host factors, suppress the host defense, and manipulate host metabolism to establish infection. We have updated the knowledge accumulated about the proteins of geminiviruses and their satellites in the context of pathogenesis in a single review. We also discuss their interactions with host factors to provide a mechanistic understanding of the infection process.

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.

Population diversity of cassava mosaic begomoviruses increases over the course of serial vegetative propagation

Cassava mosaic disease (CMD) represents a serious threat to cassava, a major root crop for more than 300 million Africans. CMD is caused by single-stranded DNA begomoviruses that evolve rapidly, making it challenging to develop durable disease resistance. In addition to the evolutionary forces of mutation, recombination and reassortment, factors such as climate, agriculture practices and the presence of DNA satellites may impact viral diversity. To gain insight into the factors that alter and shape viral diversity in planta, we used high-throughput sequencing to characterize the accumulation of nucleotide diversity after inoculation of infectious clones corresponding to African cassava mosaic virus (ACMV) and East African cassava mosaic Cameroon virus (EACMCV) in the susceptible cassava landrace Kibandameno. We found that vegetative propagation had a significant effect on viral nucleotide diversity, while temperature and a satellite DNA did not have measurable impacts in our study. EACMCV diversity increased linearly with the number of vegetative propagation passages, while ACMV diversity increased for a time and then decreased in later passages. We observed a substitution bias toward C→T and G→A for mutations in the viral genomes consistent with field isolates. Non-coding regions excluding the promoter regions of genes showed the highest levels of nucleotide diversity for each genome component. Changes in the 5' intergenic region of DNA-A resembled the sequence of the cognate DNA-B sequence. The majority of nucleotide changes in coding regions were non-synonymous, most with predicted deleterious effects on protein structure, indicative of relaxed selection pressure over six vegetative passages. Overall, these results underscore the importance of knowing how cropping practices affect viral evolution and disease progression.

Transmission parameters of pepper whitefly-borne vein yellows virus (PeWBVYV) by Bemisia tabaci and identification of an insect protein with a putative role in polerovirus transmission

Pepper crops in Israel are infected by poleroviruses, Pepper vein yellows virus 2 (PeVYV-2) and Pepper whitefly-borne vein yellows virus (PeWBVYV). Herein we characterize the transmission of PeWBVYV and the aphid-transmitted PeVYV-2, and show that PeWBVYV is specifically transmitted by MEAM1 species of the whitefly Bemisia tabaci, with a minimum latency period of 120 h, and not by the Mediterranean (MED). PeWBVYV and PeVYV-2 were detected in the hemolymph of MED and MEAM1, respectively, however, amounts of PeWBVYV in the hemolymph of MED or PeVYV-2 in MEAM1 were much lower than PeWBVYV in hemolymph of MEAM1. Moreover, we show that PeWBVYV does not interact with the GroEL protein of the symbiont Hamiltonella and thus does not account for the non-transmissibility by MED. An insect glycoprotein, C1QBP, interacting in vitro with the capsid proteins of both PeWBVYV and PeVYV-2 is reported which suggests a putative functional role in polerovirus transmission.

Insect Transmission of Plant Single-Stranded DNA Viruses

Of the approximately 1,200 plant virus species that have been described to date, nearly one-third are single-stranded DNA (ssDNA) viruses, and all are transmitted by insect vectors. However, most studies of vector transmission of plant viruses have focused on RNA viruses. All known plant ssDNA viruses belong to two economically important families, Geminiviridae and Nanoviridae, and in recent years, there have been increased efforts to understand whether they have evolved similar relationships with their respective insect vectors. This review describes the current understanding of ssDNA virus-vector interactions, including how these viruses cross insect vector cellular barriers, the responses of vectors to virus circulation, the possible existence of viral replication within insect vectors, and the three-way virus-vector-plant interactions. Despite recent breakthroughs in our understanding of these viruses, many aspects of plant ssDNA virus transmission remain elusive. More effort is needed to identify insect proteins that mediate the transmission of plant ssDNA viruses and to understand the complex virus-insect-plant three-way interactions in the field during natural infection.

Plant-virus-insect tritrophic interactions: insights into the functions of geminivirus virion-sense strand genes

The genome of the plant-infecting viruses in the family Geminiviridae is composed of one or two circular single stranded DNA of approximately 2.7-5.2 kb in length. These viruses have emerged as the most devastating pathogen infecting a large number of crops and weeds across the continents. They code for fewer open reading frames (ORFs) through the generation of overlapping transcripts derived from the bidirectional viral promoters. Members of geminiviruses code for up to four ORFs in the virion-sense strand, and their gene expression is regulated by various cis-elements located at their promoters in the intergenic region. These viral proteins perform multiple functions at every stage of the viral life cycle such as virus transport, insect-mediated virus transmission and suppression of host defence. They impede the host's multi-layered antiviral mechanisms including gene silencing (at transcriptional and post-transcriptional levels) and hypersensitive response. This review summarizes the essential role of virion-sense strand encoded proteins in transport of viral genomes within and between plant cells, countering defence in hosts (both plants and the insects), and also in the ubiquitous role in vector-mediated transmission. We highlight the significance of their pro-viral activities in manipulating host-derived innate immune responses and the interaction with whitefly-derived proteins. We also discuss the current knowledge on virus replication and transcription within the insect body.

Mutations in the coat protein of a begomovirus result in altered transmission by different species of whitefly vectors

For many crop pathogens including viruses, high genetic variation provides them with potential to adapt to and prevail in a changing environment. Understanding genetic variation in viruses and their significance is a key to elaborate virus epidemiology and evolution. While genetic variation of plant viruses has been documented to impact virus–host interactions, how it affects virus–insect vector interactions remains elusive. Here, we report the impact of mutations in the coat protein of squash leaf curl China virus (SLCCNV), a begomovirus, on the interaction between the virus and its whitefly vectors. We characterized mutations in the coat protein of SLCCNV and found that some residues exhibited higher mutation frequency than the others. We assayed the impact of mutation on infectivity using agroinoculation and found these mutations marginally affect virus infectivity. We further analyze their functions using virus acquisition and transmission trials and found some of mutations resulted in altered transmission of SLCCNV by different species of the whitefly Bemisia tabaci complex. We then identified the key amino acid residue(s) involved by constructing several mutant viruses and found that a single-residue mutation in the coat protein of SLCCNV was sufficient to significantly alter the whitefly transmission characteristics of SLCCNV. We examined the competition between different genotypes of SLCCNV in plant infection and whitefly transmission. We found that mutations in the coat protein did not alter the fitness of SLCCNV in plants, but they rendered the virus more competitive in transmission by certain species of whiteflies. Our findings indicate that mutations in the coat protein may play a key role in both the adaptation of begomoviruses to the changing vector populations and the evolution of begomoviruses.

Plant Antiviral Immunity Against Geminiviruses and Viral Counter-Defense for Survival

The family Geminiviridae includes plant-infecting viruses whose genomes are composed of one or two circular non-enveloped ssDNAs(+) of about 2.5-5.2 kb each in size. These insect-transmissible geminiviruses cause significant crop losses across continents and pose a serious threat to food security. Under the control of promoters generally located within the intergenic region, their genomes encode five to eight ORFs from overlapping viral transcripts. Most proteins encoded by geminiviruses perform multiple functions, such as suppressing defense responses, hijacking ubiquitin-proteasomal pathways, altering hormonal responses, manipulating cell cycle regulation, and exploiting protein-signaling cascades. Geminiviruses establish complex but coordinated interactions with several host elements to spread and facilitate successful infection cycles. Consequently, plants have evolved several multilayered defense strategies against geminivirus infection and distribution. Recent studies on the evasion of host-mediated resistance factors by various geminivirus proteins through novel mechanisms have provided new insights into the development of antiviral strategies against geminiviruses. This review summarizes the current knowledge concerning virus movement within and between cells, as well as the recent advances in our understanding of the biological roles of virus-encoded proteins in manipulating host-mediated responses and insect transmission. This review also highlights unexplored areas that may increase our understanding of the biology of geminiviruses and how to combat these important plant pathogens.

A distinct seed-transmissible strain of tomato leaf curl New Delhi virus infecting Chayote in India

Chayote (Sechium edule (Jacq. Sw.) is a single seeded cucurbitaceous vegetable crop mainly grown for its fruit. During 2015-2016, mosaic and leaf distortion type of symptoms were observed in chayote plants in hilly regions of Tamil Nadu. The disease incidence was 50. 3-100% and yield loss was about 69.9% in Dindigul district. The infected chayote plants showed yellow spots, yellow mosaic, leaf curling, puckering, and enations. The fruits of infected plants were malformed and were not marketable. The begomovirus causing the disease was identified as a variant of tomato leaf curl New Delhi virus. The chayote isolates of ToLCNDV share only 91 to 92% identity with other ToLCNDV isolates, deserving to be designated as distinct strain. The phylogenetic analysis on the basis of DNA A component nucleotides clearly indicated common origin of chayote, ridge gourd, ash gourd isolates of India along with Spanish isolates of ToLCNDV. This was contrasting to diverse origin of ToLCNDV isolates from other countries. The virus was sap transmissible to selected cucurbitaceous hosts. The whitefly population (Asia-I) reared in the glass house transmitted the virus to bottle gourd with 4 h of acquisition access period (AAP) and 24 h of inoculation feeding period (IFP). Heavy infestation of greenhouse whiteflies, Trialeurodes vaporariorum on infected chayote plant in Kodaikanal paved way to investigations on its role in vector transmission of ToLCNDV. The field population of T. vaporariorum was found to be viruliferous in PCR using virus specific primers. The greenhouse whitefly efficiently transmitted the virus with 4 h of AAP and 24 h IFP. Seed-borne nature of ToLCNDV was confirmed in PCR by using Roja's and ToLCNDV specific primers in different parts of the fruit viz., pericarp, mesocarp, seed coat, endosperm and embryo. This is the first report of seed transmissible nature of ToLCNDV, its implication in transboundary movement of the virus across several countries is discussed.

Differential Shape of Geminivirus Mutant Spectra Across Cultivated and Wild Hosts With Invariant Viral Consensus Sequences

Geminiviruses (family Geminiviridae) possess single-stranded circular DNA genomes that are replicated by cellular polymerases in plant host cell nuclei. In their hosts, geminivirus populations behave as ensembles of mutant and recombinant genomes, known as viral quasispecies. This favors the emergence of new geminiviruses with altered host range, facilitating new or more severe diseases or overcoming resistance traits. In warm and temperate areas several whitefly-transmitted geminiviruses of the genus Begomovirus cause the tomato yellow leaf curl disease (TYLCD) with significant economic consequences. TYLCD is frequently controlled in commercial tomatoes by using the dominant Ty-1 resistance gene. Over a 45 day period we have studied the diversification of three begomoviruses causing TYLCD: tomato yellow leaf curl virus (TYLCV), tomato yellow leaf curl Sardinia virus (TYLCSV) and tomato yellow leaf curl Malaga virus (TYLCMaV, a natural recombinant between TYLCV and TYLCSV). Viral quasispecies resulting from inoculation of geminivirus infectious clones were examined in plants of susceptible tomato (ty-1/ty-1), heterozygous resistant tomato (Ty-1/ty-1), common bean, and the wild reservoir Solanum nigrum. Differences in virus fitness across hosts were observed while viral consensus sequences remained invariant. However, the complexity and heterogeneity of the quasispecies were high, especially in common bean and the wild host. Interestingly, the presence or absence of the Ty-1 allele in tomato did not lead to differences in begomovirus mutant spectra. However, the fitness decrease of TYLCSV and TYLCV in tomato at 45 dpi might be related to an increase in CP (Coat protein) mutation frequency. In Solanum nigrum the recombinant TYLCMaV, which showed lower fitness than TYLCSV, at 45 dpi actively explored Rep (Replication associated protein) ORF but not the overlapping C4. Our results underline the importance of begomovirus mutant spectra during infections. This is especially relevant in the wild reservoir of the viruses, which has the potential to maintain highly diverse mutant spectra without modifying their consensus sequences.

The level of midgut penetration of two begomoviruses affects their acquisition and transmission by two species of Bemisia tabaci

Begomoviruses are transmitted by whiteflies in a persistent manner, but factors responsible for the variation of virus transmission by different species are poorly understood. We examined ingestion of papaya leaf curl China virus (PaLCuCNV) and tomato yellow leaf curl virus (TYLCV) by two species of the Bemisia tabaci complex, MEAM1 and MED, and then quantified the virion concentrations in different organs/tissues in each species. We found that PaLCuCNV penetrated the midgut wall of MED less efficiently than MEAM1, resulting in lower efficiency of PalCuCNV transmission by MED than that by MEAM1, while TYLCV penetrated the midgut wall of both species and was transmitted by them at similar levels of efficiency. Virus coat protein determined the virus capacity to cross the midgut wall of a given whitefly species. These data indicate that the level of midgut penetration determines virus acquisition and transmission by whiteflies in the first instance.

The Incredible Journey of Begomoviruses in Their Whitefly Vector

Begomoviruses are vectored in a circulative persistent manner by the whitefly Bemisia tabaci. The insect ingests viral particles with its stylets. Virions pass along the food canal and reach the esophagus and the midgut. They cross the filter chamber and the midgut into the haemolymph, translocate into the primary salivary glands and are egested with the saliva into the plant phloem. Begomoviruses have to cross several barriers and checkpoints successfully, while interacting with would-be receptors and other whitefly proteins. The bulk of the virus remains associated with the midgut and the filter chamber. In these tissues, viral genomes, mainly from the tomato yellow leaf curl virus (TYLCV) family, may be transcribed and may replicate. However, at the same time, virus amounts peak, and the insect autophagic response is activated, which in turn inhibits replication and induces the destruction of the virus. Some begomoviruses invade tissues outside the circulative pathway, such as ovaries and fat cells. Autophagy limits the amounts of virus associated with these organs. In this review, we discuss the different sites begomoviruses need to cross to complete a successful circular infection, the role of the coat protein in this process and the sites that balance between virus accumulation and virus destruction.

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 complete nucleotide sequence and genomic characterization of grapevine asteroid mosaic associated virus

In analyzing grapevine clones infected with grapevine red blotch associated virus, we identified a small number of isometric particles of approximately 30nm in diameter from an enriched fraction of leaf extract. A dominant protein of 25kDa was isolated from this fraction using SDS-PAGE and was identified by mass spectrometry as belonging to grapevine asteroid mosaic associated virus (GAMaV). Using a combination of three methods RNA-Seq, sRNA-Seq, and Sanger sequencing of RT- and RACE-PCR products, we obtained a full-length genome sequence consisting of 6719 nucleotides without the poly(A) tail. The virus possesses all of the typical conserved functional domains concordant with the genus Marafivirus and lies evolutionarily between citrus sudden death associated virus and oat blue dwarf virus. A large shift in RNA-Seq coverage coincided with the predicted location of the subgenomic RNA involved in coat protein (CP) expression. Genus wide sequence alignments confirmed the cleavage motif LxG(G/A) to be dominant between the helicase and RNA dependent RNA polymerase (RdRp), and the RdRp and CP domains. A putative overlapping protein (OP) ORF lacking a canonical translational start codon was identified with a reading frame context more consistent with the putative OPs of tymoviruses and fig fleck associated virus than with those of marafiviruses. BLAST analysis of the predicted GAMaV OP showed a unique relatedness to the OPs of members of the genus Tymovirus.

Identification of amino acid residues of the coat protein of Sri Lankan cassava mosaic virus affecting symptom production and viral titer in Nicotiana benthamiana

Sri Lankan cassava mosaic virus (SLCMV) is bipartite begomovirus infecting cassava in India and Sri Lanka. Interestingly, the DNA-A component of the SLCMV alone is able to infect Nicotiana benthamiana causing symptoms of upward leaf rolling and stunting. One of the differences between monopartite and bipartite begomoviruses is the requirement of Coat Protein (CP) for infectivity; CP being essential for the former, but dispensable in the latter. This investigation was aimed to determine the importance of CP in the infectivity of the bipartite SLCMV, behaving as a monopartite virus in N. benthamiana. We tested CP-null mutants, single amino acid replacement mutants and double, triple and quadruple combinations of the above in SLCMV DNA-A, for infectivity, symptom development and viral DNA accumulation in N. benthamiana. While CP-null mutants were non-infectious, a majority of the single amino acid replacement mutants and their combinations retained infectivity, some with attenuated symptoms and reduced viral titers. Some of the combined mutations restored the attenuated symptoms to wild type levels. Some of the mutations were predicted to cause changes in the secondary structure of the CP, which roughly correlated with the attenuation of symptoms and the reduction in viral titers.

Analysis of the Mild strain of tomato yellow leaf curl virus, which overcomes Ty-2 gene-mediated resistance in tomato line H24

In tomato line H24, an isolate of the Mild (Mld) strain of tomato yellow leaf curl virus (TYLCV-Mld [JR:Kis]) overcomes Ty-2 gene-mediated resistance and causes typical symptoms of tomato yellow leaf curl disease (TYLCD). No systemic infection with visible symptoms or accumulation of viral DNA in the upper leaves was observed in H24 challenged with another isolate, TYLCV-IL (TYLCV-IL [JR:Osaka]), confirming that H24 is resistant to the IL strain. To elucidate the genomic regions that cause the breakdown of the Ty-2 gene-mediated resistance, we constructed a series of chimeras by swapping genes between the two strains. A chimeric virus that had the overlapping C4/Rep region of the Mld strain in the context of the IL strain genome, caused severe TYLCD in H24 plants, suggesting that the overlapping C4/Rep region of the Mld strain is associated with the ability of this strain to overcome Ty-2 gene-mediated resistance.

Rapid accumulation and low degradation: key parameters of Tomato yellow leaf curl virus persistence in its insect vector Bemisia tabaci

Of worldwide economic importance, Tomato yellow leaf curl virus (TYLCV, Begomovirus) is responsible for one of the most devastating plant diseases in warm and temperate regions. The DNA begomoviruses (Geminiviridae) are transmitted by the whitefly species complex Bemisia tabaci. Although geminiviruses have long been described as circulative non-propagative viruses, observations such as long persistence of TYLCV in B. tabaci raised the question of their possible replication in the vector. We monitored two major TYLCV strains, Mild (Mld) and Israel (IL), in the invasive B. tabaci Middle East-Asia Minor 1 cryptic species, during and after the viral acquisition, within two timeframes (0-144 hours or 0-20 days). TYLCV DNA was quantified using real-time PCR, and the complementary DNA strand of TYLCV involved in viral replication was specifically quantified using anchored real-time PCR. The DNA of both TYLCV strains accumulated exponentially during acquisition but remained stable after viral acquisition had stopped. Neither replication nor vertical transmission were observed. In conclusion, our quantification of the viral loads and complementary strands of both Mld and IL strains of TYLCV in B. tabaci point to an efficient accumulation and preservation mechanism, rather than to a dynamic equilibrium between replication and degradation.

Persistent, circulative transmission of begomoviruses by whitefly vectors

Begomoviruses comprise an emerging and economically important group of plant viruses exclusively transmitted by the sweetpotato whitefly Bemisia tabaci in many regions of the world. The past twenty years have witnessed significant progress in studying the molecular interactions between members of this virus group and B. tabaci. Mechanisms and proteins encoded by the insect vector and its bacterial symbionts, which have been shown to be important for virus transmission, have been identified and thoroughly studied. Despite the economic importance of this group of viruses and their impact on the global agriculture, progress in investigating the virus-vector interactions is moving slowly when compared with similar virus-vector systems in plants and animals. Major advances in this field and future perspectives will be discussed in this review.

Replication of Tomato Yellow Leaf Curl Virus in Its Whitefly Vector, Bemisia tabaci

Tomato yellow leaf curl virus (TYLCV) is a begomovirus transmitted exclusively by the whitefly Bemisia tabaci in a persistent, circulative manner. Replication of TYLCV in its vector remains controversial, and thus far, the virus has been considered to be nonpropagative. Following 8 h of acquisition on TYLCV-infected tomato plants or purified virions and then transfer to non-TYLCV-host cotton plants, the amounts of virus inside whitefly adults significantly increased (>2-fold) during the first few days and then continuously decreased, as measured by the amounts of genes on both virus DNA strands. Reported alterations in insect immune and defense responses upon virus retention led us to hypothesize a role for the immune response in suppressing virus replication. After virus acquisition, stress conditions were imposed on whiteflies, and the levels of three viral gene sequences were measured over time. When whiteflies were exposed to TYLCV and treatment with two different pesticides, the virus levels continuously increased. Upon exposure to heat stress, the virus levels gradually decreased, without any initial accumulation. Switching of whiteflies between pesticide, heat stress, and control treatments caused fluctuating increases and decreases in virus levels. Fluorescence in situ hybridization analysis confirmed these results and showed virus signals inside midgut epithelial cell nuclei. Combining the pesticide and heat treatments with virus acquisition had significant effects on fecundity. Altogether, our results demonstrate for the first time that a single-stranded DNA plant virus can replicate in its hemipteran vector.

IMPORTANCE

Plant viruses in agricultural crops are of great concern worldwide. Many of them are transmitted from infected to healthy plants by insects. Persistently transmitted viruses often have a complex association with their vectors; however, most are believed not to replicate within these vectors. Such replication is important, as it contributes to the virus's spread and can impact vector biology. Tomato yellow leaf curl virus (TYLCV) is a devastating begomovirus that infects tomatoes. It is persistently transmitted by the whitefly Bemisia tabaci but is believed not to replicate in the insect. To demonstrate that TYLCV is, in fact, propagative (i.e., it replicates in its insect host), we hypothesized that insect defenses play a role in suppressing virus replication. We thus exposed whitefly to pesticide and heat stress conditions to manipulate its physiology, and we showed that under such conditions, the virus is able to replicate and significantly influence the insect's fecundity.

Alfalfa Leaf Curl Virus: an Aphid-Transmitted Geminivirus

The family Geminiviridae comprises seven genera differentiated by genome organization, sequence similarity, and insect vector. Capulavirus, an eighth genus, has been proposed to accommodate two newly discovered highly divergent geminiviruses that presently have no known vector. Alfalfa leaf curl virus, identified here as a third capulavirus, is shown to be transmitted by Aphis craccivora. This is the first report of an aphid-transmitted geminivirus.

Circulative, "nonpropagative" virus transmission: an orchestra of virus-, insect-, and plant-derived instruments

Species of plant viruses within the Luteoviridae, Geminiviridae, and Nanoviridae are transmitted by phloem-feeding insects in a circulative, nonpropagative manner. The precise route of virus movement through the vector can differ across and within virus families, but these viruses all share many biological, biochemical, and ecological features. All share temporal and spatial constraints with respect to transmission efficiency. The viruses also induce physiological changes in their plant hosts resulting in behavioral changes in the insects that optimize the transmission of virus to new hosts. Virus proteins interact with insect, endosymbiont, and plant proteins to orchestrate, directly and indirectly, virus movement in insects and plants to facilitate transmission. Knowledge of these complex interactions allows for the development of new tools to reduce or prevent transmission, to quickly identify important vector populations, and to improve the management of these economically important viruses affecting agricultural and natural plant populations.

Quantification and localization of Watermelon chlorotic stunt virus and Tomato yellow leaf curl virus (Geminiviridae) in populations of Bemisia tabaci (Hemiptera, Aleyrodidae) with differential virus transmission characteristics

Bemisia tabaci (Gennadius) is one of the economically most damaging insects to crops in tropical and subtropical regions. Severe damage is caused by feeding and more seriously by transmitting viruses. Those of the genus begomovirus (Geminiviridae) cause the most significant crop diseases and are transmitted by B. tabaci in a persistent circulative mode, a process which is largely unknown. To analyze the translocation and to identify critical determinants for transmission, two populations of B. tabaci MEAM1 were compared for transmitting Watermelon chlorotic stunt virus (WmCSV) and Tomato yellow leaf curl virus (TYLCV). Insect populations were chosen because of their high and respectively low virus transmission efficiency to compare uptake and translocation of virus through insects. Both populations harbored Rickettsia, Hamiltonella and Wolbachia in comparable ratios indicating that endosymbionts might not contribute to the different transmission rates. Quantification by qPCR revealed that WmCSV uptake and virus concentrations in midguts and primary salivary glands were generally higher than TYLCV due to higher virus contents of the source plants. Both viruses accumulated higher in insects from the efficiently compared to the poorly transmitting population. In the latter, virus translocation into the hemolymph was delayed and virus passage was impeded with limited numbers of viruses translocated. FISH analysis confirmed these results with similar virus distribution found in excised organs of both populations. No virus accumulation was found in the midgut lumen of the poor transmitter because of a restrained virus translocation. Results suggest that the poorly transmitting population comprised insects that lacked transmission competence. Those were selected to develop a population that lacks virus transmission. Investigations with insects lacking transmission showed that virus concentrations in midguts were reduced and only negligible virus amounts were found at the primary salivary glands indicating for a missing or modified receptor responsible for virus attachment or translocation.

Contrasting genetic structure between two begomoviruses infecting the same leguminous hosts

Begomoviruses are whitefly-transmitted, ssDNA plant viruses and are among the most damaging pathogens causing epidemics in economically important crops worldwide. Wild/non-cultivated plants play a crucial epidemiological role, acting as begomovirus reservoirs and as ‘mixing vessels' where recombination can occur. Previous work suggests a higher degree of genetic variability in begomovirus populations from non-cultivated hosts compared with cultivated hosts. To assess this supposed host effect on the genetic variability of begomovirus populations, cultivated (common bean, Phaseolus vulgaris, and lima bean, Phaseolus lunatus) and non-cultivated (Macroptilium lathyroides) legume hosts were sampled from two regions of Brazil. A total of 212 full-length DNA-A genome segments were sequenced from samples collected between 2005 and 2012, and populations of the begomoviruses Bean golden mosaic virus (BGMV) and Macroptilium yellow spot virus (MaYSV) were obtained. We found, for each begomovirus species, similar genetic variation between populations infecting cultivated and non-cultivated hosts, indicating that the presumed genetic variability of the host did not a priori affect viral variability. We observed a higher degree of genetic variation in isolates from MaYSV populations than BGMV populations, which was explained by numerous recombination events in MaYSV. MaYSV and BGMV showed distinct distributions of genetic variation, with the BGMV population (but not MaYSV) being structured by both host and geography.

Specific cells in the primary salivary glands of the whitefly Bemisia tabaci control retention and transmission of begomoviruses

The majority of plant viruses are vectored by arthropods via persistent-circulative or noncirculative transmission. Previous studies have shown that specific binding sites for noncirculative viruses reside within the stylet or foregut of insect vectors, whereas the transmission mechanisms of circulative viruses remain ambiguous. Here we report the critical roles of whitefly primary salivary glands (PSGs) in the circulative transmission of two begomoviruses. The Middle East Asia Minor 1 (MEAM1) species of the whitefly Bemisia tabaci complex efficiently transmits both Tomato yellow leaf curl China virus (TYLCCNV) and Tomato yellow leaf curl virus (TYLCV), whereas the Mediterranean (MED) species transmits TYLCV but not TYLCCNV. PCR and fluorescence in situ hybridization experiments showed that TYLCCNV efficiently penetrates the PSGs of MEAM1 but not MED whiteflies. When a fragment of the coat protein of TYLCCNV was exchanged with that of TYLCV, mutated TYLCCNV accumulated in the PSGs of MED whiteflies, while mutant TYLCV was nearly undetectable. Confocal microscopy revealed that virion transport in PSGs follows specific paths to reach secretory cells in the central region, and the accumulation of virions in the secretory region of PSGs was correlated with successful virus transmission. Our findings demonstrate that whitefly PSGs, in particular the cells around the secretory region, control the specificity of begomovirus transmission.

IMPORTANCE

Over 75% of plant viruses are transmitted by insects. However, the mechanisms of virus transmission by insect vectors remain largely unknown. Begomoviruses and whiteflies are a complex of viruses and vectors which threaten many crops worldwide. We investigated the transmission of two begomoviruses by two whitefly species. We show that specific cells of the whitefly primary salivary glands control viral transmission specificity and that virion transport in the glands follows specific paths to reach secretory cells in the central region and then to reach the salivary duct. Our results indicate that the secretory cells in the central region of primary salivary glands determine the recognition and transmission of begomoviruses. These findings set a foundation for future research not only on circulative plant virus transmission but also on other human and animal viruses transmitted by arthropod vectors.

Implication of the bacterial endosymbiont Rickettsia spp. in interactions of the whitefly Bemisia tabaci with tomato yellow leaf curl virus

Numerous animal and plant viruses are transmitted by arthropod vectors in a persistent, circulative manner. Tomato yellow leaf curl virus (TYLCV) is transmitted by the sweet potato whitefly Bemisia tabaci. We report here that infection with Rickettsia spp., a facultative endosymbiont of whiteflies, altered TYLCV-B. tabaci interactions. A B. tabaci strain infected with Rickettsia acquired more TYLCV from infected plants, retained the virus longer, and exhibited nearly double the transmission efficiency compared to an uninfected B. tabaci strain with the same genetic background. Temporal and spatial antagonistic relationships were discovered between Rickettsia and TYLCV within the whitefly. In different time course experiments, the levels of virus and Rickettsia within the insect were inversely correlated. Fluorescence in situ hybridization analysis of Rickettsia-infected midguts provided evidence for niche exclusion between Rickettsia and TYLCV. In particular, high levels of the bacterium in the midgut resulted in higher virus concentrations in the filter chamber, a favored site for virus translocation along the transmission pathway, whereas low levels of Rickettsia in the midgut resulted in an even distribution of the virus. Taken together, these results indicate that Rickettsia, by infecting the midgut, increases TYLCV transmission efficacy, adding further insights into the complex association between persistent plant viruses, their insect vectors, and microorganism tenants that reside within these insects.

IMPORTANCE

Interest in bacterial endosymbionts in arthropods and many aspects of their host biology in agricultural and human health systems has been increasing. A recent and relevant studied example is the influence of Wolbachia on dengue virus transmission by mosquitoes. In parallel with our recently studied whitefly-Rickettsia-TYLCV system, other studies have shown that dengue virus levels in the mosquito vector are inversely correlated with bacterial load. Our work here presents evidence of unifying principles between vectors of plant and animal viruses in a role for endosymbionts in manipulating vector biology and pathogen transmission. Our results demonstrate the influence of an interesting and prominent bacterial endosymbiont in Bemisia tabaci in TYLCV transmission, a worldwide disease infecting tomatoes. Besides its agricultural importance, this system provides interesting insights into Bemisia interaction with these newly discovered endosymbionts.

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 are a group of icosahedral single stranded DNA viruses exclusively transmitted by the sweet potato whitefly Bemisia tabaci in a persistent, circulative manner. In this mode of transmission, begomoviruses are acquired by their insect vector as intact virions from the plant phloem, move along the food canal, foregut and esophagus and reach the midgut where they are absorbed into the hemolymph via the filter chamber. The filter chamber is the site where most of the ingested food is filtered, and the first site where the majority of begomoviruses appear to be translocated into the hemolymph via unknown proteins or receptors. Transport from the filter chamber to the hemolymph is aided by a Heat Shock Protein 70. Virus particles not translocated across the filter chamber circulate in the midgut loop but it is not known whether absorption into the hemolymph occurs along this loop. Localization studies have confirmed that begomoviruses are not associated with the hindgut and absorption of virions in this organ is unlikely. In the hemolymph, virions have been shown to interact with a GroEL chaperone produced by the whitefly's endosymbiontic bacteria for ensuring their safe journey to the salivary glands. Virions penetrate the primary salivary glands via unknown proteins or receptors and are transported and secreted outside the whitefly to the plant with salivary secretions. Several recent studies have demonstrated the implications of insect and endosymbiont proteins such as the heat shock protein 70 and the bacterial GroEL protein, in the transmission of begomoviruses by B. tabaci. Additional studies attempting to identify other proteins that aid or interact with begomoviruses along their circulation pathway in the whitefly are reviewed in this paper.

Evolution of increased survival in RNA viruses specialized on cancer-derived cells

Viruses and other pathogens can diverge in their evolved host-use strategies because of exposure to different host types and conflicts between within-host reproduction and between-host survival. Most host-pathogen studies have emphasized the role of intrahost reproduction in the evolution of pathogen virulence, whereas the role of extra-host survival has received less attention. Here, we examine the evolution of free-living virion survival in RNA virus populations differing in their histories of host use. To do so, we used lineages of vesicular stomatitis virus (VSV) that were experimentally evolved in laboratory tissue culture for 100 generations on cancer-derived cells, noncancerous cells, or alternating passages of the two host types. We observed that free-living survival improved when VSV populations specialized on human epithelial carcinoma (HeLa) cells, whereas this trait was not associated with selection on noncancer cells or combinations of the cell types. We attributed this finding to shorter-lived HeLa monolayers and/or rapid cell-to-cell spread of viruses on HeLa cells in tissue culture, both of which could select for enhanced virus stability between host-cell replenishment. We also showed evidence that increases in virion survival were associated with decreases in virulence, which suggests a trade-off between survival and virulence for the VSV populations on one cell type. Our results shed new light on the causes and consequences of "sit and wait" infection strategies in RNA viruses.

Status and prospects of plant virus control through interference with vector transmission

Most plant viruses rely on vector organisms for their plant-to-plant spread. Although there are many different natural vectors, few plant virus-vector systems have been well studied. This review describes our current understanding of virus transmission by aphids, thrips, whiteflies, leafhoppers, planthoppers, treehoppers, mites, nematodes, and zoosporic endoparasites. Strategies for control of vectors by host resistance, chemicals, and integrated pest management are reviewed. Many gaps in the knowledge of the transmission mechanisms and a lack of available host resistance to vectors are evident. Advances in genome sequencing and molecular technologies will help to address these problems and will allow innovative control methods through interference with vector transmission. Improved knowledge of factors affecting pest and disease spread in different ecosystems for predictive modeling is also needed. Innovative control measures are urgently required because of the increased risks from vector-borne infections that arise from environmental change.

Progressive aggregation of Tomato yellow leaf curl virus coat protein in systemically infected tomato plants, susceptible and resistant to the virus

Tomato yellow leaf curl virus (TYLCV) coat protein (CP) accumulated in tomato leaves during infection. The CP was immuno-detected in the phloem associated cells. At the early stages of infection, punctate signals were detected in the cytoplasm, while in the later stages aggregates of increasing size were localized in cytoplasm and nuclei. Sedimentation of protein extracts through sucrose gradients confirmed that progress of infection was accompanied by the formation of CP aggregates of increasing size. Genomic ssDNA was found in the cytoplasm and in the nucleus, while the dsDNA replicative form was exclusively associated with the nucleus. CP-DNA complexes were detected by immuno-capture PCR in nuclear and cytoplasmic large aggregates. Nuclear aggregates contained infectious particles transmissible to test plants by whiteflies. In contrast to susceptible tomatoes, the formation of large CP aggregates in resistant plants was delayed. By experimentally changing the level of resistance/susceptibility of plants, we showed that maintenance of midsized CP aggregates was associated with resistance, while large aggregates where characteristic of susceptibility. We propose that sequestering of virus CP into midsized aggregates and retarding the formation of large insoluble aggregates containing infectious particles is part of the response of resistant plants to TYLCV.

In planta production of a candidate vaccine against bovine papillomavirus type 1

Bovine papillomavirus type 1 (BPV-1) is an economically important virus that induces tumourigenic pathologies in horses and cows. Given that the BPV-1 L1 major coat protein can self-assemble into highly immunogenic higher-order structures, we transiently expressed it in Nicotiana benthamiana as a prelude to producing a candidate vaccine. It was found that plant codon optimization of L1 gave higher levels of expression than its non-optimized counterpart. Following protein extraction, we obtained high yields (183 mg/kg fresh weight leaf tissue) of relatively pure L1, which had self-assembled into virus-like particles (VLPs). We found that these VLPs elicited a highly specific and strong immune response, and therefore they may have utility as a potential vaccine. This is the first report demonstrating the viable production of a candidate BPV vaccine protein in plants.

Whitefly-mediated transmission of cotton leaf curl Multan betasatellite: evidence for betasatellite encapsidation in coat protein of helper begomoviruses

Cotton leaf curl Multan betasatellite (CLCuMB) is responsible for symptom expression of a devastating disease of cotton in the Indian subcontinent. CLCuMB depends on helper virus replication-associated protein for its replication and on viral coat protein (CP) for its encapsidation. However, no direct evidence of encapsidation of CLCuMB in viral CP has been available. In the present study, non-viruliferous whiteflies were placed on tomato plants that had been agroinoculated with infectious clones of an Iranian isolate of tomato yellow leaf curl virus (TYLCV-[Ab]) and CLCuMB for an acquisition access period of 72 h and then transferred to healthy tomato seedlings at the 3- to 4-leaf stage. Typical symptoms of TYLCV-[Ab] appeared on inoculated seedlings 30-45 days post-inoculation. The presence of TYLCV-[Ab] and CLCuMB DNAs in symptomatic test plants and viruliferous whiteflies was confirmed by PCR analysis using specific primers and DIG Southern blotting. Furthermore, the possibility of CLCuMB DNA encapsidation in TYLCV-[Ab] CP within infected plants was examined by immunocapture PCR. The results showed that CLCuMB DNA was encapsidated in TYLCV-[Ab] CP. Whitefly-mediated transmission of CLCuMB in the presence of helper virus is additional evidence for encapsidation of CLCuMB by TYLCV-[Ab] CP.

Arsenophonus GroEL interacts with CLCuV and is localized in midgut and salivary gland of whitefly B. tabaci

Cotton leaf curl virus (CLCuV) (Gemininiviridae: Begomovirus) is the causative agent of leaf curl disease in cotton plants (Gossypium hirsutum). CLCuV is exclusively transmitted by the whitefly species B. tabaci (Gennadius) (Hemiptera: Alerodidae). B. tabaci contains several biotypes which harbor dissimilar bacterial endo-symbiotic community. It is reported that these bacterial endosymbionts produce a 63 kDa chaperon GroEL protein which binds to geminivirus particles and protects them from rapid degradation in gut and haemolymph. In biotype B, GroEL protein of Hamiltonella has been shown to interact with Tomato yellow leaf curl virus (TYLCV). The present study was initiated to find out whether endosymbionts of B. tabaci are similarly involved in CLCuV transmission in Sriganganagar (Rajasthan), an area endemic with cotton leaf curl disease. Biotype and endosymbiont diversity of B. tabaci were identified using MtCO1 and 16S rDNA genes respectively. Analysis of our results indicated that the collected B. tabaci population belong to AsiaII genetic group and harbor the primary endosymbiont Portiera and the secondary endosymbiont Arsenophonus. The GroEL proteins of Portiera and Arsenophonus were purified and in-vitro interaction studies were carried out using pull down and co-immunoprecipitation assays. In-vivo interaction was confirmed using yeast two hybrid system. In both in-vitro and in-vivo studies, the GroEL protein of Arsenophonus was found to be interacting with the CLCuV coat protein. Further, we also localized the presence of Arsenophonus in the salivary glands and the midgut of B. tabaci besides the already reported bacteriocytes. These results suggest the involvement of Arsenophonus in the transmission of CLCuV in AsiaII genetic group of B. tabaci.

On the alleged origin of geminiviruses from extrachromosomal DNAs of phytoplasmas

Background

Several phytoplasmas, wall-less phloem limited plant pathogenic bacteria, have been shown to contain extrachromosomal DNA (EcDNA) molecules encoding a replication associated protein (Rep) similar to that of geminiviruses, a major group of single stranded (ss) DNA plant viruses. On the basis of that observation and of structural similarities between the capsid proteins of geminiviruses and the Satellite tobacco necrosis virus, it has been recently proposed that geminiviruses evolved from phytoplasmal EcDNAs by acquiring a capsid protein coding gene from a co-invading plant RNA virus.

Results

Here we show that this hypothesis has to be rejected because (i) the EcDNA encoded Rep is not of phytoplasmal origin but has been acquired by phytoplasmas through horizontal transfer from a geminivirus or its ancestor; and (ii) the evolution of geminivirus capsid protein in land plants implies missing links, while the analysis of metagenomic data suggests an alternative scenario implying a more ancient evolution in marine environments.

Conclusion

The hypothesis of geminiviruses evolving in plants from DNA molecules of phytoplasma origin contrasts with other findings. An alternative scenario concerning the origin and spread of Rep coding phytoplasmal EcDNA is presented and its implications on the epidemiology of phytoplasmas are discussed.

Global analysis of the transcriptional response of whitefly to tomato yellow leaf curl China virus reveals the relationship of coevolved adaptations

The begomoviruses are the largest and most economically important group of plant viruses transmitted exclusively by the whitefly Bemisia tabaci in a circulative, persistent manner. The circulation of the viruses within the insect vectors involves complex interactions between virus and vector components; however, the molecular mechanisms of these interactions remain largely unknown. Here we investigated the transcriptional response of the invasive B. tabaci Middle East-Asia Minor 1 species to Tomato yellow leaf curl China virus (TYLCCNV) using Illumina sequencing technology. Results showed that 1,606 genes involved in 157 biochemical pathways were differentially expressed in the viruliferous whiteflies. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated that TYLCCNV can perturb the cell cycle and primary metabolism in the whitefly, which explains the negative effect of this virus on the longevity and fecundity of B. tabaci. Our data also demonstrated that TYLCCNV can activate whitefly immune responses, such as autophagy and antimicrobial peptide production, which might lead to a gradual decrease of viral particles within the body of the viruliferous whitefly. Furthermore, PCR results showed that TYLCCNV can invade the ovary and fat body tissues of the whitefly, and Lysotracker and Western blot analyses revealed that the invasion of TYLCCNV induced autophagy in both the ovary and fat body tissues. Surprisingly, TYLCCNV also suppressed the whitefly immune responses by downregulating the expression of genes involved in Toll-like signaling and mitogen-activated protein kinase (MAPK) pathways. Taken together, these results reveal the relationship of coevolved adaptations between begomoviruses and whiteflies and will provide a road map for future investigations into the complex interactions between plant viruses and their insect vectors.

Tomato yellow leaf curl viruses: ménage à trois between the virus complex, the plant and the whitefly vector

Tomato yellow leaf curl disease (TYLCD) is one of the most devastating viral diseases affecting tomato crops in tropical, subtropical and temperate regions of the world. Here, we focus on the interactions through recombination between the different begomovirus species causing TYLCD, provide an overview of the interactions with the cellular genes involved in viral replication, and highlight recent progress on the relationships between these viruses and their vector, the whitefly Bemisia tabaci.

TAXONOMY

The tomato yellow leaf curl virus-like viruses (TYLCVs) are a complex of begomoviruses (family Geminiviridae, genus Begomovirus) including 10 accepted species: Tomato yellow leaf curl Axarquia virus (TYLCAxV), Tomato yellow leaf curl China virus (TYLCCNV), Tomato yellow leaf curl Guangdong virus (TYLCGuV), Tomato yellow leaf curl Indonesia virus (TYLCIDV), Tomato yellow leaf curl Kanchanaburi virus (TYLVKaV), Tomato yellow leaf curl Malaga virus (TYLCMalV), Tomato yellow leaf curl Mali virus (TYLCMLV), Tomato yellow leaf curl Sardinia virus (TYLCSV), Tomato yellow leaf curl Thailand virus (TYLCTHV), Tomato yellow leaf curl Vietnam virus (TYLCVNV) and Tomato yellow leaf curl virus(TYLCV). We follow the species demarcation criteria of the International Committee on Taxonomy of Viruses (ICTV), the most important of which is an 89% nucleotide identity threshold between full-length DNA-A component nucleotide sequences for begomovirus species. Strains of a species are defined by a 93% nucleotide identity threshold.

HOST RANGE

The primary host of TYLCVs is tomato (Solanum lycopersicum), but they can also naturally infect other crops [common bean (Phaseolus vulgaris), sweet pepper (Capsicum annuum), chilli pepper (C. chinense) and tobacco (Nicotiana tabacum)], a number of ornamentals [petunia (Petuniaxhybrida) and lisianthus (Eustoma grandiflora)], as well as common weeds (Solanum nigrum and Datura stramonium). TYLCVs also infect the experimental host Nicotiana benthamiana.

DISEASE SYMPTOMS

Infected tomato plants are stunted or dwarfed, with leaflets rolled upwards and inwards; young leaves are slightly chlorotic; in recently infected plants, fruits might not be produced or, if produced, are small and unmarketable. In common bean, some TYLCVs produce the bean leaf crumple disease, with thickening, epinasty, crumpling, blade reduction and upward curling of leaves, as well as abnormal shoot proliferation and internode reduction; the very small leaves result in a bushy appearance.

Detection of environmental Vibrio parahaemolyticus using a polyclonal antibody by flow cytometry

The aim of this study was to detect and quantify Vibrio parahaemolyticus using flow cytometry (FCM) in combination with a polyclonal antibody developed in our laboratory. Experiments were carried out using V. parahaemolyticus cells in pure and mixed bacteria culture suspensions in either artificial or natural seawater. Using FCM, V. parahaemolyticus cells labelled with the polyclonal antibody and a secondary fluorescein isothiocyanate-conjugated antibody were detected and rapidly quantified at low cell densities (10(3) cells ml(-1) ) in both the pure and mixed cultures. To determine the specificity of our antibody, its cross-reactivity with other ATCC bacterial strains and some environmental Vibrio spp. and Gram-positive isolates was also assessed. Significant immunoreactivity levels above background were obtained for V. harvey 64, V. parahaemolyticus 704 and V. alginolyticus 1407, although the intensities were significantly less than for V. parahaemolyticus Conero. The experiments carried out in natural seawater confirmed the antibody specificity towards V. parahaemolyticus Conero even if a lower proportion of labelled cells was observed. The application of FCM in combination with a primary polyclonal antibody appears to be a promising technique for the detection and quantification of V. parahaemolyticus cells in aquatic environments.