William M. Wintermantel

Vector Specificity, Host Range, and Genetic Diversity of Tomato chlorosis virus

Tomato chlorosis virus (ToCV), family Closteroviridae, genus Crinivirus, causes interveinal chlorosis, leaf brittleness, and limited necrotic flecking or leaf bronzing on tomato leaves. ToCV can cause a decline in plant vigor and reduce fruit yield. It is emerging as a serious production problem for field and greenhouse tomato growers, and has been increasing in prevalence in many parts of the world. The virus is unique among known whitefly-transmitted viruses, due to its ability to be transmitted by four whitefly vectors from two genera. Studies demonstrated that transmission efficiency and virus persistence in the vector varies significantly among the different whitefly vectors. Trialeurodes abutilonea and Bemisia tabaci biotype B are highly efficient vectors of ToCV. B. tabaci biotype A and T. vaporariorum are less efficient vectors, but are fully capable of transmission. ToCV persists for up to 5 days in T. abutilonea, 2 days in B. tabaci biotype B, and only 1 day in B. tabaci biotype A and T. vaporariorum. ToCV has a moderately wide host range, infecting 24 host plant species in seven families. A portion of the coat protein coding region of five geographically diverse ToCV isolates was compared and found to be highly conserved. This information, coupled with existing information on conservation within the heat shock protein 70 homologue coding region, suggests that many ToCV isolates throughout the world are related very closely, and may have been distributed on plant material.

Co-Infection by Two Criniviruses Alters Accumulation of Each Virus in a Host-Specific Manner and Influences Efficiency of Virus Transmission

Tomato chlorosis virus (ToCV), and Tomato infectious chlorosis virus (TICV), family Closteroviridae, genus Crinivirus, cause interveinal chlorosis, leaf brittleness, and limited necrotic flecking or bronzing on tomato leaves. Both viruses cause a decline in plant vigor and reduce fruit yield, and are emerging as serious production problems for field and greenhouse tomato growers in many parts of the world. The viruses have been found together in tomato, indicating that infection by one Crinivirus sp. does not prevent infection by a second. Transmission efficiency and virus persistence in the vector varies significantly among the four different whitefly vectors of ToCV; Bemisia tabaci biotypes A and B, Trialeurodes abutilonea, and T. vaporariorum. Only T. vaporariorum can transmit TICV. In order to elucidate the effects of co-infection on Crinivirus sp. accumulation and transmission efficiency, we established Physalis wrightii and Nicotiana benthamiana source plants, containing either TICV or ToCV alone or both viruses together. Vectors were allowed to feed separately on all virus sources, as well as virus-free plants, then were transferred to young plants of both host species. Plants were tested by quantitative reverse-transcription polymerase chain reaction, and results indicated host-specific differences in accumulation by TICV and ToCV and alteration of accumulation patterns during co-infection compared with single infection. In N. benthamiana, TICV titers increased during co-infection compared with levels in single infection, while ToCV titers decreased. However, in P. wrightii, titers of both TICV and ToCV decreased during mixed infection compared with single infection, although to different degrees. Vector transmission efficiency of both viruses corresponded with virus concentration in the host in both single and mixed infections. This illustrates that Crinivirus epidemiology is impacted not only by vector transmission specificity and incidence of hosts but also by interactions between viruses and efficiency of accumulation in host plants.

The complete nucleotide sequence and genome organization of tomato chlorosis virus

Summary.The crinivirus tomato chlorosis virus (ToCV) was discovered initially in diseased tomato and has since been identified as a serious problem for tomato production in many parts of the world, particularly in the United States, Europe and Southeast Asia. The complete nucleotide sequence of ToCV was determined and compared with related crinivirus species. RNA 1 is organized into four open reading frames (ORFs), and encodes proteins involved in replication, based on homology to other viral replication factors. RNA 2 is composed of nine ORFs including genes that encode a HSP70 homolog and two proteins involved in encapsidation of viral RNA, referred to as the coat protein and minor coat protein. Sequence homology between ToCV and other criniviruses varies throughout the viral genome. The minor coat protein (CPm) of ToCV, which forms part of the “rattlesnake tail” of virions and may be involved in determining the unique, broad vector transmissibility of ToCV, is larger than the CPm of lettuce infectious yellows virus (LIYV) by 217 amino acids. Among sequenced criniviruses, considerable variability exists in the size of some viral proteins. Analysis of these differences with respect to biological function may provide insight into the role crinivirus proteins play in virus infection and transmission.

Metabolic Coevolution in the Bacterial Symbiosis of Whiteflies and Related Plant Sap-Feeding Insects

Genomic decay is a common feature of intracellular bacteria that have entered into symbiosis with plant sap-feeding insects. This study of the whitefly Bemisia tabaci and two bacteria (Portiera aleyrodidarum and Hamiltonella defensa) cohoused in each host cell investigated whether the decay of Portiera metabolism genes is complemented by host and Hamiltonella genes, and compared the metabolic traits of the whitefly symbiosis with other sap-feeding insects (aphids, psyllids, and mealybugs). Parallel genomic and transcriptomic analysis revealed that the host genome contributes multiple metabolic reactions that complement or duplicate Portiera function, and that Hamiltonella may contribute multiple cofactors and one essential amino acid, lysine. Homologs of the Bemisia metabolism genes of insect origin have also been implicated in essential amino acid synthesis in other sap-feeding insect hosts, indicative of parallel coevolution of shared metabolic pathways across multiple symbioses. Further metabolism genes coded in the Bemisia genome are of bacterial origin, but phylogenetically distinct from Portiera, Hamiltonella and horizontally transferred genes identified in other sap-feeding insects. Overall, 75% of the metabolism genes of bacterial origin are functionally unique to one symbiosis, indicating that the evolutionary history of metabolic integration in these symbioses is strongly contingent on the pattern of horizontally acquired genes. Our analysis, further, shows that bacteria with genomic decay enable host acquisition of complex metabolic pathways by multiple independent horizontal gene transfers from exogenous bacteria. Specifically, each horizontally acquired gene can function with other genes in the pathway coded by the symbiont, while facilitating the decay of the symbiont gene coding the same reaction.

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

BackgroundThe 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.ResultsWe 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.ConclusionsThe 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.

Curly top survey in the Western United States.

Curly top in sugar beet continues to be a challenging disease to control in the western United States. To aid in development of host resistance and management options, the curtovirus species composition was investigated by sampling 246 commercial fields along with nursery and field trials in the western United States. DNA was isolated from leaf samples and the species were identified using species-specific polymerase chain reaction primers for the C1 gene. Amplicons from 79 isolates were also sequenced to confirm identifications. Beet severe curly top virus (BSCTV) and Beet mild curly top virus (BMCTV) were widely distributed throughout the western United States, while only a few isolates of Beet curly top virus (BCTV) were found. In phylogenetic analysis, BSCTV, BMCTV, and BCTV isolates formed distinct groups in the dendrogram. Seven isolates not amplifiable with species-specific primers did amplify with curly top coat protein primers, indicating novel curtovirus species or strains may be present. Given the wide host range of the viruses responsible for curly top, frequent co-infections, and genetic diversity within and among species, establishing better host resistance, and controlling curly top will continue to be a challenge.

Co-infection of Beet mosaic virus with Beet Yellowing Viruses Leads to Increased Symptom Expression on Sugar Beet

Three distinct aphid-transmitted viruses associated with a yellowing disease on sugar beet were examined in single and mixed infections for the effects of virus interactions on plant weight, rate of symptom development, and virus concentration. Sugar beet lines exhibiting different degrees of susceptibility to the virus yellows complex were inoculated with either one, two, or all three viruses. Severe stunting, as measured by fresh plant biomass, was observed with mixed infections with Beet yellows virus (BYV) and Beet mosaic virus (BtMV), compared to single infections of these viruses. In addition, the overall rate of appearance of Beet western yellows virus (BWYV) symptoms increased during co-infection with BtMV. Synergistic effects on stunting severity, as measured by plant biomass, were more pronounced in susceptible beet lines, but similar patterns also were observed in lines exhibiting tolerance to virus yellows. Relative concentrations of viruses were compared among single and mixed infections using dot-blot hybridization with virus specific probes, and quantified by phosphorimage analysis. Titers of all three viruses increased as a result of co-infection compared with single infections.

A New Expanded Host Range of Cucurbit yellow stunting disorder virus Includes Three Agricultural Crops

Cucurbit yellow stunting disorder virus (CYSDV) was identified in the fall of 2006 affecting cucurbit production in the southwestern United States (California, Arizona), as well as in nearby Sonora, Mexico, resulting in nearly universal infection of fall melon crops in 2006 and 2007, and late infection of 2007 spring melons. Survival of CYSDV through the largely cucurbit-free winter months suggested the presence of weed or alternate crop hosts, although previous studies indicated a limited host range restricted to members of the Cucurbitaceae. To determine potential reservoir hosts for CYSDV in desert production, weed and crop hosts were collected from throughout the region over a period of 26 months, and were tested for the presence of CYSDV by reverse transcription-polymerase chain reaction (RT-PCR) using CYSDV HSP70h- and coat protein gene-specific primers. Many noncucurbits collected from infected melon fields and nearby areas were symptomless and virus free; however, CYSDV was detected in alfalfa (Medicago sativa), lettuce (Lactuca sativa), and snap bean (Phaseolus vulgaris), as well as in several weed species widely prevalent in the region. Typical crinivirus symptoms of interveinal yellowing and leaf brittleness were observed on CYSDV-infected snap bean, alkali mallow (Sida hederacea) and Wrights groundcherry (Physalis wrightii), while other infected crop and weed hosts were symptomless. Transmission tests demonstrated that lettuce, snap bean, alkali mallow, Wrights groundcherry, and buffalo gourd (Cucurbita foetidissima) could serve as virus reservoir hosts for transmission of CYSDV to melon and other cucurbits. These results expand the previously known host range of CYSDV, demonstrating that the virus is capable of infecting not only members of the Cucurbitaceae, but also plants in seven additional taxonomic families.

Genetic Composition of Pepino mosaic virus Population in North American Greenhouse Tomatoes

In just a few short years, pepino mosaic disease has quickly become endemic in greenhouse tomatoes around the world. Although three genotypes of Pepino mosaic virus (PepMV) were identified in the United States, genetic composition of PepMV in greenhouse tomato crops in North America has not been determined. In this study, genetic variability and population structure of PepMV were evaluated through nucleotide sequence comparison and phylogenetic analysis of two genomic regions (helicase domain and TGB2-3) derived from 91 cDNA clones that were derived from 31 field-collected samples. These samples were collected from several major greenhouse tomato facilities in five states in the United States and two provinces in Canada. All four major genotypes of PepMV (EU, US1, US2, and CH2) were found in North America. Three distinct genotypes (EU, US1, and US2) were found in mixed infection in samples collected from Arizona and Colorado, two genotypes (EU and CH2) in Texas, and a single genotype (EU) in Alabama and California and the provinces of British Columbia and Ontario in Canada. The complexity of population genetics of PepMV in the United States poses an additional challenge to the greenhouse tomato industry because a tomato cultivar with durable resistance to multiple genotypes of PepMV may be harder to develop.

Epidemiology of criniviruses: an emerging problem in world agriculture

The genus Crinivirus includes the whitefly-transmitted members of the family Closteroviridae. Whitefly-transmitted viruses have emerged as a major problem for world agriculture and are responsible for diseases that lead to losses measured in the billions of dollars annually. Criniviruses emerged as a major agricultural threat at the end of the twentieth century with the establishment and naturalization of their whitefly vectors, members of the genera Trialeurodes and Bemisia, in temperate climates around the globe. Several criniviruses cause significant diseases in single infections whereas others remain asymptomatic and only cause disease when found in mixed infections with other viruses. Characterization of the majority of criniviruses has been done in the last 20 years and this article provides a detailed review on the epidemiology of this important group of viruses.