A Temporal Diversity Analysis of Brazilian Begomoviruses in Tomato Reveals a Decrease in Species Richness between 2003 and 2016

A Tomato-Free Period Delays and Reduces Begomovirus Disease in Processing Tomato Fields in a Complex Agroecosystem in Central Brazil

A mandatory tomato-free period (TFP) was implemented in the state of Goiás, Brazil, in 2007 to help manage diseases caused by whitefly-transmitted begomoviruses. The impact of the TFP was examined in five locations across three states in Central Brazil from 2013 to 2016. Surveys revealed significant differences in begomovirus disease incidence among locations, i.e., low in Guaíra-TFP and Patos de Minas-TFP; moderate-high in Itaberaí-TFP and Morrinhos-TFP; and high in the non-TFP (NTFP) control, Cristalina-NTFP. PCR tests and DNA sequencing were used to validate the symptoms and showed that all collected symptomatic plant samples were infected with tomato severe rugose virus (ToSRV), a common indigenous bipartite begomovirus. Early season surveys (20 to 40 days after transplants [DAT]) in Itaberaí-TFP and Morrinhos-TFP revealed significantly less begomovirus disease in fields established sooner after the TFP (0 to 2 months) compared with incidences in (i) equivalent early planted fields in the Cristalina-NTFP control and (ii) fields established longer after the end of the TFP (>2 to 5 months). Whitefly infestation of crops was detected year-round in all locations and years, and all tested adults were classified in the Bemisia tabaci MEAM1 cryptic species. Infestation levels were significantly higher during the summer but did not vary significantly among locations. Results of monthly monitoring of adult whiteflies for general begomovirus and ToSRV were positively correlated and were indicators of disease incidence in the field. Notably, ToSRV was not detected in whiteflies collected from nontomato plants during the TFP, and there was a longer lag period before detection in whiteflies collected from processing tomatoes for Itaberaí-TFP and Morrinhos-TFP compared with Cristalina-NTFP. Taken together with the low levels of ToSRV infection detected in potential nontomato reservoir hosts at all locations, our results revealed low levels of primary inoculum during the TFP. Thus, even in a complex agroecosystem with year-round whitefly infestation of crops, the TFP was beneficial due to delayed and reduced begomovirus disease pressure during a critical stage of plant development (first month) and for favoring low levels of primary inoculum. Thus, we concluded that the TFP should be part of a regional integrated pest management (IPM) program targeting ToSRV in Brazil.

Tobacco Mild Green Mosaic Virus (TMGMV) Isolates from Different Plant Families Show No Evidence of Differential Adaptation to Their Host of Origin

The relevance of tobamoviruses to crop production is increasing due to new emergences, which cannot be understood without knowledge of the tobamovirus host range and host specificity. Recent analyses of tobamovirus occurrence in different plant communities have shown unsuspectedly large host ranges. This was the case of the tobacco mild green mosaic virus (TMGMV), which previously was most associated with solanaceous hosts. We addressed two hypotheses concerning TMGMV host range evolution: (i) ecological fitting, rather than genome evolution, determines TMGMV host range, and (ii) isolates are adapted to the host of origin. We obtained TMGMV isolates from non-solanaceous hosts and we tested the capacity of genetically closely related TMGMV isolates from three host families to infect and multiply in 10 hosts of six families. All isolates systemically infected all hosts, with clear disease symptoms apparent only in solanaceous hosts. TMGMV multiplication depended on the assayed host but not on the isolate's host of origin, with all isolates accumulating to the highest levels in Nicotiana tabacum. Thus, results support that TMGMV isolates are adapted to hosts in the genus Nicotiana, consistent with a well-known old virus-host association. In addition, phenotypic plasticity allows Nicotiana-adapted TMGMV genotypes to infect a large range of hosts, as encountered according to plant community composition and transmission dynamics.

Tobamoviruses Show Broad Host Ranges and Little Genetic Diversity Among Four Habitat Types of a Heterogeneous Ecosystem

Host ranges of plant viruses are poorly known, as studies have focused on pathogenic viruses in crops and adjacent wild plants. High-throughput sequencing (HTS) avoids the bias toward plant-virus interactions that result in disease. Here we study the host ranges of tobamoviruses, important pathogens of crops, using HTS analyses of an extensive sample of plant communities in four habitats of a heterogeneous ecosystem. Sequences of 17 virus operational taxonomic units (OTUs) matched references in the Tobamovirus genus, eight had narrow host ranges, and five had wide host ranges. Regardless of host range, the OTU hosts belonged to taxonomically distant families, suggesting no phylogenetic constraints in host use associated with virus adaptation, and that tobamoviruses may be host generalists. The OTUs identified as tobacco mild green mosaic virus (TMGMV), tobacco mosaic virus (TMV), pepper mild mottle virus, and Youcai mosaic virus had the largest realized host ranges that occurred across habitats and exhibited host use unrelated to the degree of human intervention. This result is at odds with assumptions that contact-transmitted viruses would be more abundant in crops than in wild plant communities and could be explained by effective seed-, contact-, or pollinator-mediated transmission or by survival in the soil. TMGMV and TMV had low genetic diversity that was not structured according to habitat or host plant taxonomy, which indicated that phenotypic plasticity allows virus genotypes to infect new hosts with no need for adaptive evolution. Our results underscore the relevance of ecological factors in host range evolution, in addition to the more often studied genetic factors. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Characterization of Soybean, Tomato, and Nicandra physalodes as Sources of Inoculum of Tomato Severe Rugose Virus to Tomato Crops

Tomato severe rugose virus (ToSRV) is one of Brazil's main begomoviruses infecting tomato (Solanum lycopersicum). Recent studies indicate that soybean (Glycine max) crops harboring the whitefly Bemisia tabaci Middle East-Asia Minor 1 (MEAM1) may have epidemiological significance by acting as an asymptomatic amplifier host for the virus. In this study, we gathered experimental greenhouse and field evidence of the role of soybean in the epidemiology of the disease caused by ToSRV. Tomato and Nicandra physalodes, known as good sources of inoculum of this begomovirus, were used as references. The infection rates of soybean, tomato, and N. physalodes with ToSRV in greenhouse no-choice transmission tests with B. tabaci MEAM1 were 50, 71.4, and 64.2%, respectively. The transmission efficiencies of ToSRV to tomato when B. tabaci MEAM1 acquired the virus in ToSRV-infected soybean, tomato, and N. physalodes were 43, 33, and 20%, respectively. Leaves of ToSRV-infected soybean, tomato, and N. physalodes used as sources of inoculum had similar virus titers. In the host preference assay, viruliferous whiteflies preferred to land on tomato rather than soybean and N. physalodes, whereas aviruliferous whiteflies landed indistinctly on these plants. Under experimental field conditions, the transmission efficiency of ToSRV to tomato was higher when tomato was used as a source of inoculum, followed by N. physalodes and soybean. Considering that soybean is extensively cultivated in several Brazilian states that also grow tomato, it can serve as an efficient asymptomatic source of inoculum and support the recent hypothesis that it can also play, under certain conditions, a relevant role as an amplifier host in the epidemiology of the disease caused by ToSRV.

Metagenomics show high spatiotemporal virus diversity and ecological compartmentalisation: Virus infections of melon, Cucumis melo, crops, and adjacent wild communities

The emergence of viral diseases results from novel transmission dynamics between wild and crop plant communities. The bias of studies towards pathogenic viruses of crops has distracted from knowledge of non-antagonistic symbioses in wild plants. Here, we implemented a high-throughput approach to compare the viromes of melon (Cucumis melo) and wild plants of crop (Crop) and adjacent boundaries (Edge). Each of the 41-plant species examined was infected by at least one virus. The interactions of 104 virus operational taxonomic units (OTUs) with these hosts occurred largely within ecological compartments of either Crop or Edge, with Edge having traits of a reservoir community. Local scale patterns of infection were characterised by the positive correlation between plant and virus richness at each site, the tendency for increased specialist host use through seasons, and specialist host use by OTUs observed only in Crop, characterised local-scale patterns of infection. In this study of systematically sampled viromes of a crop and adjacent wild communities, most hosts showed no disease symptoms, suggesting non-antagonistic symbioses are common. The coexistence of viruses within species-rich ecological compartments of agro-systems might promote the evolution of a diversity of virus strategies for survival and transmission. These communities, including those suspected as reservoirs, are subject to sporadic changes in assemblages, and so too are the conditions that favour the emergence of disease.

How To Be a Successful Monopartite Begomovirus in a Bipartite-Dominated World: Emergence and Spread of Tomato Mottle Leaf Curl Virus in Brazil

Begomoviruses are members of the family Geminiviridae, a large and diverse group of plant viruses characterized by a small circular single-stranded DNA genome encapsidated in twinned quasi-icosahedral virions. Cultivated tomato (Solanum lycopersicum L.) is particularly susceptible and is infected by >100 bipartite and monopartite begomoviruses worldwide. In Brazil, 25 tomato-infecting begomoviruses have been described, most of which are bipartite. Tomato mottle leaf curl virus (ToMoLCV) is one of the most important of these and was first described in the late 1990s but has not been fully characterized. Here, we show that ToMoLCV is a monopartite begomovirus with a genomic DNA similar in size and genome organization to those of DNA-A components of New World (NW) begomoviruses. Tomato plants agroinoculated with the cloned ToMoLCV genomic DNA developed typical tomato mottle leaf curl disease symptoms, thereby fulfilling Koch's postulates and confirming the monopartite nature of the ToMoLCV genome. We further show that ToMoLCV is transmitted by whiteflies, but not mechanically. Phylogenetic analyses placed ToMoLCV in a distinct and strongly supported clade with other begomoviruses from northeastern Brazil, designated the ToMoLCV lineage. Genetic analyses of the complete sequences of 87 ToMoLCV isolates revealed substantial genetic diversity, including five strain groups and seven subpopulations, consistent with a long evolutionary history. Phylogeographic models generated with partial or complete sequences predicted that the ToMoLCV emerged in northeastern Brazil >700 years ago, diversifying locally and then spreading widely in the country. Thus, ToMoLCV emerged well before the introduction of MEAM1 whiteflies, suggesting that the evolution of NW monopartite begomoviruses was facilitated by local whitefly populations and the highly susceptible tomato host. IMPORTANCE Worldwide, diseases of tomato caused by whitefly-transmitted geminiviruses (begomoviruses) cause substantial economic losses and a reliance on insecticides for management. Here, we describe the molecular and biological properties of tomato mottle leaf curl virus (ToMoLCV) from Brazil and establish that it is a NW monopartite begomovirus indigenous to northeastern Brazil. This answered a long-standing question regarding the genome of this virus, and it is part of an emerging group of these viruses in Latin America. This appears to be driven by widespread planting of the highly susceptible tomato and by local and exotic whiteflies. Our extensive phylogenetic studies placed ToMoLCV in a distinct strongly supported clade with other begomoviruses from northeastern Brazil and revealed new insights into the origin of Brazilian begomoviruses. The novel phylogeographic analysis indicated that ToMoLCV has had a long evolutionary history, emerging in northeastern Brazil >700 years ago. Finally, the tools used here (agroinoculation system and ToMoLCV-specific PCR test) and information on the biology of the virus (host range and whitefly transmission) will be useful in developing and implementing integrated pest management (IPM) programs targeting ToMoLCV.

Insights into Emerging Begomovirus-Deltasatellite Complex Diversity: The First Deltasatellite Infecting Legumes

Begomoviruses and associated DNA satellites are involved in pathosystems that include many cultivated and wild dicot plants and the whitefly vector Bemisia tabaci. A survey of leguminous plants, both crops and wild species, was conducted in Venezuela, an understudied country, to determine the presence of begomoviruses. Molecular analysis identified the presence of bipartite begomoviruses in 37% of the collected plants. Four of the six begomoviruses identified constituted novel species, and two others had not been previously reported in Venezuela. In addition, a novel deltasatellite (cabbage leaf curl deltasatellite, CabLCD) was found to be associated with cabbage leaf curl virus (CabLCV) in several plant species. CabLCD was the first deltasatellite found to infect legumes and the first found in the New World to infect a crop plant. Agroinoculation experiments using Nicotiana benthamiana plants and infectious viral clones confirmed that CabLCV acts as a helper virus for CabLCD. The begomovirus-deltasatellite complex described here is also present in wild legume plants, suggesting the possible role of these plants in the emergence and establishment of begomoviral diseases in the main legume crops in the region. Pathological knowledge of these begomovirus-deltasatellite complexes is fundamental to develop control methods to protect leguminous crops from the diseases they cause.

Global Advances in Tomato Virome Research: Current Status and the Impact of High-Throughput Sequencing

Viruses cause a big fraction of economically important diseases in major crops, including tomato. In the past decade (2011–2020), many emerging or re-emerging tomato-infecting viruses were reported worldwide. In this period, 45 novel viral species were identified in tomato, 14 of which were discovered using high-throughput sequencing (HTS). In this review, we first discuss the role of HTS in these discoveries and its general impact on tomato virome research. We observed that the rate of tomato virus discovery is accelerating in the past few years due to the use of HTS. However, the extent of the post-discovery characterization of viruses is lagging behind and is greater for economically devastating viruses, such as the recently emerged tomato brown rugose fruit virus. Moreover, many known viruses still cause significant economic damages to tomato production. The review of databases and literature revealed at least 312 virus, satellite virus, or viroid species (in 22 families and 39 genera) associated with tomato, which is likely the highest number recorded for any plant. Among those, here, we summarize the current knowledge on the biology, global distribution, and epidemiology of the most important species. Increasing knowledge on tomato virome and employment of HTS to also study viromes of surrounding wild plants and environmental samples are bringing new insights into the understanding of epidemiology and ecology of tomato-infecting viruses and can, in the future, facilitate virus disease forecasting and prevention of virus disease outbreaks in tomato.