Tomato Yellow Leaf Curl Thailand Virus Research Articles

Differential Effects of Two Tomato Begomoviruses on the Life History and Feeding Preference of Bemisia tabaci.

Tomato yellow leaf curl disease, caused by a group of closely related tomato yellow leaf curl viruses, is a major threat to tomato cultivation worldwide. These viruses are primarily transmitted by the sweet potato whitefly (Bemisia tabaci) in a persistent-circulative manner, wherein the virus circulates in the body of B. tabaci and infects its tissues. The complex relationship between viruses and whiteflies significantly influences virus transmission, with studies showing varying effects of the former on the life history and feeding preference of the latter. Whether these effects are direct or indirect, and whether they are negative, neutral, or positive, appears to depend on the specific interactions between virus and whitefly species. The tomato yellow leaf curl Thailand virus (TYLCTHV) and the tomato leaf curl Taiwan virus (ToLCTV) are two prevalent begomoviruses in fields in Taiwan. This study examined the direct and indirect effects of TYLCTHV and ToLCTV on the life history traits (longevity, fecundity, nymph survival, and nymph developmental time) and feeding preference of B. tabaci Middle East-Asia Minor 1 (MEAM1). The results revealed that TYLCTHV had no effects on these life history traits or the feeding preference of MEAM1 whiteflies. Although ToLCTV did not directly affect the longevity and fecundity of MEAM1 whiteflies, their fecundity and the nymph developmental time were negatively affected by feeding on ToLCTV-infected plants. In addition, ToLCTV infection also altered the feeding preference of MEAM1 whiteflies. The different effects of virus infection may contribute to the lower prevalence of ToLCTV compared to TYLCTHV in fields in Taiwan.

Seed and pollen transmission of tomato leaf curl New Delhi virus, tomato leaf curl Taiwan virus, and tomato yellow leaf curl Thailand virus in cucumbers and tomatoes.

Understanding the seed-borne nature of plant viruses is essential for developing disease control strategies and is impactful to the seed market. Here, we investigated seed transmissibility of tomato leaf curl New Delhi virus-cucumber isolate (ToLCNDV-CB) and -oriental melon isolate (ToLCNDV-OM) in cucumber and seed transmissibility of tomato leaf curl Taiwan virus (ToLCTV) and tomato yellow leaf curl Thailand virus (TYLCTHV) in tomato. Parent plants were inoculated using agroinfiltration with virus infectious clones, and virus infection was confirmed by PCR with virus-specific primers. ToLCNDV-CB and ToLCNDV-OM were detected in different parts of the female and male flowers and the fruits of cucumbers. ToLCNDV-CB and ToLCNDV-OM were also detected in cucumber seed coats and seedlings with an infection rate higher than 79%. Similar results were observed with ToLCTV and TYLCTHV as they were detected in different parts of the female and male flowers and fruits of three tomato cultivars. ToLCTV and TYLCTHV were also detected in tomato seed coats and seedlings with an infection rate higher than 36%. In addition, pollen-mediated transmission assays of these four begomoviruses were conducted with pollen derived from virus-infected plants to healthy plants. Results showed that ToLCNDV-CB and ToLCNDV-OM were detected in cross-pollinated cucumber progenies with an infection rate higher than 70%. ToLCTV and TYLCTHV were [also] detected in cross-pollinated tomato progenies with an infection rate higher than 77%. Our results indicated that ToLCNDV, ToLCTV, and TYLCTHV can be transmitted via seeds or pollens of cucumber and tomato plants. To our knowledge, this is the first report documenting the pollen-mediated transmission of begomoviruses.

Virus-virus interactions alter the mechanical transmissibility and host range of begomoviruses.

Begomoviruses are mainly transmitted by whiteflies. However, a few begomoviruses can be transmitted mechanically. Mechanical transmissibility affects begomoviral distribution in the field. In this study, two mechanically transmissible begomoviruses, tomato leaf curl New Delhi virus-oriental melon isolate (ToLCNDV-OM) and tomato yellow leaf curl Thailand virus (TYLCTHV), and two nonmechanically transmissible begomoviruses, ToLCNDV-cucumber isolate (ToLCNDV-CB) and tomato leaf curl Taiwan virus (ToLCTV), were used to study the effects of virus-virus interactions on mechanical transmissibility. Nicotiana benthamiana and host plants were coinoculated through mechanical transmission with inoculants derived from plants that were mix-infected or inoculants derived from individually infected plants, and the inoculants were mixed immediately before inoculation. Our results showed that ToLCNDV-CB was mechanically transmitted with ToLCNDV-OM to N. benthamiana, cucumber, and oriental melon, whereas ToLCTV was mechanically transmitted with TYLCTHV to N. benthamiana and tomato. For crossing host range inoculation, ToLCNDV-CB was mechanically transmitted with TYLCTHV to N. benthamiana and its nonhost tomato, while ToLCTV with ToLCNDV-OM was transmitted to N. benthamiana and its nonhost oriental melon. For sequential inoculation, ToLCNDV-CB and ToLCTV were mechanically transmitted to N. benthamiana plants that were either preinfected with ToLCNDV-OM or TYLCTHV. The results of fluorescence resonance energy transfer analyses showed that the nuclear shuttle protein of ToLCNDV-CB (CBNSP) and the coat protein of ToLCTV (TWCP) localized alone to the nucleus. When coexpressed with movement proteins of ToLCNDV-OM or TYLCTHV, CBNSP and TWCP relocalized to both the nucleus and the cellular periphery and interacted with movement proteins. Our findings indicated that virus-virus interactions in mixed infection circumstances could complement the mechanical transmissibility of nonmechanically transmissible begomoviruses and alter their host range. These findings provide new insight into complex virus-virus interactions and will help us to understand the begomoviral distribution and to reevaluate disease management strategies in the field.

Identification of begomoviruses associated with the insect vector Bemisia tabaci and various host plants on Java Island, Indonesia.

Begomoviruses are economically important plant viruses and are transmitted by Bemisia tabaci which is a complex of various cryptic species. However, it is uncertain whether most begomoviruses that infect host plants are transmitted by B. tabaci at a similar rate. We compared the begomovirus profiles that were detected in a total of 37 whitefly populations and 52 host plants on Java Island, Indonesia. Seven begomovirus species were detected in B. tabaci at different rates: pepper yellow leaf curl Indonesia virus (PepYLCIV, 56.8%), tomato yellow leaf curl Kanchanaburi virus (TYLCKaV, 46.0%), tomato leaf curl New Delhi virus (ToLCNDV, 21.6%), squash leaf curl China virus (SLCCNV, 21.6%), ageratum yellow vein China virus (AYVCNV, 2.7%), mungbean yellow mosaic India virus (MYMIV, 2.7%), and okra enation leaf curl virus (OELCuV, 2.7%). The begomoviruses were detected at different rates in three cryptic species of B. tabaci. In addition, six begomovirus species were detected in the various host plants at different rates: PepYLCIV (67.3%), TYLCKaV (53.9%), ToLCNDV (13.5%), MYMIV (11.5%), AYVCNV (3.9%), and Tomato yellow leaf curl Thailand virus (TYLCTHV) (1.9%). By comparing the virus presence between whiteflies and plants, five begomoviruses (AYVCNV, MYMIV, PepYLCIV, ToLCNDV, and TYLCKaV) were detected in both samples, but their sequence similarity was highly variable depending on the begomovirus themselves; TYLCKaV was highest (99.4%-100%) than any other viruses. Our study suggests B. tabaci acquire begomoviruses at different rates from plants. This study provides important information on the potential variation in the begomovirus transmission mechanism.

Incidence and Molecular Identification of Begomoviruses Infecting Tomato and Pepper in Myanmar.

In Myanmar, yellow mosaic and leaf curl diseases caused by whitefly-transmitted begomoviruses are serious problems for vegetables such as tomatoes and peppers. To investigate the incidence of begomoviruses in Myanmar between 2017 and 2019, a field survey of tomato and pepper plants with virus-like symptoms was conducted in the Naypyitaw, Tatkon, and Mohnyin areas of Myanmar. Among the 59 samples subjected to begomovirus detection using polymerase chain reaction, 59.3% were infected with begomoviruses. Complete genome sequences using rolling circle amplification identified five begomovirus species: tomato yellow leaf curl Thailand virus (TYLCTHV), tomato yellow leaf curl Kanchanaburi virus (TYLCKaV), tobacco leaf curl Yunnan virus (TbLCYnV), chili leaf curl Pakistan virus (ChiLCV/PK), and tobacco curly shoot Myanmar virus (TbCSV-[Myanmar]). Excluding the previously reported TYLCTHV, three begomoviruses (ChiLCV/PK, TYLCKaV, and TbLCYnV) were identified in Myanmar for the first time. Based on the 91% demarcation threshold of begomovirus species, TbCSV-[Myanmar] was identified as a new species in this study. Among these, ChiLCV/PK and TbCSV-[Myanmar] were the most predominant in tomato and pepper fields in Myanmar. Identification of begomovirus species may be helpful for predicting the origin of viruses and preventing their spread.

Fungal F8-Culture Filtrate Induces Tomato Resistance against Tomato Yellow Leaf Curl Thailand Virus

Tomato (Solanum lycopersicum) is an important economic crop worldwide. However, tomato production is jeopardized by the devastating tomato yellow leaf curl disease caused by whitefly-transmitted begomoviruses (WTBs). In this study, we evaluated the efficacy of our previously developed plant antiviral immunity inducer, fungal F8-culture filtrate, on tomato to combat tomato yellow leaf curl Thailand virus (TYLCTHV), the predominant WTB in Taiwan. Our results indicated that F8-culture filtrate treatment induced strong resistance, did not reduce the growth of tomato, and induced prominent resistance against TYLCTHV both in the greenhouse and in the field. Among TYLCTHV-inoculated Yu-Nu tomato grown in the greenhouse, a greater percentage of plants treated with F8-culture filtrate (43–100%) were healthy-looking compared to the H2O control (0–14%). We found that TYLCTHV cannot move systemically only on the F8-culture filtrate pretreated healthy-looking plants. Tracking the expression of phytohormone-mediated immune maker genes revealed that F8-culture filtrate mainly induced salicylic acid-mediated plant immunity. Furthermore, callose depositions and the expression of the pathogen-induced callose synthase gene, POWDERY MILDEW RESISTANT 4 were only strongly induced by TYLCTHV on tomato pretreated with F8-culture filtrate. This study provides an effective way to induce tomato resistance against TYLCTHV.

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

Simple SummaryTomato yellow leaf curl viruses cause disease epidemics in tomato crops in tropical and subtropical areas worldwide. The sweet potato whitefly, Bemisia tabaci, is a vector of this group of viruses. This research studied the transmission biology of tomato yellow leaf curl Thailand virus (TYLCTHV) by B. tabaci, including virus-infected tissues, virus translocation, virus replication, and transovarial transmission (i.e., transmission from mother to progeny via ovaries). We discovered that the virus first infects the alimentary gut, then the hemolymph, and finally the salivary glands of the whitefly. The virus did not replicate in the whitefly during infection. In addition, TYLCTHV was detected in only 10% of infected females’ first-generation progeny, but the progeny was unable to cause viral infection of tomato plants; therefore, there was no evidence of transovarial transmission. When combined with the current literature, our results suggest that B. tabaci transmits TYLCTHV in a persistent-circulative mode.Viruses that cause tomato yellow leaf curl disease are part of a group of viruses of the genus Begomovirus, family Geminiviridae. Tomato-infecting begomoviruses cause epidemics in tomato crops in tropical, subtropical, and Mediterranean climates, and they are exclusively transmitted by Bemisia tabaci in the field. The objective of the present study was to examine the transmission biology of the tomato yellow leaf curl Thailand virus (TYLCTHV) by B. tabaci, including virus-infected tissues, virus translocation, virus replication, and transovarial transmission. The results demonstrated that the virus translocates from the alimentary gut to the salivary glands via the hemolymph, without apparent replication when acquired by B. tabaci. Furthermore, the virus was detected in 10% of the first-generation progeny of viruliferous females, but the progeny was unable to cause the viral infection of host plants. There was no evidence of transovarial transmission of TYLCTHV in B. tabaci. When combined with the current literature, our results suggest that B. tabaci transmits TYLCTHV in a persistent-circulative mode. The present study enhances our understanding of virus–vector interaction and the transmission biology of TYLCTHV in B. tabaci.

Evaluation of phenotyping and genotyping for resistance against Tomato yellow leaf curl Thailand virus (TYLCTHV) validated by Ty-2, Ty-3, and Ty-4 genes in tomato

ISHS I International Symposium on Botanical Gardens and Landscapes Evaluation of phenotyping and genotyping for resistance against Tomato yellow leaf curl Thailand virus (TYLCTHV) validated by Ty-2, Ty-3, and Ty-4 genes in tomato

Evaluation of barrier plants for the cultural control of tomato yellow leaf curl disease

Abstract Barrier cropping plays an essential role in controlling insect pests and insect-transmitted diseases in cultural control. It has been proven efficient in suppressing the spread of nonpersistently transmitted viruses. For suppressing the spread of persistently transmitted viruses, barrier cropping is not considered an effective control strategy because barrier plants cannot act as a virus sink to purge the virus in the vector. However, few successful cases of barrier cropping suppressing the spread of persistently transmitted viruses have been reported. The objectives of the present study were to screen candidates (cucumber, okra, Chinese kale, soybean, and corn) for potential barrier plants to control tomato yellow leaf curl Thailand virus (TYLCTHV) and examine whether prefeeding on these plants can reduce the virus titer in its vector, Bemisia tabaci, thus reducing TYLCTHV transmission. The results revealed that nonviruliferous whiteflies preferred cucumber and okra to tomato, whereas viruliferous whiteflies preferred cucumber to tomato. Although prefeeding on cucumber, okra, and Chinese kale did not reduce the titer of TYLCTHV in viruliferous whiteflies, the vector transmission rate decreased after the whiteflies fed on Chinese kale. It implies that planting Chinese kale as a barrier plant for tomato cultivation may reduce the incidence of TYLCTHV. In addition, the preference to cucumber plants may reduce the incidence of whiteflies acquiring TYLCTHV from virus-infected tomato plants and of viruliferous whiteflies inoculating the virus into healthy tomato plants, thereby reducing the disease incidence. Further field trials of barrier cropping using the candidate plants are warranted.

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

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

Priming of Plant Resistance to Heat Stress and Tomato Yellow Leaf Curl Thailand Virus With Plant-Derived Materials.

Plants are often simultaneously exposed to diverse environmental stresses, and can tune suitable responses to them through hormones. Salicylic acid (SA) and jasmonic acid (JA) signaling pathways are known to enhance resistance against heat stress and tomato yellow leaf curl Thailand virus (TYLCTHV) infection. However, there is limited information regarding alternative natural priming agents against heat stress and viruses. In this study, two plant-derived priming agents, eugenol and anise oil, were tested for their roles in conferring thermotolerance and virus resistance in tomato plants. Under heat stress, the survival rates and average fresh weight were higher in plants treated with eugenol or anise oil than in control plants. These two priming agents were further tested for antiviral activities. After TYLCTHV infection, the disease incidence and relative abundance of TYLCTHV were lower in anise oil- and eugenol-treated plants than in control plants. Further analyses revealed that a few SA, JA, and RNA silencing genes were enhanced in the former. Moreover, SA, JA, and H2O2 contents increased considerably after eugenol and anise oil treatments. Our findings imply that anise oil and eugenol initiated SA- and JA-mediated defenses to promote thermotolerance and antiviral responses of tomato plants.

Detection of Tomato yellow leaf curl Thailand virus transmitted by Bemisia tabaci Asia I in tomato and pepper

Tomato yellow leaf curl Thailand virus (TYLCTHV) has caused serious yield loss of tomato in Thailand for more than three decades. Control of the virus is essential for successful integrated disease management (IDM) strategies. This study investigated the capability of whitefly (Bemisia tabaci) to transmit TYLCTHV on two host plants—tomato and pepper—and sought to detect the presence of viral DNA in the plants and whiteflies after two transmissions. The new TYLCTHV isolate used in this study (SPN-Tom1A) was 2756 nts long with typical characteristics of TYLCTHV DNA-A. Viral DNA-A (WF-SPN Tom2015) also was detected in viruliferous whiteflies; nucleotide sequence revealed 99% identity to TYLCTHV SPN-Tom1A sequence. In duplicated transmission test, vector B. tabaci Asia I was highly efficient in transmitting TYLCTHV from tomato to tomato by 90–100%, followed by the transmission from tomato to pepper by 55–90%. In contrast, the transmission rate of TYLCTHV pepper to tomato seedlings was nil. Tomato plants showed upward cupping and yellow leaves typical of TYLCTHV infection only when TYLCTHV was transmitted from tomato to tomato, but when transmitted to pepper, the infected peppers were symptomless. Bodies of viruliferous whiteflies fed on tomato and pepper were confirmed by PCR for the presence of TYLCTHV at 58.89–80% and 60–89.45%, respectively. Our results thus provide evidence that pepper infected with TYLCTHV is not a source plant for TYLCTHV transmission to tomato.

Resistance to viral yellow leaf curl in tomato through RNAi targeting two Begomovirus species strains

Tomato yellow leaf curl disease caused by different begomovirus species leads to substantial tomato production losses worldwide. In Taiwan, the monopartite tomato leaf curl Taiwan virus (ToLCTWV) and bi-partite tomato yellow leaf curl Thailand virus (TYLCTHV) are the predominant begomovirus species causing this disease. Resistance genes are available in wild tomato species and a continuous search for new resistance genes and alternative control methods is required to respond to the rapid evolution of virus strains. RNA interference is an efficient technology to induce resistance against viral pathogens. Six different sections of the ToLCTWV genome were tested in transformed tomato for their capacity to reduce symptoms and inhibit viral DNA accumulation. The two most effective constructs for ToLCTWV infection carried regions of the C1 and C2 genes, and portions of either the C3 or C4 gene of ToLCTWV. A RNAi construct containing fusions of C1, C2 and C3 sections of ToLCTWV and the corresponding sections of the TYLCTHV DNA-A genome were introgressed into tomato line CLN1621L. R1 and R2 families were challenged using viruliferous whiteflies in separate screen houses for ToLCTWV and TYLCTHV. Sixteen and 12 R2 plants derived from one primary transformant remained symptomless until at least 3 weeks after exposure to ToLCTWV and TYLCTHV, respectively, and accumulated only very low titres of viral DNA, as shown by real-time polymerase chain reaction analysis. Our results suggest that expression of bi-viral RNAi constructs in tomato can lead to resistance against two different tomato infecting begomovirus species.

Different transmission efficiencies may drive displacement of tomato begomoviruses in the fields in Taiwan

A progressive displacement of Tomato leaf curl Taiwan virus (ToLCTWV) by Tomato yellow leaf curl Thailand virus (TYLCTHV) from 2005 to 2009 has been recorded in tomato fields in Taiwan. Begomoviruses are exclusively transmitted by Bemisia tabaci complex, so we hypothesised that the displacement of tomato begomoviruses in the fields may be due to the invasion of a new virus/vector and the different transmission efficiencies of the viruses by the vectors. The objective of this research was to compare the transmission efficiency of TYLCTHV and ToLCTWV by the B and Q biotypes of B. tabaci complex. When transmission efficiency, virus retention in vector, and latent period for vector transmission were compared, the B biotype transmitted TYLCTHV and ToLCTWV more efficiently than did the Q biotype, and transmitted TYLCTHV more efficiently than ToLCTWV. The B biotype retained both viruses and remained infective throughout adulthood, but the Q biotype did not keep its infectivity, although it did retain both viruses lifelong. The B biotype transmitted TYLCTHV and ToLCTWV with the shortest latent period. In summary, B. tabaci B biotype and TYLCTHV is the best alliance for disease transmission, so we conclude that this may be one of drivers responsible for the displacement of ToLCTWV by TYLCTHV in tomato fields in Taiwan.

Emergence and diversity of begomoviruses infecting solanaceous crops in East and Southeast Asia

Over the past three decades diseases caused by whitefly-transmitted geminiviruses (begomoviruses) have emerged to be important constraints to the production of solanaceous crops, particularly tomato (Solanum lycopersicum) and peppers (Capsicum spp.), in many tropical and subtropical regions of the world. The most studied of these is Tomato yellow leaf curl virus (TYLCV), which has spread to many other areas from its likely origin in the Mediterranean basin region. The virus is usually associated with the polyphagous and virus-vectoring-efficient B-biotype of its vector whitefly (Bemisia tabaci). However, in Southeast and East Asia, a wide variety of distinct local begomovirus species have been identified from tomato and pepper crops over this period, and TYLCV was detected in Japan only in about 1996, China in 2006 and Korea in 2008, despite B-biotype whiteflies being present in several of the countries of the region since at least the early 1990s. Continental Southeast Asia appears to be a major center of diversity for begomoviruses and some species may have spread across the region; Tomato yellow leaf curl Thailand virus (TYLCTHV) appears to have spread from the Thailand–Myanmar region into southern China and is now displacing the local tomato-infecting species in Taiwan, and Tomato yellow leaf curl Kanchanaburi virus (TYLCKaV) appears to have spread from the Thailand–Vietnam region to Java, Indonesia. Since many of the native tomato- or pepper-infecting begomoviruses and associated satellite DNAs have also been detected in local weed species, it seems likely that their ancestors originated in these weed hosts, but with the expansion and intensification of tomato and pepper production in the region, there was selection for recombinant or mutant forms with greater virulence on tomato and/or pepper. Expansion and intensification of these crops may also have resulted in increased populations of local, and if present, B- or Q-biotype whiteflies, aiding the increase and spread of local begomovirus species.

Temporal distribution and pathogenicity of the predominant tomato‐infecting begomoviruses in Taiwan

Between 1998 and 2009, the four tomato‐infecting begomovirus species detected in Taiwan were Ageratum yellow vein Hualien virus (AYVHuV), Tomato leaf curl Taiwan virus (ToLCTWV), Tomato yellow leaf curl Thailand virus (TYLCTHV) and a newly defined species Tomato leaf curl Hsinchu virus (ToLCHsV). AYVHuV was detected occasionally in 2003 and ToLCHsV only in 2000–2001, whilst ToLCTWV was detected throughout the period. TYLCTHV was first detected in 2005. Between 1998 and 2005, >99% of the begomovirus‐positive samples were infected with ToLCTWV. In 2007 in western Taiwan, 16% of the positive samples were infected with ToLCTWV, 35% with TYLCTHV and 49% with mixed infection (ToLCTWV/TYLCTHV). In contrast, in eastern Taiwan the proportions were 84% ToLCTWV, 2% TYLCTHV and 14% mixed infection. However, throughout Taiwan in 2008–2009, most positive samples were either identified as TYLCTHV (51%) or mixed infection (ToLCTWV/TYLCTHV; 41%), and only 8% were ToLCTWV. This shows a clear trend of shifting from ToLCTWV to TYLCTHV and mixed infection over a short time period in Taiwan. Sequence analyses indicated that tomato‐infecting AYVHuV, an apparent recombinant between ToLCTWV and AYVHuV from Ageratum, represents a new strain Hsinchu. TYLCTHV Taiwan isolates were highly similar to each other, whereas ToLCTWV isolates had greater diversity and were classified into three strains which had one country‐wide and two local distributions. ToLCTWV and TYLCTHV were confirmed as monopartite and bipartite begomoviruses, respectively, by agroinfection followed by transmission with Bemisia tabaci biotype B. In addition, TYLCTHV was found to be mechanically transmissible together with viral DNA‐B.

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. 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. 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. 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.

Tomato yellow leaf curl Thailand virus-[Y72] from Yunnan is a monopartite begomovirus associated with DNAβ

Previous studies have shown that isolates of tomato yellow leaf curl Thailand virus (TYLCTHV) originated from Thailand are bipartite begomoviruses, while all the seven TYLCTHV isolates found in China are associated with DNAbeta molecules. In this study, infectious clones of TYLCTHV isolate Y72 (TYLCTHV-[Y72]) and its DNAbeta were constructed to verify the bipartite or monopartite nature of TYLCTHV. Agroinoculation showed that TYLCTHV-[Y72] alone was able to induce significant symptoms in Nicotiana benthamiana, Nicotiana glutinosa, and Solanum lycopersicum plants, but co-inoculation with its associated satellite DNAbeta produced more severe symptoms, which is similar to tobacco curly shoot virus. Southern blot results showed that TYLCTHV DNAbeta could increase the virus accumulation in systemically infected tissues. Thus, TYLCTHV-[Y72] is a monopartite begomovirus, which may represent an evolutionary intermediate between the begomoviruses requiring DNAbeta and begomoviruses dispensable of DNAbeta.

Application of Phi29 DNA polymerase in identification and full-length clone inoculation of tomato yellow leaf curl Thailand virus and tobacco leaf curl Thailand virus

Tomato plants grown in greenhouses in Thailand developed typical symptoms of a tomato yellow leaf curl Thailand virus (TYLCTHV) infection. After confirmation by ELISA, a Phi29 DNA polymerase approach was chosen for further molecular analysis of TYLCTHV. Total DNA purified from infected tomato leaves was subjected to rolling-circle amplification (RCA) of DNA-A and DNA-B of TYLCVTHV. In addition, a new monopartite geminivirus with a putative recombinant background was identified by RCA and tentatively named tobacco leaf curl Thailand virus (TbLCTHV). To confirm the composition of both geminiviruses, full-length clones were established and used for inoculation of Nicotiana benthamiana by particle bombardment or agroinfection. When TYLCTHV DNA-A and DNA-B were applied together by particle bombardment or agroinfection, severe stunting, yellowing, and leaf curling were observed. Whereas TYLCTHV DNA-A and TbLCTHV revealed no infection after'particle bombardment, similar symptoms in N. benthamiana, like leaf upward curling and yellowing were observed following agroinfection.DNA components of TYLCTHV DNA-A and DNA-B were excised from their respective plasmids, ligated, and amplified by Phi29 DNA polymerase. The ability of viral concatamere inoculation was evaluated in particle co-bombardment experiments on N. benthamiana. Thus, particle bombardment of RCA-derived multimeric products proved to be at least as effective as inoculation with a partial repeat construct and tenfold as effective as inoculation with excised unit-lengths of DNA-A and DNA-B of TYLCVTHV when using each DNA component in an amount of 5 ng.

Possible recombination of tomato-infecting begomoviruses in Thailand

Nucleotide sequences of the three distinct Tomato yellow leaf curl Thailand virus (TYLCTHV) strains (CM, NK, SK) were analyzed for recombination events. Recombination detection program analyses and a sequence alignment survey provided evidence of recombination between AC1 sequences of TYLCV, TYLCTHV-[MM], and TYLCTHV-[NK] as major parents and of ToLCLV, ToLCTWV, and TYLCTHV-[SK] as minor parents of TYLCTHV-[NK], -[SK], -[CM], respectively. The results further support the notion that interspecies recombination may play a significant role in geminivirus diversity and their emergence as important pathogens.