Resistance to Tomato chlorosis virus in wild tomato species that impair virus accumulation and disease symptom expression

Coinfection of Tomato Plants with Tomato yellow leaf curl virus and Tomato chlorosis virus Affects the Interaction with Host and Whiteflies

Susceptible plants infected by single or multiple viruses can differ in symptoms and other alterations influencing virus dissemination. Furthermore, behavior of viruliferous vectors may be altered in certain cases to favor acquisition and inoculation processes conductive to virus transmission. We explored single and mixed infections frequently occurring in tomato crops, caused by two viruses transmitted by the whitefly Bemisia tabaci: Tomato yellow leaf curl virus (TYLCV, Begomovirus, Geminiviridae) and Tomato chlorosis virus (ToCV, Crinivirus, Closteroviridae). Coinfection of both viruses in tomato plants showed more severe symptoms at late stages compared with single infections, although at earlier stages the interaction began with attenuation. This asymmetric synergism correlated with the dynamics of ToCV accumulation and expression of the salicylic acid responsive gene PR-P6. Visual and olfactory cues in whitefly preference were evaluated under controlled conditions in choice assays, testing viruliferous and nonviruliferous adult whiteflies. In experiments allowing both visual and olfactory cues, whiteflies preferred symptomatic leaflets from plants infected either with TYLCV alone or with TYLCV and ToCV, over those infected with ToCV alone or noninfected leaflets, suggesting that TYLCV drove host selection. Odor cues tested in Y-tube olfactometer assays showed neutral effects on whiteflies' preference, and bioassays comparing the attractiveness of colored sticky cards confirmed preference for sectors colored to mimic TYLCV symptomatic leaves compared with asymptomatic leaves. Our results show that the presence of coinfecting viruses affect the host and could alter the behavior of insect vectors.

Host Plant Resistance to Bemisia tabaci to Control Damage Caused in Tomato Plants by the Emerging Crinivirus Tomato Chlorosis Virus

Tomato chlorosis virus (genus Crinivirus, family Closteroviridae) (ToCV) is rapidly emerging, causing increased damage to tomato production worldwide. The virus is transmitted in a semipersistent manner by several whitefly (Hemiptera: Aleyrodidae) species and is expanding its geographical and host ranges associated with the emergence of whiteflies of the Bemisia tabaci complex. Control is based essentially on intensive insecticide applications against the insect vector but is largely ineffective. No virus-resistant or tolerant commercial tomato cultivars are available. Recently, a B. tabaci-resistant tomato line based on the introgression of type IV leaf glandular trichomes and secretion of acylsucroses from the wild tomato Solanum pimpinellifolium was shown to effectively control the spread of tomato yellow leaf curl virus, a begomovirus (genus Begomovirus, family Geminiviridae) persistently transmitted by B. tabaci. As short acquisition and transmission periods are associated to the semipersistent transmission of ToCV, its possible control by means of the B. tabaci-resistant tomato could be compromised. Moreover, if the antixenosis effect of the resistance trait present in those tomato plants results in increased B. tabaci mobility, an increased ToCV spread might even occur. We demonstrated, however, that the use of acylsugar-producing B. tabaci-resistant tomatoes effectively controls ToCV spread compared to a near-isogenic line without type IV trichomes and acylsugar secretion. No increase in the primary ToCV spread is observed, and secondary spread could be reduced significantly decreasing the incidence of this virus. The possible use of host plant resistance to whiteflies to limit spread of ToCV opens up new alternatives for a more effective control of this virus to reduce the damage caused in tomato crops.

Silencing of Odorant-Binding Protein Gene OBP3 Using RNA Interference Reduced Virus Transmission of Tomato Chlorosis Virus

Tomato chlorosis virus (ToCV) is widespread, seriously impacting tomato production throughout the world. ToCV is semi-persistently transmitted by Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). Currently, insect olfaction is being studied to develop novel pest control technologies to effectively control B. tabaci and whitefly-borne virus diseases. Despite current research efforts, no report has been published on the role of odorant-binding proteins (OBPs) in insect preference under the influence of plant virus. Our previous research showed that viruliferous B. tabaci preferred healthy plants at 48 h after virus acquisition. In this study, we determined the effect of OBPs on the host preference interactions of ToCV and whiteflies. Our results show that with the increase in acquisition time, the OBP gene expressions changed differently, and the OBP3 gene expression showed a trend of first rising and then falling, and reached the maximum at 48 h. These results indicate that OBP3 may participate in the host preference of viruliferous whiteflies to healthy plants. When the expression of the OBP3 gene was knocked down by an RNA interference (RNAi) technique, viruliferous Mediterranean (MED) showed no preference and the ToCV transmission rate was reduced by 83.3%. We conclude that OBP3 is involved in the detection of plant volatiles by viruliferous MED. Our results provide a theoretical basis and technical support for clarifying the transmission mechanism of ToCV by B. tabaci and could provide new avenues for controlling this plant virus and its vectors.

Tomato chlorosis virus, an emergent plant virus still expanding its geographical and host ranges

Tomato chlorosis virus (ToCV) causes an important disease that primarily affects tomato, although it has been found infecting other economically important vegetable crops and a wide range of wild plants. First described in Florida (USA) and associated with a 'yellow leaf disorder' in the mid-1990s, ToCV has been found in 35 countries and territories to date, constituting a paradigmatic example of an emergent plant pathogen. ToCV is transmitted semipersistently by whiteflies (Hemiptera: Aleyrodidae) belonging to the genera Bemisia and Trialeurodes. Whitefly transmission is highly efficient and cases of 100% infection are frequently observed in the field. To date, no resistant or tolerant tomato plants are commercially available and the control of the disease relies primarily on the control of the insect vector.

TAXONOMY

Tomato chlorosis virus is one of the 14 accepted species in the genus Crinivirus, one of the four genera in the family Closteroviridae of plant viruses.

VIRION AND GENOME PROPERTIES

The genome of ToCV is composed of two molecules of single-stranded positive-sense RNA, named RNA1 and RNA2, separately encapsidated in long, flexuous, rod-like virions. As has been shown for other closterovirids, ToCV virions are believed to have a bipolar structure. RNA1 contains four open reading frames (ORFs) encoding proteins associated with virus replication and suppression of gene silencing, whereas RNA2 contains nine ORFs encoding proteins putatively involved in encapsidation, cell-to-cell movement, gene silencing suppression and whitefly transmission.

HOST RANGE

In addition to tomato, ToCV has been found to infect 84 dicot plant species belonging to 25 botanical families, including economically important crops.

TRANSMISSION

Like all species within the genus Crinivirus, ToCV is semipersistently transmitted by whiteflies, being one of only two criniviruses transmitted by members of the genera Bemisia and Trialeurodes.

DISEASE SYMPTOMS

Tomato 'yellow leaf disorder' syndrome includes interveinal yellowing and thickening of leaves. Symptoms first develop on lower leaves and then advance towards the upper part of the plant. Bronzing and necrosis of the older leaves are accompanied by a decline in vigour and reduction in fruit yield. In other hosts the most common symptoms include interveinal chlorosis and mild yellowing on older leaves.

CONTROL

Control of the disease caused by ToCV is based on the use of healthy seedlings for transplanting, limiting accessibility of alternate host plants that can serve as virus reservoirs and the spraying of insecticides for vector control. Although several wild tomato species have been shown to contain genotypes resistant to ToCV, there are no commercially available resistant or tolerant tomato varieties to date.

Identification and Expression Analysis of Genes Induced in Response to Tomato chlorosis virus Infection in Tomato

Tomato (Solanum lycopersicum) is one of the most widely grown and economically important vegetable crops in the world. Tomato chlorosis virus (ToCV) is one of the recently emerged viruses of tomato distributed worldwide. ToCV-tomato interaction was investigated at the molecular level for determining changes in the expression of tomato genes in response to ToCV infection in this study. A cDNA library enriched with genes induced in response to ToCV infection were constructed and 240 cDNAs were sequenced from this library. The macroarray analysis of 108 cDNAs revealed that the expression of 92 non-redundant tomato genes was induced by 1.5-fold or greater in response to ToCV infection. The majority of ToCV-induced genes identified in this study were associated with a variety of cellular functions including transcription, defense and defense signaling, metabolism, energy, transport facilitation, protein synthesis and fate and cellular biogenesis. Twenty ToCV-induced genes from different functional groups were selected and induction of 19 of these genes in response to ToCV infection was validated by RT-qPCR assay. Finally, the expression of 6 selected genes was analyzed in different stages of ToCV infection from 0 to 45 dpi. While the expression of three of these genes was only induced by ToCV infection, others were induced both by ToCV infection and wounding. The result showed that ToCV induced the basic defense response and activated the defense signaling in tomato plants at different stages of the infection. Functions of these defense related genes and their potential roles in disease development and resistance to ToCV are also discussed.

Biochemical, Serological, Molecular and Natural Host Studies on Tomato Chlorosis Virus in Egypt

BACKGROUND AND OBJECTIVE

Tomato Chlorosis Virus (ToCV) is a white fly-transmitted and phloem-limited crinivirus reported in this study for the first time in Egypt. ToCV caused drastic reduction in tomato yield since 2013. The aim of this study is to characterize the virus incidence using biological, serological and molecular tools.

MATERIAL AND METHODS

The B. tabaci MEAM1 white fly was used for virus isolation and propagation. Identity of ToCV , its natural hosts were confirmed with RT-PCR using a specific primer pair for ToCV-heat shock protein 70 homologue (HSP70h) gene, sequencing and phylogenetic studies. ToCV was purified using the innovative electro-elution technique. The induced antiserum for the Egyptian isolate of the virus (ToCV-Giza) was used for DAS-ELISA and dot blotting immuno-assays to evaluate the virus presence in tomato and other natural hosts.

RESULTS

The ToCV-Giza isolate was donated an accession number "MH667315.1" from the GenBank. Blastx sequence analysis of the HSP70h gene indicated 97-99% of amino acid similarities with many tested ToCV isolates. Phylogenetic studies showed the clustering of all ToCV isolates including ToCV-Giza in a separate group from the other tested criniviruses. The virus had a UV spectrum of a nucleoprotein with Amax and Amin at 260 and 240 nm, respectively and A260/280 ratio of 1.33. Out of 52 different tested plant species within 22 families, 44 were positive hosts for ToCV. Thirty seven out of these 44 plant species were considered as new hosts for ToCV in the present study. These included Ammi majus and Coriandrum sativum (Apiaceae), cabbage (Brassicaceae), sweet potato (Convolvulaceae), melon, cucumber, luffa (Cucurbitaceae), soybean, cowpea, faba bean (Fabaceae), Egyptian and American Cotton (Malvaceae). Several ornamentals either herbal type or woody trees belonging to Acanthaceae, Amaranthaceae, Euophorbiaceae, Moraceae and Rubiaceae were also recognized for the first time as hosts for ToCV.

CONCLUSION

The obtained results confirmed the wide distribution of ToCV in its natural hosts in Egypt. Hygienic measures including control of the virus vector and removing of natural hosts should be strictly implicated.

Development of a graft inoculation method and a real-time RT-PCR assay for monitoring Tomato chlorosis virus infection in tomato

A graft inoculation method coupled with RT-qPCR was developed for monitoring ToCV infection in tomato plants. Ten seed-grown tomato seedlings were graft inoculated with phloem tissue-containing stem segments from a ToCV-infected tomato plants. Another group of tomato seedling were grafted with similar stem segments from a healthy tomato plant as mock inoculated control. The CP gene of ToCV was cloned under the control of T7 promoter and in vitro synthesized RNA was used as a standard for quantification. Total RNA was isolated from leaf samples of ToCV-inoculated and mock-inoculated control plants before the inoculation and 1-60 days post inoculation (dpi). The presence and the titer of ToCV were determined from all ToCV-inoculated or mock-inoculated control plants by RT-qPCR. After 15 dpi, ToCV was detected in 20-30% of graft-inoculated plants. The infection rate then increased progressively and reached to 70-80% by 60 dpi. Titer of ToCV was at the detectable level at 15 dpi and increased and reached to maximum level by 40 dpi and then started to decrease. The results showed that patch grafting is a simple and efficient method for experimental inoculation of ToCV and can be used as an alternative and/or complementary to vector transmission in the laboratories. The patch grafting could be combined with RT-qPCR and used for infecting and quantitative monitoring of ToCV or other phloem-limited viruses in tomato or in other plants.

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.

Detection and epidemic dynamic of ToCV and CCYV with Bemisia tabaci and weed in Hainan of China

BACKGROUND

In recent years, two of the crinivirus, Tomato chlorosis virus (ToCV) and Cucurbit chlorotic yellows virus (CCYV) have gained increasing attention due to their rapid spread and devastating impacts on vegetable production worldwide. Both of these viruses are transmitted by the sweet potato whitefly, Bemisia tabaci (Gennadius), in a semi-persistent manner. Up to now, there is still lack of report in Hainan, the south of China.

METHODS

We used observational and experimental methods to explore the prevalence and incidence dynamic of CCYV and ToCV transmitted by whiteflies in Hainan of China.

RESULTS

In 2016, the chlorosis symptom was observed in the tomato and cucumber plants with a large number of B. tabaci on the infected leaves in Hainan, China, with the incidence rate of 69.8% and 62.6% on tomato and cucumber, respectively. Based on molecular identification, Q biotype was determined with a viruliferous rate of 65.0% and 55.0% on the tomato and cucumber plants, respectively. The weed, Alternanthera philoxeroides near the tomato and cucumber was co-infected by the two viruses. Furthermore, incidence dynamic of ToCV and CCYV showed a close relationship with the weed, Alternanthera philoxeroides, which is widely distributed in Hainan.

CONCLUSION

Our results firstly reveal that the weed, A. philoxeroides is infected by both ToCV and CCYV. Besides, whiteflies showed a high viruliferous rate of ToCV and CCYV. Hainan is an extremely important vegetable production and seed breeding center in China. If the whitefly can carry these two viruses concurrently, co-infection in their mutual host plants can lead to devastating losses in the near future.

Eugenol confers resistance to Tomato yellow leaf curl virus (TYLCV) by regulating the expression of SlPer1 in tomato plants

Tomato yellow leaf curl virus (TYLCV) is one of the most devastating plant diseases, and poses a significant agricultural concern because of the lack of an efficient control method. Eugenol is a plant-derived natural compound that has been widely used as a food additive and in medicine. In the present study, we demonstrated the potential of eugenol to enhance the resistance of tomato plants to TYLCV. The anti-TYLCV efficiency of eugenol was significantly higher than that of moroxydine hydrochloride (MH), a widely used commercial antiviral agent. Eugenol application stimulated the production of endogenous nitric oxide (NO) and salicylic acid (SA) in tomato plants. The full-length cDNA of SlPer1, which has been suggested to be a host R gene specific to TYLCV, was isolated from tomato plants. A sequence analysis suggested that SlPer1 might be a nucleobase-ascorbate transporter (NAT) belonging to the permease family. The transcript levels of SlPer1 increased markedly in response to treatment with eugenol or TYLCV inoculation. The results of this study also showed that SlPer1 expression was strongly induced by SA, MeJA (jasmonic acid methyl ester), and NO. Thus, we propose that the increased transcription of SlPer1 contributed to the high anti-TYLCV efficiency of eugenol, which might involve in the generation of endogenous SA and NO. Such findings provide the basis for the development of eugenol as an environmental-friendly agricultural antiviral agent.

Allelic variation at the rpv1 locus controls partial resistance to Plum pox virus infection in Arabidopsis thaliana

BACKGROUND

Sharka is caused by Plum pox virus (PPV) in stone fruit trees. In orchards, the virus is transmitted by aphids and by grafting. In Arabidopsis, PPV is transferred by mechanical inoculation, by biolistics and by agroinoculation with infectious cDNA clones. Partial resistance to PPV has been observed in the Cvi-1 and Col-0 Arabidopsis accessions and is characterized by a tendency to escape systemic infection. Indeed, only one third of the plants are infected following inoculation, in comparison with the susceptible Ler accession.

RESULTS

Genetic analysis showed this partial resistance to be monogenic or digenic depending on the allelic configuration and recessive. It is detected when inoculating mechanically but is overcome when using biolistic or agroinoculation. A genome-wide association analysis was performed using multiparental lines and 147 Arabidopsis accessions. It identified a major genomic region, rpv1. Fine mapping led to the positioning of rpv1 to a 200 kb interval on the long arm of chromosome 1. A candidate gene approach identified the chloroplast phosphoglycerate kinase (cPGK2) as a potential gene underlying the resistance. A virus-induced gene silencing strategy was used to knock-down cPGK2 expression, resulting in drastically reduced PPV accumulation.

CONCLUSION

These results indicate that rpv1 resistance to PPV carried by the Cvi-1 and Col-0 accessions is linked to allelic variations at the Arabidopsis cPGK2 locus, leading to incomplete, compatible interaction with the virus.

Effects of the crinivirus coat protein-interacting plant protein SAHH on post-transcriptional RNA silencing and its suppression

In plants, post-transcriptional gene silencing (PTGS) is a sequence-specific mechanism of RNA degradation induced by double-stranded RNA (dsRNA), which is processed into small interfering RNAs (siRNAs). siRNAs are methylated and, thereby, stabilized by the activity of the S-adenosylmethionine-dependent RNA methyltransferase HEN1. PTGS is amplified by host-encoded RNA-dependent RNA polymerases (RDR), which generate dsRNA that is processed into secondary siRNAs. To counteract this RNA silencing-mediated response of the host, plant viruses express proteins with silencing suppression activity. Here, we report that the coat protein (CP) of crinivirus (family Closteroviridae, genus Crinivirus) Tomato chlorosis virus, a known suppressor of silencing, interacts with S-adenosylhomocysteine hydrolase (SAHH), a plant protein essential for sustaining the methyl cycle and S-adenosylmethionine-dependent methyltransferase activity. Our results show that, by contributing to an increased accumulation of secondary siRNAs generated by the action of RDR6, SAHH enhances local RNA silencing. Although downregulation of SAHH prevents local silencing, it enhances the spread of systemic silencing. Our results also show that SAHH is important in the suppression of local RNA silencing not only by the crinivirus Tomato chlorosis virus CP but also by the multifunctional helper component-proteinase of the potyvirus Potato virus Y.

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.

Major tomato viruses in the Mediterranean basin

Tomato (Solanum lycopersicum L.) originated in South America and was brought to Europe by the Spaniards in the sixteenth century following their colonization of Mexico. From Europe, tomato was introduced to North America in the eighteenth century. Tomato plants show a wide climatic tolerance and are grown in both tropical and temperate regions around the world. The climatic conditions in the Mediterranean basin favor tomato cultivation, where it is traditionally produced as an open-field plant. However, viral diseases are responsible for heavy yield losses and are one of the reasons that tomato production has shifted to greenhouses. The major tomato viruses endemic to the Mediterranean basin are described in this chapter. These viruses include Tomato yellow leaf curl virus, Tomato torrado virus, Tomato spotted wilt virus, Tomato infectious chlorosis virus, Tomato chlorosis virus, Pepino mosaic virus, and a few minor viruses as well.