Tomato Yellow Leaf Curl Virus Disease Research Articles

Establish an efficient inoculation system of tomato yellow leaf curl virus to assist tomato resistance breeding

Tomato yellow leaf curl virus (TYLCV) endangers the production of tomato in the world, which is largely overcome by cultivating resistant tomato cultivars. Since virus evolves fast and may disrupt the resistance in resistant cultivars, tomato resistance breeding against emerging TYLCV isolates is a long-term task, which requires an efficient inoculation system of TYLCV to screen and identify resistant tomato plants. Previous Agrobacterium-mediated TYLCV infectious clone is to construct the viral genome in the binary vector, and does not have high infection efficiency. In this study, we integrate TYLCV genome in pTRV2GW vector containing partial genome of Tobacco Rattle virus, and generate an Agrobacterium-mediated infectious clone pTRV2GW-TYLCV. Tomato plants are infected by vacuum infiltration with the Agrobacterium culture of infectious clone. PCR analysis with all TYLCV ORFs in infected plants reveals distinct detection of each ORF at different infection stages and a final 100 % infection rate of this inoculation method. Infected tomato plants have typical TYLCV disease symptoms including yellowing, curling, and smaller leaves. Plant growth and leaf photosynthesis are repressed in infected plants compared to non-infected plants. Moreover, the inoculation system is successfully applied for differentiating TYLCV resistant and susceptible cultivars, which can assist tomato resistance breeding.

Ecoepidemiological Model and Optimal Control Analysis of Tomato Yellow Leaf Curl Virus Disease in Tomato Plant

The purpose of this study is to analyze the impact of control strategies, namely, insecticide spray, roguing of a diseased tomato plant, and protective netting to protect tomato plant from tomato yellow leaf curl virus disease (TYLCVD). For this, we formulate and analyze a simple deterministic model for the transmission dynamics of TYLCVD that incorporates these control strategies. We initially take into account the constant control case, we calculate the basic reproduction number, and we investigate the existence and stability of the disease-free and endemic equilibria. We use the Kamgang-Sallet stability to ensure that the disease-free equilibrium point is globally asymptotically stable when the reproduction number R 0 is less than one. This indicates that TYLCVD dies out independent of the initial size of the tomato population. For R 0 < 1 , the disease-free equilibrium becomes unstable, and the endemic equilibrium is globally asymptotically stable. This indicates that TYLCVD propagates. In the nonconstant control case, we use Pontryagin’s maximum principle to derive the necessary conditions for the optimal control of the disease. Our findings show that all the combined efforts of two out of three strategies can significantly reduce the power of infectivity of the disease except the combination of the use of insecticide spray and rouging infected tomato plants. Besides our numerical simulations show, the implementation of the combination of roguing diseased plants and protective netting has a similar effect in minimizing or eliminating TYLCV as the use of all strategies. Hence, as resources are always in scarce, we recommend policymakers to adapt the combination of the use of roguing diseased tomato plants and protective netting to eradicate the disease.

Response of Physicochemical Properties and Enzyme Activities in Rhizosphere Soil to Yellow Leaf Curl Virus Disease of Cherry Tomato (<i>Lycopersicon esulentum </i>Mill.)

Tomato yellow leaf curl virus disease (TYLCVD) has become a devastating disease, which seriously threatens the healthy development of tomato industry in China. Creating a healthy soil environment may be the key to success in preventing TYLCVD. Field investigation and laboratory analysis were used to evaluate the response characteristics of macro elements, medium elements, micro elements and enzyme activities in the rhizosphere soil of cherry tomato to TYLCVD. The main objective of this study was to determine the suitable soil environment for the healthy survival of cherry tomato. Soil samples were collected from rhizosphere of healthy and diseased plants. The results showed that the occurrence of TYLCVD was related to the enzyme activities, pH value, macro elements N, P, K, medium elements Ca, Mg, S and micro elements Mn, Zn, Cu, Si in the rhizosphere soil of cherry tomato. Compared with healthy plants, the activities of urease, catalase and acid phosphatase were significantly decreased by 48.6%–77.4%, 23.7%–43.8% and 19.1%–31.0%, respectively, in rhizosphere soil of diseased cherry tomato plants (<I>P</I> < 0.05). The pH value, contents of alkaline hydrolysis N, available Mg, available S, available Mn, available Zn, available Cu and available Si in rhizosphere soil of diseased plants were significantly lower than those in healthy plants (<I>P</I> < 0.05). The pH value in rhizosphere soil of diseased plants was 0.18-0.25 units lower than that of healthy plants. And the contents of alkaline hydrolysis N, available Mg, available S, available Mn, available Zn, available Cu and available Si were reduced by 5.54%–20.0%, 29.7%–73.9%, 27.3%–48.8%, 6.95%–10.1%, 13.6%–-15.2%, 10.6%–25.0% and 4.97%-8.30%, respectively. However, the contents of available P, available K and available Ca in rhizosphere soil of diseased plants were significantly higher than those in healthy plants (<I>P</I> < 0.05), were increased by 3.77%–41.2%, 6.75%–37.4% and 16.7%–50.5%, respectively. Our findings improve our understanding of the links between the occurrence of TYLCVD and the soil environment, which have implications for developing strategies for the prevention of tomato yellow leaf curl virus disease.

Potential Adoption of the F2 Generation of Some TYLCV-Resistant Tomato Hybrids in Commercial Production under Natural Field Infection

The economic feasibility of using the tomato yellow leaf curl virus (TYLCV) resistant F1 tomato hybrids (Solanum lycopersicum L.) may be improved with the possibility of using their F2 generation in commercial production. Hence, this study was performed to evaluate the F1 and F2 generations of 15 TYLCV resistant/tolerant commercial tomato hybrids during the 2018 and 2019 fall season to identify the best hybrids and F2 populations for commercial production. Both F1 and F2 populations were evaluated separately for TYLCV resistance, yield, and fruit quality traits. Broad sense heritability (H2b) values of TYLCV-resistance were high, being > 60% in hybrids 186, Goldstone, Rabha, SV0922 and SV8320 in both seasons. F1 and F2 populations of ‘186’, ‘Dania’, ‘PS550’, ‘Rozalina’, ‘SV0922’, ‘SV8320’, and ‘Tyrmes’ had the highest total yield (TY)/plant, low TYLCV disease (TYLCVD) mean scores, acceptable fruit quality traits, and average fruit weight > 80 g. Desirable inbreeding depression (ID) values were observed for TYLCVD-mean score in F2 populations of most evaluated hybrids; for TY/plant in F2 populations of ‘SV3773’ in both seasons, and in populations of ‘186’, ‘448’, ‘Goldstone’, ‘Rabha’, ‘SV8320’ and ‘SV0922’ in the second season. Most hybrids had desirable significant ID values for fruit quality traits. Accordingly, it is concluded that the F2 populations of hybrids 186, SV0922 and SV8320 may be used in commercial tomato production under TYLCV-infection in fall season, as these populations recorded high h2b values for TYLCV-resistance trait and desirable ID for TYLCV-resistance, yield, and fruit quality traits.

Bacillus amyloliquefaciens Ba13 induces plant systemic resistance and improves rhizosphere microecology against tomato yellow leaf curl virus disease

The control effect of Bacillus against tomato yellow leaf curl virus (TYLCV) disease has not been reported. It is not clear whether adjustment of the rhizosphere microbial community structure by beneficial microbes applied as biocontrol agents is related to their role in increasing resistance of plants to viral diseases. Therefore, the objective of this study was to investigate the control effect and mechanism of Bacillus amyloliquefaciens Ba13 against TYLCV disease. Two treatments including control and Ba13 treatment followed by natural infection with whiteflies were conducted. We measured plant growth, physiological parameters, disease severity, and leaf virus quantity and also assayed resistance-related gene expression, defense enzyme activity, and rhizosphere microbial community structure. Strain Ba13 lowered the infection rate, disease severity, and leaf virus quantity in tomato plants, with the control rate of 48–52%. Root development and shoot biomass markedly improved in tomato following Ba13 treatment. Ba13 elevated expression of pathogenesis-related PR1, PR2, and PR3 genes, and enhanced activities of phenylalanine ammonia lyase, polyphenol oxidase, peroxidase, β-1,3 glucanase, and chitinase in tomato leaves. Cultivation-dependent microbiological analysis showed that Ba13 increased the bacteria-to-fungi ratio and the number of beneficial microbes while decreasing the number of pathogenic fungi in the rhizosphere. Further analysis revealed the potential of a dominant rhizosphere bacterium, B. velezensis B3, to promote plant growth and improve systemic resistance in tomato. B. amyloliquefaciens Ba13 can enhance plant resistance against TYLCV disease through direct induction of systemic resistance and also by increasing the number of beneficial microbes in the rhizosphere, thereby achieving control of TYLCV disease.

Streptomyces pactum Act12 controls tomato yellow leaf curl virus disease and alters rhizosphere microbial communities

Actinomycetes comprise a large group of biocontrol bacteria, yet no studies have reported on their effects against plant virus diseases. Here, we evaluated the control effects and the possible mechanisms of Streptomyces pactum Act12 against tomato yellow leaf curl virus disease (TYLCD) through field and pot experiments. We also analyzed changes in plant growth and rhizosphere microbial community composition following the application of Act12. The pre-inoculation of soil with the Act12 agent enhanced the activities of peroxidase and chitinase while upregulating the expression of genes related to plant systemic resistance (PR-1 and SIPI-II) and specific TYLCD resistance (SIPer1 and SIVRSLip) in tomato leaves under field conditions. The effects of Act12 at inducing a decline in TYLCD severity and promoting plant growth were deemed satisfactory in all three field experiments. In the pot experiments, Act12 induced a decline in the viral DNA level and an enhancing of peroxidase, polyphenoloxidase, and phenylalanine ammonia lyase activities in tomato leaves. Additionally, Act12 treatment reduced the accumulation of H2O2 and O·−2, increased the levels of salicylic and jasmonic acids, and upregulated the expression of genes related to plant resistance and RNA interference in tomato leaves. Applying Act12 also increased the diversity of bacteria and the abundance of potential beneficial microbes (e.g., Trichoderma and Bacillus) in the root-zone and root-surface soils, resulting in modular differentiation of co-occurrence networks. In conclusion, applying the S. pactum Act12 agent reduced TYLCD severity and promoted plant growth in tomato. Whether the changes of resistance-related gene expression and rhizosphere microbial community composition contribute to TYLCV resistance needs to be further investigated.

Farmers Agronomic Practice in Management of the Tomato (&lt;i&gt;Solanum lycopersicon&lt;/i&gt; L.) Yellow Leaf Curl Virus in Central Region of Ghana

The study assessed farmers’ awareness of the Tomato yellow leaf curl virus (TYLCV) disease and their agronomic and disease management practices in the Efutu municipality, Komenda-Edina-Eguafo-Abirem (KEEA), and Mfantseman districts which are leading tomato producing centres in the Central Region of Ghana. The study also surveyed the incidence and severity of the TYLCV disease in tomato fields across the three districts. Household data were collected using structured questionnaire from 150 respondents using multi-stage procedure, and analysed using descriptive statistics. Incidence (DI) and severity index (SI) of TYLCV disease were determined from forty (40) tomato fields selected from each of the three districts. The field data was subjected to analysis of variance (ANOVA) and the means separated with least significant difference (l.s.d) method at 5% level of probability.Majority of the farmers (92.6%) were aware of the TYLCVdisease and said it could cause yield losses rangingfrom less than 10% to over 41% but did not know the cause.Majority (60.4%) of the farmers managed the TYLCV disease in their farms mainly by applying insecticides (55.6%) and roguing of diseased plants (43.1%). About 60.7% of the farmers practiced mixed cropping, and most of them cultivated tomato in both the major and minor cropping seasons, using mainly an improved form of Solanum pimpinellifolium . The highest mean disease incidence and mean disease severity indices were recorded at KEEA (52.9±2.7%, 26.89±1.2%), followed by Efutu (49.5±1.19%, 25.29±0.9%), and Mfantseman (42.1±2.7%, 21.41±0.8%) respectively.In conclusion, TYLCV was highly prevalent in the study area, but infection was moderate due to the use of improved tomato variety.

The Contrasting Effects of Elevated CO2 on TYLCV Infection of Tomato Genotypes with and without the Resistance Gene, Mi-1.2.

Elevated atmospheric CO2 typically enhances photosynthesis of C3 plants and alters primary and secondary metabolites in plant tissue. By modifying the defensive signaling pathways in host plants, elevated CO2 could potentially affect the interactions between plants, viruses, and insects that vector viruses. R gene-mediated resistance in plants represents an efficient and highly specific defense against pathogens and herbivorous insects. The current study determined the effect of elevated CO2 on tomato plants with and without the nematode resistance gene Mi-1.2, which also confers resistance to some sap-sucking insects including whitefly, Bemisia tabaci. Furthermore, the subsequent effects of elevated CO2 on the performance of the vector whiteflies and the severity of Tomato yellow leaf curl virus (TYLCV) were also determined. The results showed that elevated CO2 increased the biomass, plant height, and photosynthetic rate of both the Moneymaker and the Mi-1.2 genotype. Elevated CO2 decreased TYLCV disease incidence and severity for Moneymaker plants but had the opposite effect on Mi-1.2 plants whether the plants were agroinoculated or inoculated via B. tabaci feeding. Elevated CO2 increased the salicylic acid (SA)-dependent signaling pathway on Moneymaker plants but decreased the SA-signaling pathway on Mi-1.2 plants when infected by TYLCV. Elevated CO2 did not significantly affect B. tabaci fitness or the ability of viruliferous B. tabaci to transmit virus regardless of plant genotype. The results indicate that elevated CO2 increases the resistance of Moneymaker plants but decreases the resistance of Mi-1.2 plants against TYLCV, whether the plants are agroinoculated or inoculated by the vector. Our results suggest that plant genotypes containing the R gene Mi-1.2 will be more vulnerable to TYLCV and perhaps to other plant viruses under elevated CO2 conditions.

Tomato yellow leaf curl virus Can Overwinter in Stellaria aquatica, a Winter-Hardy TYLCV-Reservoir Weed.

Tomato yellow leaf curl virus (TYLCV), one of the most serious plant viruses in tropical and subtropical regions, is transmitted to host plants by the vector insect Bemisia tabaci. In order to control TYLCV, it is important to identify weed hosts for overwintering TYLCV. Stellaria aquatica, a winter-hardy weed, was found growing with TYLCV-infected tomato plants in greenhouse production. TYLCV was detected in S. aquatica plants by polymerase chain reaction and Southern blot hybridization analysis. The intergenic region nucleotide sequences amplified from TYLCV-infected tomato plants, TYLCV-viruliferous whiteflies, and S. aquatica were identical. During winter (December to February), TYLCV-viruliferous whiteflies and TYLCV-infected tomato plants were removed or absent from greenhouses. However, S. aquatica plants were observed over a period of 10 months from August to May in such greenhouses, and TYLCV was consistently detected in some of these plants. To investigate the transmission of TYLCV from TYLCV-infected S. aquatica plants to healthy tomato plants by whiteflies, TYLCV-infected S. aquatica plants were transplanted to pots in cages with nonviruliferous whiteflies and healthy tomato plants. After 4 weeks, tomato plants developed typical TYLCV disease symptoms, and TYLCV was detected in both whiteflies and tomato plants. These results show that S. aquatica can act as a winter-hardy reservoir for TYLCV, and suggest that this weed could play an important role in overwintering of TYLCV in tomato greenhouses.

Natural occurrence of begomovirus recombinants associated with tomato yellow leaf curl disease co-existing with parental viruses in tomato crops and weeds in Tunisia

Tomato yellow leaf curl virus disease (TYLCVD) is the main limiting factor for tomato production in the Mediterranean Basin, and particularly in Tunisia where climatic and crop production conditions, as well as the occurrence of many weeds, provide suitable conditions for the presence and spread of TYLCVD all year round. In Tunisia, epidemics in tomato-growing regions have been associated with two begomoviruses: Tomato yellow leaf curl virus (TYLCV) and Tomato yellow leaf curl Sardinia virus (TYLCSV) but the presence of recombinants between them has never been investigated. A large-scale survey was conducted on leaves sampled from late-season tomato crops exhibiting severe curling symptoms, from beans, and from nine nearby weed species in the South and Sahel regions of Tunisia to investigate: (i) the natural occurrence of the TYLCV and TYLCSV species; (ii) the presence of the two recombinant types, RecA and RecB, derived from these species. Identification of TYLCV and TYLCSV was based on a multiplex PCR with primers generating a typical pattern on gels (750 bp and 366 bp fragments, respectively). Recombinants were identified by simplex PCR, which amplified the intergenic region, the most common recombination site described for begomoviruses, and sequencing. The occurrence of TYLCSV and TYLCV was confirmed in both Tunisian regions. Their co- existence was shown on tomato and in new weed species. This is the first time that recombinants between TYLCV and TYLCSV species showing 93% to 95% nucleotide identity with tomato yellow leaf curl recombinant viruses, are reported in Tunisia in tomato and black nightshade.

Comparative transcriptome profiling of a resistant vs. susceptible tomato (Solanum lycopersicum) cultivar in response to infection by tomato yellow leaf curl virus.

Tomato yellow leaf curl virus (TYLCV) threatens tomato production worldwide by causing leaf yellowing, leaf curling, plant stunting and flower abscission. The current understanding of the host plant defense response to this virus is very limited. Using whole transcriptome sequencing, we analyzed the differential gene expression in response to TYLCV infection in the TYLCV-resistant tomato breeding line CLN2777A (R) and TYLCV-susceptible tomato breeding line TMXA48-4-0 (S). The mixed inoculated samples from 3, 5 and 7 day post inoculation (dpi) were compared to non-inoculated samples at 0 dpi. Of the total of 34831 mapped transcripts, 209 and 809 genes were differentially expressed in the R and S tomato line, respectively. The proportion of up-regulated differentially expressed genes (DEGs) in the R tomato line (58.37%) was higher than that in the S line (9.17%). Gene ontology (GO) analyses revealed that similar GO terms existed in both DEGs of R and S lines; however, some sets of defense related genes and their expression levels were not similar between the two tomato lines. Genes encoding for WRKY transcriptional factors, R genes, protein kinases and receptor (-like) kinases which were identified as down-regulated DEGs in the S line were up-regulated or not differentially expressed in the R line. The up-regulated DEGs in the R tomato line revealed the defense response of tomato to TYLCV infection was characterized by the induction and regulation of a series of genes involved in cell wall reorganization, transcriptional regulation, defense response, ubiquitination, metabolite synthesis and so on. The present study provides insights into various reactions underlining the successful establishment of resistance to TYLCV in the R tomato line, and helps in the identification of important defense-related genes in tomato for TYLCV disease management.

Eugenol enhances the resistance of tomato against tomato yellow leaf curl virus

Tomato yellow leaf curl virus disease (TYLCVD) causes severe to economic losses in tomato crops in China. The control of TYLCVD is based primarily on the use of synthetic insecticide to control its vector whitefly (Bemisia tabaci). To look for an alternative method for disease control, we investigated the effect of eugenol on controlling TYLCVD. The potential of eugenol to trigger systemic acquired resistance (SAR) in tomato (Jiangsu 14) plants against TYLCV was also investigated. In greenhouse experiments, eugenol significantly reduced disease severity when applied as a foliar spray, thus demonstrating a systemic effect. The disease spread rapidly in control plants and by the end of the experiment almost all control plants showed severe symptoms. Eugenol also induced H₂O₂ accumulation in tomato plants. Activities of peroxidase (POD), polyphenol oxidase (PPO) and phenylalanine ammonia lyase (PAL) were significantly induced compared with those of control plants. As further consequences, increase of salicylic acid (SA) levels and expression of PR-1 proteins, a molecular marker of SAR in tomato, could also be observed. This is the first report of eugenol as an elicitor and its ability to suppress plant virus diseases under greenhouse conditions. It is suggested that eugenol has the potential to be an effective biocontrol agent against TYLCV in tomato plants.

초고속 Real-time PCR을 이용한 Tomato yellow leaf curl virus의 신속진단

Tomato yellow leaf curl virus (TYLCV), transmitted exclusively by the whitefly (Bemisia tabaci) in a circulative manner is one of the most important virus in tomato. Since the first report of TYLCV incidence in Korea in 2008, the virus has rapidly spread nationwide. TYLCV currently causes serious economic losses in tomato production in Korea. Early detection of TYLCV is one of the most important methods to allow rouging of infected tomato plants to minimize the spread of TYLCV disease. We have developed an ultra-rapid and sensitive real-time polymerase chain reaction (PCR) using a new designed real-time PCR system, GenSpectorTM TMC-1000 that is a small and portable real-time PCR machine requiring only a reaction volume on microchips. The new system provides ultra-high speed reaction (30 cycles in less than 15 minutes) and melting curve analysis for amplified TYLCV products. These results suggest that the short reaction time and ultra sensitivity of the GenSpector-based real-time PCR technique is suitable for monitoring epidemics and pre-pandemic TYLCV disease. This is the first report for plant virus detection using an ultra-rapid real-time PCR system.

04/02381 Transmission planning today: a challengingundertaking: Nadira, R. et al. The Electricity Journal, 2004, 17, (4), 24–32

Plant viral diseases are the most important pathogen attack tomato particularly of which is Tomato yellow leaf curl virus (TYLCV). Obtaining stable and durable resistances seems the most effective solution for controlling the disease. In this context, a breeding programme with two steps was conducted in the Niayes ecological area of Senegal: i) 41 tomato varieties from different origins were screened to assess their reaction to TYLCV disease, ii) 12 resistant varieties and two susceptible were tested in an experimental field. The first step was a preliminary trial in unreplicated plots, to identify promising varieties. However, disease assessments (incidence and severity) were done three times and we used longitudinal data analyses to classify the varieties for resistance. Yields in t/ha were also measured for the different varieties. Among our accessions, Roma VF and Xina known for their susceptibility, and Nadira known for its tolerance, were used as controls. Our results revealed 12 resistant varieties, 16 tolerant varieties and 8 susceptible varieties. The yield values were well correlated to the resistance measured by both incidence and by severity right from the second date of symptom observations, at the time of flowering. TYLCV disease is thus one of the main factors limiting tomato-growing the Niayes agro-ecological area. The second step of the selection process was done in a complete block design with three replications. This trial allowed a confirmation of the 12 resistant varieties. The final selection of the varieties took into account the other traits, like yield and traits related to the fruit characters.

Effect of Host Plant Resistance to Tomato yellow leaf curl virus (TYLCV) on Virus Acquisition and Transmission by Its Whitefly Vector

ABSTRACT The effect that Tomato yellow leaf curl virus (TYLCV)-infected resistant tomato plants may have on virus epidemiology was studied. Four tomato genotypes that exhibit different levels of viral resistance, ranging from fully susceptible to highly resistant, served as TYLCV-infected source plants. Viral acquisition and transmission rates by white-flies following feeding on the different source plants were evaluated. TYLCV transmission rate by whiteflies that had fed on infected source plants 21 days postinoculation (DPI), shortly after the appearance of TYLCV symptoms, was negatively correlated with the level of resistance displayed by the source plant. Therefore, the higher the resistance, the lower the transmission rate. In addition, TYLCV DNA accumulation was shown to be lower in the resistant source plants compared with the susceptible plants. Whitefly survival rate, following feeding on source plants 21 DPI, was similar for all the cultivars tested. Significant differences in whitefly survival were found, however, following feeding on the infected source plants at 35 DPI; here, whitefly survival rate increased with higher levels of resistance displayed by the source plant. At 35 DPI, the susceptible plants had developed severe TYLCV disease symptoms, and transmission rates from these plants were the lowest, presumably due to the poor condition of these plants. Transmission rates from source plants displaying a medium level of resistance level were highest, with rates declining following feeding on source plants displaying higher levels of TYLCV resistance. TYLCV DNA accumulation in whiteflies following feeding on infected source plants at both 21 and 35 DPI was directly correlated with viral DNA accumulation in source plants. Results show that, in essence, the higher the resistance expressed, the less suitable the plant was as a viral source. Consequently, following acquisition from a highly resistant plant, TYLCV transmission by whiteflies will be less efficient.

Ultraviolet-Deficient Greenhouse Environment Affects Whitefly Attraction and Flight-Behavior

The effect of a UV-deficient environment on the attraction and dispersal behavior of whiteflies, Bemisia argentifolii (Bellows & Perring), and on the transmission efficiency of the whitefly-borne tomato yellow leaf curl geminivirus, was tested under field conditions and through controlled experiments. We found that the rate of tomato yellow leaf curl virus-disease spread to tomato plants grown under walk-in tunnels covered with regular greenhouse plastic sheets increases sharply with time, whereas the virus infection-rate under UV-absorbing sheets proceeds at a very slow pace. Average number of whiteflies trapped under regular plastic sheet tunnels was significantly higher than numbers trapped in UV-absorbing plastic sheet tunnels. Similarly, the average number of whiteflies trapped on yellow-sticky traps placed on the outside walls of tunnels covered with regular plastic was higher than the number trapped on the outside walls of tunnels covered with UV-absorbing plastic sheets. No differences were found in the whitefly's ability to transmit tomato yellow leaf curl virus under the two types of plastic covers. Whitefly dispersal pattern under the two types of plastic covers was examined using a release-recapture experiment. In each type of walk-in tunnel we established a grid of yellow-sticky traps forming two concentric circles: an inner and an external. Under UV-absorbing tunnels, significantly higher numbers of whiteflies were captured on the internal circle of traps than the external circle. The fraction of whiteflies that were captured on theexternalcirclewasmuchhigherunderregularcovers,whencomparedwithUV-absorbingcovers, suggesting that filtration of UV light hindered the ability of whiteflies to disperse inside these plastic tunnels. Our results indicate that the mechanisms by which UV-deficiency protects covered crops from insect infestation and spread of viruses are that the lack of UV interferes with insect flight orientation; and that the lack of UV radiation alters the normal behavior of the invading insects, resulting in reduced dispersal activity.

Transgenic Tomato Plants Expressing the Tomato Yellow Leaf Curl Virus Capsid Protein are Resistant to the Virus

The tomato yellow leaf curl virus (TYLCV) gene that encodes the capsid protein (V1) was placed under transcriptional control of the cauliflower mosaic virus 35S promoter and cloned into an Agrobacterium Ti-derived plasmid and used to transform plants from an interspecific tomato hybrid, Lycopersicon esculentum X L. pennellii (F1), sensitive to the TYLCV disease. When transgenic F1 plants, expressing the V1 gene, were inoculated with TYLCV using whiteflies fed on TYLCV-infected plants, they responded either as untransformed tomato or showed expression of delayed disease symptoms and recovery from the disease with increasingly more resistance upon repeated inoculation. Transformed plants that were as sensitive to inoculation as untransformed controls expressed the V1 gene at the RNA level only. All the transformed plants that recovered from disease expressed the TYLCV capsid protein.

Replication of tomato yellow leaf curl virus (TYLCV) DNA in agroinoculated leaf discs from selected tomato genotypes.

The leaf disc agroinoculation system was applied to study tomato yellow leaf curl virus (TYLCV) replication in explants from susceptible and resistant tomato genotypes. This system was also evaluated as a potential selection tool in breeding programmes for TYLCV resistance. Leaf discs were incubated with a head-to-tail dimer of the TYLCV genome cloned into the Ti plasmid of Agrobacterium tumefaciens. In leaf discs from susceptible cultivars (Lycopersicon esculentum) TYLCV single-stranded genomic DNA and its double-stranded DNA forms appeared within 2-5 days after inoculation. Whiteflies (Bemisia tabaci) efficiently transmitted the TYLCV disease to tomato test plants following acquisition feeding on agroinoculated tomato leaf discs. This indicates that infective viral particles have been produced and have reached the phloem cells of the explant where they can be acquired by the insects. Plants regenerated from agroinfected leaf discs of sensitive tomato cultivars exhibited disease symptoms and contained TYLCV DNA concentrations similar to those present in field-infected tomato plants, indicating that TYLCV can move out from the leaf disc into the regenerating plant. Leaf discs from accessions of the wild tomato species immune to whitefly-mediated inoculation, L. chilense LA1969 and L. hirsutum LA1777, did not support TYLCV DNA replication. Leaf discs from plants tolerant to TYLCV issued from breeding programmes behaved like leaf discs from susceptible cultivars.