Spread of Tomato apical stunt viroid (TASVd) in Greenhouse Tomato Crops Is Associated with Seed Transmission and Bumble Bee Activity

Reverse-transcription recombinase-aided amplification and CRISPR/Cas12a-based universal detection for fast screening and accurate identification of six pospiviroids infecting Solanaceae crops

BACKGROUND

Pospiviroids, members of the genus Pospiviroid, can cause severe diseases in tomato and other Solanaceae crops, causing considerable economic losses worldwide. Six pospiviroids including potato spindle tuber viroid (PSTVd), tomato chlorotic dwarf viroid (TCDVd), tomato planta macho viroid (TPMVd), Columnea latent viroid (CLVd), pepper chat fruit viroid (PCFVd), and tomato apical stunt viroid (TASVd) are regulated in many countries and organizations. Rapid, accurate detection is thus crucial for controlling the spread of these pospiviroids.

RESULTS

For simultaneous detection of these six pospiviroids, we developed a rapid, visual method that uses a reverse transcription recombinase-aided amplification (RT-RAA) assay coupled with a clustered regularly interspaced short palindromic repeats and CRISPR-associated protein 12a (CRISPR/Cas12a) system. In particular, this technique could achieve both universal detection and specific identification of the six target pospiviroids within 40 min. The universal detection could diagnose the six target pospiviroids in a single reaction, and the specific identification could identify each target pospiviroid without cross-reactivity of other pospiviroids. The sensitivity limits for the target pospiviroids detection with the proposed detection method were higher than those of the conventional reverse transcription-polymerase chain reaction (RT-PCR) method.

CONCLUSION

We designed an RT-RAA-CRISPR/Cas12a-based universal detection method for both large-scale screening and accurate identification of the six target pospiviroids, which is appropriate for on-site detection. Our study results can aid in performing rapid, large-scale screening of multiple pests simultaneously. © 2024 Society of Chemical Industry.

Viroids of the Mediterranean Basin

There has been substantial progress in the Mediterranean countries regarding research on viroids. Twenty-nine viroid species, all belonging to Pospiviroidae and Avsunviroidae genera, have been detected in the Mediterranean Basin. Not only have detection methods, such as reverse transcription-quantitative polymerase chain reaction and next-generation sequencing, been used for viroid detection, along with molecular hybridization techniques allowing for rapid detection, identification, and characterization of known and novel viroids in these countries, but eradication measures have also been taken that allowed for the efficient elimination of certain viroids in a number of Mediterranean countries. The eradication measures were followed as recommended by the European and Mediterranean Plant Protection Organization, which is known by its abbreviation, EPPO. The Mediterranean Region has been a niche for viroids since ancient times due to the warm climate and the socio-cultural conditions that facilitate viroid transmission among different host plant species.

A critical review on bioaerosols-dispersal of crop pathogenic microorganisms and their impact on crop yield

Bioaerosols are potential sources of pathogenic microorganisms that can cause devastating outbreaks of global crop diseases. Various microorganisms, insects and viroids are known to cause severe crop diseases impeding global agro-economy. Such losses threaten global food security, as it is estimated that almost 821 million people are underfed due to global crisis in food production. It is estimated that global population would reach 10 billion by 2050. Hence, it is imperative to substantially increase global food production to about 60% more than the existing levels. To meet the increasing demand, it is essential to control crop diseases and increase yield. Better understanding of the dispersive nature of bioaerosols, seasonal variations, regional diversity and load would enable in formulating improved strategies to control disease severity, onset and spread. Further, insights on regional and global bioaerosol composition and dissemination would help in predicting and preventing endemic and epidemic outbreaks of crop diseases. Advanced knowledge of the factors influencing disease onset and progress, mechanism of pathogen attachment and penetration, dispersal of pathogens, life cycle and the mode of infection, aid the development and implementation of species-specific and region-specific preventive strategies to control crop diseases. Intriguingly, development of R gene-mediated resistant varieties has shown promising results in controlling crop diseases. Forthcoming studies on the development of an appropriately stacked R gene with a wide range of resistance to crop diseases would enable proper management and yield. The article reviews various aspects of pathogenic bioaerosols, pathogen invasion and infestation, crop diseases and yield.

Bees and thrips carry virus-positive pollen in peach orchards in South Carolina, United States

Prunus necrotic ringspot virus (PNRSV) and prune dwarf virus (PDV) are pollen-borne viruses of important stone fruit crops, including peaches, which can cause substantial yield loss. Although both horizontal and vertical (i.e., seed) transmission of both viruses occurs through pollen, the role of flower-visiting insects in their transmission is not well understood. Bees and thrips reportedly spread PNRSV and PDV in orchards and greenhouse studies; however, the field spread of PNRSV and PDV in peach orchards in the southeastern United States is not explored. We hypothesized that bees and thrips may facilitate virus spread by carrying virus-positive pollen. Our 2-yr survey results show that 75% of captured bees are carrying virus-positive pollen and moving across the orchard while a subsample of thrips were also found virus positive. Based on morphology, Bombus, Apis, Andrena, Eucera, and Habropoda are the predominant bee genera that were captured in peach orchards. Understanding the role of bees and thrips in the spread of PNRSV and PDV will enhance our understanding of pollen-borne virus ecology.

A Novel, Precise and High-Throughput Technology for Viroid Detection in Cannabis (MFDetectTM)

Hop latent viroid (HLVd) is a severe disease of cannabis, causing substantial economic losses in plant yield and crop value for growers worldwide. The best way to control the disease is early detection to limit the spread of the viroid in grow facilities. This study describes MFDetect^TM as a rapid, highly sensitive, and high-throughput tool for detecting HLVd in the early stages of plant development. Furthermore, in the largest research study conducted so far for HLVd detection in cannabis, we compared MFDetect^TM with quantitative RT-PCR in a time course experiment using different plant tissues, leaves, petioles, and roots at different plant developmental stages to demonstrate both technologies are comparable. Our study found leaf tissue is a suitable plant material for HLVd detection, with the viroid titer increasing in the infected leaf tissue with the age of plants. The study showed that other tissue types, including petiole and roots, were equally sensitive to detection via MFDetect^TM. The assay developed in this research allows the screening of thousands of plants in a week. The assay can be scaled easily to provide growers with a quick turnaround and a cost-effective diagnostic tool for screening many plants and tissue types at different stages of development.

The pollen virome: A review of pollen-associated viruses and consequences for plants and their interactions with pollinators

The movement of pollen grains from anthers to stigmas, often by insect pollinator vectors, is essential for plant reproduction. However, pollen is also a unique vehicle for viral spread. Pollen-associated plant viruses reside on the outside or inside of pollen grains, infect susceptible individuals through vertical or horizontal infection pathways, and can decrease plant fitness. These viruses are transferred with pollen between plants by pollinator vectors as they forage for floral resources; thus, pollen-associated viral spread is mediated by floral and pollen grain phenotypes and pollinator traits, much like pollination. Most of what is currently known about pollen-associated viruses was discovered through infection and transmission experiments in controlled settings, usually involving one virus and one plant species of agricultural or horticultural interest. In this review, we first provide an updated, comprehensive list of the recognized pollen-associated viruses. Then, we summarize virus, plant, pollinator vector, and landscape traits that can affect pollen-associated virus transmission, infection, and distribution. Next, we highlight the consequences of plant-pollinator-virus interactions that emerge in complex communities of co-flowering plants and pollinator vectors, such as pollen-associated virus spread between plant species and viral jumps from plant to pollinator hosts. We conclude by emphasizing the need for collaborative research that bridges pollen biology, virology, and pollination biology.

Viroids: Non-Coding Circular RNAs Able to Autonomously Replicate and Infect Higher Plants

Viroids are a unique type of infectious agent, exclusively composed of a relatively small (246-430 nt), highly base-paired, circular, non-coding RNA. Despite the small size and non-coding nature, the more-than-thirty currently known viroid species infectious of higher plants are able to autonomously replicate and move systemically through the host, thereby inducing disease in some plants. After recalling viroid discovery back in the late 60s and early 70s of last century and discussing current hypotheses about their evolutionary origin, this article reviews our current knowledge about these peculiar infectious agents. We describe the highly base-paired viroid molecules that fold in rod-like or branched structures and viroid taxonomic classification in two families, Pospiviroidae and Avsunviroidae, likely gathering nuclear and chloroplastic viroids, respectively. We review current knowledge about viroid replication through RNA-to-RNA rolling-circle mechanisms in which host factors, notably RNA transporters, RNA polymerases, RNases, and RNA ligases, are involved. Systemic movement through the infected plant, plant-to-plant transmission and host range are also discussed. Finally, we focus on the mechanisms of viroid pathogenesis, in which RNA silencing has acquired remarkable importance, and also for the initiation of potential biotechnological applications of viroid molecules.

Modes of Viroid Transmission

Studies on the ways in which viroids are transmitted are important for understanding their epidemiology and for developing effective control measures for viroid diseases. Viroids may be spread via vegetative propagules, mechanical damage, seed, pollen, or biological vectors. Vegetative propagation is the most prevalent mode of spread at the global, national and local level while further dissemination can readily occur by mechanical transmission through crop handling with viroid-contaminated hands or pruning and harvesting tools. The current knowledge of seed and pollen transmission of viroids in different crops is described. Biological vectors shown to transmit viroids include certain insects, parasitic plants, and goats. Under laboratory conditions, viroids were also shown to replicate in and be transmitted by phytopathogenic ascomycete fungi; therefore, fungi possibly serve as biological vectors of viroids in nature. The term “mycoviroids or fungal viroids” has been introduced in order to denote these viroids. Experimentally, known sequence variants of viroids can be transmitted as recombinant infectious cDNA clones or transcripts. In this review, we endeavor to provide a comprehensive overview of the modes of viroid transmission under both natural and experimental situations. A special focus is the key findings which can be applied to the control of viroid diseases.

The pollen virome of wild plants and its association with variation in floral traits and land use

Pollen is a unique vehicle for viral spread. Pollen-associated viruses hitchhike on or within pollen grains and are transported to other plants by pollinators. They are deposited on flowers and have a direct pathway into the plant and next generation via seeds. To discover the diversity of pollen-associated viruses and identify contributing landscape and floral features, we perform a species-level metagenomic survey of pollen from wild, visually asymptomatic plants, located in one of four regions in the United States of America varying in land use. We identify many known and novel pollen-associated viruses, half belonging to the Bromoviridae, Partitiviridae, and Secoviridae viral families, but many families are represented. Across the regions, species harbor more viruses when surrounded by less natural and more human-modified environments than the reverse, but we note that other region-level differences may also covary with this. When examining the novel connection between virus richness and floral traits, we find that species with multiple, bilaterally symmetric flowers and smaller, spikier pollen harbored more viruses than those with opposite traits. The association of viral diversity with floral traits highlights the need to incorporate plant-pollinator interactions as a driver of pollen-associated virus transport into the study of plant-viral interactions.

Rapid diagnostic detection of tomato apical stunt viroid based on isothermal reverse transcription-recombinase polymerase amplification

Tomato apical stunt viroid (TASVd) is a serious threat to tomato plants that can cause a considerable yield loss. In the present study, two isothermal molecular diagnostic assays based on reverse transcription-recombinase polymerase amplification (RT-RPA) utilizing the AmplifyRP® platform for plant pathogen detection were developed. The results of this research demonstrated distinct specificity of both developed assays, AmplifyRP® Acceler8™ and AmplifyRP® XRT, expressed in the absence of any cross-reaction activity to all total RNA extracts obtained from plants infected with other pospiviroids. The RT-RPA assays detected viroid RNA in 81- and 27-fold dilutions of the original TASVd-infected crude extract for AmplifyRP® Acceler8™ and AmplifyRP® XRT, respectively. The sensitivity tests in serial water dilutions showed the ability of AmplifyRP® Acceler8™ and AmplifyRP® XRT to detect 8 and 80 fg of pure TASVd RNA transcript, respectively. The influence of crude extract on viroid RNA transcript detection was also examined and a decrease of sensitivity of approximately 100-fold for both RT-RPA assays was revealed. To our knowledge, this is the first report describing development of RT-RPA assays to detect TASVd in plants using the AmplifyRP® platform that can be further employed both in laboratory conditions and in the field for on-site diagnosis.

Development of a one-step RT-qPCR detection assay for the newly described citrus viroid VII

An apscaviroid, tentatively named citrus viroid VII (CVd-VII), was recently discovered in citrus in Australia. A diagnostic assay using real-time reverse transcription polymerase chain reaction was developed and validated to detect the viroid in citrus plants. The assay showed a high level of sensitivity, reliably detecting 2000 plasmid copies per reaction, while down to 20 plasmid copies per reaction were occasionally detected. The assay showed high specificity, producing no false positives or cross-reactivity with a range of other citrus graft-transmissible pathogens, including viroids, viruses and bacteria. The real-time assay was also found to be more sensitive than the available end-point reverse transcription polymerase chain reaction assay by a factor of 100,000 and could be a useful tool for the rapid detection of CVd-VII in diagnostic and research environments.

Universal Primers for Rapid Detection of Six Pospiviroids in Solanaceae Plants Using One-Step Reverse-Transcription PCR and Reverse-Transcription Loop-Mediated Isothermal Amplification

A number of viruses and viroids infect solanaceous plants causing severe yield losses. Several seed-borne viroids are listed as quarantine pathogens in many countries. Among them, columnea latent viroid, pepper chat fruit viroid, potato spindle tuber viroid, tomato apical stunt viroid, tomato chlorotic dwarf viroid, and tomato planta macho viroid are of major concerns. The objective of this study was to design and test universal primers that could be used to detect six viroids in solanaceous plants using one-step reverse transcription PCR (RT-PCR) and reverse transcription loop-mediated isothermal amplification (RT-LAMP). Results revealed that a pair of degenerate primers could be used in a one-step RT-PCR to amplify six pospiviroids from Solanaceae seeds and plants. Moreover, five primers were designed and used in RT-LAMP to amplify six pospiviroids. The minimal concentration of viroid RNA required for a successful detection varied, ranging from 1 fg to 10 ng, depending on the species of viroid and detection method. In general, RT-LAMP was more sensitive than RT-PCR, but both assays were rapid and highly sensitive tools to detect six pospiviroids. Detection methods in use for these viroids require at least two different sets of primers. The assays developed in this research could facilitate the ability to screen a large number of solanaceous plants and seeds intended for import and export.

Possible Overestimation of Seed Transmission in the Spread of Pospiviroids in Commercial Pepper and Tomato Crops Based on Large-Scale Grow-Out Trials and Systematic Literature Review

Several outbreaks of pospiviroids have been reported in pepper and tomato crops worldwide. Tracing back the origin of the infections has led to different sources. In some cases, the infections were considered to result from seed transmission. Other outbreaks were related to transmission from ornamental crops and weeds. Pospiviroids, in particular potato spindle tuber viroid, are regulated by many countries because they can be harmful to potatoes and tomatoes. Seed transmission has been considered an important pathway of introduction and spread. However, the importance of this pathway can be questioned. This paper presents data on seed transmission from large-scale grow-out trials of infested pepper and tomato seed lots produced under standard seed-industry conditions. In addition, it presents the results of a systematic review of published data on seed transmission and outbreaks in commercial pepper and tomato crops. Based on the results of the grow-out trials and review of the literature, it was concluded that the role of seed transmission in the spread of pospiviroids in practice is possibly overestimated.

Transmission studies of the newly described apple chlorotic fruit spot viroid using a combined RT-qPCR and droplet digital PCR approach

The transmission of the apscaviroid tentatively named apple chlorotic fruit spot viroid (ACFSVd) was investigated using a one-step reverse-transcription (RT) droplet digital PCR assay for absolute quantification of the viroid, followed by quantification of relative standard curves by RT-qPCR. Our results indicate that ACFSVd is effectively transmitted by grafting, budding and seeds. No transmission has yet been observed to the viroid-inoculated pome fruit species Pyrus sp. and Cydonia sp. ACFSVd was detected in viruliferous aphids (Myzus persicae, Dysaphis plantaginea) and in codling moths (Cydia pomonella). The viroid was also detected systemically in the infected hemiparasitic plant Viscum album subsp. album (mistletoe).

Delivering Integrated Pest and Pollinator Management (IPPM)

The need to reduce pollinator exposure to harmful pesticides has led to calls to expedite the adoption of integrated pest management (IPM). We make the case that IPM is not explicitly 'pollinator friendly', but rather must be adapted to reduce impacts on pollinators and to facilitate synergies between crop pollination and pest control practices and ecosystem services. To reconcile these diverse needs, we introduce a systematic framework for 'integrated pest and pollinator management' (IPPM). We also highlight novel tools to unify monitoring and economic decision-making processes for IPPM and outline key policy actions and knowledge gaps. We propose that IPPM is needed to promote more coordinated, ecosystem-based strategies for sustainable food production, against the backdrop of increasing pesticide regulation and pollinator dependency in agriculture. VIDEO ABSTRACT.

Rasta Disease of Tomato in Ghana is Caused by the Pospiviroids Potato spindle tuber viroid and Tomato apical stunt viroid

Rasta is a virus-like disease of unknown etiology affecting tomato (Solanum lycopersicum) plants in Ghana. Symptoms include stunting; epinasty, crumpling, and chlorosis of leaves; and necrosis of leaf veins, petioles, and stems. Leaf samples with rasta symptoms were collected from commercial tomato fields in Ghana in October 2012 and applied to FTA cards, and RNA extracts were prepared. Reverse-transcription polymerase chain reaction (RT-PCR) tests with primers for Columnea latent viroid, which causes rasta-like symptoms in tomato plants in Mali, were negative, whereas tests with degenerate viroid primer pairs were inconclusive. However, tomato seedlings (Early Pak 7) mechanically inoculated with RNA extracts of 10 of 13 samples developed rasta-like symptoms. In RT-PCR tests with RNA from leaves of the 10 symptomatic seedlings and primers for Potato spindle tuber viroid (PSTVd) or Tomato apical stunt viroid (TASVd), the expected size (approximately 360 bp) of DNA fragment was amplified from eight and two seedlings, respectively. Sequence analyses confirmed that these fragments were from PSTVd and TASVd isolates, and revealed a single PSTVd haplotype and two TASVd haplotypes. The PSTVd and TASVd isolates from Ghana had high nucleotide identities (>94%) with isolates from other geographic regions. In a host range study, PSTVd and TASVd isolates from Ghana induced rasta symptoms in the highly susceptible tomato cultivar Early Pak 7 and mild or no symptoms in Glamour, and symptomless infections in a number of other solanaceous species. PSTVd and TASVd isolates were seed associated and possibly seed transmitted.

Viroid-infected Tomato and Capsicum Seed Shipments to Australia

Pospiviroid species are transmitted through capsicum and tomato seeds. Trade in these seeds represents a route for the viroids to invade new regions, but the magnitude of this hazard has not been adequately investigated. Since 2012, tomato seed lots sent to Australia have been tested for pospiviroids before they are released from border quarantine, and capsicum seed lots have been similarly tested in quarantine since 2013. Altogether, more than 2000 seed lots have been tested. Pospiviroids were detected in more than 10% of the seed lots in the first years of mandatory testing, but the proportion of lots that were infected declined in subsequent years to less than 5%. Six pospiviroid species were detected: Citrus exocortis viroid, Columnea latent viroid, Pepper chat fruit viroid, Potato spindle tuber viroid, Tomato chlorotic dwarf viroid and Tomato apical stunt viroid. They were detected in seed lots exported from 18 countries from every production region. In many seed lots, the detectable fraction (prevalence) of infected seeds was estimated to be very small, as low as 6 × 10⁻⁵ (~1 in 16,000; CI 5 × 10⁻⁶ to 2.5 × 10⁻⁴) for some lots. These findings raise questions about seed production practices, and the study indicates the geographic distributions of these pathogens are uncertain, and there is a continuing threat of invasion.

The bumblebee Bombus terrestris carries a primary inoculum of Tomato brown rugose fruit virus contributing to disease spread in tomatoes

The bumblebee Bombus terrestris is a beneficial pollinator extensively used in tomato production. Our hypothesis was that bumblebee hives collected from a Tomato brown rugose fruit virus (ToBRFV) infected tomato greenhouse, preserve an infectious primary inoculum. Placing a bumblebee hive collected from a ToBRFV contaminated greenhouse, in a glass-/net-house containing only uninfected healthy tomato plants, spread ToBRFV disease. Control uninfected tomato plants grown in a glass-/net-house devoid of any beehive remained uninfected. ToBRFV-contaminated hives carried infectious viral particles as demonstrated in a biological assay on laboratory test plants of virus extracted from hive components. Viral particles isolated from a contaminated hive had a typical tobamovirus morphology observed in transmission electron microscopy. Assembly of ToBRFV genome was achieved by next generation sequencing analysis of RNA adhering to the bumblebee body. Bumblebee dissection showed that ToBRFV was mostly present in the abdomen suggesting viral disease spread via buzz pollination. These results demonstrate that bumblebee hives collected from ToBRFV-contaminated greenhouses carry a primary inoculum that reflects the status of viruses in the growing area. This new mode of ToBRFV spread by pollinators opens an avenue for detection of viruses in a growing area through analysis of the pollinators, as well as emphasizes the need to reevaluate the appropriate disease management protocols.

Differences in dynamics of horizontal transmission of Tomato planta macho viroid and Potato spindle tuber viroid after pollination with viroid-infected pollen

For viroids, pollen transmission is an important transmission pathway to progeny seeds and new hosts. In the current study, we found that Tomato planta macho viroid (TPMVd)-but not Potato spindle tuber viroid (PSTVd)-was horizontally transmitted by pollen from petunia plants. Using tissue-printing hybridization to track the changes in viroid distribution after pollination, we noted that TPMVd was present in petunia stigma, styles, and eventually ovaries, whereas PSTVd was detected in the stigma and upper style but not the ovary. These findings suggest that horizontal transmission of viroids depends on the infection of the lower style and ovary during the elongation of pollen tubes after pollination. Additionally, TPMVd was transmitted horizontally, leading to systematic infection, when we used TPMVd-infected petunia pollen to pollinate the flowers of healthy tomato plants. Fertilization typically does not occur after heterologous pollination and thus likely is not required to accomplish horizontal transmission of viroids.

Plant and Insect Viruses in Managed and Natural Environments: Novel and Neglected Transmission Pathways

The capacity to spread by diverse transmission pathways enhances a virus' ability to spread effectively and survive when circumstances change. This review aims to improve understanding of how plant and insect viruses spread through natural and managed environments by drawing attention to 12 novel or neglected virus transmission pathways whose contribution is underestimated. For plant viruses, the pathways reviewed are vertical and horizontal transmission via pollen, and horizontal transmission by parasitic plants, natural root grafts, wind-mediated contact, chewing insects, and contaminated water or soil. For insect viruses, they are transmission by plants serving as passive "vectors," arthropod vectors, and contamination of pollen and nectar. Based on current understanding of the spatiotemporal dynamics of virus spread, the likely roles of each pathway in creating new primary infection foci, enlarging previously existing infection foci, and promoting generalized virus spread are estimated. All pathways except transmission via parasitic plants, root grafts, and wind-mediated contact transmission are likely to produce new primary infection foci. All 12 pathways have the capability to enlarge existing infection foci, but only to a limited extent when spread occurs via virus-contaminated soil or vertical pollen transmission. All pathways except those via parasitic plant, root graft, contaminated soil, and vertical pollen transmission likely contribute to generalized virus spread, but to different extents. For worst-case scenarios, where mixed populations of host species occur under optimal virus spread conditions, the risk that host species jumps or virus emergence events will arise is estimated to be "high" for all four insect virus pathways considered, and, "very high" or "moderate" for plant viruses transmitted by parasitic plant and root graft pathways, respectively. To establish full understanding of virus spread and thereby optimize effective virus disease management, it is important to examine all transmission pathways potentially involved, regardless of whether the virus' ecology is already presumed to be well understood or otherwise.

Tomato yellow leaf curl virus (TYLCV-IL): a seed-transmissible geminivirus in tomatoes

Tomato yellow leaf curl virus (TYLCV) is one of the most well-known tomato-infecting begomoviruses and transmitted by Bemisia tabaci. Seed transmission has previously been reported for some RNA viruses, but TYLCV has not previously been described as a seed-borne virus. In 2013 and 2014, without whitefly-mediated transmission, TYLCV was detected in young tomato plants germinated from fallen fruits produced from TYLCV-infected tomato plants in the previous cultivation season. In addition, TYLCV-Israel (TYLCV-IL) was also detected in seeds and their seedlings of TYLCV-infected tomato plants that were infected by both viruliferous whitefly-mediated transmission and agro-inoculation. The seed infectivity was 20-100%, respectively, and the average transmission rate to seedlings was also 84.62% and 80.77%, respectively. TYLCV-tolerant tomatoes also produced TYLCV-infected seeds, but the amount of viral genome was less than seen in TYLCV-susceptible tomato plants. When tomato plants germinated from TYLCV-infected seeds, non-viruliferous whiteflies and healthy tomato plants were placed in an insect cage together, TYLCV was detected from whiteflies as well as receiver tomato plants six weeks later. Taken together, TYLCV-IL can be transmitted via seeds, and tomato plants germinated from TYLCV-infected seeds can be an inoculum source of TYLCV. This is the first report about TYLCV seed transmission in tomato.

Apple scar skin viroid naked RNA is actively transmitted by the whitefly Trialeurodes vaporariorum

Nucleic acid transfer between plants is a phenomenon which is likely to occur in many ways in nature. We report here the active transmission of Apple scar skin viroid (ASSVd) naked ssRNA species by the whitefly Trialeurodes vaporariorum (Tv). Not only the viroid RNA, its DNA form was also identified from the insect. The viroid transfer efficiency was enhanced with the help of Cucumis sativus Phloem protein 2 (CsPP2), a plant protein known to translocate viroid RNAs. This PP2/ASSVd complex is stably present in the viroid infected cucumber plants, as was identified with the help of immunological reaction. As viroid-like secondary structures are found in some plant RNAs, and PP2 is known to bind and translocate several RNAs, the results have huge implications in transfer of these RNA species between plants visited by the whitefly.

Potato spindle tuber viroid: Stability on Common Surfaces and Inactivation With Disinfectants

The length of time Potato spindle tuber viroid (PSTVd) remained infective in extracted tomato leaf sap on common surfaces and the effectiveness of disinfectants against it were investigated. When sap from PSTVd-infected tomato leaves was applied to eight common surfaces (cotton, wood, rubber tire, leather, metal, plastic, human skin, and string) and left for various periods of time (5 min to 24 h) before rehydrating the surface and rubbing onto healthy tomato plants, PSTVd remained infective for 24 h on all surfaces except human skin. It survived best on leather, plastic, and string. It survived less well after 6 h on wood, cotton, and rubber and after 60 min on metal. On human skin, PSTVd remained infective for only 30 min. In general, rubbing surfaces contaminated with dried infective sap directly onto leaves caused less infection than when the sap was rehydrated with distilled water but overall results were similar. The effectiveness of five disinfectant agents at inactivating PSTVd in sap extracts was investigated by adding them to sap from PSTVd-infected leaves before rubbing the treated sap onto leaves of healthy tomato plants. Of the disinfectants tested, 20% nonfat dried skim milk and a 1:4 dilution of household bleach (active ingredient sodium hypochlorite) were the most effective at inactivating PSTVd infectivity in infective sap. When reverse-transcription polymerase chain reaction was used to test the activity of the five disinfectants against PSTVd in infective sap, it detected PSTVd in all instances except in sap treated with 20% nonfat dried skim milk. This study highlights the stability of PSTVd in infective sap and the critical importance of utilizing hygiene practices such as decontamination of clothing, tools, and machinery, along with other control measures, to ensure effective management of PSTVd and, wherever possible, its elimination in solanaceous crops.

Molecular biology of viroid-host interactions and disease control strategies

Viroids are single-stranded, covalently closed, circular, highly structured noncoding RNAs that cause disease in several economically important crop plants. They replicate autonomously and move systemically in host plants with the aid of the host machinery. In addition to symptomatic infections, viroids also cause latent infections where there is no visual evidence of infection in the host; however, transfer to a susceptible host can result in devastating disease. While there are non-hosts for viroids, no naturally occurring durable resistance has been observed in most host species. Current effective control methods for viroid diseases include detection and eradication, and cultural controls. In addition, heat or cold therapy combined with meristem tip culture has been shown to be effective for elimination of viroids for some viroid-host combinations. An understanding of viroid-host interactions, host susceptibility, and non-host resistance could provide guidance for the design of viroid-resistant plants. Efforts to engineer viroid resistance into host species have been underway for several years, and include the use of antisense RNA, antisense RNA plus ribozymes, a dsRNase, and siRNAs, among others. The results of those efforts and the challenges associated with creating viroid resistant plants are summarized in this review.

Quantitation and localization of pospiviroids in aphids

In this paper, the potential role of aphids in viroid transmission was explored. Apterous aphids were fed on pospiviroid-infected plants and viroid targets in the aphids were consequently quantified through RT-qPCR and localized within the aphid body using fluorescence in situ hybridization (FISH). Based on the analytical sensitivity test, the limit of detection (LOD) was estimated at 1.69×10(6) viroid copies per individual aphid body. To localize the viroids in the aphids, a pospiviroid-generic Cy5-labelled probe was used and the fluorescent signal was determined by confocal microscopy. Viroids were clearly observed in the aphid's stylet and stomach, but not in the embryos. Viroids were detected in 29% of the aphids after a 24h feeding period, which suggests only a partial and low concentration viroid uptake by the aphid population including viroid concentrations under the LOD. However, these results show that viroids can be ingested by aphids while feeding on infected plants, thus potentially increasing the transmission risk. The combination of FISH and RT-qPCR provides reliable and fast localization and quantitation of viroid targets in individual aphids and thus constitutes a valuable tool in future epidemiological research.

Distribution of potato spindle tuber viroid in reproductive organs of petunia during its developmental stages

Embryo infection is important for efficient seed transmission of viroids. To identify the major pattern of seed transmission of viroids, we used in situ hybridization to histochemically analyze the distribution of Potato spindle tuber viroid (PSTVd) in each developmental stage of petunia (flowering to mature seed stages). In floral organs, PSTVd was present in the reproductive tissues of infected female × infected male and infected female × healthy male but not of healthy female × infected male before embryogenesis. After pollination, PSTVd was detected in the developed embryo and endosperm in all three crosses. These findings indicate that PSTVd is indirectly delivered to the embryo through ovule or pollen during the development of reproductive tissues before embryogenesis but not directly through maternal tissues as cell-to-cell movement during embryogenesis.