Elimination of two viruses which interact synergistically from sweetpotato by shoot tip culture and cryotherapy

Genetic engineering and genome editing technologies as catalyst for Africa's food security: the case of plant biotechnology in Nigeria

Many African countries are unable to meet the food demands of their growing population and the situation is worsened by climate change and disease outbreaks. This issue of food insecurity may lead to a crisis of epic proportion if effective measures are not in place to make more food available. Thus, deploying biotechnology towards the improvement of existing crop varieties for tolerance or resistance to both biotic and abiotic stresses is crucial to increasing crop production. In order to optimize crop production, several African countries have implemented strategies to make the most of this innovative technology. For example, Nigerian government has implemented the National Biotechnology Policy to facilitate capacity building, research, bioresource development and commercialization of biotechnology products for over two decades. Several government ministries, research centers, universities, and agencies have worked together to implement the policy, resulting in the release of some genetically modified crops to farmers for cultivation and Commercialization, which is a significant accomplishment. However, the transgenic crops were only brought to Nigeria for confined field trials; the manufacturing of the transgenic crops took place outside the country. This may have contributed to the suspicion of pressure groups and embolden proponents of biotechnology as an alien technology. Likewise, this may also be the underlying issue preventing the adoption of biotechnology products in other African countries. It is therefore necessary that African universities develop capacity in various aspects of biotechnology, to continuously train indigenous scientists who can generate innovative ideas tailored towards solving problems that are peculiar to respective country. Therefore, this study intends to establish the role of genetic engineering and genome editing towards the achievement of food security in Africa while using Nigeria as a case study. In our opinion, biotechnology approaches will not only complement conventional breeding methods in the pursuit of crop improvements, but it remains a viable and sustainable means of tackling specific issues hindering optimal crop production. Furthermore, we suggest that financial institutions should offer low-interest loans to new businesses. In order to promote the growth of biotechnology products, especially through the creation of jobs and revenues through molecular farming.

Application of Biotechniques for In Vitro Virus and Viroid Elimination in Pome Fruit Crops

Sustainable production of pome fruit crops is dependent upon having virus-free planting materials. The production and distribution of plants derived from virus- and viroid-negative sources is necessary not only to control pome fruit viral diseases but also for sustainable breeding activities, as well as the safe movement of plant materials across borders. With variable success rates, different in vitro-based techniques, including shoot tip culture, micrografting, thermotherapy, chemotherapy, and shoot tip cryotherapy, have been employed to eliminate viruses from pome fruits. Higher pathogen eradication efficiencies have been achieved by combining two or more of these techniques. An accurate diagnosis that confirms complete viral elimination is crucial for developing effective management strategies. In recent years, considerable efforts have resulted in new reliable and efficient virus detection methods. This comprehensive review documents the development and recent advances in biotechnological methods that produce healthy pome fruit plants. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Evaluation of Virus-Free Chrysanthemum 'Hangju' Productivity and Response to Virus Reinfection in the Field: Molecular Insights into Virus-Host Interactions

The shoot apical meristem culture has been used widely to produce virus-free plantlets which have the advantages of strong disease resistance, high yield, and prosperous growth potential. However, this virus-free plant will be naturally reinfected in the field. The physiological and metabolic responses in the reinfected plant are still unknown. The flower of chrysanthemum 'Hangju' is a traditional medicine which is unique to China. In this study, we found that the virus-free 'Hangju' (VFH) was reinfected with chrysanthemum virus B/R in the field. However, the reinfected VFH (RVFH) exhibited an increased yield and medicinal components compared with virus-infected 'Hangju' (VIH). Comparative analysis of transcriptomes was performed to explore the molecular response mechanisms of the RVFH to CVB infection. A total of 6223 differentially expressed genes (DEGs) were identified in the RVFH vs. the VIH. KEGG enrichment and physiological analyses indicated that treatment with the virus-free technology significantly mitigated the plants' lipid and galactose metabolic stress responses in the RVFH. Furthermore, GO enrichment showed that plant viral diseases affected salicylic acid (SA)-related processes in the RVFH. Specifically, we found that phenylalanine ammonia-lyase (PAL) genes played a major role in defense-related SA biosynthesis in 'Hangju'. These findings provided new insights into the molecular mechanisms underlying plant virus-host interactions and have implications for developing strategies to improve plant resistance against viruses.

Micrografting: An Old Dog Plays New Tricks in Obligate Plant Pathogens

Micrografting, which was developed almost 50 years ago, has long been used for virus eradication, micropropagation, regeneration, rejuvenation, and graft compatibility. Recently, micrografting has been used for studies of long-distance trafficking and signaling of molecules between scions and rootstocks. The graft transmissiveness of obligate plant pathogens, such as viruses, viroids, and phytoplasmas, facilitated the use of micrografting to study biological indexing and pathogen transmission, pathogen-induced graft incompatibility, and screening for the pathogen resistance during the past 20 years. The present study provides comprehensive information on the latter subjects. Finally, prospects are proposed to direct further studies.

Occurrence of Grapevine Leafroll-Associated Virus-3 (GLRaV-3), Complete Nucleotide Sequence and Cultivar Susceptibility to a GLRaV-3 Isolate from Shaanxi Province of China

Grapevine (Vitis spp.) is globally one of the most economically important fruit crops. China is the largest grapevine-growing country of the world and Shaanxi province is one of the major grapevine-growing provinces in the country. A survey of GLRaV-3 found it widespread, with 57–100% infection frequencies, in both wine and table grapevine cultivars of three grapevine-growing regions of Shaanxi province. The virus infection frequencies varied with cultivars and regions. In order to obtain the full genomic length of a new GLRaV-3 isolate, GLRaV-3-Sau (accession number MK988555), was sequenced. This isolate has a genome of 18026 nucleotides, and 14 open reading frames (ORFs). The full-genome of the isolate GLRaV-3-Sau shared 85.88% nucleotide identity to GLRaV-3-LN, another isolate found in China. Coat protein (CP) genes of GLRaV-3 isolates were identical (99%) to the Vitis vinifera isolate (accession number HQ185608.1) from the USA. Immunohistochemistry for virus localization found that distribution patterns were similar in red-berried cultivar ‘Cabernet Sauvignon’ and white-berried cultivar ‘Chardonnay’, and GLRaV-3 is restricted in phloem tissue of vascular bundles. Virus transmission by micrografting found virus transmission efficiency was higher in ‘Chardonnay’ and ‘Thompson Seedless’ than in ‘Hunan-1’, indicating that ‘Hunan-1’ was less sensitive to GLRaV-3. As far as we know, these are the most comprehensive comparisons on the genome and CP genes of GLRaV-3 worldwide and the first to have found that the grapevine ‘Hunan-1’ is less susceptible to GLRaV-3.

Long-Term Preservation of Plant Viruses in Cryopreserved Shoot Tips

Availability of the methods for long-term virus preservation facilitates easy acquirement of viruses, which are needed in many basic and applied virological studies. Cryopreservation is currently considered an ideal means for long-term preservation of plant germplasm. Recent studies have shown that cryopreservation provided an efficient and reliable method for long-term preservation of plant viruses. Here, we describe the detailed procedures of droplet vitrification for long-term preservation of apple stem grooving virus (ASGV), which represents a type of viruses that can invade meristematic cells of the shoot tips, and potato leafroll virus (PLRV), which is a phloem-limited virus that does not infect the apical meristem. Shoot tip cryopreservation provides an advantageous strategy for the long-term preservation of plant viruses.

Production of Virus-Free Rose Plants using Meristem-Tip Culture and in vitro Thermotherapy

<b>Background and Objective:</b> Tissue culture and thermotherapy were proved to be suitable in eliminating viruses of many plants. This study was designed in an attempt to produce virus-free Al-Taif rose plants (<i>Rosa damascena</i> Trigintipetala Dieck) through the practical application of the tissue culture approach and thermotherapy. <b>Materials and Methods:</b> Double Antibody Sandwich-Enzyme-Linked Immunosorbent Assay ( DAS-ELISA) and Reverse Transcription-Polymerase Chain Reaction (RT-PCR) techniques were used to detect the presence of <i>Apple mosaic virus</i> (ApMV) and <i>Strawberry latent ringspot virus</i> (SLRV) in rose plant materials collected from Taif, KSA. RT-PCR was more sensitive than DAS-ELISA in detecting the 2 viruses. <b>Results:</b> Three different meristem-tip sterilization methods were compared and results revealed that treatment 3 (T<sub>3</sub>: 70% Ethanol for 1.0 min and 15% Clorox (Sodium hypochlorite 5.25%) for 10 min) was the most suitable as 97.78% of cleaned meristem tips survived. Meristem tips with different lengths were thermotherapy-treated for different durations. It was indicated that meristem tips of 0.5 or 1.0 cm and heat-treated at 37<sup>o</sup>C for four weeks gave the highest percentage of meristems that were able to differentiate into micro-shoots. <b>Conclusion:</b> RT-PCR detection of ApMV and SLRV revealed that using thermotherapy-treatment, for 4 weeks, of 0.5 cm long meristem tips was successfully applied to eliminate the 2 viruses in 92 and 96% of regenerated plantlets, respectively.

Grapevine Shoot Tip Cryopreservation and Cryotherapy: Secure Storage of Disease-Free Plants

Grapevine (Vitis spp.) is one of the most economically important temperate fruit crops. Grapevine breeding programs require access to high-quality Vitis cultivars and wild species, which may be maintained within genebanks. Shoot tip cryopreservation is a valuable technique for the safe, long-term conservation of Vitis genetic resources that complements traditional field and in vitro germplasm collections. Vitis is highly susceptible to virus infections. Virus-free plants are required as propagation material for clonally propagated germplasm, and also for the global exchange of grapevine genetic resources. Shoot tip cryotherapy, a method based on cryopreservation, has proven to be effective in eradicating viruses from infected plants, including grapevine. This comprehensive review outlines/documents the advances in Vitis shoot tip cryopreservation and cryotherapy that have resulted in healthy plants with high regrowth levels across diverse Vitis species.

In Vitro Identification and In Vivo Confirmation of Inhibitors for Sweet Potato Chlorotic Stunt Virus RNA Silencing Suppressor, a Viral RNase III

Sweet potato virus disease (SPVD), caused by synergistic infection of Sweet potato chlorotic stunt virus (SPCSV) and Sweet potato feathery mottle virus (SPFMV), is responsible for substantial yield losses all over the world. However, there are currently no approved treatments for this severe disease. The crucial role played by RNase III of SPCSV (CSR3) as an RNA silencing suppressor during the viruses' synergistic interaction in sweetpotato makes it an ideal drug target for developing antiviral treatment. In this study, high-throughput screening (HTS) of small molecular libraries targeting CSR3 was initiated by a virtual screen using Glide docking, allowing the selection of 6,400 compounds out of 136,353. We subsequently developed and carried out kinetic-based HTS using fluorescence resonance energy transfer technology, which isolated 112 compounds. These compounds were validated with dose-response assays including kinetic-based HTS and binding affinity assays using surface plasmon resonance and microscale thermophoresis. Finally, the interference of the selected compounds with viral accumulation was verified in planta In summary, we identified five compounds belonging to two structural classes that inhibited CSR3 activity and reduced viral accumulation in plants. These results provide the foundation for developing antiviral agents targeting CSR3 to provide new strategies for controlling sweetpotato virus diseases.IMPORTANCE We report here a high-throughput inhibitor identification method that targets a severe sweetpotato virus disease caused by coinfection with two viruses (SPCSV and SPFMV). The disease is responsible for up to 90% yield losses. Specifically, we targeted the RNase III enzyme encoded by SPCSV, which plays an important role in suppressing the RNA silencing defense system of sweetpotato plants. Based on virtual screening, laboratory assays, and confirmation in planta, we identified five compounds that could be used to develop antiviral drugs to combat the most severe sweetpotato virus disease.

Antifreeze Protein Improves the Cryopreservation Efficiency of Hosta capitata by Regulating the Genes Involved in the Low-Temperature Tolerance Mechanism

In this study, whether the addition of antifreeze protein (AFP) to a cryopreservative solution (plant vitrification solution 2 (PVS2)) is more effective in reducing freezing injuries in Hosta capitata than PVS2 alone at different cold exposure times (6, 24, and 48 h) is investigated. The upregulation of C-repeat binding factor 1 (CBF1) and dehydrin 1 (DHN1) in response to low temperature was observed in shoots. Shoots treated with distilled water (dH2O) strongly triggered gene expression 6 h after cold exposure, which was higher than those expressed in PVS2 and PVS2+AFP. However, 24 h after cold exposure, gene expressions detected in dH2O and PVS2 treatments were similar and higher than PVS2 + AFP. The expression was highest in PVS2+AFP when the exposure time was extended to 48 h. Similarly, nitric reductase activities 1 and 2 (Nia1 and Nia2) genes, which are responsible for nitric oxide production, were also upregulated in low-temperature-treated shoots, as observed for CBF1 and DHN1 expression patterns during cold exposure periods. Based on the gene expression patterns, shoots treated with PVS2+AFP were more likely to resist cold stress, which was also associated with the higher cryopreservation efficiency of PVS2+AFP compared to PVS2 alone. This finding suggests that the improvement of cryopreservation efficiency by AFP could be due to the transcriptional regulation of CBF1, DHN1, Nia1, and Nia2, which might reduce freezing injuries during cryopreservation. Thus, AFP could be potentially used as a cryoprotectant in the cryopreservation of rare and commercially important plant germplasm.

Phytotoxicity and Other Adverse Effects on the In Vitro Shoot Cultures Caused by Virus Elimination Treatments: Reasons and Solutions

In general, in vitro virus elimination is based on the culture of isolated meristem, and in addition thermotherapy, chemotherapy, electrotherapy, and cryotherapy can also be applied. During these processes, plantlets suffer several stresses, which can result in low rate of survival, inhibited growth, incomplete development, or abnormal morphology. Even though the in vitro cultures survive the treatment, further development can be inhibited; thus, regeneration capacity of treated in vitro shoots or explants play also an important role in successful virus elimination. Sensitivity of genotypes to treatments is very different, and the rate of destruction largely depends on the physiological condition of plants as well. Exposure time of treatments affects the rate of damage in almost every therapy. Other factors such as temperature, illumination (thermotherapy), type and concentration of applied chemicals (chemo- and cryotherapy), and electric current intensity (electrotherapy) also may have a great impact on the rate of damage. However, there are several ways to decrease the harmful effect of treatments. This review summarizes the harmful effects of virus elimination treatments applied on tissue cultures reported in the literature. The aim of this review is to expound the solutions that can be used to mitigate phytotoxic and other adverse effects in practice.

Osmo- and cryotherapy of sugarcane (Saccharum spp. L.) shoot-tips infected with sugarcane mosaic virus (SCMV)

In this work, we investigated the effect of different osmoprotective treatments and of cryopreservation using a droplet-vitrification (D-V) protocol to eliminate sugarcane mosaic virus (SCMV) of shoot-tips excised from in vitro propagated infected plantlets. Shoot-tips of sugarcane (Saccharum spp. L.) were precultured on semisolid MS medium supplemented with 0.3 M sucrose for 1 day, loaded in solution with 0.4 M sucrose and 2 M glycerol for 30 min and exposed to plant vitrification solution 2 for 15 min at room temperature prior to ultra-rapid cooling in liquid nitrogen. Virus indexing was performed by the DAS-ELISA immunoenzymatic test. The presence of SCMV was confirmed in the donor-plantlets derived of infected field material. No virus was detected in the regenerated plantlets from shoot-tips subjected to cryopreservation protocol. The progressive decrease in absorbances occurred from the first preculture treatment and no significant differences (P ≤ 0.05) were found with respect to following steps of D-V protocol. These results indicate that the osmotic dehydration treatments (osmotherapy) and cryopreservation (cryotherapy) may be potentially effective strategies to remove the SCMV from infected plants.

Molecular Viral Diagnosis and Sanitation of Yam Genetic Resources: Implications for Safe Yam Germplasm Exchange

Yam (Dioscorea spp.) is an important crop in tropical and subtropical regions. Many viruses have been recently identified in yam, hampering genetic conservation and safe international exchanges of yam germplasm. We report on the implementation of reliable and cost-effective PCR-based detection tools targeting eight different yam-infecting viruses. Viral indexing of the in vitro yam collection maintained by the Biological Resources Center for Tropical Plants (BRC-TP) in Guadeloupe (French West Indies) unveiled a high prevalence of potyviruses, badnaviruses, Dioscorea mosaic associated virus (DMaV) and yam asymptomatic virus 1 (YaV1) and a high level of coinfections. Infected yam accessions were subjected to a combination of thermotherapy and meristem culture. Sanitation levels were monitored using PCR-based and high-throughput sequencing-based diagnosis, confirming the efficacy and reliability of PCR-based detection tools. Sanitation rates were highly variable depending on viruses. Sixteen accessions were successfully sanitized, paving the way to safe yam germplasm exchanges and the implementation of clean seed production programs worldwide.

Cryobiotechnology of Plants: A Hot Topic Not Only for Gene Banks

Agriculture has always been an important part of human evolution. Traditionally, farming is changing and developing with regard to challenges it faces. The major challenges of modern agriculture are food and nutrition safety for the growing world population. Promoting species and genetic diversity in agriculture appears to be an important approach to dealing with those challenges. Gene banks all around the world play a crucial role in preserving plant genetic resources for future crop improvements. The plant germplasm can be preserved in different ways, depending on the species or form of stored plant tissue. This review focuses on a special preservation method—cryopreservation. Cryopreservation is an effective technique for storing living systems at ultra-low temperatures, usually in liquid nitrogen or its vapor phase. This conservation method is crucial for plants that do not produce seeds or that produce non germinating seeds, as well as for plants that propagate vegetatively. Moreover, based on the cryopreservation method, a novel plant biotechnology tool for pathogen eradication called cryotherapy has been developed. The use of liquid nitrogen eliminates plant pathogens such as viruses, phytoplasmas, and bacteria. Our article reviews recent advances in cryo-biotechnologies such as cryopreservation and cryotherapy, with special focus on studies concerning fruit plants.

Detection and Phylogenetic Analysis of Viruses Linked with Fig Mosaic Disease in Seventeen Fig Cultivars in Palestine

Fig mosaic is a viral disease (FMD) that spreads in Palestinian common fig ( Ficus carica L.) orchards. Recognizing the economic value of fig plants and the harmful nature of FMD, the disease poses a significant threat to the economy of the fig production in Palestine. We applied the reverse transcription and amplification (RT-PCR) and PCR technique to leaf samples of 77 trees and 14 seedlings of 17 fig cultivars. The samples were collected from orchards in the main fig-growing provinces of the Palestinian West Bank, to assess the prevalence of viruses associated with FMD, and confirm a possible link of symptoms with viruses detected. Four viruses were detected: Fig mosaic virus (FMV), Fig badnavirus-1 (FBV-1), Fig leaf mottle-associated virus 2 (FLMaV-2), and Fig fleck-associated virus (FFkaV). FMV and FBV-1 were found in all tested fig plants (100%), while FLMaV-2 and FFkaV were detected in 61.5% and 33% of the fig samples, respectively. The high incidence of FBV-1 in the newly propagated symptomatic and symptomless seedlings from different cultivars may be an indication that FBV-1 is integrated into the genome of the fig in a cultivar nondiscriminatory manner. Very weak or no association was detected between FMD symptoms severity in the 17 Palestinian fig cultivars with the various viruses' combinations observed (i.e., number of the viruses infecting the plant). These results support the notion that FMD symptom severity expression is likely to be controlled by a combination of FMV infection, cultivars, and environmental factors, rather than the number of viruses infecting the plant.

A New Perspective on Cryotherapy: Pathogen Elimination Using Plant Shoot Apical Meristem via Cryogenic Techniques

Plant pathogens cause different diseases on crops and industrial plant species that result in economic losses. Pathogen-free plant material has usually been obtained by traditional procedures such as meristem culture, thermotherapy, and chemotherapy. However, there are many limitations of these procedures such as mechanical challenges of meristem excision and low regeneration rate, low resistance to high temperatures, phytotoxicity, and mutagenic effects of the chemicals used in the procedures. Cryotherapy is a newly developed biotechnological tool that has been very effective in virus elimination from economically important plant species. This tool has overcome the abovementioned limitations. This chapter aims to highlight the importance of the cryogenic procedures (vitrification, encapsulation-vitrification, droplet vitrification, two-step freezing, dehydration, encapsulation-dehydration) in order to generate virus-free germplasm.

Phenotyping viral infection in sweetpotato using a high-throughput chlorophyll fluorescence and thermal imaging platform

BACKGROUND

Virus diseases caused by co-infection with Sweet potato feathery mottle virus (SPFMV) and Sweetpotato chlorotic stunt virus (SPCSV) are a severe problem in the production of sweetpotato (Ipomoea batatas L.). Traditional molecular virus detection methods include nucleic acid-based and serological tests. In this study, we aimed to validate the use of a non-destructive imaging-based plant phenotype platform to study plant-virus synergism in sweetpotato by comparing four virus treatments with two healthy controls.

RESULTS

By monitoring physiological and morphological effects of viral infection in sweetpotato over 29 days, we quantified photosynthetic performance from chlorophyll fluorescence (ChlF) imaging and leaf thermography from thermal infrared (TIR) imaging among sweetpotatoes. Moreover, the differences among different treatments observed from ChlF and TIR imaging were related to virus accumulation and distribution in sweetpotato. These findings were further validated at the molecular level by related gene expression in both photosynthesis and carbon fixation pathways.

CONCLUSION

Our study validated for the first time the use of ChlF- and TIR-based imaging systems to distinguish the severity of virus diseases related to SPFMV and SPCSV in sweetpotato. In addition, we demonstrated that the operating efficiency of PSII and photochemical quenching were the most sensitive parameters for the quantification of virus effects compared with maximum quantum efficiency, non-photochemical quenching, and leaf temperature.

Cryobiotechnology: A Double-Edged Sword for Obligate Plant Pathogens

Pathogen-free stock plants are required as propagation materials in nurseries and healthy materials are needed in germplasm exchange between countries or regions through quarantine programs. In addition, plant gene banks also prefer to maintain pathogen-free germplasm collections. Shoot tip cryotherapy is a novel biotechnology method whereby cryopreservation methods are used to eradicate obligate pathogens from vegetatively propagated plants. Long-term preservation of pathogens is necessary in all types of virus-related basic research and applications such as antigen preparation for virus detection by immunology-based methods, production of plant-based vaccines, genetic transformation to produce virus-derived resistant transgenic plants, and bionanotechnology to produce nano drugs. Obligate plant pathogens such as viruses and viroids are intracellular parasites that colonize only living cells of the hosts. Therefore, their long-term preservation is difficult. Cryotreatments cannot completely eradicate the obligate pathogens that do not infect meristematic cells and certain proportions of plants recovered from cryotreatments are still pathogen-infected. Furthermore, cryotreatments often fail to eradicate the obligate pathogens that infect meristematic cells. Cryopreservation can be used for the long-term cryopreservation of the obligate plant pathogens. Thus, cryobiotechnology functions as a double-edged sword for plant pathogen eradication and cryopreservation. This review provides updated a synthesis of advances in cryopreservation techniques for eradication and cryopreservation of obligate plant pathogens.

In vitro thermotherapy-based methods for plant virus eradication

Production of virus-free plants is necessary to control viral diseases, import novel cultivars from other countries, exchange breeding materials between countries or regions and preserve plant germplasm. In vitro techniques represent the most successful approaches for virus eradication. In vitro thermotherapy-based methods, including combining thermotherapy with shoot tip culture, chemotherapy, micrografting or shoot tip cryotherapy, have been successfully established for efficient eradication of various viruses from almost all of the most economically important crops. The present study reviewed recent advances in in vitro thermotherapy-based methods for virus eradication since the twenty-first century. Mechanisms as to why thermotherapy-based methods could efficiently eradicate viruses were discussed. Finally, future prospects were proposed to direct further studies.

Long-term preservation of potato leafroll virus, potato virus S, and potato spindle tuber viroid in cryopreserved shoot tips

Availability of and easy access to diverse plant viruses and viroids is a prerequisite in applied and basic studies related to viruses and viroids. Long-term preservation of viruses and viroids is difficult. A protocol was described for long-term preservation of potato leafroll virus (PLRV), potato virus S (PVS), and potato spindle tuber viroid (PSTVd) in cryopreserved shoot tips of potato cv. Zihuabai. Shoot regrowth levels following cryopreservation were higher in 1.5 mm-shoot tips (58-60%) than in 0.5-mm-ones (30-38%). All shoots recovered from 0.5-mm-shoot tips were PVS- and PSTVd-preserved, but none of them were PLRV-preserved. Cryopreservation of 1.5-mm-shoot tips resulted in 35% and 100% of PLRV- and PVS- and PSTVd-preserved shoots. Studies on cell survival patterns and virus localization provided explanations to the varying PLRV-preservation frequencies produced by cryopreservation of the two sizes of shoot tips. Although micropropagation efficiencies were low after 12 weeks of subculture following cryopreservation, similar efficiencies were obtained after 16 weeks of subculture in pathogen-preserved shoots recovered from cryopreservation, compared with the diseased in vitro stock shoots (the control). Pathogen concentrations in the three pathogens-preserved shoots analyzed by qRT-PCR were similar to those in micropropagated shoots. The three pathogens cryopreserved in shoot tips were readily transmitted by grafting and mechanical inoculation to potato plants. PLRV, PVS, and PSTVd represent a diverse range of plant viruses and viroid in terms of taxonomy and infectious ability. Therefore, shoot tip cryopreservation opens a new avenue for long-term preservation of the virus and viroid.

Cryopreservation of virus: a novel biotechnology for long-term preservation of virus in shoot tips

BACKGROUND

Preservation of plant virus is a fundamental requirement in all types of virus-related research and applied applications. Development of efficient, reliable strategies for long-term preservation of plant virus would largely assist these studies.

RESULTS

The present study reported a novel biotechnology allowing cryopreservation of Apple stem grooving virus (ASGV) in living shoot tips. Following cryopreservation by droplet-vitrification or encapsulation-dehydration, about 62-67% of shoot regrowth and 100% of ASGV cryopreservation were obtained. Although shoot proliferation and virus concentration were reduced in cryopreserved diseased shoots after 8 weeks of shoot regeneration, continuous subculture for 4 times (16 weeks) increased shoot proliferation and virus concentration to comparative levels as those produced by shoot tip culture (as a control to shoot tip cryopreservation). Cryopreserved ASGV was efficiently transmitted to a woody plant by micrografting and to a herbaceous indicator by mechanical inoculation. Gene sequencing in three fragments of ASGV genome including coat protein and movement protein showed that cryopreserved ASGV shared 99.87% nucleotide identities with shoot tip culture-preserved virus, indicating cryopreserved virus is genetically stable.

CONCLUSIONS

The present study demonstrates ASGV, a representative virus that can infect meristematic cells of shoot tips, can be efficiently cryopreserved in shoot tips. To the best of our knowledge, this is the first report on plant virus cryopreservation in living tissues, and has great potential applications to long-term preservation of plant viruses.

Elimination of viruses through thermotherapy and meristem culture in apple cultivar 'Oregon Spur-II'

In the present study, an attempt was made to eliminate apple chlorotic leaf spot virus, apple mosaic virus, apple stem grooving virus and apple stem pitting virus from apple cultivar 'Oregon Spur-II'. Thermotherapy was carried out at 37-40 °C for 4 weeks followed by culturing of meristems of different sizes. During establishment of explants, highest survival percentage (62.35%) and proliferation (30.68%) was recorded during summer season. However, size of meristems and position of buds from where meristems were excised also influenced their survival. The meristems of size 0.6-0.7 mm were found to be the most appropriate for maximum establishment. Meristems excised from buds positioned on distil portions of actively growing shoots showed better results. MS medium supplemented with BA (1.0 mg/l), IBA (0.05 mg/l) and GA₃ (0.1 mg/l) resulted in 56.62% establishment of explants, while maximum number of meristems proliferated with low BA (0.5 mg/l), IBA (0.08 mg/l) and same GA₃ concentration. Two to fourfold multiplication was observed. Virus indexing of shoots raised from different sizes of meristems was carried out and found that 0.3-0.6 mm size was able to eliminate ACLSV, ApMV, ASGV and ASPV. However, some of 0.5-0.6 mm sized shoots were found infected with ACLSV. Larger meristems could not completely eliminate the viruses under study.

Cryotherapy by encapsulation-dehydration is effective for in vitro eradication of latent viruses from 'Marubakaido' apple rootstock

Apple stem pitting virus (ASPV), Apple chlorotic leaf spot virus (ACLSV) and Apple stem grooving virus (ASGV) are several major viral pathogens of apple trees, responsible for substantial damage to the world's apple industry. This study aimed to evaluate the effectiveness of the encapsulation-dehydration cryopreservation technique to eradicate these viral pathogens from in vitro shoot tips excised from 'Marubakaido' apple rootstock cultures. Axillary shoot tips were excised from in vitro cultures, encapsulated in alginate beads, precultured in MS salts, dehydrated in a laminar flow hood, immersed in liquid nitrogen, then warmed and recovered on medium. After LN exposure, in vitro rooting and acclimatization, recovered 'Marubakaido' plants exhibited 52% survival and 35% regrowth without callus formation. After 8 months of regrowth, PCR analyses revealed that all the plants were free of ACLSV and ASPV, but 2 out of 20 recovered plants were still infected with ASGV. This is the first report in Brazil of the application of cryotherapy to eradicate viral complexes in Malus. Cryotherapy can facilitate the production of virus-free plants by producing high quality plant material.

Cryotherapy: A Novel Method for Virus Eradication in Economically Important Plant Species

Virus diseases have been a great threat to production of economically important crops. In practice, the use of virus-free planting material is an effective strategy to control viral diseases. Cryotherapy, developed based on cryopreservation, is a novel plant biotechnology tool for virus eradication. Comparing to the traditional meristem culture for virus elimination, cryotherapy resulted in high efficiency of pathogen eradication. In general, cryotherapy includes seven major steps: (1) introduction of infected plant materials into in vitro cultures, (2) shoot tip excision, (3) tolerance induction of explants to dehydration and subsequent freezing in liquid nitrogen (LN), (4) a short-time treatment of explants in LN, (5) warming and post-culture for regeneration, (6) re-establishment of regenerated plants in greenhouse conditions, and (7) virus indexing.

Virus Incidence of Sweet Potato in Korea from 2011 to 2014

A nationwide survey was performed to investigate the current incidence of viral diseases in Korean sweet potatoes for germplasm and growing fields from 2011 to 2014. A total of 83.8% of the germplasm in Korea was infected with viruses in 2011. Commercial cultivars that were used to supply growing fields were infected at a rate of 62.1% in 2012. Among surveyed viruses, the incidence of five Potyvirus species that infect sweet potato decreased between 2012 and 2013, and then increased again in 2014. Representatively, the incidence of Sweet potato feathery mottle virus (SPFMV) was 87.0% in 2012, 20.7% in 2013 and then increased to 35.3% in 2014. Unlike RNA viruses, DNA viruses were shown to decrease continuously. The incidence of Sweet potato leaf curl virus (SPLCV) was 5.5% in 2003, 59.5% in 2011, and 47.4% in 2012. It then decreased continuously year by year to 33.2% in 2013, and then 25.6% in 2014. While the infection rate of each virus species showed a tendency to decline, the virus infection status was more variable in 2013 and 2014. Nevertheless, the high rate of single infections and mixed infection combinations were more variable than the survey results from 2012. As shown in the results from 2013, the most prevalent virus infection was a single infection at 27.6%, with the highest rate of infection belonging to sweet potato symptomless virus-1 (SPSMV-1) (12.9%). Compared to 2013, infection combinations were more varied in 2014, with a total of 122 kinds of mixed infection.

Synergies and antagonisms in virus interactions

Metagenomic surveys and data from next generation sequencing revealed that mixed infections among plant viruses are probably a rule rather than an exception in natural pathosystems. The documented cases may range from synergism to antagonism, which may depend from the spatiotemporal order of arrival of the viruses on the host and upon the host itself. In synergistic interactions, the measurable differences in replication, phenotypic and cytopathological changes, cellular tropism, within host movement, and transmission rate of one of the two viruses or both are increased. Conversely, a decrease in replication, or inhibition of one or more of the above functions by one virus against the other, leads to an antagonistic interaction. Viruses may interact directly and by transcomplementation of defective functions or indirectly, through responses mediated by the host like the defense mechanism based on RNA silencing. Outcomes of these interactions can be applied to the risk assessment of transgenic crops expressing viral proteins, or cross-protected crops for the identification of potential hazards. Prior to experimental evidence, mathematical models may help in forecasting challenges deriving from the great variety of pathways of synergistic and antagonistic interactions. Actually, it seems that such predictions do not receive sufficient credit in the framework of agriculture.

Elimination of chrysanthemum stunt viroid and chrysanthemum chlorotic mottle viroid from infected chrysanthemum by cryopreservation

Chrysanthemum morifolium 'Borami' and 'Secret Pink' showing symptoms of stunt disease caused by chrysanthemum stunt viroid (CSVd) and 'Yellow Cap' showing chlorotic mottle disease caused by chrysanthemum chlorotic mottle viroid (CChMVd) were confirmed to be infected by the respective viroids by using reverse transcription polymerase chain reaction (RT-PCR). Real-time PCR results showed that the viroid concentrations in the infected cultivars varied between the different regions of origin (Chilgok, Gumi, and Gyeongsan). We applied a cryopreservation protocol for elimination of CSVd from naturally infected 'Borami' collected from Gumi, showing the lowest concentration of CSVd, by varying several factors such as plant vitrification solutions (PVS2 and PVS3), duration of exposure to liquid nitrogen, shoot-tip size, and low-temperature treatment. The solution (PVS2) and low-temperature treatment were found to be critical factors determining the efficacy of viroid elimination. We optimized the protocol by combining of all resulted optimal factors and tested the applicability of the protocol in 'Borami' collected from Chilgok and Gyeongsan and in 'Secret Pink' from Chilgok, Gumi, and Gyeongsan, which displayed different viroid concentrations. We found that the elimination rates varied depending on the cultivar and region of origin. Similar results were observed when the protocol was applied to eliminate CChMVd from the 'Yellow Cap' collected from the same regions. Finally, we found that nested PCR is more reliable for viroid detection than RT-PCR. Overall, cryopreservation can be used to eliminate viroids from infected chrysanthemums; however, the efficacy depends on genotype and initial viroid concentration.

An integrated badnavirus is prevalent in fig germplasm

Fig mosaic occurs worldwide and is the most common and important viral disease of fig. In the quest to identify the causal agent of the disease, several new viruses have been identified, including a new DNA virus, the subject of this communication. Phylogenetic analysis placed the virus, provisionally named Fig badnavirus-1 (FBV-1), in the genus Badnavirus, family Caulimoviridae. The experimental host range of FBV-1 was evaluated and the virus was mechanically transmitted to several herbaceous hosts. FBV-1 was detected in the National Clonal Germplasm Repository fig collection and additional samples from Arkansas, California, Florida, Michigan, Ohio, Oregon, and South Carolina, suggesting its wide distribution in the United States. Further tests revealed the presence of FBV-1 in seedlings and meristem tissue culture plants. Forty-four isolates were used in a study evaluating the population structure of the virus in the United States. Evidence that FBV-1 is integrated in the fig genome is presented and discussed.

Production of pathogen-free horticultural crops by cryotherapy of in vitro-grown shoot tips

Horticultural crops are economically valuable for sustainable agricultural production. Plant diseases caused by Pathogens including virus, phytoplasma and bacterium have been a great threat to production of horticultural crops. The efficient use of pathogen-free plant materials has overcome the menace of plant diseases and has sustained crop production. Cryotherapy of shoot tips, a novel application of cryopreservation technique, has become a new plant biotechnology tool for plant pathogen eradication. When compared with the traditional methods, cryotherapy of shoot tips produces high frequency of pathogen-free plants, which is independent of shoot tip size and cryogenic methods. Cryotherapy of shoot tips has six major steps to produce pathogen-free plants: (1) introduction of infected plant materials into in vitro cultures; (2) excision of shoot tips; (3) cryotherapy; (4) post-culture for plant regeneration; (5) indexing of pathogens in regenerated plants after cryotherapy; and (6) establishment of pathogen-free nuclear stock plants. The key steps 2, 3, and 4 are similar to cryopreservation, and play a major role in obtaining high pathogen eradication frequency.

New dimensions for VIGS in plant functional genomics

Virus-induced gene silencing (VIGS) is an efficient tool for gene function studies. It has been used to perform both forward and reverse genetics to identify plant genes involved in several plant processes. However, this technology has not yet been used to its full potential. This can be attributed to several of its limitations such as inability to silence genes during seed germination and the non-stable nature of silencing. However, several recent studies have shown that these limitations can now be overcome. In this review, we will discuss the limitations of VIGS and suitable solutions. In addition, we also describe the recent improvements and future prospects of using VIGS in plant biology.

Detection and Classification of SPLCV Isolates in the U.S. Sweetpotato Germplasm Collection via a Real-Time PCR Assay and Phylogenetic Analysis

The United States Department of Agriculture-Agricultural Research Service sweetpotato (Ipomoea batatas) germplasm collection contains accessions that were initially collected from various countries worldwide. These materials have been maintained and distributed as in vitro plantlets since the mid-1980s. The status of viral infection by the emerging Sweet potato leaf curl virus (SPLCV) and other Begomovirus spp. in this germplasm has yet to be determined. In order to minimize the potential distribution of virus-infected clones, all accessions in the collection were tested for SPLCV using a real-time polymerase chain reaction assay. In total, 47 of 701 accessions of in vitro plantlets tested positive for SPLCV. The presence of SPLCV detected in these materials was confirmed via biological indexing using the indicator plants I. nil and I. muricata. Symptoms appeared more rapidly on I. muricata than on I. nil. Nucleotide polymorphisms among the isolates were evaluated by sequencing the AV1 coat protein gene from 24 SPLCV-infected accessions. The results revealed that the SPLCV isolates shared high sequence identity. Ten nucleotide substitutions were identified, most of which were synonymous changes. Phylogenetic analysis was conducted on those 24 SPLCV isolates in combination with six described SPLCV species and various SPLCV strains from GenBank to evaluate the relationships among viral species or strains. The results from this analysis indicated that most of the AV1 genes derived from previously classified SPLCV species clustered together, some of which formed well-supported monophyletic clades, further supporting the current taxonomy. Overall, identification of SPLCV-infected germplasm will allow approaches to be employed to eliminate the virus from the collection and limit the distribution of infected materials.

Cryotherapy of shoot tips: novel pathogen eradication method

Cryotherapy is a novel application of plant cryopreservation techniques that allows pathogen eradication at a high frequency. It eliminates plant pathogens such as viruses, phytoplasmas and bacteria by briefly treating shoot tips in liquid nitrogen using cryopreservation protocols. Healthy plants are regenerated from the surviving pathogen-free meristematic tissue. The method facilitates treatment of large numbers of samples and is independent of shoot tip size. It has the potential to replace more traditional methods like meristem culture.