Occurrence and Distribution of Actinidia Viruses in Shaanxi Province of China

Combining Thermotherapy with Shoot Tip Culture or Cryotherapy for Improved Virus Eradication from In Vitro Actinidia macrosperma

In recent years, kiwifruit viral diseases have become increasingly prevalent in kiwifruit-producing regions of China, significantly impacting both the yield and quality of kiwifruit. This has emerged as a significant constraint on the healthy and sustainable development of the kiwifruit industry. The use of virus-free propagation materials has been proven the most effective strategy for controlling plant viral diseases. In the present study, shoot tip culture, shoot tip cryotherapy, and their combinations with thermotherapy were established to eradicate Actinidia virus A (AcVA), Actinidia virus B (AcVB), and Actinidia chlorotic ringspot-associated virus (AcCRaV) from Actinidia macrosperma. Additionally, the impact of shoot tip size on virus eradication was evaluated. Among the three confirmed viruses, regardless of the procedure, AcVB was the easiest to eradicate, followed by AcVA and AcCRaV. Combining thermotherapy with shoot tip culture or cryotherapy resulted in a higher virus-free frequency (up to 27.3 and 50%, respectively) than shoot tip culture or cryotherapy alone (0 to 20%). Notably, the combination of thermotherapy and 0.5- to 1-mm shoot tip cryotherapy was shown to be the most effective protocol for virus eradication from A. macrosperma, which produced 50% of regenerated shoots free from all the tested viruses. To the best of our knowledge, this is the first report on virus elimination from kiwifruit infected with multiple viruses based on conventional shoot tip culture and shoot tip cryotherapy.

Identification of a novel waikavirus infecting Pittosporum tobira in China

A novel waikavirus, tentatively named "Pittosporum tobira waikavirus" (PtWV), was identified in Pittosporum tobira plants exhibiting mosaic and ringspot symptoms on foliage in Yunnan, China. The full-length genomic sequence was determined by high-throughput sequencing and rapid amplification of cDNA ends. The genome of PtWV is 12,709 nt in length and has a large open reading frame (ORF) of 11,010 nt, encoding a polyprotein, and a small ORF that encodes a 13.2-kDa bellflower vein chlorosis virus (BVCV)-like protein. Phylogenetic analysis and sequence alignment revealed that PtWV is closely related to actinidia yellowing virus 1 (AcYV1), which shares the highest amino acid (aa) sequence similarity (50.1% identity) in the Pro-RdRp region. To the best of our knowledge, this is the first report of a novel waikavirus in P. tobira.

Rapid and Visual Detection of Actinidia Chlorotic Ringspot-Associated Virus Using One-Step Reverse-Transcription Recombinase Polymerase Amplification Combined with Lateral Flow Dipstick Assay

Actinidia chlorotic ringspot-associated virus (AcCRaV) occurs widely in major kiwifruit producing areas of China and is often accompanied by coinfecting viruses, affecting the growth, yield, and quality of kiwifruit. Therefore, a rapid and sensitive detection method is crucial for diagnosing and developing effective AcCRaV management strategies. In this study, a one-step reverse-transcription recombinase polymerase amplification combined with a lateral flow dipstick (RT-RPA-LFD) assay was developed for rapid detection of AcCRaV. Specific primers and a probe were designed based on the conserved region of the coat protein gene sequence of AcCRaV. The one-step RT-RPA reaction can be performed at 35 and 40°C within 10 to 30 min, and the amplification results can be read directly on the LFD within 5 min. The detection limit of the one-step RT-RPA-LFD assay was 10-8 ng (about 20 viral copies), which was equal with one-step RT-qPCR and 100 times more sensitive than one-step RT-PCR. Moreover, the one-step RT-RPA-LFD assay was successfully applied to detect AcCRaV from crude extracts, and the entire detection process can be completed within 40 min. These results indicate that the RT-RPA-LFD assay is a simple, rapid, and sensitive strategy that can be used for accurate diagnosis of AcCRaV-infected kiwifruit plants in the field. To our knowledge, this is the first study applying the one-step RT-RPA-LFD assay to detect a kiwifruit virus.

Complete nucleotide sequence of hackberry virus A, a tentative member of the genus Waikavirus

The complete genomic sequence of a waikavirus from Chinese hackberry in Zhejiang province, China, named "hackberry virus A" (HVA), was determined using high-throughput sequencing (HTS) combined with reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) PCR. The bicistronic genomic RNA of HVA was found to consist of 12,691 nucleotides (nt), excluding the 3'-terminal poly(A) tail, and to encode a large polyprotein of 3783 amino acids (aa) and an additional 10.3-kDa protein. The aa sequences of the Pro-Pol and the CP regions of this virus share 39.8-44.2% and 25.5-36.4% identity, respectively, with currently known waikaviruses. These values are significantly below the current species demarcation threshold (< 75% and < 80% aa identity for the CP and Pro-Pol region, respectively) for the family Secoviridae, indicating that HVA represents a new species in the genus Waikavirus. This is the first report of a virus infecting Chinese hackberry.

Efficient Elimination of Actinidia Chlorotic Ringspot-Associated Virus from Infected Kiwifruit Shoots Cultured In Vitro

In this study, methods of Actinidia chlorotic ringspot-associated virus (AcCRaV) elimination by shoot tip culture, thermotherapy followed by shoot tip culture, and chemotherapy followed by shoot tip culture were explored. The results showed that the AcCRaV elimination rate was 23.3% when the secondary shoot tip culture method was used and when the shoot tip length was less than 0.5 mm. The AcCRaV elimination rate was 100% when thermotherapy (36°C [day] and 32°C [night]) was applied for 20 days followed by shoot tip culture (shoot tip length less than 1.0 mm). When shoot segments were treated with ribavirin at 15 µg/ml for 2 months followed by shoot tip culture, the elimination rate of AcCRaV was 100% (shoot tip length less than 1.0 mm). When shoot segments were treated with ribavirin at 25 µg/ml for 2 months followed by shoot tip culture, the elimination rate of AcCRaV was 100% (shoot tip length less than 1.5 mm). This is the first report on kiwifruit virus elimination methods.

A Novel Coronavirus and a Broad Range of Viruses in Kenyan Cave Bats

BACKGROUND AND METHODS

To investigate virus diversity in hot zones of probable pathogen spillover, 54 oral-fecal swabs were processed from five bat species collected from three cave systems in Kenya, using metagenome sequencing.

RESULTS

Viruses belonging to the Astroviridae, Circoviridae, Coronaviridae, Dicistroviridae, Herpesviridae and Retroviridae were detected, with unclassified viruses. Retroviral sequences were prevalent; 74.1% of all samples were positive, with distinct correlations between virus, site and host bat species. Detected retroviruses comprised Myotis myotis, Myotis ricketti, Myotis daubentonii and Galidia endogenous retroviruses, murine leukemia virus-related virus and Rhinolophus ferrumequinum retrovirus (RFRV). A near-complete genome of a local RFRV strain with identical genome organization and 2.8% nucleotide divergence from the prototype isolate was characterized. Bat coronavirus sequences were detected with a prevalence of 24.1%, where analyses on the ORF1ab region revealed a novel alphacoronavirus lineage. Astrovirus sequences were detected in 25.9%of all samples, with considerable diversity. In 9.2% of the samples, other viruses including Actinidia yellowing virus 2, bat betaherpesvirus, Bole tick virus 4, Cyclovirus and Rhopalosiphum padi virus were identified.

CONCLUSIONS

Further monitoring of bats across Kenya is essential to facilitate early recognition of possibly emergent zoonotic viruses.

Viromics Reveals the Viral Diversity in Cultivated and Wild Kiwifruit

China, the center of origin of kiwifruit, has the largest kiwifruit cultivation and production area worldwide, and Shaanxi Province is the major kiwifruit-growing region in China. However, our knowledge of kiwifruit viruses is largely skewed toward their pathology in cultivated orchards, and little is known about viral diversity in wild kiwifruit. To determine the viral diversity in cultivated and wild kiwifruit, 32 cultivated kiwifruit samples from Shaanxi Province and 30 wild kiwifruit samples from the Qinling Mountains were collected and subjected to high-throughput sequencing in this study. Eleven known viruses were found among the 32 cultivated kiwifruit samples, and 8 known viruses and 2 new viruses were found among the 30 wild kiwifruit samples. One of the two new viruses, Actinidia yellowing virus 3 (AcYV3), a member of the genus Idaeovirus, may be associated with severe yellowing of kiwifruit leaves. In addition, more than 50 nearly full-length genome sequences of known viruses were obtained. The detection rates, recombination, and molecular variation of these viruses were further analyzed. The results obtained in this study provide valuable information for understanding the virome of cultivated and wild kiwifruit.

Occurrence, Distribution, and Transmission of Alfalfa Viruses in China

Alfalfa (Medicago sativa L.) is one of the most important quality forages worldwide and is cultivated throughout China. Alfalfa is susceptible to a variety of viral diseases during its growth, which has caused huge amounts of commercial losses. However, the profile of the alfalfa virus in China remains ambiguous and the viruses transmitted by Odontothrips loti (Haliday), dominant insect pests in alfalfa, are also poorly understood. In the present study, virus diversity was investigated in the primary alfalfa-growing areas in China. A total of 18 alfalfa viruses were identified through RNA-sequencing (RNA-seq) and reverse transcription-polymerase chain reaction (RT-PCR). Two new plant viruses, Medicago sativa virus 1 (MsV1) and Medicago sativa luteovirus 1 (MsLV1), were detected for the first time. Another four viruses, including the Alfalfa ringspot-associated virus (ARaV), Alfalfa virus F (AVF), Alfalfa enamovirus 1 (AEV1), and Alfalfa deltaparitivirus (ADPV), were reported in China for the first time as well. Both Alfalfa mosaic virus (AMV) and Medicago sativa alphapartitivirus 2 (MsAPV2) are the dominant pathogens, with an infection incidence of 91.7-100%, and 74.4-97.2%, respectively. Additionally, O. loti with first- and second-instar nymphs were shown to acquire the AMV within 0.25 h of feeding on a virus-infected alfalfa. Transmission by thrips to healthy alfalfa plants was also demonstrated. Additionally, we clarified the dynamic changes in the AMV in pre-adult stages of O. loti, which indicated that the AMV is propagated in the nymph stage of O. loti. These findings provide valuable information for understanding the alfalfa virome, confirm the role thrips O. loti plays in alfalfa virus transmission, and improve our fundamental knowledge and management of diseases in China.

Advances in and Prospects for Actinidia Viruses

Kiwifruit (Actinidia spp.) is an economically important fruit crop worldwide. Before 2010, kiwifruit viruses had not received much attention; since then, more than 20 viruses infecting kiwifruit have been discovered. Some of these viruses cause severe yellowing, mosaic, necrosis, ringspots, and other symptoms on leaves, seriously impacting yield and quality. Many of these viruses are widely distributed. This review summarizes recent research advances in the identification, genomic variation, distribution, transmission, detection, incidence, prevention, and control of kiwifruit viruses and proposes directions for future research. Using virus-tested propagation material is the most economical and effective method for controlling kiwifruit viruses.

Effects of Actinidia Yellowing Ringspot Virus on the Yield and Quality of Kiwifruit

China is the origin and distribution center of kiwifruit, as well as the country with the largest cultivated area and output of kiwifruit. A previous study found that a new kiwifruit virus, Actinidia yellowing ringspot virus (AYRSpV), has been detected in kiwifruit samples with yellowed leaves. The incidence of this virus was high in kiwifruit plantings in Shaanxi Province. To determine the symptoms of this viral infection and the effects of this virus on the yield and quality of kiwifruit, we measured leaf chlorophyll levels and the fruit yield, total sugar, total acid, and dry matter contents of 'Hayward' kiwifruit grafted with AYRSpV-infected scions. The results showed that, after AYRSpV infection, symptoms including chlorotic ringspots were mainly observed in the spring and gradually recovered with high summer temperatures. A few of the leaves that did not recover showed symptoms of albinism, which lasted until the leaves fell. We found that AYRSpV infection could reduce the chlorophyll content of Hayward kiwifruit by 74.61 to 76.64%, fruit yield by 14.50 to 24.10%, sugar-to-acid ratio by 50.09 to 50.57%, and fruit dry matter content by 1.67 to 1.78%. Our results showed that AYRSpV infection could significantly affect the yield and quality of Hayward kiwifruit.

Complete nucleotide sequence of sweetbriar rose curly-top associated virus, a tentative member of the genus Waikavirus

A novel virus, tentatively named "sweetbriar rose curly-top associated virus" (SRCTaV), was identified in sweetbriar rose (Rosa rubiginosa) using high-throughput sequencing. The complete genome sequence of SRCTaV was determined and characterized. Phylogenetic analysis revealed that SRCTaV is closely related to members of the genus Waikavirus.