Discovery and molecular characterization of a new cryptovirus dsRNA genome from Japanese persimmon through conventional cloning and high-throughput sequencing

Identification and Characterization of a Novel Quanzhou Mulberry Virus from Mulberry (Morus alba)

In this study, we discovered a new virus named Quanzhou mulberry virus (QMV), which was identified from the leaves of an ancient mulberry tree. This tree is over 1300 years old and is located at Fujian Kaiyuan Temple, a renowned cultural heritage site in China. We obtained the complete genome sequence of QMV using RNA sequencing followed by rapid amplification of complementary DNA ends (RACE). The QMV genome is 9256 nucleotides (nt) long and encodes five open reading frames (ORFs). Its virion was made of icosahedral particles. Phylogenetic analysis suggests that it belongs to the unclassified Riboviria. An infectious clone for QMV was generated and agroinfiltrated into Nicotiana benthamiana and mulberry, resulting in no visible disease symptoms. However, systemic movement of the virus was only observed in mulberry seedlings, suggesting that it has a host-specific pattern of movement. Our findings provide a valuable reference for further studies on QMV and related viruses, contributing to the understanding of viral evolution and biodiversity in mulberry.

Complete nucleotide sequence of a novel partitivirus from Brassica campestris L. ssp. chinensis

In the present work, we report the discovery and complete genome sequence of a novel partitivirus identified from Brassica campestris L. ssp. chinensis, which we have named "Brassica campestris chinensis cryptic virus 1" (BCCV1). Next-generation sequencing (NGS) combined with adapter-ligation-mediated amplification allowed assembly of the full-length genome sequence of BCCV1. The genome of BCCV1 contains two dsRNA segments, dsRNA1 (1595 bp) and dsRNA2 (1591 bp), which encode a conserved RNA-dependent RNA polymerase (RdRp) and a putative capsid protein (CP), respectively. Homology searches and phylogenetic analysis of the 479-aa RdRp and 438-aa CP showed that BCCV1 is a new member of the genus Deltapartitivirus, family Partitiviridae. This is the first report of the identification of a member of the family Partitiviridae in Brassica campestris L. ssp. chinensis.

Is There a "Biological Desert" With the Discovery of New Plant Viruses? A Retrospective Analysis for New Fruit Tree Viruses

High throughput sequencing technologies accelerated the pace of discovery and identification of new viral species. Nevertheless, biological characterization of a new virus is a complex and long process, which can hardly follow the current pace of virus discovery. This review has analyzed 78 publications of new viruses and viroids discovered from 32 fruit tree species since 2011. The scientific biological information useful for a pest risk assessment and published together with the discovery of a new fruit tree virus or viroid has been analyzed. In addition, the 933 publications citing at least one of these original publications were reviewed, focusing on the biology-related information provided. In the original publications, the scientific information provided was the development of a detection test (94%), whole-genome sequence including UTRs (92%), local and large-scale epidemiological surveys (68%), infectivity and indicators experiments (50%), association with symptoms (25%), host range infection (23%), and natural vector identification (8%). The publication of a new virus is cited 2.8 times per year on average. Only 18% of the citations reported information on the biology or geographical repartition of the new viruses. These citing publications improved the new virus characterization by identifying the virus in a new country or continent, determining a new host, developing a new diagnostic test, studying genome or gene diversity, or by studying the transmission. Based on the gathered scientific information on the virus biology, the fulfillment of a recently proposed framework has been evaluated. A baseline prioritization approach for publishing a new plant virus is proposed for proper assessment of the potential risks caused by a newly identified fruit tree virus.

Host Plant Strategies to Combat Against Viruses Effector Proteins

Viruses are obligate parasites that exist in an inactive state until they enter the host body. Upon entry, viruses become active and start replicating by using the host cell machinery. All plant viruses can augment their transmission, thus powering their detrimental effects on the host plant. To diminish infection and diseases caused by viruses, the plant has a defence mechanism known as pathogenesis-related biochemicals, which are metabolites and proteins. Proteins that ultimately prevent pathogenic diseases are called R proteins. Several plant R genes (that confirm resistance) and avirulence protein (Avr) (pathogen Avr gene-encoded proteins [effector/elicitor proteins involved in pathogenicity]) molecules have been identified. The recognition of such a factor results in the plant defence mechanism. During plant viral infection, the replication and expression of a viral molecule lead to a series of a hypersensitive response (HR) and affect the host plant's immunity (pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity). Avr protein renders the host RNA silencing mechanism and its innate immunity, chiefly known as silencing suppressors towards the plant defensive machinery. This is a strong reply to the plant defensive machinery by harmful plant viruses. In this review, we describe the plant pathogen resistance protein and how these proteins regulate host immunity during plant-virus interactions. Furthermore, we have discussed regarding ribosome-inactivating proteins, ubiquitin proteasome system, translation repression (nuclear shuttle protein interacting kinase 1), DNA methylation, dominant resistance genes, and autophagy-mediated protein degradation, which are crucial in antiviral defences.

Recent Advances on Detection and Characterization of Fruit Tree Viruses Using High-Throughput Sequencing Technologies

Perennial crops, such as fruit trees, are infected by many viruses, which are transmitted through vegetative propagation and grafting of infected plant material. Some of these pathogens cause severe crop losses and often reduce the productive life of the orchards. Detection and characterization of these agents in fruit trees is challenging, however, during the last years, the wide application of high-throughput sequencing (HTS) technologies has significantly facilitated this task. In this review, we present recent advances in the discovery, detection, and characterization of fruit tree viruses and virus-like agents accomplished by HTS approaches. A high number of new viruses have been described in the last 5 years, some of them exhibiting novel genomic features that have led to the proposal of the creation of new genera, and the revision of the current virus taxonomy status. Interestingly, several of the newly identified viruses belong to virus genera previously unknown to infect fruit tree species (e.g., Fabavirus, Luteovirus) a fact that challenges our perspective of plant viruses in general. Finally, applied methodologies, including the use of different molecules as templates, as well as advantages and disadvantages and future directions of HTS in fruit tree virology are discussed.

Genome segments encoding capsid protein-like variants of Pyrus pyrifolia cryptic virus

According to previous studies, three double-stranded (ds) RNA molecules (dsRNA1, 2, and 3) detected in Japanese pear are transmitted to the next generation with high frequency through both ovules and pollen. Nucleotide sequence analysis of dsRNA1-encoding RNA-dependent RNA polymerase (RdRp) has suggested that these dsRNAs are related to a cryptovirus named Pyrus pyrifolia cryptic virus (PpCV). In this study, purified dsRNA prepared from a PpCV-infected Japanese pear cultivar was subjected to next-generation deep sequencing. This sequencing generated two de novo assembled contigs corresponding to dsRNA2 and 3, with BLAST analysis of the predicted amino acid sequences indicating homology to capsid proteins (CPs) of the cryptoviruses persimmon cryptic virus and Sinapis alba cryptic virus 1, respectively. Relationships between the two contigs and dsRNA2 and 3 were confirmed by northern blot hybridization with probes generated using primers designed from the assembled contigs. Rapid amplification of cDNA ends analyses of 5'- and 3'-terminal sequences of dsRNA2 and 3 revealed that these two dsRNAs consist of 1523 and 1481bp, respectively. The 5'-terminal sequences (AGAAUUUC) of dsRNA1, 2 and 3 were found to be conserved. Phylogenetic analysis of deduced amino acid sequences of the two CP-like variants indicated that PpCV belongs to Deltapartitivirus (Partitiviridae). Our results imply that PpCV is tri-segmented.