Nucleotide sequence and genome organization of Dweet mottle virus and its relationship to members of the family Betaflexiviridae

Complete genome sequence of citrus yellow spot virus, a newly discovered member of the family Betaflexiviridae

A novel plant virus with a positive single-stranded (+ss) RNA genome was detected in Taibei pomelo (Citrus grandis (L.) Osbeck cv. Taibeiyou) in China by high-throughput sequencing (HTS). Tentatively named "citrus yellow spot virus" (CiYSV), it has 8,061 nucleotides (nt) excluding the poly(A) tail and contains three open reading frames (ORFs). ORF1 is predicted to encode a replicase polyprotein (RP) with conserved domains typical of members of the family Betaflexiviridae. ORF2 encodes a protein sharing the highest sequence identity with the putative movement protein (MP) found in the negative-stranded RNA virus Trifolium pratense virus B (TpVB, MH982249, genus Cytorhabdovirus). ORF3 overlaps ORF2 by 137 nt and encodes a predicted coat protein (CP) that is distantly related to those of betaflexiviruses. Phylogenetic analysis based on the MP amino acid sequence showed that the CiYSV clustered with cytorhabdoviruses rather than betaflexiviruses, whilst trees based on the whole genome, RP, and CP showed it to belong to the family Betaflexiviridae but to be distinct from any other known betaflexiviruses. These results suggest that the CiYSV should be considered the first member of a tentative new genus in the family Betaflexiviridae.

One-step multiplex RT-qPCR detects three citrus viroids from different genera in a wide range of hosts

A one-step multiplex reverse transcription real-time quantitative polymerase chain reaction (RT-qPCR) based on species-specific minor groove binding (MGB) probes, was developed for the simultaneous detection, identification, and quantification of three citrus viroids belonging to different genera. Citrus exocortis viroid (Pospiviroid), Hop stunt viroid (Hostuviroid), and Citrus bark cracking viroid (Cocadviroid) cause a variety of maladies in agriculturally significant crops. Therefore, reliable assays for their detection are essential tools for various government and industry organizations implementing disease management programs. Singleplex qPCR primers and MGB probes were designed individually for the detection of the three targeted viroids, and subsequently combined in a one-step multiplex RT-qPCR reaction. A wide host range of woody plants, including citrus, grapevines, apricots, plums and herbaceous plants such as tomato, cucumber, eggplant and chrysanthemum different world regions were used to validate the assay. Single, double and triple viroid infections were identified in the tested samples. The developed multiplex RT-qPCR assay was compared with a previously reported SYBR Green I RT-qPCR for the universal detection of citrus viroids. Both assays accurately identified all citrus viroid infected samples. The multiplex assay complemented the SYBR Green I universal detection assay by differentiating among citrus viroid species in the positive samples. The developed multiplex RT-qPCR assay has the potential to simultaneously detect each targeted viroid and could be used in high throughput screenings for citrus viroids in field surveys, germplasm banks, nurseries and other viroid disease management programs.

Development and validation of a multiplex reverse transcription quantitative PCR (RT-qPCR) assay for the rapid detection of Citrus tristeza virus, Citrus psorosis virus, and Citrus leaf blotch virus

A single real-time multiplex reverse transcription quantitative polymerase chain reaction (RT-qPCR) assay for the simultaneous detection of Citrus tristeza virus (CTV), Citrus psorosis virus (CPsV), and Citrus leaf blotch virus (CLBV) was developed and validated using three different fluorescently labeled minor groove binding qPCR probes. To increase the detection reliability, coat protein (CP) genes from large number of different isolates of CTV, CPsV and CLBV were sequenced and a multiple sequence alignment was generated with corresponding CP sequences from the GenBank and a robust multiplex RT-qPCR assay was designed. The capacity of the multiplex RT-qPCR assay in detecting the viruses was compared to singleplex RT-qPCR designed specifically for each virus and was assessed using multiple virus isolates from diverse geographical regions and citrus species as well as graft-inoculated citrus plants infected with various combination of the three viruses. No significant difference in detection limits was found and specificity was not affected by the inclusion of the three assays in a multiplex RT-qPCR reaction. Comparison of the viral load for each virus using singleplex and multiplex RT-qPCR assays, revealed no significant differences between the two assays in virus detection. No significant difference in Cq values was detected when using one-step and two-step multiplex RT-qPCR detection formats. Optimizing the RNA extraction technique for citrus tissues and testing the quality of the extracted RNA using RT-qPCR targeting the cytochrome oxidase citrus gene as an RNA specific internal control proved to generate better diagnostic assays. Results showed that the developed multiplex RT-qPCR can streamline viruses testing of citrus nursery stock by replacing three separate singleplex assays, thus reducing time and labor while retaining the same sensitivity and specificity. The three targeted RNA viruses are regulated pathogens for California's mandatory "Section 3701: Citrus Nursery Stock Pest Cleanliness Program". Adopting a compatible multiplex RT-qPCR testing protocol for these viruses as well as other RNA and DNA regulated pathogens will provide a valuable alternative tool for virus detection and efficient program implementation.

Control of virus diseases of citrus

Citrus is thought to have originated in Southeast Asia and horticulturally desirable clonal selections have been clonally cultivated for hundreds of years. While some citrus species have nucellar embryony, most cultivation of citrus has been by clonal propagation to ensure that propagated plants have the same traits as the parent selection. Clonal propagation also avoids juvenility, and the propagated plants produce fruit sooner. Because of the clonal propagation of citrus, citrus has accumulated a large number of viruses; many of these viruses are asymptomatic until a susceptible rootstock and/or scion is encountered. The viruses reported to occur in citrus will be summarized in this review. Methods of therapy to clean selected clones from viruses will be reviewed; the use of quarantine, clean stock, and certification programs for control of citrus viruses and other strategies to control insect spread citrus viruses, such as mild strain cross-protection and the use of pest management areas will be discussed.

Characterization of the complete genome of a novel citrivirus infecting Actinidia chinensis

A ssRNA virus from kiwifruit (Actinidia spp.) was identified as a member of the family Betaflexiviridae. It was mechanically transmitted to the herbaceous indicators Nicotiana benthamiana, N. clevelandii, N. glutinosa and N. occidentalis. The complete genome was comprised of three ORFs and a 3'poly (A) tail. Phylogenetic analysis of the entire genome indicated it was a novel member of the genus Citrivirus (family Betaflexiviridae). The complete nucleotide sequence differed from that of citrus leaf blotch virus (CLBV) by ~ 26 %. The movement protein (ORF2) and coat protein (ORF3) shared 95-96 % and 90-92 % amino acid sequence identity, respectively, with CLBV. The replicase polyprotein (ORF1) was distinctly different from published CLBV sequences, with 78-79 % amino acid sequence identity, while the 5' UTR and 3' UTR differed from CLBV by 28 % and 29 %, respectively. The sequence differences indicate that the citrivirus from Actinidia is either a divergent strain of CLBV or a member of a new citrivirus species.