Complete nucleotide sequence and genome organization of a Chinese isolate of tobacco bushy top virus

Synergism Among the Four Tobacco Bushy Top Disease Casual Agents in Symptom Induction and Aphid Transmission

Tobacco bushy top disease (TBTD), caused by multiple pathogens including tobacco bushy top virus (TBTV), tobacco vein distorting virus (TVDV), TBTV satellite RNA (TBTVsatRNA), and TVDV-associated RNA (TVDVaRNA), is a destructive disease in tobacco fields. To date, how these causal agents are co-transmitted by aphid vectors in field and their roles in disease symptom induction remain largely unknown, due mainly to the lack of purified causal agents. In this study, we have constructed four full-length infectious clones, representing the Yunnan Kunming isolates of TVDV, TBTV, TBTVsatRNA, and TVDVaRNA (TVDV-YK, TBTV-YK, TBTVsatRNA-YK, and TVDVaRNA-YK), respectively. Co-inoculation of these four causal agents to tobacco K326 plants caused typical TBTD symptoms, including smaller leaves, necrosis, and plant stunting. In addition, inoculation of tobacco K326 plants with TBTV alone caused necrosis in systemic leaves by 7 dpi. Tobacco K326 and Nicotiana benthamiana plants infected by single virus or multiple viruses showed very different disease symptoms at various dpi. RT-PCR results indicated that co-infection of TVDVaRNA-YK could increase TVDV-YK or TBTV-YK accumulation in N. benthamiana plants, suggesting that TVDVaRNA-YK can facilitate TVDV-YK and TBTV-YK replication and/or movement in the infected plants. Aphid transmission assays showed that the successful transmission of TBTV-YK, TBTVsatRNA-YK, and TVDVaRNA-YK by Myzus persicae depended on the presence of TVDV-YK, while the presence of TBTVsatRNA-YK increased the aphid transmission efficiency of TBTV and TVDV. We consider that these four new infectious clones will allow us to further dissect the roles of these four causal agents in TBTD induction as well as aphid transmission.

Identification of two novel poleroviruses and the occurrence of Tobacco bushy top disease causal agents in natural plants

Tobacco bushy top disease (TBTD) is a devastating tobacco disease in the southwestern region of China. TBTD in the Yunnan Province is often caused by co-infections of several plant viruses: tobacco bushy top virus (TBTV), tobacco vein distorting virus (TVDV), tobacco bushy top virus satellite RNA (TBTVsatRNA) and tobacco vein distorting virus-associated RNA (TVDVaRNA). Through this study, two new poleroviruses were identified in two TBTD symptomatic tobacco plants and these two novel viruses are tentatively named as tobacco polerovirus 1 (TPV1) and tobacco polerovirus 2 (TPV2), respectively. Analyses of 244 tobacco samples collected from tobacco fields in the Yunnan Province through RT-PCR showed that a total of 80 samples were infected with TPV1 and/or TPV2, and the infection rates of TPV1 and TPV2 were 8.61% and 29.51%, respectively. Thirty-three TPV1 and/or TPV2-infected tobacco samples were selected for further test for TBTV, TVDV, TBTVsatRNA and TVDVaRNA infections. The results showed that many TPV1 and/or TPV2-infected plants were also infected with two or more other assayed viruses. In this study, we also surveyed TBTV, TVDV, TBTVsatRNA and TVDVaRNA infections in a total of 1713 leaf samples collected from field plants belonging to 29 plant species in 13 plant families and from 11 provinces/autonomous regions in China. TVDV had the highest infection rates of 37.5%, while TVDVaRNA, TBTV and TBTVsatRNA were found to be at 23.0%, 12.4% and 8.1%, respectively. In addition, TVDV, TBTV, TBTVsatRNA and TVDVaRNA were firstly detected of co-infection on 10 plants such as broad bean, pea, oilseed rape, pumpkin, tomato, crofton weed etc., and 1 to 4 of the TBTD causal agents were present in the samples collected from Guizhou, Hainan, Henan, Liaoning, Inner mongolia and Tibet autonomous regions. The results indicated that TBTD causal agents are expanding its host range and posing a risk to other crop in the field.

Detection and diversity of maize yellow mosaic virus infecting maize in Nigeria

Maize yellow mosaic virus (MaYMV; genus Polerovirus; family Luteoviridae) was recently characterized from maize in China and subsequently detected in mixed infection with sugarcane mosaic virus (genus Potyvirus; family Potyviridae) in sugarcane and itch grass in Nigeria. This study was conducted to understand the status and genetic diversity of MaYMV in maize fields in the northern guinea savannah region of Nigeria. A survey was conducted in 2017 and maize (n = 90) and itch grass (n = 10) plants suspected of virus infection based on appearance of mosaic and/or yellowing symptoms were sampled in Kaduna (n = 65) and Katsina (n = 35) states. The samples were screened individually by reverse transcription polymerase chain reaction using the genus-specific primers targeting poleroviruses and potyviruses Pol-G-F and Pol-G-R primers encompassing the partial P1-P2 fusion protein and coat protein genes of poleroviruses and primer pair CI-For & CI-Rev encompassing the partial cylindrical inclusion proteins of most potyviruses. A subset of amplified DNA fragments was cloned, Sanger-sequenced, and the obtained sequences were bioinformatically analyzed along with corresponding sequences from GenBank. The ~ 1.1 Kb polerovirus fragment was detected in 32.2% (29/90) of the maize samples while all 10 itch grass samples tested negative. BLASTN analysis of sequences derived from six polerovirus samples confirmed the virus identity as MaYMV. In pairwise comparisons, the MaYMV sequences derived in this study shared 97-99% nucleotide identity with sequences of other MaYMV isolates available in the NCBI GenBank. Phylogenetic analysis revealed the segregation of global MaYMV sequences into three host-independent clusters with pattern of geographic structuring.

Complete genome sequence of a tentative new umbravirus isolated from Patrinia scabiosaefolia

The complete genomic RNA sequence of a tentative new umbravirus from Patrinia scabiosaefolia, tentatively named "patrinia mild mottle virus" (PatMMoV), was determined. The genome of PatMMoV consists of 4,214 nucleotides and has a typical umbravirus genome organization with four open reading frames. BLAST searches showed that the complete nucleotide sequence of PatMMoV had the highest identity (72%; 50% query coverage) to Ixeridium yellow mottle-associated virus 2 (IxYMaV-2; an unclassified umbravirus). In addition, phylogenetic analysis and pairwise comparisons showed that PatMMoV and IxYMaV-2 were the most closely related and placed in the same clade within a group of umbraviruses. These results suggest that PatMMoV is a putative new member of the genus Umbravirus in the family Tombusviridae.

Structural alteration of a BYDV-like translation element (BTE) that attenuates p35 expression in three mild Tobacco bushy top virus isolates

To identify the molecular effects of Tobacco bushy top virus (TBTV) evolution on the degeneration of tobacco bushy top disease, three TBTV isolates with mild virulence were compared with wild-type TBTV to assess the translation of p35, which relies on a BYDV-like translation element (BTE) in a cap-independent manner. The in vitro expression of p35 in the mild isolates was only 20% to 40% of the expression observed in wt TBTV. Based on translation data from chimeric TBTV RNA, low-level p35 expression in the three mild isolates was associated with two regions: the 5' terminal 500 nt region (RI) and the 3' internal region (RV), which included the BTE. For the RV region, low level p35 expression was mainly associated with structural alterations of the BTE instead of specific sequence mutations within the BTE based on SHAPE structural probing and mutation analysis. Additionally, structural alteration of the TBTV BTE resulted from mutations outside of the BTE, implying structural complexity of the local region surrounding the BTE. This study is the first report on the structural alteration of the 3' cap-independent translation element among different isolates of a given RNA virus, which is associated with variations in viral virulence.

Opium poppy mosaic virus, a new umbravirus isolated from Papaver somniferum in New Zealand

A novel virus, tentatively named "opium poppy mosaic virus" (OPMV), was isolated from Papaver somniferum (opium poppy) with leaf mosaic and mottling symptoms in Auckland, New Zealand, in 2006. The virus was mechanically transmitted to herbaceous plants of several species, in which it induced local and/or systemic symptoms. No virus particles were observed by electron microscopy in the diseased P. somniferum or any of the symptomatic herbaceous plants. The complete genomic sequence of 4230 nucleotides contains four open reading frames (ORF) and is most closely related (59.3 %) to tobacco bushy top virus, a member of the genus Umbravirus. These data suggest that OPMV is a new umbravirus.

Phylogenetic and recombination analysis of Tobacco bushy top virus in China

BACKGROUND

During the past decade, tobacco bushy top disease, which is mainly caused by a combination of Tobacco bushy top virus (TBTV) and Tobacco vein-distorting virus (TVDV), underwent a sudden appearance, extreme virulence and degeneration of the epidemic in the Yunnan province of China. In addition to integrative control of its aphid vector, it is of interest to examine diversity and evolution among different TBTV isolates.

METHODS

5' and 3' RACE, combined with one step full-length RT-PCR, were used to clone the full-length genome of three new isolates of TBTV that exhibited mild pathogenicity in Chinese fields. Nucleotide and amino acid sequences for the TBTV isolates were analyzed by DNAMAN. MEGA 5.0 was used to construct phylogenetic trees. RDP4 was used to detect recombination events during evolution of these isolates.

RESULTS

The genomes of three isolates, termed TBTV-JC, TBTV-MD-I and TBTV-MD-II, were 4152 nt in length and included one distinctive difference from previously reported TBTV isolates: the first nucleotide of the genome was a guanylate instead of an adenylate. Diversity and phylogenetic analyses among these three new TBTV isolates and five other available isolates suggest that ORFs and 3'UTRs of TBTV may have evolved separately. Moreover, the RdRp-coding region was the most variable. Recombination analysis detected a total of 29 recombination events in the 8 TBTV isolates, in which 24 events are highly likely and 5 events have low-level likelihood based on their correlation with the phylogenetic trees. The three new TBTV isolates have individual recombination patterns with subtle divergences in parents and locations.

CONCLUSIONS

The genome sizes of TBTV isolates were constant while different ORF-coding regions and 3'UTRs may have evolved separately. The RdRp-coding region was the most variable. Frequent recombination occurred among TBTV isolates. Three new TBTV isolates have individual recombination patterns and may have different progenitors.

A novel combination of a new umbravirus, a new satellite RNA and potato leafroll virus causes tobacco bushy top disease in Ethiopia

Etiological studies of a recently emerged bushy top disease of tobacco in Ethiopia indicated that a ~4.5-kb dsRNA from infected plants represents an umbravirus, whereas a smaller band (~0.5 kb) is that of a new satellite RNA. Potato leafroll virus was also consistently associated with the disease. The three agents, whose experimental host ranges are restricted to members of the family Solanaceae, always occurred together in field samples and are transmitted together by the aphid Myzus persicae nicotianae. The umbravirus, which represents a new species, is most closely related to groundnut rosette virus, and the name Ethiopian tobacco bushy top virus is proposed.

Simultaneous detection of four causal agents of tobacco bushy top disease by a multiplex one-step RT-PCR

Tobacco bushy top disease is a complex disease caused by mixed infection of Tobacco bushy top virus (TBTV), Tobacco vein distorting virus (TVDV), satellite RNA of TBTV (Sat-TBTV) and Tobacco vein distorting virus associate RNA (TVDVaRNA). A one-tube multiplex reverse transcription-PCR (RT-PCR) assay was developed for simultaneous detection of the four causal agents of the disease. Four pairs of specific primers based on the conserved regions of each of the four disease agents were used in the one-tube RT-PCR. The RT-PCR products consisted of fragments of 1049 base pairs (bp) for TBTV, 792bp for TVDVaRNA, 598bp for Sat-TBTV and 357bp for TVDV, and their origins were confirmed by sequencing. Primer concentrations and cycling condition were optimized for the multiplex RT-PCR. The detection limit of the assay was up to 10(-4) dilution. The assay was evaluated using tobacco plants infected naturally with one to four target viruses, transmission vector of aphids and field samples collected from Yunnan, Hunan, and Guizhou province, China. The results show that the multiplex RT-PCR is reliable and sensitive as a simple, rapid and cost-effective method to detect these pathogens in tobacco and aphid. This assay will be useful for virus surveys when large numbers of samples are tested.

Complete nucleotide sequence and genome organization of a Chinese isolate of Tobacco vein distorting virus

Tobacco bushy top disease is caused by tobacco bushy top virus (TBTV, a member of the genus Umbravirus) which is dependent on tobacco vein-distorting virus (TVDV) to act as a helper virus encapsidating TBTV and enabling its transmission by aphids. Isometric virions from diseased tobacco plants were purified and disease symptoms were reproduced after experimental aphid transmission. The complete genome of TVDV was determined from cloned RT-PCR products derived from viral RNA. It was 5,920 nucleotides (nts) long and had the six major open reading frames (ORFs) typical of a member of the genus Polerovirus. Sequence comparisons showed that it differed significantly from any of the other species in the genus and this was confirmed by phylogenetic analyses of the RdRp and coat protein. SDS-PAGE analysis of purified virions gave two protein bands of about 26 and 59 kDa both of which reacted strongly in Western blots with antiserum produced to prokaryotically expressed TVDV CP showing that the two forms of the TVDV CP were the only protein components of the capsid.

Structural plasticity of Barley yellow dwarf virus-like cap-independent translation elements in four genera of plant viral RNAs

The 3' untranslated regions (UTRs) of many plant viral RNAs contain cap-independent translation elements (3' CITEs). Among the 3' CITEs, the Barley yellow dwarf virus (BYDV)-like translation elements (BTEs) form a structurally variable and widely distributed group. Viruses in three genera were known to harbor 3' BTEs, defined by the presence of a 17-nt consensus sequence. To understand BTE function, knowledge of phylogenetically conserved structure is essential, yet the secondary structure has been determined only for the BYDV BTE. Here we show that Rose spring dwarf-associated luteovirus, and two viruses in a fourth genus, Umbravirus, contain functional BTEs, despite deviating in the 17nt consensus sequence. Structure probing by selective 2'-hydroxyl acylation and primer extension (SHAPE) revealed conserved and highly variable structures in BTEs in all four genera. We conclude that BTEs tolerate striking evolutionary plasticity in structure, while retaining the ability to stimulate cap-independent translation.

Molecular biology of umbraviruses: phantom warriors

The genomes of umbraviruses differ from those of most other viruses in that they do not encode a coat protein, and thus no virus particles are formed in infected plants. Protection of umbraviral RNA outside the host plant, during vector transmission, utilizes the coat protein of an assistor luteovirus, but this review focuses on the mechanisms that compensate for the lack of a coat protein in processes within the host plant. As well as an RNA-dependent RNA polymerase, umbravirus genomes encode two other proteins from almost completely overlapping open reading frames. One of these is a cell-to-cell movement protein that can mediate the transport of homologous and heterologous viral RNAs through plasmodesmata without the participation of a coat protein. The other, the ORF3 protein, binds to viral RNA to form filamentous ribonucleoprotein particles that have elements of helical structure. It serves to stabilize the RNA and facilitates its transport through the vascular system of the plant. It may also be involved in protection of the viral RNA from the plant's defensive RNA-silencing response, although it is not a suppressor of silencing. The ORF3 protein also enters the cell nucleus, specifically targeting the nucleolus. Although the function of this localization is unknown, the ORF3 protein may provide a valuable tool for investigating plant nucleolar function.