Taro Vein Chlorosis Virus Research Articles

Incidence of RNA viruses infecting taro and tannia in East Africa and molecular characterisation of dasheen mosaic virus isolates.

Taro (Colocasia esculenta) and tannia (Xanthosoma sp.) plants growing in 25 districts across Ethiopia, Kenya, Tanzania and Uganda were surveyed for four RNA viruses. Leaf samples from 392 plants were tested for cucumber mosaic virus (CMV), dasheen mosaic virus (DsMV), taro vein chlorosis virus (TaVCV) and Colocasia bobone disease‐associated virus (CBDaV) by RT‐PCR. No samples tested positive for TaVCV or CBDaV, while CMV was only detected in three tannia samples with mosaic symptoms from Uganda. DsMV was detected in 40 samples, including 36 out of 171 from Ethiopia, one out of 94 from Uganda and three out of 41 from Tanzania, while none of the 86 samples from Kenya tested positive for any of the four viruses. The complete genomes of nine DsMV isolates from East Africa were cloned and sequenced. Phylogenetic analyses based on the amino acid sequence of the DsMV CP‐coding region revealed two distinct clades. Isolates from Ethiopia were distributed in both clades, while samples from Uganda and Tanzania belong to different clades. Seven possible recombination events were identified from the analysis carried out on the available 15 full‐length DsMV isolates. Nucleotide substitution ratio analysis revealed that all the DsMV genes are under strong negative selection pressure.

A Review on Viruses Infecting Taro (Colocasia esculenta (L.) Schott).

Taro is an important crop in parts of the world, especially in the Pacific Islands. Like all plants, it is also susceptible to virus infections that could result in diseases, which negatively affects the source of food and trade revenue. Understanding the biology of taro viruses could improve current knowledge regarding the relationship between viruses and taro, thus allowing for a better approach towards the management of the diseases that are associated with them. By compiling and discussing the research on taro and its four major viruses (Dasheen mosaic virus, Taro bacilliform virus, Colocasia bobone disease virus, and Taro vein chlorosis virus) and a relatively new one (Taro bacilliform CH virus), this paper explores the details of each virus by examining their characteristics and highlighting information that could be used to mitigate taro infections and disease management.

Molecular characterization of Morogoro maize-associated virus, a nucleorhabdovirus detected in maize (Zea mays) in Tanzania.

RNAtag-seq of maize samples collected in Tanzania revealed the presence of a previously undescribed nucleorhabdovirus, tentatively named "Morogoro maize-associated virus" (MMaV), in three samples. The MMaV genome is 12,185-12,187 nucleotides long and shares a 69-70% nucleotide sequence identity with taro vein chlorosis virus. Annotation of the genomes showed a typical nucleorhabdovirus gene organization. PCR was unable to detect the same virus in the remaining 35 samples collected in the region.

First Report of Dasheen Mosaic Virus Infecting Taro (Colocasia esculenta) in Bangladesh

Taro (Colocasia esculenta [L.] Schott) is a root crop widely grown in tropical and subtropical countries (Banjaw 2017). It makes up about 80% of the aroids (family Araceae) cultivated in Bangladesh (Bhuiyan et al. 2011). Dasheen mosaic virus (DsMV) is a potyvirus transmitted by aphids to many aroids (Zettler and Hartman 1987). The vegetative propagation of taro contributes to the widespread distribution of viruses in this crop (Kazmi et al. 2015). In December 2016, virus-like symptoms including feathery chlorotic mottling and distortions in leaves were observed on taro (cultivar Panikachu) growing in Chandpur (23.2321° N, 90.6631° E) in Bangladesh. Thirteen taro samples with feathery mottling and distorted leaves and three nonsymptomatic leaves were collected and tested for potyvirus infection using universal potyvirus-specific ELISA (Agdia, Elkhart, IN) and universal potyvirus-specific reverse transcription polymerase chain reaction (RT-PCR) (Zheng et al. 2010), respectively. Eight of the 13 symptomatic taro samples tested positive for potyvirus in both tests, whereas the three nonsymptomatic taro leaves tested negative. To identify the specific potyvirus involved, amplicons (332 bp) of the NIb region that were generated by RT-PCR using universal potyvirus-specific primers were sequenced (accession MH036416) (Wang et al. 2017). BLASTn analysis of the sequences showed that this virus shared 87% nucleotide identity with a DsMV isolate from Florida (AF048981). Two additional sets of primers were also used to further characterize the isolate: DsMV gene CI-specific primers DMV 5708-5731-F/DMV 6131-6154-R (Wang et al. 2017); and DsMV gene CP-specific primers DMV 9398-9419-F (5′-CTTGCGTCAGATAATGCATCAC-3′)/DMV 9965-9985-R (5′-CACCGTGCACGAAGCATCTCG-3′). BLASTn analysis of the 407-bp CI amplicon (accession MH036417) shared 91% identity with a Chinese isolate of DsMV (AJ298033), and the 564-bp CP amplicon (accession MH036418) shared 92% identity with the same Chinese isolate of DsMV. Eight PCR-positive samples also tested positive for DsMV by triple-antibody sandwich ELISA using a DsMV-specific antibody (Agdia). Moreover, all 13 samples were tested for taro vein chlorosis virus (Atibalentja et al. 2018), cucumber mosaic virus (Wang et al. 2014), and taro bacilliform CH virus (Wang et al. 2017) by RT-PCR, and the results were negative. To our knowledge, this is the first report of DsMV infecting taro plants in Bangladesh. Our results will be useful for further genome characterization and management of DsMV in Bangladesh to mitigate potential threats to taro production there.

First Report of Taro vein chlorosis virus on Taro (Colocasia esculenta) in the U.S. Territory of American Samoa

Virus diseases of taro (Colocasia esculenta) are a serious cause for concern wherever taro is grown; not only for the extent of yield losses they inflict, but also for the restrictions imposed to the international exchange of taro commodities and germplasm in order to limit the spread of new viruses in countries where they have not yet been reported. The stakes are even higher for small South Pacific island communities like American Samoa where taro is the most important staple food crop and a highly valued gift in many cultural functions. Dasheen mosaic virus (DsMV, family: Potyviridae) and Taro bacilliform virus (TaBV, family: Caulimoviridae) are the only two viruses previously known to infect taro in American Samoa (Revill et al., 2005a). In August of 2016, taro plants (cv. Samoa red 2) exhibiting leaf-vein chlorosis and streaks along the petioles, similar to the symptoms caused by Taro vein chlorosis virus (TaVCV, family: Rhabdoviridae, genus: Nucleorhabdovirus) (Revill et al., 2005b), were observed o...

Completed sequence and corrected annotation of the genome of maize Iranian mosaic virus.

Maize Iranian mosaic virus (MIMV) is a negative-sense single-stranded RNA virus that is classified in the genus Nucleorhabdovirus, family Rhabdoviridae. The MIMV genome contains six open reading frames (ORFs) that encode in 3΄ to 5΄ order the nucleocapsid protein (N), phosphoprotein (P), putative movement protein (P3), matrix protein (M), glycoprotein (G) and RNA-dependent RNA polymerase (L). In this study, we determined the first complete genome sequence of MIMV using Illumina RNA-Seq and 3'/5' RACE. MIMV genome ('Fars' isolate) is 12,426 nucleotides in length. Unexpectedly, the predicted N gene ORF of this isolate and of four other Iranian isolates is 143 nucleotides shorter than that of the MIMV coding-complete reference isolate 'Shiraz 1' (Genbank NC_011542), possibly due to a minor error in the previous sequence. Genetic variability among the N, P, P3 and G ORFs of Iranian MIMV isolates was limited, but highest in the G gene ORF. Phylogenetic analysis of complete nucleorhabdovirus genomes demonstrated a close evolutionary relationship between MIMV, maize mosaic virus and taro vein chlorosis virus.

First Report of Taro vein chlorosis virus Infecting Taro (Colocasia esculenta) in the United States.

In March 2013, taro plants (Colocasia esculenta [L.] Schott cv. Iliuaua) with leaves displaying veinal chlorosis and necrosis were observed on the island of Molokai. These symptoms were similar to those of taro vein chlorosis, a disease of taro caused by Taro vein chlorosis virus (TaVCV; family Rhabdoviridae, genus Nucleorhabdovirus). To explore this possibility, RNA was isolated from both symptomatic and asymptomatic taro leaves using the NucleoSpin RNA II extraction kit (Macherey-Nagel, Bethlehem, PA) according to the provided protocol, except that RLT Buffer (Qiagen Inc., Valencia, CA) was used as the initial extraction buffer. The RNAs were converted to cDNA using random primers and MMLV-RT reverse transcriptase (Promega, Madison, WI). The cDNA underwent PCR assays using primer sets Pol2A1/Pol2A2 and Cap2A/Cap2B which target the RNA-dependent RNA polymerase (RdRp) and putative nucleocapsid genes of TaVCV, respectively (1). Amplification products of the correct size were obtained for both primer sets, and these underwent molecular cloning using pGEM-T Easy (Promega). Three clones were selected and their sequences determined by dye-terminator sequencing. After primer sequence removal, the Pol2A1/Pol2A2 product (952 bp; GenBank Accession No. KF921085) and Cap2A/Cap2B product (1,050 bp; KF921086) were found to be 79 and 84% identical to a Fijian strain of TaVCV (AY674964), respectively. Samples from 328 plants with and without taro vein chlorosis symptoms were collected from 35 sites on five of the Hawaiian islands and assayed for TaVCV using the Pol2A1/Pol2A2 primer set as described above. The incidence of TaVCV in these samples was 21.6%, with positive samples coming from each island. Although a very strong association between symptoms and the presence of TaVCV was observed, eight asymptomatic plants were also positive, suggesting the detection assay was able to detect the virus before the onset of symptoms. Conversely, three symptomatic plants were found to be negative, suggesting the Pol2A1/Pol2A2 PCR assay might not detect all strains of TaVCV in Hawaii. A digoxygenin-labeled probe (Roche Applied Science, Indianapolis, IN) derived from the Pol2A1/Pol2A2 amplification product of one sample hybridized with the cDNA of only four of nine TaVCV-infected samples collected from three different islands in a dot-blot hybridization assay performed at high stringency. This probe did not hybridize with the cDNA of five TaVCV-negative samples. TaVCV exhibits a great deal of genetic diversity in the South Pacific nations where it is found; nucleotide divergence of up to 27% in regions of the RdRp gene has been reported (1). The high genetic divergence between the TaVCV isolate characterized in Hawaii and the TaVCV accession in GenBank, as well as the dot blot hybridization assay results support this observation. The widespread distribution of TaVCV in Hawaii suggests it is not a recent introduction. However, the common practice of farmers sharing taro propagules has likely accelerated its spread. An arthropod vector of TaVCV has yet to be identified, so it is unknown whether natural spread is also occurring in Hawaii. Taro has both economic and cultural importance to Hawaii. These findings, representing the first detection of TaVCV in Hawaii and the United States, illustrate the need to develop virus-free germplasm for local, national, and international distribution of this important staple crop. Reference: (1) P. Revill et al. J. Gen Virol. 86:491, 2005.

Characterisation of a proposed Nucleorhabdovirus new to South Africa

A previously uncharacterised plant rhabdovirus, infecting Bermuda grass (Cynodon dactylon) in the North West Province, South Africa, has been found. To determine the morphology and virion size of this virus, embedded ultra-thin sections of infected plant samples were observed under a transmission electron microscope. The virion distribution within the cell, its bullet-shaped morphology and its size (240 × 63 nm) indicated that this might be a rhabdovirus of the genus Nucleorhabdovirus. Degenerate polymerase chain reaction (PCR) primers were designed by alignment of the polymerase gene sequences of several plant rhabdoviruses in order to identify conserved regions. Standard PCR and sequencing protocols were used to determine a partial polymerase gene sequence of this virus sample which was then compared to the most closely related sequences available on Genbank. The analysis indicated that the virus was indeed most closely related to known nucleorhabdoviruses, with the highest nucleotide sequence similarities being to Maize mosaic virus and Taro vein chlorosis virus (70% and 69.7% respectively). Serological testing indicated that the South African Cynodon rhabdovirus had a close serological relationship with the nucleorhabdovirus Cynodon chlorotic streak virus.

Analysis of nucleotide sequence of Iranian maize mosaic virus confirms its identity as a distinct nucleorhabdovirus

The nucleotide sequence of the Iranian maize mosaic rhabdovirus (IMMV) was obtained using a random-PCR method (rPCR) followed by PCR with specific primers. Analysis of the complete nucleotide sequence of the IMMV genes and intergenic regions comprising a total of 12,381 nucleotides (including the partial sequences of leader and trailer regions) revealed six open reading frames (ORF) on the viral complementary RNA (vcRNA). On the basis of its similarities to other rhabdovirus sequences, the IMMV genome consists of 3'-leader-N-P-3-M-G-L-5'-trailer. The intergenic regions contained a characteristic consensus sequence, 3'-AAUUCUUUUUGGGUUU/G-5'. The IMMV gene products showed a high similarity to those of maize mosaic virus and taro vein chlorosis virus and a more distant relationship to other rhabdoviruses. Together with the biological, serological and morphological features described earlier, our molecular data provide evidence that IMMV is a distinct member of the genus Nucleorhabdovirus in the family Rhabdoviridae.

Cloning and subcellular localization of the phosphoprotein and nucleocapsid proteins of Potato yellow dwarf virus, type species of the genus Nucleorhabdovirus

We have cloned and characterized mRNAs corresponding to the phosphoprotein (P) and nucleocapsid (N) genes of the sanguinolenta strain of Potato yellow dwarf virus (PYDV). The P and N messenger RNAs both begin with a common AAACA pentanucleotide and are 1546 nt and 962 nt in length, and capable of encoding 52 kDa and 31 kDa proteins, respectively. The N mRNA contains a 12 nt 5′ non-translated sequence (NTS) and a 83 nt 3′-NTS. Similarly, the P mRNA has a 19 nt 5′-NTS and a 125 nt 3′-NTS. Primary structure analyses revealed three potential phosphorylation sites in the P protein and six in the N protein. Despite a lack of predictable nuclear localization signals (NLSs) in either protein, transient expression of the P and N proteins in N. benthamiana showed that both proteins are targeted exclusively to nuclei. Phylogenetic analyses showed that PYDV is most closely related to Maize mosaic virus and Taro vein chlorosis virus, which also lack predictable NLSs in their N proteins. The present data further distinguish PYDV from SYNV and suggest that, together, these viruses serve to provide a more comprehensive view of rhabdovirus cell biology, which can be studied in a common host plant.

Taro vein chlorosis virus: characterization and variability of a new nucleorhabdovirus

Sequencing of the monopartite RNA genome of a Fijian isolate of Taro vein chlorosis virus (TaVCV) confirmed that it is a definitive rhabdovirus with most similarity to members of the genus Nucleorhabdovirus. The TaVCV 12 020 nt negative-sense RNA genome contained six ORFs in the antigenomic sequence, equivalent to the N, P, 3, M, G and L genes that have been identified in other rhabdoviruses. The putative gene products had highest similarity to those of the nucleorhabdovirus Maize mosaic virus. A characteristic 3'-AAUUCUUUUUGGGUUGU/A-5' sequence was identified in each of the intergenic regions and the TaVCV leader and trailer sequences comprised 140 and 61 nt, respectively. Assignment of TaVCV to the genus Nucleorhabdovirus was supported by thin-section electron microscopy of TaVCV-infected taro leaves, which identified virions budding from nuclear membranes into the perinuclear space. Variability studies identified high levels of TaVCV sequence diversity. Within the L gene of 20 TaVCV isolates from Fiji, the Federated States of Micronesia, New Caledonia, Papua New Guinea, Solomon Islands and Vanuatu, maximum variability at the nucleotide level was 27.4 %. Within the N gene, maximum variability among 15 isolates at the nucleotide level was 19.3 %. The high level of TaVCV variability observed suggested that the introduction of TaVCV to the Pacific Islands was not a recent occurrence.

Incidence and distribution of viruses of Taro (Colocasia esculenta) in Pacific Island countries

Four viruses have been reported from taro; Dasheen mosaic virus (DsMV), Taro bacilliform virus (TaBV) and two putative rhabdoviruses, Colocasia bobone disease virus (CBDV) and Taro vein chlorosis virus (TaVCV). A fifth virus, tentatively named Taro reovirus (TaRV), has also been recently identified. The distribution of these viruses throughout the Pacific Islands, and the symptoms associated with their infection, are unknown in many cases due to a lack of sensitive diagnostic tests. We have used recently developed PCR-based diagnostic tests to survey taro growing in 11 Pacific Island countries for the presence of known viruses. DsMV and TaBV were widespread, whereas TaVCV and TaRV were more restricted in their distribution. CBDV was restricted to PNG and Solomon Islands and was always associated with the two most serious viral diseases of taro; alomae disease and bobone disease, but the causal agent of these two diseases remains unclear.

Comparison of the growth of in vitro produced pathogen‐tested Colocasia taro and field‐collected planting material

In Fiji, taro (Colocasia esculenta Schott.) varieties grown in vitro from shoot tips, and pathogen‐tested (PT) to meet guidelines for international transfer, were compared with field‐collected (FC) plants over 3 years. For three varieties, ‘Toakula’, ‘Samoa’ hybrid, and ‘Tausala ni Samoa’, there were no significant differences between heights and corm yields of PT and FC plants. For a fourth variety, ‘Samoa’, PT plants outgrew FC plants and yields were more than double. This variety, considered best adapted to hill slopes, grew well in alluvial coastal soils after in vitro culture. A major change appeared to be in response to Dasheen mosaic virus infection. By the third trial, an average 13.2 of 18 FC plants per plot showed virus symptoms compared to 2.6 in PT plots; and whereas FC plants were stunted (0.75 m) and low yielding (5.4 t/ha) PT plants were similar in height and yield to the other varieties (1.11 m and 11.8 t/ha). The results could not be explained by the elimination of a second virus, Taro vein chlorosis virus from PT plants.

Evidence for two rhabdoviruses in taro ( Colocasia escudenta ) in the Pacific region

Colocasia bobone disease virus, a rhabdovirus associated with Bobone and Alomae diseases oftaro in the Solomon Islands and Papua New Guinea, was compared with a rhabdovirus associated with veinal chlorosis of taro in several Pacific Islands. Differences in serological reaction, particle size and disease symptoms indicate the existence of two distinct strains or viruses. The name taro vein chlorosis virus is suggested for the second strain/virus.