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Two of the four genera of plant rhabdoviruses (Cytorhabdovirus and Nucleorhabdovirus) were established based on the sites of member virus replication and morphogenesis. Cytorhabdoviruses replicate and mature in the cytoplasm of infected cells. They form a distinct monophyletic group based on well-supported Maximum Likelihood trees inferred from complete L sequences. The genus classification based on monophyly has thus far correlated with classification by intracytoplasmic virus maturation.
Enveloped virions are bacilliform, 60–75 nm in diameter and 200–350 nm long (Jackson et al., 2005, Dietzgen 2002).
The buoyant density in sucrose or potassium tartrate is 1.19–1.20 g cm−3 (Dietzgen 2002). The lipid envelope is derived from the cytoplasmic membranes of plant or insect host cells (Jackson et al., 2005).
Negative-sense, single-stranded RNA genomes are unsegmented and 12.8–14.5 kb in length. Six to ten mRNAs, one for each of the encoded proteins, have been identified in infected plants.
N, P, M, G and L represent the five canonical rhabdovirus structural proteins. P of lettuce necrotic yellows virus (LNYV) and alfalfa dwarf virus (ADV) have RNA silencing suppressor activity (Mann et al., 2015, Mann et al., 2016, Bejerman et al., 2016). The P3 proteins of LNYV and ADV localize to plasmodesmata and trans-complement movement of heterologous viruses between plant cells (Mann et al., 2016). Colocasia bobone disease-associated virus (CBDaV), ADV, strawberry crinkle virus (SCV), northern cereal mosaic virus (NCMV) and barley yellow striate mosaic virus (BYSMV) encode small proteins that have the structural characteristics of class 1a viroporins, similar to those detected commonly in mammalian rhabdoviruses. The functions of other accessory proteins are unknown.
The lipoprotein envelope is derived from the host plant or the insect vector (Jackson et al., 2005). Lipid composition is unknown.
LNYV G is glycosylated with a complex network of oligosaccharides containing N-acetylchitobiose, N-linked to asparagine residues. The carbohydrate binds to concanavalin A and can be removed by endoglycosidase F (Dietzgen and Francki 1988).
The LNYV genome organisation (12.8 kb) is similar to that of sonchus yellow net virus (SYNV, see genus Nucleorhabdovirus). Preceded by a non-coding 84 nt leader sequence, the gene order is 3′ N-P-4b-M-G-L 5′ (Figure 1.Cytorhabdovirus). The N gene encodes the nucleoprotein (or nucleocapsid protein), and the P, M, G and L genes encode the phosphoprotein (polymerase cofactor), matrix protein, glycoprotein and RNA polymerase, respectively. The 4b gene encodes a cell-to-cell movement protein (P3). The intergenic regions contain highly conserved consensus sequences. The 5′ non-coding trailer sequence of 187 nt has extensive complementarity to the 3′ leader. The NCMV genome is 13.2 kb with a gene order similar to that of LNYV, except for the presence of four additional small genes of unknown function between the P and M genes, and an additional gene between the G and L genes that encodes a class 1a viroporin-like protein. The genome of BYSMV (12.7 kb) is similar to that of NCMV, except that one of the additional small ORF (P5) located in an alternative frame within the P4 gene and expressed by a leaky scanning mechanism. The genome organisations of lettuce yellow mottle virus (LYMoV) (12.9 kb), CBDaV (12.2 kb), ADV (14.5 kb) and SCV (14.5 kb) are similar to that of LNYV, except for the presence in SCV and ADV of one additional gene between the G and L genes encoding a class 1a viroporin-like protein. Alternative ORFs within the P gene also occur in several cytorhabdoviruses but it is not known if they are expressed.
Cytorhabdoviruses replicate in the cytoplasm of infected cells in association with masses of thread-like structures (viroplasms) (Jackson et al., 2005). Virions bud in association with the endoplasmic reticulum (ER) and accumulate in ER-derived vesicles (Wolanski and Chambers 1971, Chambers et al., 1965). A nuclear phase has been suggested but not proven in the replication of some cytorhabdoviruses (e.g., LNYV) (Wolanski and Chambers 1971). Evidence of the nuclear involvement in the replication of others is lacking (e.g., BYSMV). Endogenous transcriptase activity is readily detectable in cytorhabdovirus preparations (Francki and Randles 1972).
Figure 1.Cytorhabdovirus. Schematic representation of cytorhabdovirus genome organisations. N, P, M, G and L represent ORFs encoding the structural proteins. ORFs encoding viral cell-to-cell movement proteins (blue) and predicted class 1a viroporin-like proteins (yellow) are shown. Other ORFs encode putative accessory proteins of unknown function, some of which occur as homologous sets (purple, green orange).
A wide variety of monocot and dicot plant species are susceptible to cytorhabdoviruses, although each virus usually has a restricted host range. Cytorhabdoviruses are transmitted by aphids (LNYV, ADV), planthoppers (NCMV, BYSMV) or leafhoppers (rice stripe mosaic virus (RSMV), related, unclassified)). Some viruses are also transmitted during vegetative propagation, and some can also be transmitted mechanically from infected sap. Seed transmission has not been reported. In all carefully examined cases, viruses replicate in cells of the insect vector as well as in the plant host (Jackson et al., 2005, Redinbaugh and Hogenhout 2005).
Viruses assigned to different species within the genus Cytorhabdovirus have several of the following characteristics: A) minimum nucleotide sequence divergence >50 % in cognate genes; B) occupy different ecological niches as evidenced by differences in hosts and/or arthropod vectors; and C) can be clearly distinguished in serological tests or by nucleic acid hybridisation. Cytorhabdovirus species assignments are primarily determined by plant host range and vector specificity. Nucleic acid hybridisation and RT-PCR have been used to provide confirmation of species assignments and serological criteria have enabled verification of common viruses that infect different hosts. However, no virus strains have been defined unambiguously using serology. The complete genome sequence is available for seven of 11 viruses currently assigned to the genus (LNYV, LYMoV, NCMV, BYSMV, ADV, CBDaV and SCV) and for 12 unclassified cytorhabdovirus-like viruses (see below). Hybridisation using cloned probes and conserved L gene polymerase motif sequences has been used to differentiate viruses within the genus and to identify some strains.
The genus name is derived from the cytoplasmic localisation of virus replication complexes (Cyto: from Greek kytos, “cell”).
Iranian citrus ringspot-associated virus
ivy vein banding virus
maize yellow striate virus
persimmon virus A
raspberry vein chlorosis virus
rice stripe mosaic virus
soybean blotchy mosaic virus
tomato yellow mottle-associated virus
Wuhan insect virus 4**
Wuhan insect virus 5**
Wuhan insect virus 6**
yerba mate chlorosis-associated virus
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