Species: Cassava vein mosaic virus
UMR DGPC, Résistance des Plantes, Montpellier, France
ILTAB, Donald Danforth Plant Science Center, St. Louis, USA
Host Range and Symptomatology
Transmission by Vectors
Transmission through Seed
Transmission by Grafting
Transmission by Dodder
Nucleic Acid Hybridization
Stability in Sap
Properties of Particles
Properties of Infective Nucleic Acid
Relations with Cells and Tissues
Ecology and Control
The virus has been reported from many states in Brazil and is prevalent in the northeastern states. Only Manihot esculenta is known to be affected. The virus has isometric particles 50 nm in diameter and its genome is composed of DNA. Dense inclusion bodies can be observed in the cytoplasm of infected cells. The virus is transmitted by vegetative propagation.
Cassava vein mosaic disease was first reported in Manihot esculenta by Costa (1940). So far, cassava is the only known host for CsVMV.
Expression of symptoms is variable and symptoms may fade with age or not be expressed during certain times of the year. Cassava plants infected with CsVMV display a range of symptoms including a chlorosis following the veins; the chlorosis can coalesce to form a mosaic pattern (Fig.1 & Fig.2). Some leaves show a mosaic pattern over the entire leaf. Leaf distortion and epinasty are often observed on young leaves.
The two complete sequences of CsVMV (an unnamed isolate, accession number U20341, Calvert et al., 1995; an infectious clone, accession number U59751, de Kochko et al., 1998) are slightly different regarding the position of two open reading frames (ORF). They are however similar for the first three ORFs that contain essential genes for structure and replication of the virus.
The particularity of CsVMV is the arrangement of genes in the genome, which is distinct from that of caulimoviruses and badnaviruses. It was proposed that CsVMV be the type species of the genus Cavemovirus, a new genus of the plant pararetrovirus family Caulimoviridae (Hull et al., 2005). The genomic organization of accession number U59751 (Fig.4) was adopted by the International Committee on the Taxonomy of Viruses as being typical of the genus.
As in the genomes of other pararetroviruses, CsVMV contains a sequence that is complementary to a plant cytoplasmic tRNA. The numbering of the CsVMV genome starts at the 5'-end of this putative primer-binding site.
ORF 1 potentially encodes a protein of 1372 amino acids with a molecular mass of 186 kDa. A putative RNA-binding site domain (CXCX2CX4HX4C), found in all pararetroviruses capsid proteins, is found at the location aa 739-754. This motif is followed by a consensus sequence (aa 972-1105) similar to the intercellular transport proteins of the caulimoviruses. This arrangement of MP with respect to CP is unique.
ORF 2 overlaps ORF 1 by 14 nucleotides. The putative encoded protein of 71 amino acids, with a molecular mass of 8.8 kDa, has no homology with other proteins; its role is unknown.
The putative protein encoded by ORF 3 is 652 amino acids long with a molecular mass of 77 kDa. It contains sequences similar to protease, reverse transcriptase and RNase H consensus regions. The presence of such proteins makes CsVMV similar to other plant pararetroviruses.
Calvert et al. (1995) found two more ORFs. ORF 4 and ORF 5 overlap the preceding ORF. ORF 4 and ORF 5 encode putative proteins of 201 and 220 amino acids, with molecular masses of 24 and 26 kDa, respectively. There is no similarity with known proteins encoded by caulimoviruses or badnaviruses.
The sequence described by de Kochko et al. (1998), derived from the complete sequence of an infectious clone, differs from the one described by Calvert et al. (1995) by the addition of a single nucleotide. Two adenines were found at positions 6817-18, instead of only one at 6817. The consequences are that the previously described ORFs 4 and 5 become one ORF (ORF 4). This ORF 4 would encode a predicted protein of 46.3 kDa, with a low homology of sequence with the inclusion body proteins of the caulimoviruses. De Kochko et al. (1998) also mention a possible ORF 5 (nts 7973-8136), which would encode a protein of 6.3 kDa with no apparent sequence homology with any protein in the sequence data bases.
The large intergenic region (736 nts long) contains an eucaryotic consensus TATA box and the promoter of transcription. A promoter fragment was isolated from CsVMV, comprising nucleotides -443 to +72. It was shown to direct strong constitutive gene expression in transgenic plants (Verdaguer et al., 1996). The functional architecture of the CsVMV promoter fragment has been reported (Verdaguer et al., 1998). The promoter is made up of different regions that confer distinct tissue-specific expression of the gene. For example, the region encompassing nucleotides -222 to -173 contains cis elements that control promoter expression in green tissues and root tips. The region encompassing nucleotides -178 to -63 is responsible for expression in vascular elements.
Symptoms in a CsVMV infected cassava plant. Photograph courtesy of Pr E.W. Kitajima.
Infected cassava leaf showing mosaic pattern symptoms. Photograph courtesy of Pr E.W. Kitajima.
Electron micrograph of CsVMV particles. The bar represents 0.1 µm. Photograph courtesy of Pr E.W. Kitajima.
Predicted genomic organization of CsVMV according to the nucleotide sequence determined by de Kochko et al. (1998). The positions of the five open reading frames (ORFs) are indicated by arrows. Boxes represent conserved motifs of predicted proteins.