Potato mop-top pomovirus (PMTV) is one of a few viruses that can move systemically in plants in the absence of the capsid protein (CP). Pomoviruses encode the triple gene block genetic module of movement proteins (TGB 1, 2, and 3) and recent research suggests that PMTV RNA is transported either as ribonucleoprotein (RNP) complexes containing TGB1 or encapsidated in virions containing TGB1. Furthermore, there are different requirements for local or systemic (long-distance) movement. Research suggests that nucleolar passage of TGB1 may be important for the long-distance movement of both RNP and virions. Moreover, and uniquely, the long-distance movement of the CP-encoding RNA requires expression of both major and minor CP subunits and is inhibited when only the major CP sub unit is expressed. This paper reviews pomovirus research and presents a current model for RNA movement.
For a better understanding of the functionality and pathogenicity of beet soil-borne virus (BSBV), full-length cDNA clones have been constructed for the three genomic RNAs. With the aim of assessing their effectiveness and relative contribution to the virus housekeeping functions, transcripts were inoculated on Chenopodium quinoa and Beta macrocarpa leaves using five genome combinations. Both RNAs-1 (putative replicase) and -3 (putative movement proteins) proved to be essential for virus replication in planta and symptom production on C. quinoa, whereas RNA-2 (putative coat protein, CP, and a read-through domain, RT) was not. No symptoms were recorded on B. macrocarpa, but viral RNAs were detected. In both host plants, the 19 kDa CP was detected by Western blotting as well as a 115 kDa protein corresponding to the CP-RT.Beet soil-borne virus (BSBV) is a pomovirus transmitted to Chenopodiaceae by the protist Polymyxa betae (Ivanović et al., 1983), which is also the vector of the aetiological agent of the rhizomania syndrome of sugar beet, beet necrotic yellow vein virus (BNYVV) (Tamada & Baba, 1973). Originally reported in Italy (Canova, 1959), rhizomania disease is now widespread in most countries where sugar beet is grown (McGrann et al., 2009) and BSBV is often found in beet infected with BNYVV (Meunier et al., 2003). However, the pathogenicity of BSBV and its contribution to the rhizomania syndrome remain unclear, with opinions still divided on this (Prillwitz & Schlösser, 1992;Kaufmann et al., 1993;Lindsten, 1993;Rush & Heidel, 1995).The BSBV genome consists of three single-stranded RNAs of positive polarity, packaged into rod-shaped particles (Koenig et al., 1996(Koenig et al., , 1997Koenig & Loss, 1997). RNA-1 (5.8 kb) encodes the putative viral replicase. The 19 kDa coat protein (CP) and a putative 85 kDa read-through (RT) domain are encoded by RNA-2 (3.5 kb). RNA-3 (3.0 kb) comprises three open reading frames (ORFs) encoding three putative proteins (48, 13 and 22 kDa) thought to be responsible for the viral cell-to-cell movement, resembling the well-known triple gene block proteins (TGBs) (Fig. 1).In this study, the contribution of each RNA component to virus survival and symptom expression was investigated through the use of full-length cDNA clones on two host plants, Chenopodium quinoa and Beta macrocarpa. This last plant species was preferred to the natural host Beta vulgaris with a view of developing the basis for further molecular analysis of viral systemicity, following the example of previous studies on BNYVV (Lauber et al., 1998).Full-length cDNA sequences were generated from an Iranian BSBV isolate (Nyshabour, Khorasan Razavi Province), which was trapped from infested soil in roots of B. vulgaris. After total RNA extraction from sugar beet roots using the SV total RNA isolation kit (Promega), the three genomic RNAs were reverse transcribed and amplified by PCR (RT-PCR) using expand reverse transcriptase and the expand long template PCR System (Roche). Primers matching the extremities of the t...
Ten beet virus Q (BVQ) strains from six different countries were sequenced to characterize the readthrough (RT) domain of the coat protein (CP). The GenBank/EMBL/DDBJ accession numbers for the sequences reported in this paper are FM244643-FM244652. With three nucleotide additions of 5, 285 and 1 nt, the common RT of 76 kDa was found to be longer than the single reference available to date (35 kDa). It is hypothesized that multiple inoculation cycles on Chenopodium quinoa were responsible for these three deletions in the C-terminal part of the BVQ RNA-2 previously described. Two putative transmembrane domains, TM1 and TM2, were predicted in the consensus amino acid sequence of the ten BVQ strains, and the putative BVQ TM2 was aligned with that of potato mop-top virus.
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