Tubules containing virions are present in plant tissues infected with Commelina yellow mottle badnavirus.

Abstract

In establishing a systemic infection, plant viruses must overcome the barrier to cell-to-cell movement that the plant cell wall presents. Two mechanisms of cell-to-cell movement that take advantage of plasmodesmatal connections between cells have been recognized (for reviews see Maule, 1991 ;Lucas & Gilbertson, 1994). One mechanism involves a specific virus movement protein that increases the gating limit of plasmodesmata and is thought to participate in the formation of an RNA-movement protein-nucleoprotein complex that is capable of passing through the modified plasmodesmata. The second mechanism involves the formation of virus-conducting tubular structures that extend between the cytoplasms of adjacent cells. These tubules are composed, at least partly, of virus-encoded movement proteins and their formation is not dependent on the presence of the cell wall, plasmodesmata or other virus gene products (Perbal et al., 1993 ; Kasteel et al., 1996Kasteel et al., , 1997.Badnaviruses are plant pararetroviruses that encapsidate 7n5-8 kb circular double-stranded DNA in non-enveloped bacilliform virions. Commelina yellow mottle badnavirus (CoYMV) is the type member for the genus (Lockhart & Olszewski, 1994). Generally, badnaviruses encode three proteins. The largest of these proteins is a polyprotein that yields several proteins including the virus coat, an aspartic protease involved in processing the polyprotein, and enzymes with reverse transcriptase and ribonuclease H activities that are involved in virus replication. Although little is known about the mechanism of badnavirus cell-to-cell movement, there is limited sequence similarity between the N terminus of the polyprotein and known movement proteins (Bouhida et al., 1993), and mutations affecting this region block cell-to-cell spread of the virus but do not affect its replication (Tzafrir et al., 1997), suggesting that this portion of the polyprotein gives rise to the CoYMV movement protein. Here we report the observation of CoYMV-containing tubules both in fractionated cell-free extracts of infected leaves and in situ. In addition, immunogold labelling demonstrates that the tubules are composed, at least partly, of CoYMV movement protein. This report describes a likely mechanism of cell-to-cell movement of badnaviruses, and provides additional evidence that this mechanism, which has been reported primarily for viruses with isometric particles, may also operate in the case of viruses with elongated particles (Kasteel et al., 1997).Cell-free extracts of CoYMV-infected Commelina diffusa leaf tissues were prepared in phosphate buffer (200 mM phosphate buffer pH 6n0, 4 % PEG 8000, 0n5 % 2-mercaptoethanol, 0n5 % sodium sulfite, 10 mM EDTA). The extracts were layered over a 5 ml cushion of 30 % sucrose in 20 mM phosphate buffer pH 6n0 containing 250 mM NaCl (PN buffer) and the virions and tubules were pelleted by centrifugation in a Beckman 50.2 Ti rotor at 35 000 r.p.m. for 70 min. The resulting pellet was resuspended in 0n5-1 ml PN buffer and layered onto a prefor...

The spread of Tobacco mosaic virus infection: insights into the cellular mechanism of RNA transport

“…For instance, cell-to-cell transport of several ssRNA viruses (i. e. como-, nepo-, olea-and trichoviruses; [106, 108 -111] and dsDNA plant viruses (i. e. caulimoviruses; [112,113]) involves 'tubule-guided' transport of mature virions through Pd that are structurally modified by a tubule made of virus-encoded MP [114,115]. In contrast, TMV and other RNA viruses are believed to move from cell to cell in the form of a ribonucleoprotein complex (vRNP).…”

Identification of Inhibitory Mutants of Cauliflower mosaic virus Movement Protein Function after Expression in Insect Cells

“…The MPs of several virus genera and families contribute to this process by forming tubular structures that traverse plasmodesmata to provide a channel through which virus nucleocapsids can pass from cell to cell. These include MPs from the Caulimoviridae (11,12,16,18), Badnaviridae (2), Comoviridae (9,25,26), Tospoviridae (21), and Nepoviridae (6,19,20,28,29), among other families. Despite the significance of these MPs for virus infection and pathogenesis, very little is understood about how the tubules are formed and how virus particles translocate through them.…”

Citrus Psorosis Virus Movement Protein Contains an Aspartic Protease Required for Autocleavage and the Formation of Tubule-Like Structures at Plasmodesmata

“…Ophiovirus particles have been described as open circular CP-decorated flexuous filaments with a thickness of 3 to 4 nm (55). Since tubule-forming viruses usually have an icosahedral particle morphology, such as CPMV (15,37), CaMV (11,27,32,33), GFLV (27,(34)(35)(36), alfalfa mosaic virus, brome mosaic virus (39), and tomato spotted wild virus (13), the use of a tubule-guided mechanism by CPsV may be unexpected. Tubule-guided movement by icosahedral viruses depends on CP-MP interactions that may allow the viral particles to be guided along the inner tubule wall (56)(57)(58)(59).…”

“…PDLPs require the ER-Golgi pathway for their targeting to PD (27,29), which may explain the observed sensitivity of tubule formation and tubule-mediated virus movement to secretory pathway inhibitors (27,30,31). DNA and RNA viruses that move from cell to cell by a tubule-guided mechanism have been found in the Caulimoviridae (11,27,32,33), Secoviridae (15,27,(34)(35)(36)(37), Bunyaviridae (13,38), and Bromoviridae (39) families.…”