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...