In cowpea plant cells infected with cowpea mosaic virus, tubular structures containing virus particles are formed in the plasmodesmata between adjacent cells; these structures are supposedly involved in cell-to-cell spread of the virus. Here we show that similar tubular structures are also formed in cowpea protoplasts, from which the cell wall and plasmodesmata are absent. Between 12 and 21 h post-inoculation, tubule formation starts in the periphery of the protoplast at the level of the plasma membrane. Upon assembly, the virus-containing tubule is enveloped by the plasma membrane and extends into the culture medium. This suggests that the tubule has functional polarity and makes it likely that a tubule ‘grows’ into a neighbouring cell . On average, 75% of infected protoplasts were shown to possess tubular structures extending from their surface. The tubule wall was 3 to 4 nm thick and they were up to 20 µm in length, as shown by fluorescent light microscopy and negative staining electron microscopy. By analogy to infected plant cells, both the viral 58K/48K movement and capsid proteins were located in these tubules, as determined by immunofluorescent staining and immunogold labelling using specific antisera against these proteins. These results demonstrate that the formation of tubules is not necessarily dependent on the presence of plasmodesmata or the cell wall, and that they are composed, at least in part, of virus-encoded components.


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