Ultrastructural and temporal observations of the potyvirus cylindrical inclusions (Cls) show that the Cl protein acts transiently in aiding virus movement.

A systematic ultrastructural study across the edge of an advancing infection in pea seed-borne mosaic potyvirus-infected pea cotyledons showed the cylindrical inclusion (CI) protein to exist in transient functional states. Initially, the characteristic CI pinwheel inclusion bodies were positioned centrally over the plasmodesmal apertures (including those of plasmodesmata connected to the previously infected cell), in agreement with a proposed role in virus movement (Carrington et al., 1998, Plant J., 13, in press). The viral coat protein was associated with these structures and was seen within the modified plasmodesma, most notably in a continuous channel that passed along the axis of the pinwheel and through the plasmodesma. The CI protein was not detected within the plasmodesmal cavities. Later in the infection (i.e., behind the zone of active virus replication) the CI was no longer associated with cell walls, or with coat protein, and showed signs of structural degeneration. In contrast, the coat protein remained within plasmodesmal cavities. The role of the CI in assisting virus movement is not known but the presence of the CI was linked with an apparent transient reduction in callose in the vicinity of the plasmodesmata.