Dye-coupling in Tobacco Mesophyll Cells Surrounding Growing Tobacco Mosaic Tobamovirus-induced Local Lesions

Some factors related to cessation of the movement of tobacco mosaic tobamovirus (TMV) in the late phase of a defence response were examined. Mesophyll cells surrounding the local lesions induced by TMV in N. tabacum cv. Xanthi‐nc were micro‐injected with fluorescent dye 2, 3 and 7 days post‐inoculation. At 7 days post‐inoculation, twelve out of 20 injections into cells adjacent to the lesion, showed the expected dye‐coupling (outflow of fluorescent dye from injected cell to adjacent ones via plasmodesmata) whereas 17–20 out of 20 injections were successful in other cases. Callose inhibitor (tunicamycin), dark treatment and incubation of plants with ascorbic acid, which play a role in blocking plasmodesmata or induction of defence responses, did not seem to have an effect on lesion growth. These data imply that defects in the plasmodesmal function, although not total, may account for the formation of late local defence reaction together with other factors that restrict viral spread in the continuous cell‐to‐cell mode in mesophyll tissue.