Evidence for concurrent spread of tobacco mosaic virus from infected epidermal cells to neighboring epidermal and mesophyll cells

Abstract

The upper surfaces of tobacco (Nicotiana tabacum L.) leaves were mechanically inoculated with tobacco mosaic virus (TMV) and, at various times thereafter, protoplasts from upper epidermal and pallisade mesophyll cells were isolated. The percentage of each type of protoplast infected with TMV was then determined by fluorescent antibody analysis. The results provided an estimation of the numbers of infections initially established and of the direction and rate of spread of virus to neighboring leaf cells. From 0.4 to 2% of the epidermal protoplasts, but none of the mesophyll protoplasts, isolated during the first 5–6 h after inoculation of leaves were infected. With leaves sampled during the next 1–2 h, the percentage of infected epidermal protoplasts increased; at the same time, the percentage of infected mesophyll protoplasts increased from zero to values approximating the initial percentage of infected epidermal protoplasts. This suggests that only epidermal cells were initially infected and that virus later spread from these cells to both epidermal and mesophyll cells in the inoculated leaves.