Tobacco mosaic virus replication in resistant and susceptible plants: in some resistant species virus is confined to a small number of initially infected cells.

Only small amounts of tobacco mosaic virus (TMV) are recoverable from directly inoculated leaves of some plant species, a phenomenon investigated by P. C. Cheo (1970, Phytopathology 60, 41-46) and termed subliminal infection. To interpret this phenomenon in two varieties of cowpea (Vigna sinensis Emil.), primary leaves were inoculated on their lower surfaces with TMV (common strain), and at various times postinoculation, mesophyll protoplasts were isolated, incubated for 36 hr, and stained with a TMV-specific fluorescent-labeled antibody. It was determined that only 1 in 50,000 to 150,000 protoplasts contained TMV antigen; this number remained essentially unchanged for experimental periods of from immediately after inoculation to up to 11 days postinoculation (the longest period examined). Cytological staining of epidermis from another subliminally infected host, cotton, also revealed infection of only a few cells. These data suggest that leaves of subliminally infected plants support TMV replication in those cells which receive virus during mechanical inoculation, but that the infectious principle is unable to move from those original centers in these hosts. Control experiments with tobacco (Nicotiana tabacum L. cv. Turkish Samsun), in which virus spreads extensively in the inoculated leaves, suggest that a rapid cell-to-cell movement of the infectious entity begins after about 6 hr following inoculation. An unexpected observation was that some cowpea and tobacco mesophyll cells become infected immediately upon mechanical inoculation, suggesting that mesophyll cells can be primary sites of viral ingress into the leaf.