Two cases of non-host resistance against potyviruses operating at the single cell level

Nonhost resistance of plants against viruses is a very common but poorly understood phenomenon. In this study Nicotiana tabacum L. and pea seed-borne mosaic potyvirus (PSbMV) were chosen as a model system for a closer examination of nonhost resistance. Nicotiana tabacum was confirmed to be a nonhost for PSbMV by mechanical inoculations followed by analysis by enzyme-linked immunosorbent assay and by immunofluorescence staining of mesophyll and epidermal protoplasts isolated from inoculated leaves. When tobacco protoplasts were inoculated with PSbMV virions or genomic RNA, only very few cells became infected. On the other hand, both PSbMV virions and genomic RNA could infect protoplasts from the host plant Pisum sativum L. In parallel experiments, virions of another potyvirus, potato virus Y (PVY), infected tobacco protoplasts but not protoplasts from pea, a nonhost for PVY. The results suggest, that the inability of PSbMV to infect tobacco, and of PVY to infect pea, is at least partly caused by a failure of virus replication in the inoculated cells.