Expression of resistance to potato virus X in isolated potato protoplasts.

Resistance to Potato Virus X (PVX) in potato cultivars is conferred by the genes Nx and/or Nb. The Nx gene is activated by group 3 strains of PVX. When protoplasts from the cultivars King Edward containing the Nx gene and Pentland Crown containing neither resistance gene were inoculated with a group 3 strain of PVX resistance was expressed in the King Edward protoplasts. Replication of PVX occurred in King Edward protoplasts but only at a very low level as compared with the level observed in the susceptible Pentland Crown protoplasts. Similar results were obtained when protoplasts obtained from leaf discs mechanically inoculated with PVX were used. Virus levels were assayed by immunofluorescence and ELISA. Maintenance of resistance in protoplasts of King Edward following inoculation with PVX RNA was confirmed suggesting that resistance probably operates at the transcription stage in replication. These results clearly demonstrate single gene resistance in potato protoplasts inoculated with PVX.