Potato gene X super(i) confers inoculum dependent resistance to potato virus X replication in protoplasts

Protoplasts were obtained from genetically related Argentine potato (Solanum tuberosum tuberosum ) cultivars: Serrana INTA and Huinkul MAG and from an European cultivar: Spunta. Serrana INTA is resistant to potato virus X (PVX) infection because it carries the major reaction gene X super(i) (believed to the same as Rx sub(ac1)) while Huinkul MAG and Spunta are susceptible. Protoplasts were inoculated with a purified preparation of PVXcp (a South American isolate) and then assayed for PVX concentration at different times postinfection by enzyme-linked immunosorbent assay (ELISA), Immunofluorescence and nucleic acid hybridization. PVX multiplication rates in Serrana INTA were about fifteen time slower than in susceptible genotypes when cell-bound virus levels just after infection were in the range of 0.01 to 0.1 ng PVX per viable protoplast.