Potato Gene Xi Confers Inoculum Dependent Resistance to Potato Virus X Replication in Protoplasts

Protoplasts were obtained from genetically related Argentinepotato (Solanum tuberosum tuberosum) cultivars: Serrana INTAand Huinkul MAG and from an European cultivar: Spunta. SerranaINTA is resistant to potato virus X (PVX) infection becauseit carries the major reaction gene Xⁱ (believed to be the sameas Rx_ac1) while Huinkul MAG and Spunta are susceptible. Protoplastswere inoculated with a purified preparation of PVXcp (a SouthAmerican isolate) and then assayed for PVX concentration atdifferent times postinfection by enzyme-linked immunosorbentassay (ELISA), immunofluorescence and nucleic acid hybridization.PVX multiplication rates in Serrana INTA were about fifteentimes slower than in susceptible genotypes when cell-bound viruslevels just after infection were in the range of 0.01 to 0.1ng PVX per viable protoplast. However, when inoculum levelswere raised to 1 ng PVX per viable protoplast, PVX multiplicationwas about the same in all three genotypes. To rule out geneticbackground effects in this behaviour, protoplasts of an ArgentineS. acaule clone (PI: 320277) likely carrying the same resistancegene, were infected with PVX in similar conditions, reproducingthose results obtained with Serrana INTA. The comparison ofPVX replication in protoplasts and whole plants indicate thatalthough Xⁱ gene confers resistance at the cell level it necessitatesof tissue structure to fully express immunity. (Received January 16, 1990; Accepted April 23, 1990)