Potato Gene X<sup>i</sup> Confers Inoculum Dependent Resistance to Potato Virus X Replication in Protoplasts

Abstract

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 Xi (believed to be the same as Rxac1) 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 times 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. However, when inoculum levels were raised to 1 ng PVX per viable protoplast, PVX multiplication was about the same in all three genotypes. To rule out genetic background effects in this behaviour, protoplasts of an Argentine S. acaule clone (PI: 320277) likely carrying the same resistance gene, were infected with PVX in similar conditions, reproducing those results obtained with Serrana INTA. The comparison of PVX replication in protoplasts and whole plants indicate that although Xi gene confers resistance at the cell level it necessitates of tissue structure to fully express immunity.