R. A. C. Jones

Expression of Potato Virus X Resistance Gene Rx in Potato Leaf Protoplasts

SummarynProtoplasts derived from shoot cultures of potato cv. Cara, which carries immunity gene Rx, supported only limited virus multiplication after inoculation with particles or RNA of isolate DX, a group 3 strain of potato virus X, as compared to similarly inoculated protoplasts of cultivars King Edward and Pentland Ivory which lack Rx. The Cara protoplasts were, however, able to support extensive replication of the resistance-breaking strain HB. This strain-specific resistance did not appear to be mediated by a failure or inhibition of the uncoating mechanism or of virus assembly.

Infection of Protoplasts Derived from Potato Shoot Cultures with Potato Virus X

Summary Because it contains the Nx gene, potato cv. ‘Maris Piper’ reacts hypersensitively to inoculation with group three strains of potato virus X (PVX). However, protoplasts prepared from aseptically grown shoot cultures of potato cv. ‘Maris Piper’ were infected reproducibly with either PVX or PVX RNA using inocula containing polyethylene glycol. The extent of infection of protoplasts was assessed by fluorescent antibody staining, and by assaying extracts for antigen by ELISA, for infectivity and for PVX RNA content by cDNA hybridization. Infection occurred in 43 to 65% of the protoplasts infected with PVX and in 57 to 70% of the protoplasts infected with PVX RNA. In comparison, when high quality protoplasts were obtained from leaves of pot-grown plants 55 to 65% became infected when inoculated with PVX RNA. However, protoplasts obtained from leaves of whole plants varied greatly in both quantity and quality, whereas shoot cultures reproducibly gave high quality protoplast preparations in large numbers. In protoplasts from either source, yields of progeny virus were between 10 and 30 pg per infected protoplast. The multiplication of a group 3 strain of PVX in protoplasts containing the Nx gene did not induce necrosis and confirms that hypersensitivity is not always expressed in isolated protoplasts.

A Comparison between the Capsid Proteins and the Products of in vitro Translation of Three Strains of Potato Virus X

SummarynThe capsid proteins of three potato virus X isolates that belong to different strain groups on the basis of their reactions with host resistance genes have been compared by means of SDS-PAGE and partial proteolysis mapping. Considerable differences were observed in both the migration rates in SDS-PAGE and the peptide patterns generated after partial digestion with V8 protease of the capsid proteins of the isolates. The high molecular weight proteins synthesized by in vitro translation of genomic RNAs extracted from the three isolates were electrophoretically indistinguishable and antigenically unrelated to capsid proteins.