A POTATO VIRUS-X RESISTANCE GENE MEDIATES AN INDUCED, NONSPECIFIC RESISTANCE IN PROTOPLASTS

The Rx locus in potato controls extreme resistance to most isolates of potato virus X (PVX). The resistance is expressed in whole plants and in protoplasts. Rx-mediated resistance in protoplasts causes reduced accumulation of all PVX RNA species, including the (-) strand RNA afte r a lag of 8 hr postinoculation. In work reported elsewhere, we have s hown that the Rx-breaking property of PVX(HB) was associated with the coat protein gene of PVX(UK3) and PVX(CP4). Here, we describe how a fr ameshift mutation in the coat protein gene had no effect on Rx resista nce breaking but compromised the Rx-mediated resistance to PVX(CP4). W e also describe how in coinoculation experiments, the Rx-mediated resi stance could be induced to affect PVX(HB) or cucumber mosaic virus (CM V). in these experiments, PVX(HB) or CMV was coinoculated to protoplas ts (Rx genotype) together with an isolate of PVX, which is affected by Rx. We interpret this data to indicate that Rx-mediated resistance is induced when the PVX coat protein is produced in the infected cells a nd that the induced resistance mechanism is effective against viruses unrelated to PVX.