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 after a lag of 8 hr postinoculation. In work reported elsewhere, we have shown that the Rx-breaking property of PVXHB was associated with the coat protein gene of PVXuK3 and PVXcp4. Here, we describe how a frameshift mutation in the coat protein gene had no effect on Rx resistance breaking but compromised the Rx-mediated resistance to PVXcp4. We also describe how in coinoculation experiments, the Rx-mediated resistance could be induced to affect PVXHB or cucumber mosaic virus (CMV). In these experiments, PVXnB or CMV was coinoculated to protoplasts (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 and that the induced resistance mechanism is effective against viruses unrelated to PVX
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