THE COAT PROTEIN OF POTATO-VIRUS-X IS A STRAIN-SPECIFIC ELICITOR OF RX1-MEDIATED VIRUS-RESISTANCE IN POTATO

The Rx1 gene in potato confers extreme resistance to potato virus X (P VX). To investigate the mechanism and elicitation of Rx resistance, pr otoplasts of potato cv. Cara (Rx1 genotype) and Maris Bard (nn genotyp e) were inoculated with PVX and tobacco mosaic virus (TMV). At 24 h po st inoculation in Maris Bard protoplasts there was at least 100-fold m ore PVX RNA than in protoplasts of Cara. TMV RNA accumulated to the sa me level in both types of protoplast. However, when the TMV was inocul ated together with PVX the accumulation of TMV RNA was suppressed in t he Cara (Rx1 genotype) protoplasts to the same extent as PVX. The Rx1 resistance also suppressed accumulation of a recombinant TMV in which the coat protein gene was replaced with the coat protein gene of PVX. It is therefore concluded that Rx1-mediated resistance is elicited by the PVX coat protein, independently of any other proteins encoded by P VX. The domain of the coat protein with elicitor activity was localize d by deletion and mutation analysis to the structural core of a non-vi rion form of the coat protein.