1.12.5S
THE Rx LOCUS IN POTATO CONTROL DISEASE RESISTANCE AND CELL DEATH

A BENDAHMANE, K KANYUKA and D BAULCOMBE

The Sainsbury Laboratory, Colney, Norwich NR4 7UH, UK

Background and objectives
The Rx locus in potato confers extreme resistance to potato virus X (PVX) through a mechanism which, like that controlled by many disease-resistance genes, can be described in terms of an elicitor/receptor model involving a two-stage process: the recognition and the response. The recognition phase is highly strain-specific and requires the elicitor (PVX coat protein) and the resistance gene Rx [1]. The response stage includes mechanisms that suppress viruses unrelated to PVX. However, there are several features of the Rx-mediated mechanism that are different from the virus resistance conferred by other well characterized virus resistance genes, including the N gene in tobacco which confers resistance to TMV. The Rx response is effective in the initially infected cell and is not associated with death of the infected cells. In contrast, the resistance response of N and many other virus resistance genes is not active until the virus has spread into several cells and is associated with the formation of necrotic lesions around the site of inoculation.

In previous work we showed that Rx-mediated resistance is elicited by the PVX coat protein, independently of any other proteins encoded by PVX [2]. Here we describe the isolation of Rx and the comparative analysis between extreme resistance and HR-mediated resistance.

Results and conclusions
A high-resolution physical map of the Rx locus has been produced and a BAC clone carrying the Rx locus was identified. The sub-cloning of Rx was carried out using a complementation assay based on a biolistic transient expression system and agro-infiltration transient assay. Stable transformation was used to confirm the complementation.

From the stable transformation we concluded that Rx is functional in the heterologous species Nicotiana tabacum and Nicotiana benthamiana . Sequence analysis of Rx locus and comparative analysis between extreme resistance and HR-mediated resistance will be discussed. Experiments that address how Rx-mediated resistance to disease resistance without cell death will be also presented. We also developed a screening system for genes required for Rx-mediated resistance. The output of this screen will be presented.

References
1. Bendahmane A, Kanyuka K, Baulcombe D, 1997. Theoretical and Applied Genetics 94, 153-162.
2. Bendahmane A, Kohm B, Dedi C, Baulcombe D, 1995. Plant Journal 8, 933-941.