Agrobacterium transient expression system as a tool for the isolation of disease resistance genes: application to the Rx2 locus in potato

Rx2 confers resistance against potato virus X (PVX). To clone Rx2, we devel oped a system based on Agrobacterium-mediated transient expression of candi date R genes in transgenic tobacco leaves expressing the PVX coat protein e licitor of Rx2-mediated resistance. Using this system, a potato gene elicit ing HR specifically in the presence of the elicitor was identified. Based o n genetical and functional analysis, it is concluded that the cloned gene i s Rx2. The transient expression system is potentially adaptable to cloning of any other resistance gene. The Rx2 locus is on chromosome V of potato an d the encoded protein is highly similar to the products of Rx1 and Rxh1 enc oded on potato chromosome XII. Rxh1 has been shown elsewhere to encode a po tato cyst nematode resistance gene Gpa2. All three proteins are in the leuc ine zipper-nucleotide binding site-leucine rich repeat class of resistance gene products. Rx1 and Rx2 are functionally identical and are almost identi cal in the C terminal region consistent with a role of the leucine rich rep eats in recognition of the PVX coat protein. In the N terminal, half there are some regions where the Rx1 and Rx2 proteins are more similar to each ot her than to the Rxh1 protein. However, in other regions these proteins are more similar to Rxh1 than to each other. Based on this mosaic pattern of se quence similarity, we conclude that sequence exchange occurs repeatedly bet ween genetically unlinked disease resistance genes through a process of gen e conversion.