L DESLANDES¹, L GODIARD¹, L SAUVIAC¹, I ROGISTER¹, S CAMUT¹, E HOLUB², J BEYNON², M ARLAT¹ and Y MARCO¹

¹Laboratoire de Biologie Moleculaire des Relations Plantes/Micro-organismes CNRS-INRA, BP 27, 31326 Castanet-Tolosan, France; ²Department of Plant Genetics and Biotechnology, Horticulture Research International, Wellesbourne, Warwick, CV34 9EF, UK

Background and objectives
Bacterial wilt, one of the most important bacterial diseases worldwide, is caused by Ralstonia solanacearum, a soilborne vascular pathogen found mostly in tropical and subtropical areas. In tomato, resistance to the pathogen is polygenic and several loci governing resistance to bacterial wilt have been identified. The goal of this study was to test whether Arabidopsis thaliana could be used to analyse the genetic bases of resistance to this pathogen.

Results and conclusions
The interactions between Arabidopsis and various strains of R. solanacearum were examined. By root inoculation, a wild-type strain, GMI1000, wilted the ecotype Col-5 in an hrp-dependent process. After penetration in the host plant by the root system, the bacteria invaded via the xylem vessels and spread systemically throughout the plant. One Arabidopsis ecotype, Nd-1, was found to be resistant to most strains of R. solanacearum, including strain GMI1000. Recently, the wilt disease was also detected in western Europe. A strain isolated from tomato in France, 14.25, induced the development of the wilt disease on the Arabidopsis ecotype La-er, while ecotype Col-0 did not cause any symptoms after root inoculation.

Altogether, these data suggest the existence of several loci governing resistance to bacterial wilt in Arabidopsis. Genetic analysis of segregating populations generated from crosses between susceptible and resistant ecotypes indicated that a single recessive locus in Nd-1 confers resistance to strain GMI1000. F₈ recombinant inbred lines generated between the two ecotypes Col-5 and Nd-1 were used to map this disease resistance locus, RRS1, on chromosome V. In addition, resistance to strain 14.25 was shown to be conferred by a major locus, RRS2, present on chromosome II. The fine mapping of these loci will be presented.

Our data demonstrate that the interactions between Arabidopsis and R. solanacearum constitute a novel model system to study the genetic and molecular basis of plant-pathogen recognition.