PAG-V: S10 - WHAT CAN TOMATO <i>Cf</i> GENES TELL US ABOUT EVOLUTION OF NEW DISEASE RESISTANCE GENES?

Plant & Animal Genome V Conference

Town & Country Hotel, San Diego, CA, January 12-16, 1997.

S10

WHAT CAN TOMATO Cf GENES TELL US ABOUT EVOLUTION OF NEW DISEASE RESISTANCE GENES?

DIXON, MARK S, Martin Parniske, Kostas Hatzixanthis, David A Jones, Colwyn M Thomas, Kim Hammond-Kosack, Catherine Golstein, Kate Harrison, Jonathan D G Jones
The Sainsbury Laboratory, John Innes Institute, Norwich, NR4 7UH, UK

Towards understanding how plants protect themselves against pathogens, several tomato Cf resistance genes, which confer resistance to distinct races of the fungal pathogen Cladosporium fulvum, were cloned (Cf-9, Cf-2, Cf-4 and Cf-5). The sequence of each predicts that they are structurally very similar, in that, each is largely extracellular and carries many leucine-rich repeats (LRRs). These genes were originally introgressed from wild species of tomato and map to two distinct loci. Cf-4 and Cf-9 map to the short arm of chromosome 1 and Cf-2 and Cf-5 map near to the centromere of chromosome 6. Many homologues of Cf resistance genes (Hcr) have been cloned from the corresponding introgression lines and their sequences compared. Homologues of Cf-9 map in clusters on the short arm of chromosome 1, one cluster carrying Cf-9 and 4 Hcrs. An allelic cluster carries Cf-4 and 4 Hcrs. Comparison of both clusters shows that intergenic regions are non syntenic, thus homologous recombination between clusters results in new loci of variable Hcr number. On chromosome 6, allelic clusters from different species contain different numbers of Hcrs of Cf-2. Additionally, each Hcr has a different number of LRRs within the region hypothesised to be involved in recognition of avirulence determinants. These data reveal the complex nature of Cf loci and indicate ways in which novel resistance genes may evolve. Supportive evidence from analysis of 5 recombinants between the Cf-4 and Cf-9 loci and 1 recombinant between the Cf-2 and Cf-5 loci show the type of events which can occur. Evidence that Hcrs of Cf-9 play an active role in disease resistance will be presented.