XVI International Botanical Congess

Disease resistance (R) genes confer recognition of pathogen races carrying the corresponding avirulence (Avr) genes. This has prompted research into R gene-dependent recognition, and subsequent signalling and defence responses. Four classes of gene-for-gene R genes exist. All except the protein kinase-encoding Pto gene, encode leucine rich repeats (LRRs), a protein motif associated with protein/protein interactions. The largest class is cytoplasmic, with LRRs at the C terminus and a consensus nucleotide binding site (NBS) for interaction with ATP or GTP towards the N terminus. NBS/LRR genes confer resistance to viruses, bacteria, fungi and nematodes. This class probably recognizes Avr gene-dependent molecules that enter the plant cell. Two other classes of R gene carry extracytoplasmic LRRs and probably recognize Avr gene specified molecules delivered from outside the plant cell. Xa21 encodes eLRRs and a cytoplasmic protein kinase domain, whereas Cf-2. Cf-4. Cf-5 and Cf-9 carry membrane-anchored eLRRs. Many R genes have been isolated, attention has shifted to R gene evolution, to what and how their products recognize, and how signals are transduced to initiate defence mechanisms. We study tomato Cf-2, Cf-4, Cf-5 and Cf-9 genes conferring resistance to specific races of Cladosporium fulvum. Cf-4 and Cf-9 genes also confer responsiveness to the C fulvum Avr4 and Avr9 peptides respectively. We also study the Arabidopsis RPP1 and RPP5 genes for Peronospora resistance. Analysis of the Cf-9, Cf-4 and Cf0 loci permitted comparison of 11 Cf-9 genes. Most variation is seen at the x amino acids that flank the conserved leucines in the LxxLxLxx beta strand/beta turn. These amino acids are likely to be solvent exposed and may confer recognition. R gene sequence novelty has partly been generated by sequence exchange between Hcr9s, either by crossovers or gene conversion, and partly by mutation and divergent selection (Parniske et al, Cell 91:821-832). This interpretation has now been confirmed at several other loci. R gene loci are often extremely polymorphic, we propose that this polymorphism might be sustained by frequency dependent selection. Cf- gene function is now being investigated through a combination of genetic and biochemical approaches. Suppressor screens have revealed a gene (Rcr3) which is required for Cf-2 function, but not Cf-9 or even Cf-5 function. Map based isolation of this gene is in progress. Domain swap analyses are starting to provide insights into Cf-4 and Cf-9 domains involved in recognition. Cf-9 is active on transfer to tobacco, and transgenic tobacco suspension cultures respond to Avr9 by rapidly producing active oxygen species (AOS). This has permitted an investigation into the role of various signalling molecules, particularly protein kinases and calcium, in Cf-9 - dependent AOS production. The availability of active suspension culture lines carrying a c-myc-tagged Cf-9 should facilitate the search for proteins or peptides that associate with Cf-9 during or before activation. Cf-9cmyc is predominantly localized in the plasma membrane. Gene expression profiling has revealed a number of genes that are rapidly and specifically induced on Cf-9 elicitation in cell cultures, but which are induced by non-specific stimuli in leaves. Overlap between the R gene-dependent signalling, and signalling elicited by other stresses will be discussed.