Characterization of the tomato Cf-4 gene for resistance to Cladosporium fulvum identifies sequences that determine recognitional specificity in Cf-4 and Cf-9

In many interactions between plants and their pathogens, resistance to infection is specified by plant resistance (R) genes and corresponding pathogen avirulence (Avr) genes, In tomato, the Cf-4 and Cf-9 resistance genes map to the same location but confer resistance to Cladosporium fulvum through recognition of different avirulence determinants (AVR4 and AVR9) by a molecular mechanism that has yet to be determined, Here, we describe the cloning and characterization of Cf-4, which also encodes a membrane-anchored extracellular glycoprotein, Cf-4 contains 25 leucine-rich repeats, which is two fewer than Cf-9, The proteins have >91% identical amino acids, DNA sequence comparison suggests that Cf-4 and Cf-9 are derived from a common progenitor sequence. Amino acid differences distinguishing Cf-4 and Cf-9 are confined to their N termini, delimiting a region that determines the recognitional specificity of ligand binding. The majority of these differences are in residues interstitial to those of the leucine-rich repeat consensus motif, Many of these residues are predicted to form a solvent-exposed surface that can interact with the cognate ligand, Both Cf-4 and Cf-9 are located within a 36-kb region comprising five tandemly duplicated homologous genes, These results provide further insight into the molecular basis of pathogen perception by plants and the organization of complex R gene loci.