Identification of distinct specificity determinants in resistance protein Cf-4 allows construction of a Cf-9 mutant that confers recognition of avirulence protein Avr4.
The tomato resistance genes Cf-4 and Cf-9 confer specific, hypersensitive response-associated recognition of Cladosporium carrying the avirulence genes Avr4 and Avr9, respectively. Cf-4 and Cf-9 encode type I transmembrane proteins with extracellular leucine-rich repeats (LRRs). Compared with Cf-9, Cf-4 lacks two LRRs and differs in 78 amino acid residues. To investigate the relevance of these differences for specificity, we exchanged domains between Cf-4 and Cf-9, and mutant constructs were tested for mediating the hypersensitive response by transient coexpression with either Avr4 or Avr9. We show that the number of LRRs is essential for both Cf-4 and Cf-9 function. In addition, Cf-9 specificity resides entirely in the LRR domain and appears to be distributed over several distant LRRs. In contrast, Cf-4 specificity determinants reside in the N-terminal LRR-flanking domain and three amino acid residues in LRRs 13, 14, and 16. These residues are present at putative solvent-exposed positions, and all are required for full Cf-4 function. Finally, we show that Cf-9 carrying the specificity determinants of Cf-4 has recognitional specificity for AVR4. The data indicate that diversifying selection of solvent-exposed residues has been a more important factor in the generation of Cf-4 specificity than has sequence exchange between Cf-4 progenitor genes. The fact that most variant residues in Cf-4 are not essential for Cf-4 specificity indicates that the diverse decoration of R proteins is not fully adapted to confer recognition of a certain avirulence determinant but likely provides a basis for a versatile, adaptive recognition system.
Laboratory of Phytopathology, Wageningen University, 6709 PD Wageningen, The Netherlands.
R A
RA
Van der Hoorn
R
R
Roth
P J
PJ
De Wit
D000595:Amino Acid Sequence; D001483:Base Sequence; D002963:Cladosporium / Q000235:genetics / Q000472:pathogenicity; D018744:DNA, Plant / Q000235:genetics; D005656:Fungal Proteins / Q000235:genetics; D017343:Genes, Plant; D014644:Genetic Variation; D018551:Solanum lycopersicum / Q000235:genetics* / Q000382:microbiology* / Q000502:physiology; D008562:Membrane Glycoproteins / Q000737:chemistry / Q000235:genetics* / Q000502:physiology; D008969:Molecular Sequence Data; D009154:Mutation; D010935:Plant Diseases / Q000235:genetics / Q000382:microbiology; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics* / Q000502:physiology; D017434:Protein Structure, Tertiary; D020449:Repetitive Sequences, Amino Acid; D017384:Sequence Deletion; D017386:Sequence Homology, Amino Acid; D014774:Virulence / Q000235:genetics
C085987:AVR4 protein, Cladosporium fulvum; C109609:Cf protein, Lycopersicon esculentum; D018744:DNA, Plant; D005656:Fungal Proteins; D008562:Membrane Glycoproteins; D010940:Plant Proteins