A Suppressor/Avirulence Gene Combination in Hyaloperonospora arabidopsidis Determines Race Specificity in Arabidopsis thaliana .

The pathosystem of Arabidopsis thaliana and diploid biotrophic oomycete Hyaloperonospora arabidopsidis ( Hpa ) has been a model for investigating the molecular basis of Flor's gene-for-gene hypothesis. The isolates Hpa -Noks1 and Hpa -Cala2 are virulent on Arabidopsis accession RMX-A02 whilst an F1 generated from a cross between these two isolates was avirulent. The F2 progeny segregated 3,1 (avirulent, virulent), indicating a single major effect AVR locus in this pathogen. SNP-based linkage mapping confirmed a single AVR locus within a 14 kb map interval containing two genes encoding putative effectors. The Hpa -Cala2 allele of one gene, designated H . a rabidopsidis c ryptic1 ( HAC1 ), encodes a protein with a signal peptide and an RxLR/dEER motif, and triggers a defense response in RMX-A02. The second gene is heterozygous in Hpa -Cala2. One allele, designated S uppressor of HAC1 Cala2 ( S-HAC1 Cala2 ) encodes a protein with a signal peptide and a dKEE motif with no RxLR motif; the other allele ( s-hac1 Cala2 ) encodes a protein with a signal peptide, a dEEE motif and is divergent in sequence from the S-HAC1 Cala2 allele. In selfed progeny from Hpa -Cala2, dominant S-HAC1 Cala2 allele carrying progeny correlates with virulence in RMX-A02, whereas homozygous recessive s-hac1 Cala2 carrying progeny were avirulent. Genetic investigations suggested other heterozygous suppressor loci might exist in the Hpa -Cala2 genome.