Abstract
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. arabidopsidis cryptic1 (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 Suppressor of HAC1Cala2 (S-HAC1Cala2) 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-HAC1Cala2 allele. In selfed progeny from Hpa-Cala2, dominant S-HAC1Cala2 allele carrying progeny correlates with virulence in RMX-A02, whereas homozygous recessive s-hac1Cala2 carrying progeny were avirulent. Genetic investigations suggested other heterozygous suppressor loci might exist in the Hpa-Cala2 genome.
Highlights
Oomycetes are diploid microorganisms that resemble fungi in morphology and lifestyle, having filamentous mycelia and developing specialized structures such as haustoria during infection
At least seven downy mildew R-genes have been characterized in this pathosystem (Botella et al, 1998; McDowell et al, 1998; Bittner-Eddy et al, 1999; van der Biezen et al, 2002; Sinapidou et al, 2004; Eulgem et al, 2007), and all encode cytoplasmic NLR proteins that contain a conserved nucleotide-binding site (NBS) and a variable leucine-rich repeat (LRR) domain
The susceptible accessions were screened with an F1 isolate that was generated from a laboratory mating of Hyaloperonospora arabidopsidis (Hpa)-Cala2 and Hpa-Noks1 (Bailey et al, 2011)
Summary
Oomycetes are diploid microorganisms that resemble fungi in morphology and lifestyle, having filamentous mycelia and developing specialized structures such as haustoria during infection. At least seven downy mildew R-genes have been characterized in this pathosystem (Botella et al, 1998; McDowell et al, 1998; Bittner-Eddy et al, 1999; van der Biezen et al, 2002; Sinapidou et al, 2004; Eulgem et al, 2007), and all encode cytoplasmic NLR proteins that contain a conserved nucleotide-binding site (NBS) and a variable leucine-rich repeat (LRR) domain. Three of the seven predicted AVR elicitors have been characterized (Allen et al, 2004; Rehmany et al, 2005; Bailey et al, 2011) and all encode secreted effector-like proteins
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