Identification of Hyaloperonospora arabidopsidis Transcript Sequences Expressed during Infection Reveals Isolate-Specific Effectors

Abstract

Biotrophic plant pathogens secrete effector proteins that are important for infection of the host. The aim of this study was to identify effectors of the downy mildew pathogen Hyaloperonospora arabidopsidis (Hpa) that are expressed during infection of its natural host Arabidopsis thaliana. Infection-related transcripts were identified from Expressed Sequence Tags (ESTs) derived from leaves of the susceptible Arabidopsis Ws eds1-1 mutant inoculated with the highly virulent Hpa isolate Waco9. Assembly of 6364 ESTs yielded 3729 unigenes, of which 2164 were Hpa-derived. From the translated Hpa unigenes, 198 predicted secreted proteins were identified. Of these, 75 were found to be Hpa-specific and six isolate Waco9-specific. Among 42 putative effectors identified there were three Elicitin-like proteins, 16 Cysteine-rich proteins and 18 host-translocated RXLR effectors. Sequencing of alleles in different Hpa isolates revealed that five RXLR genes show signatures of diversifying selection. Thus, EST analysis of Hpa-infected Arabidopsis is proving to be a powerful method for identifying pathogen effector candidates expressed during infection. Delivery of the Waco9-specific protein RXLR29 in planta revealed that this effector can suppress PAMP-triggered immunity and enhance disease susceptibility. We propose that differences in host colonization can be conditioned by isolate-specific effectors.

Nep1-like proteins from three kingdoms of life act as a microbe-associated molecular pattern in Arabidopsis

“…Candidate effectors of H. arabidopsidis for this suppression are the well-known host-translocated RXLR proteins that are encoded by an estimated 130-150 genes in this oomycete (37). A large number of these RXLRs have been identified as effective suppressors of defense responses and MTI (38)(39)(40)(41)(42)(43), and could suppress the early responses induced by NLPs.…”

Characterization of the secretome of Plasmopara viticola by de novo transcriptome analysis

“…This suggests that heterozygosity exists in these loci or there are two polymorphic copies in different loci within the genome. Different effector variants have also been described for effectors of other pathogens, including Avr2 from Cladosporium fulvum [47], Avr4 from P. infestans [48] and HaRxLs from H. arabidopsidis [49]. The apparent absence of the specific effector alleles and the sequence diversity in some isolates suggests that effector genes may have been lost or mutated during host-pathogen co-evolution to avoid recognition by host resistance proteins.…”

Mechanisms and Evolution of Virulence in Oomycetes

“…An unexpected finding from transcriptomic analysis of RxLR effectors is that a relatively small percentage (approximately 10% to 15%) of the effector genes are moderately to highly transcribed during infection (16,39,122). Furthermore, resequencing surveys revealed that only approximately 10% to 15% of predicted P. sojae RxLR effectors show evidence of significant positive selection (122).…”