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.
Highlights
Plant pathogens secrete an arsenal of effector molecules that modulate host responses to enable successful infection
Expressed Sequence Tags (ESTs) sequencing of Hyaloperonospora arabidopsidis (Hpa)-infected Arabidopsis tissue Hpa isolates Emoy2, Emwa1, Noco2 and Waco9 were assessed for growth on the Arabidopsis enhanced disease susceptibility 1 mutant Ws eds1-1 [40]
A higher level of colonization of Arabidopsis leaves, with abundant hyphal growth and haustoria projections formed in adjacent plant cells, was observed for Hpa isolate Waco9 compared to Emoy2 (Fig. S1) and other isolates tested
Summary
Plant pathogens secrete an arsenal of effector molecules that modulate host responses to enable successful infection. PAMPtriggered immunity (PTI) is a resistance response that generally protects plants against a broad range of non-adapted microorganisms. A second layer of defense (effector-triggered immunity, ETI) can be activated through recognition of particular pathogen effectors or their actions on host targets by Resistance (R) proteins. ETI is a more acute plant reaction often involving programmed cell death at infection sites. Effectors can modulate the ETI response or mutate to circumvent recognition resulting in a co-evolutionary battle in which the pathogen attempts to evade host resistance and new plant R genes evolve to restrict further pathogen growth [10,11]. Pathogen effectors with high rates of gene loss, duplication or diversification are likely to be elicitors and/or modulators of plant immunity depending on the host genetic background they encounter [12,13]
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