Identification and functional analysis of downy mildew effectors in lettuce and Arabidopsis

Abstract

Oomycete pathogens cause large losses in many crop plants. Chemical control measures are undesirable and are becoming less effective. Resistance breeding has traditionally made use of dominant resistance genes to which oomycete pathogens have often been quick to adapt. Novel approaches towards resistance breeding are, therefore, required to introduce new and more durable forms of resistance in crops. A better understanding of host-pathogen interactions at the molecular level is required to design new breeding approaches. Important molecular players, that are the main subject of this study, are the effector proteins that downy mildews use to manipulate their host and cause disease. Sequencing of Expressed Sequence Tags of Hyaloperonospora arabidopsidis proved a powerful method of identifying effectors that are active during infection. This revealed isolate-specific effectors that increase susceptibility of the host. Scaling up this approach using massively parallel transcriptome sequencing allowed for an extensive overview of genes that are active during downy mildew infection of lettuce, caused by Bremia lactucae. Bremia transcript sequences were identified from the mixed set of plant and pathogen transcripts based on mapping of spore-derived short reads of Bremia genomic DNA. Effector candidates were then identified based on sequence characteristics known from effectors of other oomycetes. A set of 34 potential host-translocated effectors with an RXLR or RXLR-like motif was cloned for further analysis. The role of these effectors in promoting disease susceptibility on the lettuce host was investigated, revealing two effectors, BLR16 and BLR27 that show a strong and robust susceptibility-enhancing effect when transiently expressed. Many other tested effectors showed a trend towards enhancing susceptibility, though a single effector, BLR03 consistently reduced susceptibility. BLR16 and BLR27 were found to be expressed throughout Bremia infection of lettuce, whilst a strong reduction in the expression of BLR03 was observed already at one day post inoculation. Lettuce markers of biotic stress-responses were developed based on high abundance and homology to Arabidopsis biotic stress-induced genes. However, no evidence was found that susceptibility is enhanced by a general repression of biotic-stress responses of the host. Finally, 129 lettuce breeding lines were tested for their response to the set of cloned effectors by Agrobacterium-mediated transient transformation assays to reveal new resistance specificities. Two effectors containing a GKLR-variant of the RXLR-motif were found to be recognised, resulting in the induction of cell death; BLG01 in wild lettuce lines, and BLG03 in cultivated lettuce lines that contain the known resistance locus Dm2. Recognition of BLG01 is dependent on a region on the short arm of chromosome 9 in L. saligna, and recognition of BLG03 is linked to the Dm2 resistance locus. Bremia isolates carrying these effectors were still able to cause disease, suggesting that recognition or subsequent defence is suppressed by other effectors. The generation of an overview of the Bremia effector arsenal, identification of susceptibility-enhancing activities of Bremia effectors, and uncovering of recognition specificities are important steps forward towards understanding the molecular mechanisms underlying disease outcome.