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. 2017 Nov;216(3):915-926.
doi: 10.1111/nph.14741. Epub 2017 Aug 21.

Effector-mediated Discovery of a Novel Resistance Gene Against Bremia Lactucae in a Nonhost Lettuce Species

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Free PMC article

Effector-mediated Discovery of a Novel Resistance Gene Against Bremia Lactucae in a Nonhost Lettuce Species

Anne K J Giesbers et al. New Phytol. .
Free PMC article

Abstract

Candidate effectors from lettuce downy mildew (Bremia lactucae) enable high-throughput germplasm screening for the presence of resistance (R) genes. The nonhost species Lactuca saligna comprises a source of B. lactucae R genes that has hardly been exploited in lettuce breeding. Its cross-compatibility with the host species L. sativa enables the study of inheritance of nonhost resistance (NHR). We performed transient expression of candidate RXLR effector genes from B. lactucae in a diverse Lactuca germplasm set. Responses to two candidate effectors (BLR31 and BLN08) were genetically mapped and tested for co-segregation with disease resistance. BLN08 induced a hypersensitive response (HR) in 55% of the L. saligna accessions, but responsiveness did not co-segregate with resistance to Bl:24. BLR31 triggered an HR in 5% of the L. saligna accessions, and revealed a novel R gene providing complete B. lactucae race Bl:24 resistance. Resistant hybrid plants that were BLR31 nonresponsive indicated other unlinked R genes and/or nonhost QTLs. We have identified a candidate avirulence effector of B. lactucae (BLR31) and its cognate R gene in L. saligna. Concurrently, our results suggest that R genes are not required for NHR of L. saligna.

Keywords: Lactuca saligna; R gene; avirulence; effector-triggered immunity (ETI); lettuce downy mildew; nonhost resistance; oomycete; quantitative trait locus (QTL).

Figures

Figure 1
Figure 1
Characteristics of Bremia lactucae effectors BLR31 and BLN08, and responsiveness of Lactuca saligna accessions. (a) Signal peptide, RXLR motif and EER motif within the amino acid sequences. (b) Bremia lactucae Bl:24 growth and candidate effector gene expression during infection of Lactuca sativa cv Olof. Growth is inferred by the increase in Bremia lactucae actin relative to lettuce actin. Candidate effector gene expression is determined relative to Bremia lactucae actin. The y‐axis has been reversed to ease interpretation, as lower ∆Ct values indicate higher expression. dpi, days post‐inoculation. (c) Unrooted neighbour‐joining tree constructed of 423 AFLP fragments from 45 Lactuca saligna and two Lactuca sativa accessions. Three main branches are distinguished: European, Israeli and Southwest Asian. Bootstrap values > 60% (based on 1000 replicates) are indicated in red at the nodes. Triangles, BLR31‐responsive accession; circles, BLN08‐responsive accession.
Figure 2
Figure 2
Five BLR31‐ and BLN08‐responsive Lactuca saligna accessions and their mapping populations. (a) Plant symptoms (left) after infiltration with BLR31 and BLN08 and their response values (right) in the five selected parental Lactuca saligna accessions. Numbers on the leaves indicate infiltrations with (1) BLR31, (2) BLN08 and (3) yellow fluorescent protein (YFP). (b) Crossing scheme to create backcross (BC1), backcross inbred lines (BILs) and F2 mapping populations. Allele notation: w, wild lettuce (Lactuca saligna) allele; c, cultivated lettuce (Lactuca sativa) allele.
Figure 3
Figure 3
BLR31 responsiveness, Bl:24 infection severity and genotype on the chromosome 2 (C2) mapping locus for segregating populations of the three BLR31‐responsive Lactuca saligna parents. (a–c) BLR31‐responsive plants and relative infection severity (RIS) level to race Bl:24 per segregating population. Black bars, plants with a response to BLR31; grey bars, plants without a response to BLR31. Note: all BLR31‐responsive plants in RIS class ≤ 10% did not show any sporulation, and so their exact RIS was 0% (completely resistant). (d–f) Venn diagrams depicting four conditions: BLR31 (yellow), BLR31‐responsive plants; C2‐w (blue), plants with at least one wild lettuce (Lactuca saligna) allele on the C2 mapping locus (most closely linked co‐dominant marker depicted); RIS ≤ 10% (green), plants highly resistant to Bl:24; RIS > 10% (red), plants susceptible to Bl:24. *, plants with missing genotype data.
Figure 4
Figure 4
Map interval of responsiveness to BLR31 on chromosome 2 in Lactuca saligna CGN05947, CGN05304 and CGN05310. Alignment with the Lactuca sativa cv Salinas physical map and MRC2 and Dm3 intervals is shown below.
Figure 5
Figure 5
Map interval of responsiveness to BLN08 on C8 in Lactuca saligna CGN05318 and CGN05271. Alignment with the Lactuca sativa cv Salinas physical map and MRC8(A–C) is shown below.
Figure 6
Figure 6
BLN08 responsiveness, Bl:24 infection severity and genotype on the chromosome 8 (C8) mapping locus for the BC1 population of Lactuca saligna CGN05318. (a) Response to BLN08 and relative infection severity (RIS) level to race Bl:24. Black bars, plants with a response to BLN08; grey bars, plants without a response to BLN08. (b) Venn diagram depicting four conditions: BLN08 (yellow), BLN08‐responsive plants; C8‐w (blue), plants with one wild lettuce (Lactuca saligna) allele on the C8 mapping locus (most closely linked co‐dominant marker depicted); RIS ≤ 10% (green), plants highly resistant to Bl:24; RIS > 10% (red), plants susceptible to Bl:24. *, plants with missing genotype data.

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