Identification of candidate genes required for susceptibility to powdery or downy mildew in cucumber

Henk J Schouten,Richard G F Visser,Julian Krauskopf,Yuling Bai

doi:10.1007/s10681-014-1216-z

Henk J Schouten, Richard G F Visser + Show 2 more

Open Access

https://doi.org/10.1007/s10681-014-1216-z

Copy DOI

Journal: Euphytica	Publication Date: Aug 9, 2014
Citations: 64	License type: CC BY 4.0

Affiliation: Wageningen University & Research

Abstract

AbstractPowdery mildew (PM, caused by Podosphaera fusca) and downy mildew (DM, caused by Pseudoperonospora cubensis) are important diseases of cucumber (Cucumis sativus). Breeding for resistance has been undertaken since the 1940s, but underlying resistance genes have not been functionally analysed yet. The published genome sequence of cucumber catalyses the search for such genes. Genetic studies have indicated that resistances to PM and DM in cucumber are often inherited recessively, which indicates the presence of susceptibility genes (S-genes). Therefore we analyzed the cucumber genome for homologs of functionally proven S-genes known from other plant species. We identified 13 MLO-like genes in cucumber, three of which cluster in Clade V, the clade that contains all known MLO-like susceptibility genes to powdery mildews in other dicots. The expression of one of these three genes, CsaMLO1, located on chromosome 1, was upregulated after PM inoculation. It co-localizes with a QTL for PM resistance previously identified. Also homologs of the susceptibility genes PMR4 and PMR5 are located at this QTL. The second MLO-like gene from Clade V (CsaMLO8) resides in a recessively inherited major QTL for PM resistance at the bottom of chromosome 5, together with a PMR6-like gene. Two major QTL for DM recessive resistance at the top of chromosome 5 co-localize with CsaDMR6-2, which is homologous to the DMR6 susceptibility gene in Arabidopsis. This study has identified several candidate genes for susceptibility to PM and DM in cucumber that may explain QTL for recessively inherited resistance, reported earlier.

Highlights

Disease resistant crops are commonly bred by the introgression of resistance (R) genes from wild relatives
We searched for MLO-like genes in the cucumber genome, and found 13 putative MLO-like genes, scattered among the chromosomes (Table 1; Fig. 1). Out of these 13 MLO-like genes, three (CsaMLO1, CsaMLO8, and CsaMLO11) code for predicted proteins that belong to Clade V (Fig. 2)
Not all MLO-like genes that cluster in this clade are susceptibility genes (Zheng et al 2013), but until now all MLO-like genes of dicots that have shown to be required for susceptibility to powdery mildew (PM), belong to Clade V (Fig. 2)

Summary

Introduction

Disease resistant crops are commonly bred by the introgression of resistance (R) genes from wild relatives. Race-specific resistance conferred by R-genes asserts selective pressure on pathogen populations, therewith overcoming resistance. Resistance to powdery mildew (PM) in cucumber (Podosphaera fusca (Fr.) Braun & Shishkoff, syn. Sphaerotheca fuliginea Schlech ex Fr. Poll) has been bred through introgression of race specific resistance genes, which led to emergence of virulent races (Cohen et al 2004; Tores et al 2009). Pyramiding of R-genes has been proposed as a solution to this problem. An alternative strategy for achieving durable resistance is disabling genes that are required for susceptibility, namely susceptibility genes (S-genes) (Pavan et al 2009)

Methods

Results

Conclusion