Abstract
One of the biggest problems in cucumber cultivation is cucurbit downy mildew (DM), caused by the obligate biotroph Pseudoperonospora cubensis. Whereas DM in cucumber was previously efficiently controlled by the dm-1 gene from Indian cucumber accession PI 197087, this resistance was broken by new DM strains, prompting the search for novel sources of resistance. A promising source of resistance is the wild cucumber accession PI 197088. It was previously shown that DM resistance in this genotype inherits polygenically. In this paper, we put the focus on one of the QTL, DM4.1 that is located on chromosome 4. QTL DM4.1 was shown to consist of three subQTL: DM4.1.1 affected pathogen-induced necrosis, DM4.1.2 was shown to have an additive effect on sporulation, and DM4.1.3 had a recessive effect on chlorosis as well as an effect on sporulation. Near-isogenic lines (NILs) were produced by introgressing the subQTLs into a susceptible cucumber line (HS279) with good horticultural traits. Transcriptomic analysis revealed that many genes in general, and defense pathway genes in particular, were differentially expressed in NIL DM4.1.1/.2 compared to NIL DM4.1.3 and the susceptible parent HS279. This indicates that the resistance from subQTL DM4.1.1 and/or subQTL DM4.1.2 likely involves defense signaling pathways, whereas resistance due to subQTL DM4.1.3 is more likely to be independent of known defense pathways. Based on fine-mapping data, we identified the RLK gene CsLRK10L2 as a likely candidate for subQTL DM4.1.2, as this gene was found to have a loss-of-function mutation in the susceptible parent HS279, and was strongly upregulated by P. cubensis inoculation in NIL DM4.1.1/.2. Heterologous expression of this gene triggered necrosis, providing further evidence that this gene is indeed causal for subQTL DM4.1.2.
Highlights
The oomycete Pseudoperonospora cubensis [(Berk. and Curt.) Rost.] belongs to the family Peronosporaceae
In order to fine-map QTL DM4.1, a QTL isogenic introgression line was selected originating from a cross between the downy mildew (DM) resistant cucumber accession PI 197088 and the susceptible line HS279, followed by three generations of backcrossing with HS279 as the recurrent parent
It is well possible that a causal gene underlying QTL DM4.1.1 contributes to the increase of defense gene expression observed in Near-isogenic lines (NILs) DM4.1.1/.2, and it might be assumed that such a gene would have a function similar to that of the receptorlike kinases (RLKs) gene we identified as candidate for subQTL DM4.1.2, more fine-mapping studies are required to identify this gene, as well as transcriptomic studies of a NIL with subQTL DM4.1.1 in absence of DM4.1.2
Summary
The oomycete Pseudoperonospora cubensis [(Berk. and Curt.) Rost.] belongs to the family Peronosporaceae. 20 genera and at least 50 species within the Cucurbitaceae family, including economically important crops such as cucumber (Cucumis sativus), melon (Cucumis melo), watermelon (Citrullus lanatus), and squash (Cucurbita spp.) (Lebeda and Cohen, 2011). DM is considered to be the most important disease in cucumber worldwide, as it causes up to 100% of yield loss, and strains of P. cubensis have become resistant against fungicides as well as have overcome resistance in cucumber germplasm (Savory et al, 2011). The primary source of disease resistance is in (semi)wild cucumber accessions, maintained by gene banks, as these often carry resistance alleles that might have been lost during cucumber domestication. DM in cucumber was efficiently controlled by the recessive dm gene, introgressed in modern cultivars from Indian C. sativus var. Due to a more virulent strain of P. cubensis, dm-1 resistance provides only some level of intermediate resistance (Cohen et al, 2015; Holmes et al, 2015)
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