Abstract
An earlier identified major quantitative trait locus for resistance towards the willow leaf rust fungus Melampsora larici-epitea in a Salix viminalis x (S. viminalis × S. schwerinii) population was used to identify potential resistance genes to the rust pathogen. Screening a genomic bacterial artificial chromosome library with markers from the peak position of the QTL region revealed one gene with TIR-NBS-LRR (Toll Interleukin1 Receptor-Nucleotide Binding Site-Leucine-Rich Repeat) domain structure indicative of a resistance gene. The resistance gene analog was denoted RGA1 and further analysis revealed a number of non-synonymous single nucleotide polymorphisms in the LRR domain between the resistant and susceptible Salix genotypes. Gene expression levels under controlled conditions showed a significantly lower constitutive expression of RGA1 in the susceptible genotype. In addition, the susceptible genotype showed a significantly reduced expression level of the RGA1 gene at 24 hours post inoculation with M. larici-epitea. This indicates that the pathogen may actively suppress RGA1 gene expression allowing a compatible plant-pathogen interaction and causing infection.
Highlights
The rapid growth, ease of clonal propagation from cuttings, phytoremediation properties, and ability to grow on land unsuitable for food crops make willow (Salix spp.) in short-rotation cultivation an excellent bioenergy crop
To identify potential defence-related genes in the quantitative trait loci (QTL) region we screened the S. viminalis × S. schwerinii BAC library using three markers from the genetic map used in the QTL mapping
Using a genomic BAC library we identified a complete putative resistance gene, RGA1, of type TIR-NBS-LRR close to a major QTL for rust resistance in a S. viminalis × (S. viminalis × S. schwerinii) family
Summary
The rapid growth, ease of clonal propagation from cuttings, phytoremediation properties, and ability to grow on land unsuitable for food crops make willow (Salix spp.) in short-rotation cultivation an excellent bioenergy crop. One of the major threats to a high and stable production of short-rotation willow is leaf rust, caused by the fungus Melampsora larici-epitea Kleb. In recent breeding programs a number of willow varieties with complete or almost complete resistance have been produced [2]. Like many other rust fungi, M. larici-epitea has a high capability to overcome resistance [3,4] and cases of resistance breakdown in willow have already been experienced [5,6]
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