Abstract
The interaction of roses with the leaf spot pathogen Diplocarpon rosae (the cause of black spot on roses) is an interesting pathosystem because it involves a long-lived woody perennial, with life history traits very different from most model plants, and a hemibiotrophic pathogen with moderate levels of gene flow. Here we present data on the molecular structure of the first monogenic dominant resistance gene from roses, Rdr1, directed against one isolate of D. rosae. Complete sequencing of the locus carrying the Rdr1 gene resulted in a sequence of 265,477 bp with a cluster of nine highly related TIR–NBS–LRR (TNL) candidate genes. After sequencing revealed candidate genes for Rdr1, we implemented a gene expression analysis and selected five genes out of the nine TNLs. We then silenced the whole TNL gene family using RNAi (Rdr1–RNAi) constructed from the most conserved sequence region and demonstrated a loss of resistance in the normally resistant genotype. To identify the functional TNL gene, we further screened the five TNL candidate genes with a transient leaf infiltration assay. The transient expression assay indicated a single TNL gene (muRdr1H), partially restoring resistance in the susceptible genotype. Rdr1 was found to localize within the muRdr1 gene family; the genes within this locus contain characteristic motifs of active TNL genes and belong to a young cluster of R genes. The transient leaf assay can be used to further analyze the rose black spot interaction and its evolution, extending the analyses to additional R genes and to additional pathogenic types of the pathogen.
Highlights
Black spot caused by the hemibiotrophic ascomycete Diplocarpon rosae Wolf is the most devastating disease that impacts field-grown roses (Drewes-Alwarez, 2003; Horst, 2008)
In this study we show that the black spot resistance gene Rdr1 is a member of a highly conserved family of TNL (TIR-NBS-LRR) resistance genes
Copy numbers of Hcr9 ranging from one to five were observed in tomato (Parniske and Jones, 1999); 12–32 copies of RGC2 were estimated in lettuce (Kuang et al, 2004); 8–10 copies of RPP5 were found in Arabidopsis (Noël et al, 1999) and four Vf paralogs were detected in apple (Xu and Korban, 2002)
Summary
Black spot caused by the hemibiotrophic ascomycete Diplocarpon rosae Wolf is the most devastating disease that impacts field-grown roses (Drewes-Alwarez, 2003; Horst, 2008). Though roses are important ornamental crops with high economic importance, little progress has been made in the field of resistance breeding because genetic information about important disease-resistance traits is scarce (Debener and Linde, 2009). Two monogenic dominant genes providing race-specific resistance to black spot have been genetically characterized (Debener et al, 1998; Whitaker et al, 2010a). Rdr was introgressed into cultivated roses from the diploid Asian species Rosa multiflora and leads to an arrest of mycelia development 2–3 days after germination of conidia. The resistance phenotype is accompanied by a hypersensitive response (HR; Gachomo and Kotchoni, 2010), it is difficult to separate this from cell death that occurs at later stages of compatible interactions
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