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Im Gegensatz zu Pflanzenarten die als Modelle für die Grundlagenforschung genutzt werden oder den wichtigsten landwirtschaftlichen Kulturpflanzen hat die Züchtungsforschung an Zierpflanzen mit einer Reihe von spezifischen Problemen zu kämpfen. Eines der Probleme ist die hohe Diversität an Zierpflanzenkulturen, die zur Folge hat, dass es, im Vergleich zu den landwirtschaftlichen Hauptkulturen, nur relative geringe Investitionen in Forschung und Entwicklung für die einzelne Kulturart gibt. Weitere Probleme entstehen durch Polyploidie, große Genome, lange Generationszeiten und schlechte Transformierbarkeit. Als Beispiel werden Arbeiten zur Resistenz an Rosen vorgestellt. In tetrasom spaltenden Nachkommenschaften mit Aufspaltung von Resistenz gegenüber einzelnen pathogenen Rassen eines Erregers ist die Markerentwicklung schwierig, da der Nachweis von Repulsionskopplungen sehr große Nachkommenschaften erfordert. In einem zweiten, alternativen Ansatz wurden in einem Panel von 96 überwiegend tetraploiden Rosensorten Assoziationsstudien zur Resistenz gegen verschiedene Pathogene und zu verschiedenen anderen Merkmalen durchgeführt. Als Markersysteme wurden hauptsächlich SNPs von einem Axiom SNP array sowie KASP Marker verwendet. Beispielhaft werden einzelne mit QTLs assoziierte Marker und erste Validierungsexperimente in unabhängigen Populationen sowie die Problematik der Dosisermittlung in tetraploiden Genotypen vorgestellt. Abschließend werden Ansätze und erste Ergebnisse zur Modifizierung von MLO-Genen in tetraploiden Rosen mit Hilfe von TALEN und CRISPR gezeigt.

Powdery mildew is a major disease of economic importance in cut and pot roses. As an alternative to conventional resistance breeding strategies utilizing single-dominant genes or QTLs, mildew resistance locus o (MLO)-based resistance might offer some advantages. In dicots such as Arabidopsis, pea, and tomato, loss-of-function mutations in MLO genes confer high levels of broad-spectrum resistance. Here, we report the isolation and characterization of four MLO homologs from a large rose EST collection isolated from leaves. These genes are phylogenetically closely related to other dicot MLO genes that are involved in plant powdery mildew interactions. Therefore, they are candidates for MLO genes involved in rose powdery mildew interactions. Two of the four isolated genes contain all of the sequence signatures considered to be diagnostic for MLO genes. We mapped all four genes to three linkage groups and conducted the first analysis of alternative alleles. This information is discussed in regards to a reverse genetics approach aimed at the selection of rose plants that are homozygous for loss-of-function in one or more MLO genes.

Background: The resistance of plants to pathogens relies on two lines of defense: a basal defense response and a pathogen-specific system, in which resistance (R) genes induce defense reactions after detection of pathogen-associated molecular patterns (PAMPS). In the specific system, a so-called arms race has developed in which the emergence of new races of a pathogen leads to the diversification of plant resistance genes to counteract the pathogens' effect. The mechanism of resistance gene diversification has been elucidated well for short-lived annual species, but data are mostly lacking for long-lived perennial and clonally propagated plants, such as roses. We analyzed the rose black spot resistance gene, Rdr1, in five members of the Rosaceae: Rosa multiflora, Rosa rugosa, Fragaria vesca (strawberry), Malus x domestica (apple) and Prunus persica (peach), and we present the deduced possible mechanism of R-gene diversification.Results: We sequenced a 340.4-kb region from R. rugosa orthologous to the Rdr1 locus in R. multiflora. Apart from some deletions and rearrangements, the two loci display a high degree of synteny. Additionally, less pronounced synteny is found with an orthologous locus in strawberry but is absent in peach and apple, where genes from the Rdr1 locus are distributed on two different chromosomes. An analysis of 20 TIR-NBS-LRR (TNL) genes obtained from R. rugosa and R. multiflora revealed illegitimate recombination, gene conversion, unequal crossing over, indels, point mutations and transposable elements as mechanisms of diversification.A phylogenetic analysis of 53 complete TNL genes from the five Rosaceae species revealed that with the exception of some genes from apple and peach, most of the genes occur in species-specific clusters, indicating that recent TNL gene diversification began prior to the split of Rosa from Fragaria in the Rosoideae and peach from apple in the Spiraeoideae and continued after the split in individual species. Sequence s

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.