Analysis of the Rdr1 gene family in different Rosaceae genomes reveals an origin of an R-gene cluster after the split of Rubeae within the Rosoideae subfamily.

The Rdr1 gene confers resistance to black spot in roses and belongs to a large TNL gene family, which is organized in two major clusters at the distal end of chromosome 1. We used the recently available chromosome scale assemblies for the R. chinensis 'Old Blush' genome, re-sequencing data for nine rose species and genome data for Fragaria, Rubus, Malus and Prunus to identify Rdr1 homologs from different taxa within Rosaceae. Members of the Rdr1 gene family are organized into two major clusters in R. chinensis and at a syntenic location in the Fragaria genome. Phylogenetic analysis indicates that the two clusters existed prior to the split of Rosa and Fragaria and that one cluster has a more recent origin than the other. Genes belonging to cluster 2, such as the functional Rdr1 gene muRdr1A, were subject to a faster evolution than genes from cluster 1. As no Rdr1 homologs were found in syntenic positions for Prunus persica, Malus x domestica and Rubus occidentalis, a translocation of the Rdr1 clusters to the current positions probably happened after the Rubeae split from other groups within the Rosoideae approximately 70-80 million years ago during the Cretaceous period.

Interaction of roses with a biotrophic and a hemibiotrophic leaf pathogen leads to differences in defense transcriptome activation.

KEY MESSAGE: Transcriptomic analysis resulted in the upregulation of the genes related to common defense mechanisms for black spot and the downregulation of the genes related to photosynthesis and cell wall modification for powdery mildew. Plant pathogenic fungi successfully colonize their hosts by manipulating the host defense mechanisms, which is accompanied by major transcriptome changes in the host. To characterize compatible plant pathogen interactions at early stages of infection by the obligate biotrophic fungus Podosphaera pannosa, which causes powdery mildew, and the hemibiotrophic fungus Diplocarpon rosae, which causes black spot, we analyzed changes in the leaf transcriptome after the inoculation of detached rose leaves with each pathogen. In addition, we analyzed differences in the transcriptomic changes inflicted by both pathogens as a first step to characterize specific infection strategies. Transcriptomic changes were analyzed using next-generation sequencing based on the massive analysis of cDNA ends approach, which was validated using high-throughput qPCR. We identified a large number of differentially regulated genes. A common set of the differentially regulated genes comprised of pathogenesis-related (PR) genes, such as of PR10 homologs, chitinases and defense-related transcription factors, such as various WRKY genes, indicating a conserved but insufficient PTI [pathogen associated molecular pattern (PAMP) triggered immunity] reaction. Surprisingly, most of the differentially regulated genes were specific to the interactions with either P. pannosa or D. rosae. Specific regulation in response to D. rosae was detected for genes from the phenylpropanoid and flavonoid pathways and for individual PR genes, such as paralogs of PR1 and PR5, and other factors of the salicylic acid signaling pathway. Differently, inoculation with P. pannosa leads in addition to the general pathogen response to a downregulation of genes related to photosynthesis and cell wall modification.

The TNL gene Rdr1 confers broad-spectrum resistance to Diplocarpon rosae.

Black spot disease, which is caused by the ascomycete Diplocarpon rosae, is the most severe disease in field-grown roses in temperate regions and has been distributed worldwide, probably together with commercial cultivars. Here, we present data indicating that muRdr1A is the active Rdr1 gene, a single-dominant TIR-NBS-LRR (Toll/interleukin-1 receptor-nucleotide binding site-leucine rich repeat) (TNL)-type resistance gene against black spot disease, which acts against a broad range of pathogenic isolates independent of the genetic background of the host genotype. Molecular analyses revealed that, compared with the original donor genotype, the multiple integrations that are found in the primary transgenic clone segregate into different integration patterns in its sexual progeny and do not show any sign of overexpression. Rdr1 provides resistance to 13 different single-spore isolates belonging to six different races and broad field mixtures of conidia; thus far, Rdr1 is only overcome by two races. The expression of muRdr1A, the active Rdr1 gene, leads to interaction patterns that are identical in the transgenic clones and the non-transgenic original donor genotype. This finding indicates that the interacting avirulence (Avr) factor on the pathogen side must be widespread among the pathogen populations and may have a central function in the rose-black spot interaction. Therefore, the Rdr1 gene, pyramided with only a few other R genes by sexual crosses, might be useful for breeding roses that are resistant to black spot because the spread of new pathogenic races of the fungus appears to be slow.

Strigolactone pathway genes and plant architecture: association analysis and QTL detection for horticultural traits in chrysanthemum.

Chrysanthemums are important ornamental plants with abundant phenotypic diversity. Especially in cut-flower breeding, shoot branching is important for the success of new varieties. To assess the genetic regulation of shoot branching and other horticultural important traits, we phenotyped and genotyped two types of chrysanthemum populations: a genotype collection of 86 varieties and a biparental F1-population (MK11/3) of 160 individuals. Using two different statistical approaches, a genome-wide association analysis and a single marker ANOVA, with AFLP marker data and candidate gene markers for shoot branching, we tried to identify markers correlated to the traits of interest. As expected for the outcrossing hexasomic chrysanthemums most of the phenotypic traits showed a continuous variation in both populations. With the candidate gene approach we identified 11 significantly associated marker alleles for all 4 strigolactone pathway genes BRC1, CCD7, CCD8 and MAX2 regulating shoot branching in the genotype collection. In the MK11/3 we detected seven markers for all candidate genes except MAX2 explaining a large proportion of the variation. Using anonymous AFLP markers in the GWA with the 86 genotypes and the single locus analysis with the F1-population we could detect 15 and 17 additional marker-trait associations, respectively. Our analyses indicate a polygenic inheritance of the shoot branching in the chrysanthemum, with a fundamental role of the strigolactone pathway genes BRC1, CCD7, CCD8 and MAX2 and we identified 50 associated markers to all traits under study. These markers could be used in the selection of the parental plants for breeding chrysanthemums to enrich them for positive alleles influencing plant architecture traits.