Fine mapping and characterization of the Rpv2 locus for resistance to grapevine downy mildew
Downy mildew caused by the oomycete Plasmopara viticola is one of the most devastating diseases affecting grapevine. The current strategies for control rely mostly on fungicides, with negative effects on the environment and human health. Using resistant varieties is an environment-friendly alternative. Because elite varieties are susceptible, resistance needs to be introduced through breeding programs aiming to combine durable resistance with good agronomical and oenological characteristics. Vitis rotundifolia is an American species showing resistance to downy mildew. Genetic analysis of resistance to downy mildew in a population derived from V. rotundifolia cv. Trayshed revealed two loci, named Rpv1 and Rpv2, which mapped respectively to chromosomes 12 and 18, in regions of the genome rich in NLR-type (NOD-Like receptor; Nucleotide-binding and Leucine-rich Repeat) genes. Rpv1, confers partial resistance to P. viticola leading to reduced pathogen sporulation, while Rpv2 confers total resistance, abolishing pathogen sporulation. Rpv1 has been cloned and it encodes for an NLR protein, but nothing is known about the molecular basis underlying Rpv2. Understanding the Rpv2 resistance mechanism is crucial for optimizing its use in breeding programs. Here we describe the fine mapping of the Rpv2 locus. Successive rounds of backcrossing, identification of recombinant individuals, sequencing of recombinants and chromosome painting, allowed reducing the interval containing Rpv2. Manual annotation of the genes found in the interval identified two promising candidate genes for Rpv2. Functional validation of the candidate genes, as well as search for the avirulence gene corresponding to Rpv2, are in progress.  Keywords: disease resistance, Vitis rotundifolia, downy mildew, NLR-type gene, resistant variety.