Genetic and physiological characterization of sunflower resistance provided by the wild-derived OrDeb2 gene against highly virulent races of Orobanche cumana Wallr

Or_Deb2 confers post-attachment resistance to Orobanche cumana and is located in a 1.38 Mbp genomic interval containing a cluster of receptor-like kinase and receptor-like protein genes with nine high-confidence candidates. Sunflower broomrape is a holoparasitic angiosperm that parasitizes on sunflower roots, severely constraining crop yield. Breeding for resistance is the most effective method of control. Or_Deb2 is a dominant resistance gene introgressed into cultivated sunflower from a wild-related species that confers resistance to highly virulent broomrape races. The objectives of this study were as follows: (i) locate Or_Deb2 into the sunflower genome and determine putative candidate genes and (ii) characterize its underlying resistance mechanism. A segregating population from a cross between the sunflower resistant line DEB2, carrying Or_Deb2, and a susceptible line was phenotyped for broomrape resistance in four experiments, including different environments and two broomrape races (F_GV and G_TK). This population was also densely genotyped with microsatellite and SNP markers, which allowed locating Or_Deb2 within a 0.9 cM interval in the upper half of Chromosome 4. This interval corresponded to a 1.38 Mbp genomic region of the sunflower reference genome that contained a cluster of genes encoding LRR (leucine-rich repeat) receptor-like proteins lacking a cytoplasmic kinase domain and receptor-like kinases with one or two kinase domains and lacking an extracellular LRR region, which were valuable candidates for Or_Deb2. Rhizotron and histological studies showed that Or_Deb2 determines a post-attachment resistance response that blocks O. cumana development mainly at the cortex before the establishment of host-parasite vascular connections. This study will contribute to understand the interaction between crops and parasitic weeds, to establish durable breeding strategies based on genetic resistance and provide useful tools for marker-assisted selection and Or_Deb2 map-based cloning.