Abstract
Key messageOrDeb2 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. OrDeb2 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 OrDeb2 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 OrDeb2, and a susceptible line was phenotyped for broomrape resistance in four experiments, including different environments and two broomrape races (FGV and GTK). This population was also densely genotyped with microsatellite and SNP markers, which allowed locating OrDeb2 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 OrDeb2. Rhizotron and histological studies showed that OrDeb2 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 OrDeb2 map-based cloning.
Highlights
Sunflower broomrape (Orobanche cumana Wallr.) is a holoparasitic plant that parasitizes the roots of sunflower (Helianthus annuus L.) and constraints its production in large areas of the Old World
From these 232, a total of 220 F2:3 families were evaluated for race FGV, which resulted in 49 families consistently resistant, 99 segregating, and 72 consistently susceptible
The classification of the F2 genotypes from the mapping population (n = 232) based on the evaluation of their F3 families was slightly different from a 1:2:1 ratio in the evaluations conducted with race GTK (n = 232; ratio 49:108:75; χ2 = 6.93, P = 0.03) and FGV (n = 220, ratio 49: 99: 72; χ2 = 7.00, P = 0.03)
Summary
Sunflower broomrape (Orobanche cumana Wallr.) is a holoparasitic plant that parasitizes the roots of sunflower (Helianthus annuus L.) and constraints its production in large areas of the Old World. A sunflower postvascular connection resistance to O. cumana races F and G named as “System II resistance” has been reported to be controlled by the single partially dominant gene OrSII (Hassan et al 2008; Martín-Sanz et al 2020). In addition to this major gene effect, a quantitative component of broomrape resistance determined by quantitative trait loci (QTLs) that contribute with small-to-moderate effects to decrease the number of emerged broomrapes has been described (Pérez-Vich et al 2004; Akhtouch et al 2016; Louarn et al 2016; Imerovski et al 2019)
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