Fine mapping and genomic analyses reveal a tandem kinase-pseudokinase candidate underlying OrDeb2-mediated resistance to Orobanche cumana in sunflower

Sunflower broomrape (Orobanche cumana Wallr.) is a holoparasitic weed that parasitizes sunflower (Helianthus annuus L.) and severely constrains yield. The OrDeb2 gene confers resistance to broomrape race G in the sunflower line DEB2. It has been previously located within a 1.38-Mbp physical region in the H. annuus XRQ reference genome that contains a cluster of receptor-like proteins and receptor-like kinases (RLKs). In this study, an OrDeb2 candidate has been proposed though high-resolution genetic and physical mapping, multi-allelic comparisons across resistant and susceptible lines, and exclusion of other potential candidates based on their function and genetic features. OrDeb2 was delimited to a 219.4-kbp region in the DEB2 genome assembly that contained six annotated protein-coding genes belonging to two groups: three small heat shock proteins (HSP20) and three RLKs. Given that the only sunflower broomrape resistance gene cloned to date encodes an RLK, the key role of RLKs in resistance to biotic stresses in plants, and the similar genetic features between RLK-type resistance genes and OrDeb2 as major dominant genes in gene-for-gene interactions, RLKs in the interval emerged as the most plausible candidates. Within them, the RLK-2 gene encoding a root-expressed full-length kinase-pseudokinase protein belonging to the tandem kinase proteins (TKPs) family was prioritized, because the other two RLKs corresponded to a truncated TKP with a stop codon within the kinase domain. RLK-2 also carried sequence variants unique to DEB2 relative to multiple susceptible genotypes. The results of this research reveal the potential role of TKP genes as new players in resistance to parasitic plants and provide a framework to dissect the genetic dialog between O. cumana and sunflower in gene-for-gene interactions.