Orobanche cumana (Orobanchaceae), commonly known as broomrape, is an obligate root holoparasitic plant that infests sunflower (Helianthus annuus), causing substantial yield losses in many regions of Europe and Asia. Its seed germination is triggered by chemical signals exuded by sunflower roots, including strigolactones (SLs) and sesquiterpene lactones (STLs) such as dehydrocostus lactone (DCL). In non-parasitic plants, SL perception by the α/β-hydrolase DWARF14 (D14) initiates a signaling cascade via the SCFMAX2 ubiquitin ligase complex. SL recognition and hydrolysis by D14 destabilize the D14 lid domain, which is required for the association of D14 with the MORE AXILLARY GROWTH 2 (MAX2) F-box protein. In Orobanchaceae, perception of SL and STL signals is mediated by KARRIKIN INSENSITIVE2–divergent (KAI2d) receptors. KAI2d evolved from canonical KAI2 α/β-hydrolases, D14 homologs that respond to karrikins and unknown endogenous ligands. In Striga hermonthica, SL recognition by the KAI2d protein ShHTL7 also induces lid domain destabilization and MAX2 binding, suggesting that this mode of SL signaling is conserved in at least some KAI2ds. However, the structural basis of STL recognition in O. cumana is unknown, and we lack selective inhibitors that block germination without interfering with SL signaling in the sunflower host.
