Strigolactones initiate the formation of haustorium-like structures in Castilleja

Strigolactones serve as germination signals for several root-parasitic plant species within the Orobanchaceae family. Yet, their role in the life cycle of the facultatively parasitic genus Castilleja has remained elusive. Here, we demonstrate that strigolactones initiate the formation of haustorium-like structures in Castilleja. We present the genome of Castilleja foliolosa, which reveals an abundance of KAI2d paralogs that act as strigolactone receptors. Castilleja foliolosa's KAI2d proteins include high-turnover enzymes, in particular KAI2d15, which features a substrate binding pocket with a methionine cluster at its entrance that is involved in the trade-off between substrate turnover and affinity. Our findings provide insights into strigolactone perception in Castilleja and refine our perspective on their role in facultative parasitism.