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 <i>Castilleja</i> has remained elusive. Here, we demonstrate that strigolactones initiate the formation of haustorium-like structures in <i>Castilleja</i>. We present the genome of <i>Castilleja foliolosa</i>, which reveals an abundance of KAI2d paralogs that act as strigolactone receptors. <i>Castilleja foliolosa</i>'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 <i>Castilleja</i> and refine our perspective on their role in facultative parasitism.