A receptor kinase senses sterol by coupling with elicitins in auxotrophic .

Sterols are vital nutrients and signals for eukaryotic organisms. Mammalian cells are known to sense and respond to sterol status changes to maintain them within strict limits, a process associated with various human diseases. However, this process is not understood in oomycete pathogens, most of which are sterol auxotrophic and must obtain sterols from host plants. Here, we report that SSRK1 (sterol-sensing receptor kinase 1) detects host sterols by coupling with elicitins, thereby controlling signaling and sterol absorption. Sterols are recruited by extracellular soluble elicitins, and these complexes then bind to SSRK1 to form trimolecular complexes. These complexes subsequently trigger downstream calcium influx, activation of mitogen-activated protein kinase, and transcriptome reprogramming through the receptor's kinase activity. Our data demonstrate a unique sterol sensing pathway where elicitins and a transmembrane receptor kinase SSRK1 act as coreceptors for extracellular sterols. These findings also portray a sterol-based war between oomycetes and plants, providing a unique perspective on how a pathogen detects host signals during infection.