P-278 - Unraveling the molecular mechanism of a C2-Domain Abscisic Acid-Related protein in ROP signaling in barley disease susceptibility

Rho of Plants (ROP) GTPases are master regulators of cell polarization, hormonal signaling, and either plant immunity or susceptibility to diseases. In barley (Hordeum vulgare), RACB, a plant ROP GTPase, is required for full susceptibility to the biotrophic fungus Blumeria hordei (Bh). To understand RACB signaling during barley–Bh interaction, we study how RACB supports fungal haustoria accommodation and which other host proteins coordinate this process. RACB functions are mediated by scaffold proteins such as ROP Interactive Partner b (RIPb), which interact with RACB via it’s coiled-coil CC2 domain (RIPbCC2) at the plasma membrane and is sufficient to increase susceptibility. We have identified a barley C2-Domain Abscisic Acid-Related protein (CARa) that physically interacts with RIPb in planta. Additionally, CARa relocalizes to fungal attack sites and the extrahaustorial membrane. While both CARa and RIPb are localized around the haustorial neck, CARa and RIPbCC2 additionally localized to the extrahaustoroial membrane. Single cell knock-down of CARa renders barley epidermal cells less susceptible to Bh, while overexpression increases susceptibility. We also show that CARa binds to phospholipids in vitro in a calcium-dependent manner. These findings suggest that Bh exploits host ROP signaling to reprogram cellular organization for its own needs, with the interplay of RACB, RIPb, and CARa potentially involving calcium signaling to facilitate fungal haustoria accommodation.