Phase-separating MoSpa2 Complex Organizes Actin Nucleation Center for M.oryzae Plant Infection

Polarized actin cable from Spitzenkorper at the hyphal tip fuels filamentous growth in diverse biphasic fungal pathogens. This multi-component complex, featuring the actin nucleator Bni1 and other factors, initiates actin polymerization, guiding biphasic fungal growth and host infection. How dynamic assembly of Spitzenkorper and actin cable are achieved to support filamentous fungi that undergo multistage morphogenesis for host invasion remains unclear, including Magnaporthe oryzae, which undergoes multistage morphological transition during rice infection. Here, we identified that the scaffolder MoSpa2 remodeling actin cable networks, in space and time, by assembling the polarisome complex via phase separation, supporting Magnaporthe oryzaes polarized growth. Via N-terminal intrinsically disordered regions (IDRs), MoSpa2 first stimulates actin cable assembly through multivalent interactions with MoBni1 nucleator, and then also creates polarized actin cable bundles by F-actin association and a concurrent inhibition of cofilin-mediated F-actin depolymerization. MoSpa2 mutants exhibit impaired hyphal growth and reduced rice infection, underling its significance. This work elucidates the fundamental mechanisms underlying fungal morphogenesis, offering the potential for targeted interventions in pathogenesis.