Magnaporthe oryzae causes rice blast disease, but little is known about the dynamic restructuring of the actin cytoskeleton during its polarized tip growth and pathogenesis. Here, we used super-resolution live-cell imaging to investigate the dynamic organization of the actin cytoskeleton in M. oryzae during hyphal tip growth and pathogenesis. We observed a dense actin network at the apical region of the hyphae and actin filaments originating from the Spitzenkö rper (Spk, the organizing center for hyphal growth and development) that formed branched actin bundles radiating to the cell membrane. The actin cross-linking protein Fimbrin (MoFim1) helps organize this actin distribution. MoFim1 localizes to the actin at the subapical collar, the actin bundles, and actin at the Spk. Knockout of MoFim1 resulted in impaired Spk maintenance and reduced actin bundle formation, preventing polar growth, vesicle transport, and the expansion of hyphae in plant cells. Finally, transgenic rice (Oryza sativa) expressing RNA hairpins targeting MoFim1 exhibited improved resistance to M. oryzae infection, indicating that MoFim1 represents an excellent candidate for M. oryzae control. These results reveal the dynamics of actin assembly in M. oryzae during hyphal tip development and pathogenesis, and they suggest a mechanism in which MoFim1 organizes such actin networks.

Rice blast disease caused by the fungus Magnaporthe oryzae is a devastating rice (Oryza sativa) disease that causes severe losses (Dean et al., 2012;Li et al., 2012;Wilson & Talbot, 2009). M. oryzae builds elaborate infection structures and secrets effector proteins into plant tissues to suppress immunity and support pathogen growth (Giraldo et al., 2013). However, the molecular mechanisms in these processes are not totally understood.The actin cytoskeleton is an important component of eukaryotic cells and is mainly composed of polymeric filamentous actin (F-actin), motor proteins, actin-binding proteins, and actin-scaffolding septins; actin also occurs as monomeric globular actin (G-actin) (Berepiki