The penetration ring is a novel infection structure formed by the penetration peg for invading plant cell membrane in rice blast fungus

Many fungal pathogens develop specialized infection structures such as appressoria to penetrate plant cells. However, it is not clear whether special structures are formed after cell wall penetration before invading host cytoplasm membrane in hemibiotrophic pathogens. Here, we showed that a penetration ring consisting of Ppe1 secreted proteins is formed after appressorium-mediated cell wall penetration and remained at the base of penetration site after invading plant cytoplasm membrane in the rice blast fungus Magnaporthe oryzae. The same persistent Ppe1 ring is formed after the penetration of neighboring cells by transpressoria. PPE1 is specifically expressed during plant infection and the Delta ppe1 mutant is defective in penetration and invasive growth. Blockage of penetration peg formation impedes the development of the Ppe1 ring. Close examinations showed that the penetration ring is formed at the collar of penetration pegs between plant cell wall and cytoplasm membrane and it is persistent as a fixed ring even after invasive hyphae invaded neighboring cells. Furthermore, Ppe1 is a member of an expanded family of secreted proteins that are unique to fungal pathogens using extreme appressorium turgor for plant penetration. Other members of the Ppe1 family also localize to the penetration ring for anchoring on cytoplasm membrane during plant infection. Taken together, a penetration ring consisting of a family of secreted proteins is formed between plant cell wall and cytoplasm membrane, which may function as a novel physical structure at the interface between the tip of penetration pegs and plant cytoplasm membrane before the differentiation of invasive hyphae.