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 plasma 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 plasma membrane by 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 the plant cell wall and plasma 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 plasma membrane during plant infection. Taken together, a penetration ring consisting of a family of secreted proteins is formed between plant cell wall and plasma membrane, which may function as a novel physical structure at the interface between the tip of penetration pegs and plant plasma membrane before the differentiation of invasive hyphae. Significance StatementLike many other plant pathogens, the rice blast fungus forms melanized appressoria (specialized infection structures) to penetrate plant cells. In this study, we showed that a penetration ring is formed by penetration pegs after appressorium-mediated penetration of plant cell wall. This ring of secreted proteins is persistent at the point of penetration pegs invading plant plasma membrane to form invasive hyphae. Therefore, after the penetration of plant cell wall, the rice blast fungus forms a penetration ring that consists of a family of secreted proteins unique to pathogens using extreme appressorium turgor for penetration and may function as a physical structure for anchoring onto plant plasma membrane and developing invasive hyphae in penetrated cells.