According to their lifestyle, plant pathogens are divided into biotrophic and necrotrophic organisms. Biotrophic pathogens exclusively nourish on living host cells, whereas necrotrophic pathogens rapidly kill host cells and nourish on cell walls and cell contents. To this end, the necrotrophic fungus Botrytis cinerea secretes large amounts of phytotoxic proteins and cell wall degrading enzymes. However, the precise role of these proteins during infection is unknown. Here, we report on the identification and characterization of the previously unknown toxic protein hypersensitive response inducing protein 1 (Hip1), which induces plant cell death. We found the adoption of a structurally conserved folded Alternaria alternata Alt a 1 protein structure to be a prerequisite for Hip1 to exert its necrosis-inducing activity in a host-specific manner. Localization and the induction of typical plant defense responses by Hip1 indicate recognition as a pathogen-associated molecular pattern at the plant plasma membrane. In contrast to other secreted toxic Botrytis proteins, activity of Hip1 does not depend on the presence of the receptor-associated kinases BRI1-ASSOCIATED KINASE 1 (BAK1) and SUPPRESSOR OF BIR1-1 (SOBIR1) Our results demonstrate that recognition of Hip1, even in the absence of obvious enzymatic or pore-forming activity, induces strong plant defense reactions eventually leading to plant cell death. Botrytis hip1 overexpression strains generated by CRISPR/Cas9 displayed enhanced infection, indicating the virulence-promoting potential of Hip1. Taken together, Hip1 induces a non-canonical defence response which might be a common feature of structurally conserved fungal proteins from the Alt a 1 family.

According to their lifestyle, plant pathogens are divided into biotrophic and necrotrophic organisms. While biotrophic pathogens establish a relationship with living host cells, necrotrophic pathogens rapidly kill host cells and feed on the cell debris. To this end, the necrotrophic ascomycete fungus Botrytis cinerea secretes large amounts of phytotoxic proteins and cell wall degrading enzymes. However, the precise role of these proteins during the infection process is unknown. Here we report on the identification and characterization of the previously unknown toxic protein hypersensitive response inducing protein 1 (Hip1), which induces plant cell death. We found the adoption of a folded protein structure to be a prerequisite for Hip1 to exert its necrosis-inducing activity in Nicotiana benthamiana. Localization and the induction of specific plant responses by Hip1 indicate recognition as pathogen-associated molecular pattern at the plant plasma membrane. Our results demonstrate that recognition of Hip1, even in the absence of obvious enzymatic or poreforming activity, induces strong plant defense reactions eventually leading to plant cell death.