The Hemibiotrophic Apple Scab Fungus Venturia inaequalis Induces a Biotrophic Interface but Lacks a Necrotrophic Stage

Microscopic evidence demonstrated a strictly biotrophic lifestyle of the scab fungus <i>Venturia inaequalis</i> on growing apple leaves and characterised its hemibiotrophy as the combination of biotrophy and saprotrophy not described before. The pathogen-host interface was characterised by the formation of knob-like structures of the fungal stroma appressed to epidermal cells as early as 1 day after host penetration, very thin fan-shaped cells covering large parts of the host cell lumen, and enzymatic cuticle penetration from the subcuticular space limited to the protruding conidiophores. The <i>V. inaequalis</i> cell wall had numerous orifices, facilitating intimate contact with the host tissue. Pathogen-induced modifications of host cells included partial degradation of the cell wall, transition of epidermal cells into transfer cells, modification of epidermal pit fields to manipulate the flow of nutrients and other compounds, and formation of globular protuberances of mesophyll cells without contact with the pathogen. The non-haustorial biotrophy was characterised by enlarged areas of intimate contact with host cells, often mediated by a matrix between the pathogen and plant structures. The new microscopic evidence and information on the pathogens' biochemistry and secretome from the literature gave rise to a model of the lifestyle of <i>V. inaequalis</i>, lacking a necrotrophic stage that covers and explains its holomorphic development.