Appressoria

An appressorium (plural appressoria) is a specialized fungal or oomycete infection structure formed on the surface of a potential plant host. It develops at the tip of a germ tube or at a swollen hyphal cell after initial contact with the plant surface. The appressorium functions to breach the plant epidermis, allowing invasive hyphae to enter the host tissue.

Development and structure: The appressorium forms in response to surface cues such as hydrophobicity, chemical signals, and light. It often becomes dome- or bulb-shaped and frequently contains high concentrations of osmolytes, such as glycerol, to generate turgor. In many pathogenic fungi, the appressorium’s wall is melanized, which strengthens it and helps withstand the mechanical stress of penetration. A penetration peg then emerges from the base to breach the cuticle and underlying cell wall.

Mechanism: Penetration relies on a combination of mechanical force and enzymatic activity. The elevated internal pressure within the appressorium drives the penetration peg through the plant cuticle, while secreted enzymes (for example, cutinases and polygalacturonases) assist by weakening structural barriers. This coordinated action enables successful ingress without widespread tissue damage from the initial entry.

Distribution and significance: Appressoria are characteristic of many plant-pathogenic fungi, including Magnaporthe oryzae (rice blast) and various Colletotrichum species. They are a key determinant of pathogenicity, and mutants unable to form functional appressoria often exhibit greatly reduced virulence. Because of their central role in infection, appressoria are a major focus in plant pathology research and disease management strategies.