appressorium

An appressorium is a specialized infection structure produced by certain fungi and oomycetes to initiate host penetration. It typically forms at the tip of a germ tube after contact with a potential host surface and adheres firmly to the surface before penetrating deeper tissues.

Formation and structure typically involve recognition of surface cues such as hydrophobicity, plant waxes, or cutin components. In many plant-pathogenic fungi, the germ tube differentiates into a rounded, often pigmented appressorium. The cytoplasm becomes concentrated and osmolytes such as glycerol accumulate, generating high internal turgor pressure. Melanin deposition in the appressorium cell wall increases rigidity and helps the structure withstand the generated pressure.

The elevated turgor drives the formation of a penetration peg that mechanically breaches the plant cuticle and underlying cell wall, allowing invasive hyphae to enter and colonize tissues. In Magnaporthe oryzae (rice blast fungus), for example, the appressorium produces a highly pressurized penetration peg that breaches the rice leaf epidermis. While most commonly associated with plant pathogens, appressoria are also observed in some oomycetes and other fungal species in various ecological contexts.

Formation and development are regulated by signaling pathways, including cyclic AMP-dependent signaling and mitogen-activated protein kinase pathways, which coordinate surface sensing, cytoskeletal reorganization, and appressorium maturation.

Significance lies in its role as a key step in host invasion and disease initiation for many phytopathogens. Understanding appressorium biology informs approaches to disease control, such as breeding for resistance or targeting penetration-related processes.