Haustorium

The etymology of the name corresponds to the Latin word haustor meaning the one who draws, drains or drinks, and refers to the action performed by the outgrowth.

In fungi

Fungi in all major divisions form haustoria. Haustoria take several forms. Generally, on penetration, the fungus increases the surface area in contact with host plasma membrane releasing enzymes that break up the cell walls, enabling greater potential movement of organic carbon from host to fungus. Thus, an insect hosting a parasitic fungus such as Cordyceps may look as though it is being "eaten from the inside out" as the haustoria expand inside of it.

The simplest forms of haustoria are small spheres. The largest are complex formations adding significant mass to a cell, expanding between the cell wall and cell membrane. In the Chytridiomycota, the entire fungus may become enclosed in the cell, and it is arguable whether this should be considered analogous to a haustorium.

Haustoria arise from intercellular hyphae, appressoria, or external hyphae. The hypha narrows as it passes through the cell wall and then expands on invaginating the cell. A thickened, electron-dense collar of material is deposited around the hypha at the point of invagination. Further, the host cell wall becomes highly modified in the invaginated zone. Inclusions normally present in plasma membrane are absent, and the outer layer contains more polysaccharide. The wall of both partners is severely reduced.

Functional exchange takes place within the haustorial complex. The host supplies organic carbon to the fungus, and the metabolic activity within the complex is considerably greater than outside. Carbon from the host is absorbed by the fungus, and transported to the rest of the thallus. The host plant appears to be functioning according to signals from the fungus and the complex appears to be under the control of the invader.

The haustorium may be mycelium or root-shaped.