A wide range of fungi are biotrophic on plant tissue. The associations range from pathogenic, in the case of rust fungi, to beneficial in the case of mycorrhizal fungi. In many cases of biotrophy, the fungi invaginate the host cell. The invaginating structure formed by pathogens is generally called a haustorium. Haustoria are commonly found in leaves or stems. Where the fungus is a mutualist, the invagination may also be given a more specialised name, such as arbuscule, or peloton, and these commonly occur in roots or root-like structures.
Fungi in all major divisions form haustoria. Haustoria take several forms. Generally, on penetration, the fungus increases the surface area in contact with host plasmamembrane, increasing the potential movement of organic carbon from host to fungus. The simplest form of haustorium is a small sphere. The largest haustoria are complex hand-like formations occupying a significant proportion of the host cell.
In Olpidium, the entire fungus may become enclosed in the cell. While the function is analogous, it is not calles a haustorium.
Haustoria arise from intercellular hyphae, appressoria, or external hyphae. The hypha narrows (penetration peg) as it passes through the wall of the cell and then expands on invaginating the cytoplasm. A thickened, electron-dense collar of material is deposited around the hypha at the point of invagination. Further, the host wall becomes highly modified in the invaginated zone. Inclusions normally present in the plasmamembrane are absent, and the outer layer contains more polysaccharide. The walls of both partners are severely thinned, resulting in an interfacial matrix.
Functional exchange takes place across the interfacial matrix within the haustorial complex. The metabolic activity within the complex is considerably greater than outside.The host supplies organic carbon to the interfacial matrix surrounding the fungus. Carbon from the host is absorbed by the fungus, and immediately transported, probably as trehalose, to the rest of the thallus. Note that delivery of sugar to the intercellular space might be regulated by diffusion gradients if it is immediately removed by the fungus. 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.
In mycorrhizas, the fungus is completely surrounded by host membrane, again establishing an interfacial region in which energy from the plant can be accessed by the fungus. Arbuscules are highly branched structures in cortical cells of the root. LINK The fungi forming the arbuscule are all from the Glomeromycota. Glomalean fungi are found in a wide range of plant roots. While arbuscules are common, Paris-type arbuscular mycorrhiza also have coils of hyphae usually called pelotons in the cortical cells in addition to arbuscules. Pelotons are also found in orchid and ericoid mycorrhizas. Analogous intracellular coiling structures are also found in ectendomycorrhizas. LINK
In some predacious fungi, a captured animal may be penetrated. The infection bulb is usually only the initial stage of colonisation and the structure is not considered analogous to haustoria.
Haustoria, arbuscules and pelotons appear to have the same function, that is acquisition of organic carbon by the fungus, through the huge surface area in contact with the host. The haustorium has a significantly increased relative surface area of contact with the host. The intercellular mycelium can also take up organic carbon. In some cases, the haustorium is necessary for fungal uptake of amino acids and sulphur/phosphate rich molecules.
In plants, colonisation by pathogens may initiate host-cell death. Immediate cell death is unusual in biotrophic interactions: cell death immediately cuts access to host nutrients and the resultant death of the biotroph.
Some fungi in a diverse array of taxa may invaginate of plant cells. The increased surface area of contact appears to be important for the transfer of energy (and minerals in leaf associations) to the fungus.
Jennings DH & Lysek G 1996 Fungal Biology. Bios Scientific Publishers