Mycelial cords are linear aggregations of parallel-oriented

hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...

e. The mature cords are composed of wide, empty vessel hyphae surrounded by narrower sheathing hyphae. Cords may look similar to

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

root In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often bel ...

s, and also frequently have similar functions; hence they are also called rhizomorphs (literally, "root-forms"). As well as growing underground or on the surface of trees and other plants, some fungi make mycelial cords which hang in the air from vegetation. Mycelial cords are capable of conducting nutrients over long distances. For instance, they can transfer nutrients to a developing fruiting body, or enable wood-rotting fungi to grow through soil from an established food base in search of new food sources. For parasitic fungi, they can help spread infection by growing from established clusters to uninfected parts. The cords of some wood-rotting fungi (like ''

Serpula lacrymans ''Serpula lacrymans'' is a species of fungi known for causing dry rot. It is a basidiomycete in the order Boletales. It has the ability to rapidly colonise sites through unique and highly specialised mycelium which also leads to greater degrada ...

'') may be capable of penetrating

masonry Masonry is the craft of building a structure with brick, stone, or similar material, including mortar plastering which are often laid in, bound, and pasted together by mortar (masonry), mortar. The term ''masonry'' can also refer to the buildin ...

. The mechanism of the cord formation is not yet precisely understood. Mathematical models suggest that some fields or

gradient In vector calculus, the gradient of a scalar-valued differentiable function f of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p gives the direction and the rate of fastest increase. The g ...

s of signalling chemicals, parallel to the cord axis, may be involved. Rhizomorphs can grow up to in length and in diameter.

Rhizomorph

Rhizomorphs are a special morphological adaptation root-like structures found in fungi. These root-like structures are composed of parallel-oriented

hyphae A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...

that can be found in several species of wood-decay and ectomycorrhizal

basidiomycete Basidiomycota () is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya (often referred to as the "higher fungi") within the kingdom Fungi. Members are known as basidiomycetes. More specifically, Basid ...

as well as

ascomycete Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The def ...

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

. Rhizomorphs can facilitate the colonization of some dry-rot fungi such as ''

'' and '' Meruliporia incrassata'' and cause damage to homes in Europe and North America, respectively, by decaying wood. Another genus that is very well studied for their abundance of rhizomorphs production is '' Armillaria'', with some species being pathogens and others saprotrophs of trees and shrubs. Known for their role in facilitating the spread and colonization of fungi in the environment, rhizomorphs are the most complex organs produced by fungi. They are made up of highly specialized hyphae that are different in size, orientation, and function. Fungi that possess these structures can compete and grow in harsh conditions. Rhizomorphs are sometimes called mycelial cords, although they are structurally different: mycelial cords are less complex and have a loose network of

giving an appearance of a fan-like mat, while rhizomorphs are more complex organs that have apically dominant growth tips, water-resistant surfaces, and can transport oxygen. Rhizomorphs and mycelial cords both function in nutrient transport, water absorption, translocation and colonization of substrates.

Development and morphology

The development of rhizomorphs begins with a submerged

thallus Thallus (: thalli), from Latinized Greek (), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. A thallus usually names the entir ...

that produces

mycelium Mycelium (: mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Its normal form is that of branched, slender, entangled, anastomosing, hyaline threads. Fungal colonies composed of mycelium are fo ...

(hyphae biomass) that when deprived of nutrients and exposed to increasing oxygen, morphogenesis occurs giving rise to pseudo or microsclerotia (survival structures of some fungi), which precede rhizomorph development. Concentrations of oxygen play an important role in the production of rhizomorphs. When there is a high concentration of oxygen in the atmosphere, soil moisture, temperature and pH, rhizomorph production increases. Rhizomorphs contain four differentiated types of tissues: #The outer layers are a compact growing point that make up the

mucilage Mucilage is a thick gluey substance produced by nearly all plants and some microorganisms. These microorganisms include protists which use it for their locomotion, with the direction of their movement always opposite to that of the secretion of ...

#The melanized wall that serves as protection against colonization by another microorganisms (bacteria or fungi) #The medulla that serves for conduction of water and dissolved nutrients #The central line used as an air conducting channel. Rhizomorphs can be of a cylindrical or flat type, and melanized or unmelanized, respectively. The flat unmelanized type is more common under the bark of trees and the cylindrical melanized rhizomorph can be found in the root systems of trees. For example, species of ''Armillaria'' form melanized (dark or brown due to the formation of

melanin Melanin (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are ...

) rhizomorphs in nature with the exception of ''Desarmillaria tabescens'' (formerly, '' Armillaria tabescens'') which produces unmelanized rhizomorphs in culture.

Function

Rhizomorphs act as a system of underground absorption and growth structures that invade and decay roots and wood, as well as sometimes propagating through the air. They can access places where food resources are not available, giving certain advantages to the fungi that produce them in terms of competition. They act as an extension of the fungal body and allow the fungus to infect, disseminate and survive for long periods of time. Rhizomorphs are composed of a medulla and central line which are responsible for water, nutrient, and gas transportation. The transportation of oxygen occurs from the base of rhizomorphs to the terminal growing part (tips). Rhizomorphs that live under free oxygen conditions are able to absorb and transport nutrients.

Example taxa

Evolution of rhizomorphs in ''Armillaria'' species

The genus ''Armillaria'' is a well-studied and widely distributed

mushroom A mushroom or toadstool is the fleshy, spore-bearing Sporocarp (fungi), fruiting body of a fungus, typically produced above ground on soil or another food source. ''Toadstool'' generally refers to a poisonous mushroom. The standard for the n ...

-forming genus with rhizomorph production abundant in most species. One of the more common morphological characteristics for the genus is the presence of an annulus, which is a ring-like structure in the stem of the fruiting body with exception of the species ''Desarmillaria tabescens''. This species is known to produce unmelanized rhizomorphs ''in-vitro'', but it does not produce them in nature. In a controlled environment study with high levels of oxygen and saturated soil moisture content, ''Desarmillaria'' species produces melanized rhizomorphs However, these two conditions are difficult to find in the climate of today and could explain the lack of melanized rhizomorphs in nature and could be a carryover from previous evolutionary periods. Rhizomorph traits can be found in all species of the ''Armillaria'' as well as other fungi but it appears that the most recently diverged species are adapted to form melanized rhizomorphs.

Melanin Melanin (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are ...

in rhizomorphs are known for the absorption of metal ions from the soil and can be found in different structures such as

spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

s and cell walls of fungi among others. Functions of

also include protecting against UV radiation and moisture stress. Thus melanin production aids in longevity and survival of rhizomorphs in the soil.

Aerial rhizomorphs in the Marasmiaceae

'' Brunneocorticium corynecarpon'' is a fungus known only from its branched white aerial rhizomorphs which grow in tropical forest canopies. DNA analysis has shown it to belong in the Marasmiaceae (normally a mushroom-forming family), but no fruiting bodies or other fertile structures of it have been found. ''Marasmius crinis-equi'' ("Horse-hair fungus") is another species which generates aerial rhizomorphs, but these often have tiny mushrooms branching out from them.

References

{{reflist Fungal morphology and anatomy