Rhizomorph in Fungi Structure and Biological Role

The term Rhizomorph, is known to be a threadlike or cordlike structure in fungi (Kingdom Fungi) composed of parallel hyphae, branched tubular filaments that comprise the body of a typical fungus. Rhizomorphs serve as a nutrient absorption and translation organ.

A rhizomorph is a hyphal strand aggregation that looks like a rope. The term "rhizomorph" literally translates to "root form." Rhizomorphs get their name from their resemblance to plant roots. Rhizomorphs in the backyard or forest indicate a thriving fungal community. You've probably seen them in the ground, under the bark of dying trees, or wrapped around decaying stumps.

Are Rhizomorphs Good or Bad? 

Rhizomorph-forming fungi can be plant allies, plant enemies.They can also be neutral decomposers. Finding a rhizomorph in your garden is neither good nor bad in and of itself. It all depends on which fungal species is causing the rhizomorph and whether the plants nearby are healthy or sick. The bootlace fungus is a plant enemy that forms rhizomorphs (Armillaria mellea). This Armillaria species is a major cause of root rot, which often leads to the death of trees and shrubs.

It has the ability to infect previously healthy trees of susceptible species, as well as attack already weakened specimens of other tree species.

This species' black or reddish-brown rhizomorphs grow just beneath the infected tree's bark and in the surrounding soil. They look like bootlaces and can be up to 0.2 inch (5 mm.) in diameter. If you find one of these rhizomorphs on a tree, it is infected and should be removed.

What Do Rhizomorphs Do? 

The rhizomorph's functions for the fungus include branching out to search for additional food sources as well as transporting nutrients over long distances. Individual hyphae in fungi cannot travel as far as rhizomorphs. Some rhizomorphs have hollow centres similar to plant xylem, allowing the fungus to transport more water and water-soluble nutrients.

Rhizomorph-forming mycorrhizal fungi use these structures to find new trees with which to collaborate. The bootlace fungus uses rhizomorphs to travel through soil and infect new trees. This is how the fungus spreads through forests of various susceptible trees.

Consider the rhizomorph information in this article the next time you see root-like strings in your garden soil or growing on a fallen log, and consider that they could be a manifestation of the often-invisible fungal world rather than roots.

Development and Morphology of Rhizomorphs

Rhizomorph development begins with a submerged thallus that produces mycelium (hyphae biomass), which when deprived of nutrients and exposed to increasing oxygen undergoes morphogenesis, giving rise to pseudo or microsclerotia (that is the survival structures of some fungi), which precede rhizomorph development. Oxygen concentrations are critical in the development of rhizomorphs. Rhizomorph production increases when there is a high concentration of oxygen in the atmosphere, soil moisture, temperature, and pH.

Rhizomorphs have four distinct types of tissues.

1. The mucilage is made up of the outer layers, which are compact growing points.

2. The melanized wall protects against colonisation by other microorganisms (bacteria or fungi).

3. The medulla is responsible for water conduction and nutrient dissolution.

4. The central line is basically used as an air conducting channel

How do Rhizomorphs Act?

Rhizomorphs function as an underground absorption as well as growth system that invades and decays roots and wood. They can reach areas where food resources are scarce, giving the fungi that produce them an advantage in terms of competition.

The use of fungal rhizomorphs of marasmius fungi (also known as the ‘horsehair fungi') as nesting material, particularly by tropical bird species, is a common but little-studied phenomenon. The physical properties of ‘horsehair fungi,' which show that the fungal rhizomorphs used by Icterus pustulatus have a significantly higher tensile strength and lower water uptake than alternative fibres used to build nests. These findings point to a possible adaptive advantage for the use of marasmius fungi in nest construction.

Where are Rhizomorphs Found?

The rhizomorph frequently grows between the bark and the tree's woody tissue. The image below shows a log along Old Main Trail. The large black splotch in the centre is formed by several rhizomorphs clustered side by side. When the bark of the log fell off, this rhizomorph became visible.