How Do Rhizomorphs Support Growth and Survival in Fungi?
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.
The mucilage is made up of the outer layers, which are compact growing points.
The melanized wall protects against colonisation by other microorganisms (bacteria or fungi).
The medulla is responsible for water conduction and nutrient dissolution.
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.
FAQs on Rhizomorph: Structure, Development, and Role in Fungi
1. What is a rhizomorph in the context of fungi?
A rhizomorph is a complex, root-like structure formed by the aggregation of many fungal hyphae. Unlike a simple mycelial cord, a rhizomorph is highly organised with specialised tissues. Its primary purpose is to act as an efficient transport and invasion system, allowing the fungus to explore its environment, seek out new food sources like wood or plant roots, and survive in harsh conditions.
2. What is the primary function of rhizomorphs?
The primary function of rhizomorphs is to give the fungus a competitive advantage in its habitat. Their key functions include:
- Exploration and Foraging: They can grow long distances through soil to find new food sources, such as dead wood or living plant roots.
- Nutrient and Water Transport: They act like pipelines, moving water and nutrients from established food bases to the growing tips of the fungus.
- Infection: In parasitic fungi, rhizomorphs can penetrate the roots of host plants, leading to diseases like root rot.
- Survival: Their tough, often melanized outer layer protects the fungus from environmental stress and microbial attack.
3. Can you provide an example of a fungus that produces rhizomorphs?
A classic example is the Armillaria genus, commonly known as the honey fungus. Armillaria produces extensive networks of black, bootlace-like rhizomorphs that can spread underground for vast distances, making it one of the largest living organisms on Earth. These rhizomorphs are responsible for the fungus's ability to cause Armillaria root rot in many species of trees and shrubs.
4. How is a rhizomorph structurally different from a mycelium?
While both are composed of hyphae, a rhizomorph is far more complex and organised than a simple mycelium. A mycelium is a loose, undifferentiated network of fungal hyphae. In contrast, a rhizomorph shows tissue differentiation, typically featuring a compact outer rind for protection and a central core of wider hyphae specialised for transporting water, air, and nutrients. Think of a mycelium as a network of individual roads, while a rhizomorph is a multi-lane, protected superhighway.
5. How do rhizomorphs help fungi survive and infect plants?
Rhizomorphs are critical for survival and infection. For survival, their tough, often dark outer layer (rind) provides physical protection and resistance to drying out or being attacked by other microbes. For infection, they act as an invasive organ. The growing tip can generate significant mechanical pressure, allowing it to penetrate the bark and roots of a host plant. Once inside, the rhizomorph's transport system efficiently channels nutrients from the host back to the main fungal body, fuelling further growth and decay.
6. What is the difference between a rhizomorph and a sclerotium?
Both are survival structures, but they have different forms and primary functions. A rhizomorph is a linear, root-like structure primarily built for foraging, transport, and invasion. A sclerotium (plural: sclerotia) is a hard, dense, and compact mass of hyphae that serves as a dormant food reserve. While a rhizomorph actively grows and explores, a sclerotium remains inactive, waiting for favourable conditions to germinate and produce a new mycelium or fruiting body. The sclerotium is a survival pod, whereas the rhizomorph is an exploration and supply line.
7. Under what environmental conditions do rhizomorphs typically form?
Rhizomorph formation is often triggered by specific environmental cues, particularly related to nutrient availability. Fungi tend to develop these complex structures under conditions of low nutritional availability or when the existing food source is nearly depleted. This stimulates the fungus to invest energy into creating these exploratory organs to find new resources. Factors like adequate moisture, suitable temperature, and oxygen levels are also necessary for their growth and development once initiated.
