Mycorrhiza

root hairs

What is it ?

A Mycorrhiza is what we call the association between a fungus and plant root cell – but not the actual fungus. These symbiotic arrangements have been found in about 90% of all land plants and have been around for approximately 400 million years.

Some of the fungi can invade the cell directly and live partly inside it - something we call intracellular. These are named arbuscular mycorrhizal fungi.

Others live wrapped around the cell – or extracellularly – called ectomycorrhizal fungi. Most of the associations are symbiotic or mutualistic – where both organisms benefit. However, a few fungi actually have a parasitic association with host plants. 

diagrammatic view of mycorrhizal relationship

What are the benefits to the fungus?

Plant roots are hospitable sites for the fungi to anchor and produce their threads (hyphae). The plant roots also provide sugars and essential nutrients or complex compounds for the growth of the fungi. 

mycorrhiza forming root nodules

What are the benefits to the plant ?

In return for the sugars, the large mass of fungal hyphae acts as a virtual root system for the plants, vastly increasing the amount of water and minerals that the plant may obtain from the surrounding soil. Not only do the fungal threads help to bring water and nutrition into the plant, but in some environments (deserts) they also can store them for use when rainfall is sparse and temperatures are high. When mycorrhizae are present, plants are less susceptible to water stress.

When organic matter decomposes in the soil, mycorrhizae are important in making its nutrients available. Recent research indicates that the fungi even help break down rock, increasing availability of the essential nutrients within, such as potassium, calcium, zinc and magnesium.

Nutrients can be shown to move between different plants through the fungal network. Carbon has been shown to move from paper birch seedlings into adjacent Douglas-fir seedlings. 

Disease resistance

Mycorrhizae help the plant resist infection by other fungi and even bacteria. This may be because the plant, being better nourished, is healthier and has better resistance to the invader. It may also be that the large physical presence of one fungus impedes infection by others.

Under current investigation is if either the plant or the fungus produces compounds that prevent infection by pathogens – think antibiotics ! 

Function

Nutrient exchanges and communication between a mycorrhizal fungus and plants is the main function. 95% of the plant families investigated are predominantly mycorrhizal either in the sense that most of their species associate beneficially with them or are totally dependent on them.

The Orchids are notorious as a family in which the absence of the correct mycorrhizae is fatal even to germinating seeds. 

Plant Specialists always inoculates new trees

Will any fungus form mycorrhizae?

These interactions appear to be plant AND fungus specific. Not all mycorrhizae-forming fungi will work with all plants. There are research reports which show that association with the "wrong" fungus actually decreases the health and vigor of the plant.

Introducing mycorrhizal fungal spores (inoculation) is sometimes suggested to improve yields and plant vigor, particularly for container and landscape ornamentals. This may not be a benefit unless it is specific to the plant, because there is a requirement for a specific fungus-plant interaction for optimum benefit. 

But the best inoculation is soil from the native environment that contains the fungi ! 

mycorrhiza in association with a rice root

TYPES

Ectomycorrhiza

Ectomycorrhizae are distinct in that they do not penetrate into plant cells. Instead, they form a structure called a Hartig Net that penetrates between cells. They consist of a hyphal sheath, or mantle, covering the root tip and the Hartig net of hyphae surrounding the plant cells within the root cortex. They actually nestle into the inter cellular spaces of the plant! Leccinum aurantiacum, is this type of fungus in Beech. 

All are in a symbiotic association in 10% of plant families, mostly woody plants including the birch, dipterocarp, eucalyptus, oak, pine, and rose. Thousands of ectomycorrhizal fungal species exist, hosted in over 200 genera. A recent study has conservatively estimated global ectomycorrhizal fungal species richness at approximately 7750 species. 

Arbutoid mycorrhiza

This type of mycorrhiza involves plants of the Ericaceae subfamily Arbutoideae. The mediterranean strawberry tree and all its related plants species. 

Arbuscular mycorrhiza

Wheat has arbuscular mycorrhiza. These have hyphae that penetrate plant cells, producing branching, tree-like structures called arbuscules within the plant cells for nutrient exchange. Often, balloon-like storage structures, termed vesicles, are also produced.

The structure of the arbuscules greatly increases the contact surface area between the hypha and the host cell cytoplasm to facilitate the transfer of nutrients between them. Arbuscular mycorrhizas are obligate biotrophs, meaning that they depend upon the plant host for both growth and reproduction. They have also lost the ability to sustain themselves by decomposing dead plant material. 

Ericoid mycorrhiza

A fungus called Woollsia pungens found only some Ericacea plants are the most recently evolved of the major mycorrhizal relationships. Plants that form ericoid mycorrhizae are mostly woody understory shrubs; hosts include blueberries, bilberries, cranberries, mountain laurels, rhododendrons, heather, neinei, and giant grass tree.

The bog environments in which these plants live are acidic and nutrient-poor. The fungi can extract nitrogen from dead matter like cellulose, hemicellulose, lignin, pectin, and chitin. This increases the benefit they can provide to their plant symbiotic partners. 

fine as dust ! - orchid seeds

Orchid mycorrhiza

All orchids are myco-heterotrophic at some stage during their lifecycle, meaning that they can survive only if they form orchid mycorrhizae. Orchid seeds are so small that they contain no nutrition to sustain the germinating seedling, and instead must gain the energy to grow from their fungal symbiont. 

Monotropastrum humile

Monotropoid mycorrhiza

This type of mycorrhiza occurs in the subfamily Monotropoideae of the Ericaceae, as well as several genera in the Orchidaceae. These plants usually have no chlorophyll and derive their carbon (sugars) from the fungus partner. The carbon comes from other plants nearby that is taken up by the fungi.

Monotropastrum humile lacks chlorophyl making no sugars of its own,  It lives off the mycorrhiza it is associated with - ghostly looking ! 

Keep an eye out for our next related article NFB – nitrogen-fixing bacteria !

Here in NYC we inoculate all our garden plants upon installation  !

Call us - we know our Mycorrhiza !  

Keep an eye out for our next related article NFB – nitrogen-fixing bacteria !

PLANT SPECIALISTS

GREENING NEW YORK FOR OVER 53 YEARS !

Article written by our Staff Horticulturist, Peter B Morris, BSc, MSc, MBA

All photographs used with permission @SHUTTERSTOCK