NAMA

Mycorrhizae Explained

By David Rust

Author's Note:  This is the first in a series of basic mushroom information in a new section of the NAMA website.  We start here by describing the three most common types of mycorrhizal relationships.  More to follow!
 

The first single-celled fungi appeared on planet Earth nearly a billion years ago.  By the time plants colonized land in the Silurian (443 to 416 million years ago) and Devonian periods (419 to 359 million years ago), fungi in the phylum Mucoromycota had developed an arbuscule, a tree-like structure capable of colonizing plant cells and exchanging nutrients. This evolutionary step resulted in the first plant-fungal symbiosis, arbuscular mycorrhiza. Arbuscules can be found in fossils from 450 million years ago; these ancient structures are very, very similar to arbuscules we see today.

As evolution continued, new mycorrhizal types appeared: ectomycorrhizas (about 200 million years ago) and ericoid mycorrhizas (about 100 million years ago) evolved subsequently as the organic matter content of some ancient soils increased and sclerophyllous vegetation arose as a response to nutrient-poor soils respectively (Cairney, 2000).

Mycorrhizae are present in 92% of plant families (80% of species).  Plants allow, and indeed require, mycorrhizal fungi to colonize their roots. In this symbiotic relationship, fungal hyphae greatly expand the ability of plants to obtain nutrients and water. Fungi break down organic matter and weather mineral surfaces, and in so doing collect essential nutrients such as nitrogen, phosphorus, potassium, and nearly a dozen minerals.

 

 

The majority of vascular plants are mycorrhizal: 72% are arbuscular mycorrhizal (AM), 2.0% are ectomycorrhizal (ECM), 1.5% are ericoid mycorrhizal and 10% are orchid mycorrhizal. Just 8% are completely nonmycorrhizal (NM), whereas 7% have inconsistent NM–AM associations. Most NM and NM–AM plants are nutritional specialists (e.g. carnivores and parasites) or habitat specialists (e.g. hydrophytes and epiphytes) (Brundrett, 2018). 


 
 

Let’s look at three major types of mycorrhizal relationships: Ectomycorrhizal, Arbuscular (often formally referred to as Vesicular-Arbuscular), and Ericoid. There are structural and functional differences between these types: how they colonize roots; the resources they provide; and the types of plants they partner with.

Three Common Mycorrhizal Types

 
EctomycorrhizalArbuscularEricoid
Hartig net and mantle, intracellular colonization
 
Arbuscules and vesicles in root cortical cells
 
Colonize epidermal cell, forming dense hyphal coils
Connections last 2-4 years or moreArbuscules last 4-15 daysHair root associations are ephemeral
Mainly provides Nitrogen and Phosphorus, and 12 other nutrientsMainly provides Phosphorus and Nitrogen, Sulfur, Copper, Iron and ZincMainly provides Nitrogen, Phosphorus and Iron
Colonizes 2 Gymnosperm lineages and 28 Angiosperm lineagesColonizes vascular plants, mainly found in grasslandsColonizes dense fibrous roots of ericaceaous plants
Basidiomycetes and some AscomycetesGlomeromycotaAscomycetes and some Basidiomycetes
Long branching hyphaeThin hyphaeShort hyphae
Temperate, boreal, Mediterranean, and some tropical forestsTropical and temperate forests, grasslands, agricultural cropsHeathlands, tundra, boreal and temperate forest understory
   

Ectomycorrhizae

Ectomycorrhizae (ECM) are generally host specific, having evolved over the past 200 million years with their plant associates. Most ECM fungi are Basidiomycetes, forming fleshy fungal bodies. In this mycorrhizal type, hyphae cover the root tip with a mantle structure, and then form a Hartig net which penetrates into the space between the root’s cortical cells. Smith and Read describe the Hartig net as a labyrinthine inward growth of hyphae between the epidermal and cortical cells.  As the colonized root hairs grow and harden, the association becomes untenable in 2-5 years’ time. The hyphae constantly need to renew connections on new root growth.

ECM associations are generally with pines and hardwoods (gymnosperms and angiosperms). Even though they only represent 2% of mycorrhizae, ECM fungi colonize commercially valuable trees and produce edible fungi, vastly expanding their importance to humans. In the northern temperate regions, plants such as pine (Pinus), spruce (Picea), fir (Abies), poplar (Populus), willow (Salix), beech (Fagus), birch (Betula) and oak (Quercus) typify the ECM association. In total, 140 genera in 43 plant families have been identified as forming ECM.

 

Ectendo- and Arbutoid Mycorrhizaes

Two additional types of mycorrhizas with characteristics of Ectomycorrhizae (plus five more minor variations) with a high degree of intracellular penetration, should be noted.

Ectendomycorrhiza: occurs primarily on Pinus and Larix and is distinguished by the fact that, in addition to a usually thin fungal mantle and well-developed Hartig net of the ECM type, the epidermal and cortical cells are occupied by intracellular hyphae (Smith and Read, 2008).

Arbutoid: found in ericaceous genera Arbutus (a genus of 12 accepted species of flowering plants in the family Ericaceae) and Arctostaphylos (a genus of plants comprising the manzanitas and bearberries) and genera of the ericaceous subfamily Pyrolae. It is distinguished from the Ectendomycorrhizal category by the restriction of intracellular penetration to the epidermal layers of the root and by the involvement of a distinct suite of largely basidiomycetous fungi more normally found as ECM symbionts of trees (Smith and Read, 2008). 
 

Arbuscular Mycorrhizae

Fossil evidence from the Devonian Period reveals a structure which looks exactly like modern AM fungi. Although the current theory is that plants and fungi arrived on land about the same time, Dr. Joey Spatafora postulates that arbuscular fungal bodies may have preceded plants.  AM is thought to have a monophyletic origin in the Ordovician, approximately 480 million years ago (Redecker et al., 2000; Delaux, 2017), and is found in the majority of land plants and virtually all ecological niches (Read, 2002; Wang and Qiu, 2006).


Arbuscule, Mark Brundrett 2008 
          Mature arbuscule of Glomus. Photo courtesy Mark Brundrett, © 2008.

 

AM fungi are characterized by the formation of unique structures—arbuscules and vesicles—by fungi of the phylum Glomeromycota. AM fungi help plants to capture nutrients such as phosphorus, sulfur, nitrogen and micronutrients from the soil. AM fungi deliver about 90% of the phosphorus and 50% of the nitrogen needed by the host plant.

Arbuscules are short-lived structures (4-15 days) which provide nutrient transfer. Phosphorus is actively transferred to the plant throughout the life of the arbuscule. One reason for the short life of arbuscules could be that because their host plants grow so quickly, fungi constantly need to re-colonize new roots.

After the first arbuscules degrade, structures called vesicles often form within the colonized root. Vesicles, the second characteristic structure of AM mycorrhizae, are thick-walled structures. Vesicles contain large amounts of lipids, and often numerous nuclei. In dead root fragments, vesicles can act as propagules, and re-grow hyphae to colonize new roots (Smith and Read 1997). The hyphal network is relatively long-lived and is able to colonize new roots as they enter its domain.

No convincing evidence has been presented to suggest that AM associations are specific. Lack of specificity is indicated by the fact that globally only around 300–1,600 AM fungal taxa associate with about 200,000 plant spe­cies. In fact, in nature, plant roots are usually simultaneously colonized by multiple AM, which colonize several plant individuals at the same time, often from different species (Walder, 2015).  AM fungi colonize grasslands and cultivated agricultural plants, but also colonize trees such as Maples, Cedar, Redwood and Sequoia.

 

Ericoid Mycorrhizae

Ericaceous plants are generally found in the understory of a forest. They can also be found in alpine ecosystems and bogs in nutrient poor soils. We know ericaceous plants along the West Coast as huckleberry, manzanita, madrone and native rhododendron. In northern states, they’re represented by cranberries and blueberries. On the East Coast, ericaceous plants include magnolia and catalpa.

Plants in Ericaceae have dense fibrous roots which terminate in a structure called “hair roots”.  Because hair roots are “delicate structures”, these mycorrhizal associations are relatively short lived.  Ericoid mycorrhizae (ERM) are characterized by the formation of intracellular hyphal coils in the epidermis of hair roots and hyphae extended up to 1 cm from the root surface (Read, 1984). ERM typically lack multilayered hyphal mantles.

 
Glossary Arbuscule – a highly branched (“little tree”) structure which grows inside the plant cell wall but outside the plasma membrane
Ascomycete - a fungus whose spores develop within asci. The ascomycetes include the fungal component of most lichens, and a few large forms such as morels and truffles. Ascomycota is the largest phylum of Fungi, with over 64,000 species.
Basidiomycete – a fungus whose spores develop on a basidium; a group of higher fungi that have septate hyphae, including rusts, smuts, mushrooms, and puffballs
Epiphyte - a plant that grows above the ground, supported non-parasitically by another plant or object, and deriving its nutrients and water from the air
Ericoid – resembling heath.  The Ericaceae are a family of flowering plants, commonly known as the heath or heather family, found most commonly in acid and infertile growing conditions.
Hartig net - a network of inward growing hyphae, that extends into the root, penetrating between the epidermis and cortex. The Hartig net is named after Theodor Hartig, a 19th-century German forest biologist and botanist.
Hypha (plural – hyphae) - a long, branching filamentous structure of a fungus. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium
Hydrophyte - a plant that grows in water or very moist ground; an aquatic plant
Monophyletic – a group of organisms descended from a common evolutionary ancestor
Mycorrhiza (plural - mycorrhizae) - a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's root system.
Rhizosphere – a plant’s root system, and the. narrow region of soil that is directly influenced by root secretions
Vesicle – a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer