David Moore's World of Fungi: where mycology starts

Ectomycorrhizas

Compared to arbuscular mycorrhizas (AM), the range of plants colonised by ectomycorrhizas (ECM) is relatively small. A mere 3% of phanerogams (seed plants) are ECM, all of which are woody species; trees and shrubs. Despite this small percentage, the worldwide importance of the ECM association is greatly increased by the large area covered by the plants globally, and their economic value as the source of timber.

ECM symbioses are believed to be more common in the temperate zones of the world than the tropics.  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 the southern hemisphere Eucalyptus and Northofagus (Southern Beech) are important genera as is the Dipterocarpaceae family, found in the monsoon forests of south-east Asia. In total, 140 genera, in 43 plant families have been identified as forming ECM.

In contrast to the AM association, a wide range of fungi form ECM.  At least 65 genera have been identified, the majority of which (45 of 65) are Basidiomycota.  Eighteen genera of Ascomycota are represented and also the Zygomycete genus Endogone, which can form AM as well. Classification has been by identification of the fruiting body in the past, and since these have not always been observed, some fungi remain unclassified.

It has been estimated that between 5,000 and 6,000 species of fungi form ecto- or ectendo-mycorrhizas.  Around 4,500 of these are epigeous (have above-ground fruiting bodies), but up a quarter are hypogeous (with underground fruiting bodies) such as truffles (Fig. 1).  Members of the Ascomycota are particularly well represented by hypogeous forms.

The ECM fungi do not show a high degree of host specificity, either in natural or artificial conditions.  It is common to find mycorrhizas belonging to several different fungi on the root system of a single tree. Norway spruce (Picea albies) can form ECM symbioses with over 100 different fungal species and the fly agaric (Amanita muscaria; Fig. 2), can infect the roots of trees as varied as birch, eucalyptus, spruce and Douglas fir.

Fig. 1. White Truffles (Tuber magnatum).  © Assignments Photographers/Corbis with permission.	Fig. 2. Fly agaric (Amanita muscaria).  © Nathan Wilson with permission.

The pattern and succession of mycorrhizal development varies during the life of a tree.  Some fungi are successful primary colonisers, the 'early-stage' fungi, whilst different fungi dominate as the tree ages and reaches maturity, the 'late stage' fungi. However, some fungi will infect regardless of stage, and seedlings germinating in a mature forest are usually colonised by the dominant 'late' or 'multi' stage fungi. The 'early stage' fungi seems to have more of a role where their plant partner is a pioneer colonist, and there are few other soil fungi present.

The 'early stage' fungi act like ruderal or r-selected species, and those of the 'late stage' are k-selected organisms, as they are unable to survive in soil not already inhabited by other mycelia. The difference between these types of fungi is important commercially, as the 'early stage' fungi can be used as inoculants to stimulate new forest growth, whereas the 'late stage' fungi cannot. ECM infection has been shown to be necessary for the successful establishment of some trees, such as Pinus, and can allow seedlings to compete against mature trees in less favourable conditions, such as shading.

The characteristic features of the ECM association are (Figs 3 & 4):

• an extensive network of intercellular hyphae penetrating between epidermal and cortical cells, called the Hartig net (Fig. 3);
• the presence of a substantial sheath of fungal tissue around the absorbing root (Fig. 4);
• roots which are shorter and wider than uninfected ones (Fig. 4).

Ectomycorrhizas start to develop when hyphae infect the secondary or tertiary roots of woody species, which they seem to prefer, especially on trees. Hyphae grow back up the root from just behind the root cap and meristem, forming a weft, that may later become a bulky sheath. Hyphae from the sheath grow inwards between epidermal cells and cortical cells, by forcing their way mechanically and by excreting pectinases. These invasive hyphae form the Hartig net.

Fig. 3. Light micrographs of hand sections of ectomycorrhizal forest tree roots cleared and stained with Chlorazol Black E and viewed with (Nomarski) interference contrast microscopy. Fungal hyphae penetrate between host cells and branch to form a labyrinthine structure called the Hartig net. Host responses may include polyphenols (tannin) production in cells and the deposition of secondary metabolites in walls. Ectomycorrhizal gymnosperms, such as Pinus and Picea, have Hartig net hyphae extending deep into the cortex; this contrasts with the typical situation in angiosperms (such as Populus, Betula, Fagus, Eucalyptus, etc.), which usually have a one-cell-layer Hartig net confined to the epidermis. A, transverse section of ectomycorrhizal Tsuga canadensis (hemlock) with labyrinthine Hartig net hyphae (arrows) penetrating between the cortical cells of the root and completely surrounding many cells; note tannin-filled epidermal cells in the inner mantle (asterisk); scale bar = 100 µm). B, root cross-section of ectomycorrhizal Populus tremuloides (quaking or trembling aspen) showing labyrinthine Hartig net hyphae (arrows) around elongated epidermal cells. This complex hyphal branching pattern is thought to increase the fungal surface area in contact with the root. The active mycorrhizal zone occurs several millimetres behind the root tip (because of the time required for mycorrhizal formation), but Hartig net hyphae senesce in older regions further from the root tip. For more information and illustration of mycorrhizas visit Mark Brundrett’s website at http://mycorrhizas.info. The images are modified from Brundrett et al., 1990 and were kindly supplied by Dr Mark Brundrett, School of Plant Biology, University of Western Australia. Figure and caption from Moore et al., 2011 (URL).

Hyphae never penetrate into cells or into the stele. The intercellular Hartig net may surround each cell completely, so they have no contact with any other cells.  The extensive surface area of the Hartig net is the main interface for exchange of substances between plant and fungus.

Fungal infection changes the growth pattern of the root. The fungal sheath reduces the rate of cell division at the root tip, slowing cell elongation and hence reducing the root growth lengthways. Cortical cells also elongate radially (shown in Fig. 3B) and so the infected root becomes short and thick compared to uninfected ones (Fig. 4).  Hence, they are often called short roots. The fungal sheath also suppresses the development of root hairs, which essentially means that all nutrients entering the plant must be channelled through the fungal tissues of the sheath.

Fig. 4. Ectomycorrhizal roots; small parts of the root systems various forest tree species to illustrate the morphological diversity of ectomycorrhizal roots. A - D are all roots of Douglas fir (Pseudotsuga menziesii), but mycorrhizal with different fungi (photographs by B. Zak): A, with the basidiomycetous truffle Hysterangium (a truffle fruit body is shown); B, mycorrhizal fungus Rhizopogon vinicolor (Boletales, Basidiomycota); C, mycorrhizal fungus Poria terrestris (Byssoporia terrestris, Polyporales); D, mycorrhizal fungus Lactarius sanguifluus (Russulales). E and F show mycorrhizas involving the same fungus (Amanita muscaria) but different hosts (photographs by R. Molina): E, Sitka spruce (Picea sitchensis) and F, Monterey pine (Pinus radiata). (Images A to F prepared from graphics files kindly provided by Dr Randy Molina, Pacific Northwest Research Station, USDA Forest Service, Oregon, USA.) Illustration G shows a pine seedling grown in symbiosis with the ectomycorrhizal fungus Suillus bovinus in an experimental microcosm (where the seedling is grown in soil placed in a container made from two sheets of glass separated by a 1 cm thick former). In this case two soils were used: 1, is from a podzol E horizon soil (explained below), and 2 is a loamy organic soil. The extraradical fungal mycelia (m; evident mainly as hyphal strands) extend from the colonised root tips (r) into both soil substrates and are far more extensive than the roots. Podzols are the typical soils of coniferous, or boreal, forests in the northern hemisphere and eucalypt forests and heathlands in the southern. In podzols organic material and soluble minerals are leached from the upper layers (horizons) to the lower; the E horizon is a 4 to 8-cm thick layer of heavily leached soil and is largely composed of insoluble minerals. The high abundance of mycelium in this mineral soil shows that it is an important growth substrate for ectomycorrhizal fungi. Ectomycorrhizal fungi modify their chemical environment through local acidification around the hyphae and by exuding metal-complexing weathering agents such as organic acids and play a central role in mineral weathering of boreal forest soils (this image reproduced with permission from Elsevier). For more information and illustration of mycorrhizas visit Mark Brundrett’s website at http://mycorrhizas.info. Figure and caption from Moore et al., 2011 (URL).

A fungus will extend its sheath with a growing root, ensuring the root is always covered, and colonisation by other fungi does not occur. However, fungi can be replaced as roots resume growth after dormancy. The fungal sheath may not resume growth straight away, and the root can be exposed to other fungal symbionts. This interactive replacement contrasts with the 'non-interactive' replacement mentioned before with reference to early and late stage colonisers.

The advantages of the ECM association go beyond the mutual exchange of nutrients (click on the hyperlinked phrase for more information on nutrient exchange). Ectomycorrhizas can also confer pathogen resistance to their plant partner, and they are more effective at this, compared to AM roots.  Click for more information on the effects of mycorrhizas.