Ericoid Mycorrhizas

Introduction

Definition

Ericoid mycorrhizas represent a unique type of mycorrhizas confined to several families in the large angiosperm order Ericales. A unifying feature of plants developing this type of mycorrhiza is the formation of very specialized lateral roots, ‘ roots hair ’ (Figure 161).

These are very narrow in diameter, possess a simple anatomy, have limited extension growth, and lack secondary growth. Each root consists of a narrow vascular cylinder, one or two rows of cortical cells (including the endodermis), and an epidermal layer of enlarged cells (Figure 162).

The mycorrhizal association involves the colonization of epidermal cells by fungal hyphae followed by the formation of a branched hyphal complex in each colonized cell (Figure 163). Although ericoid mycorrhizas are confined to the order Ericales, species in two main families, Ericaceae and Epacridaceae, contribute significantly to ecosystems in the Northern and Southern hemisphere respectively.

In the Northern hemisphere, Ericaceae species with typical ericoid mycorrhizas often dominate heath lands; they are also well represented in the subalpine and alpine floras of both Europe and North America. Acidic heathlands and sandplains in the Southern hemisphere (notably in Australia) are often dominated by species in the family Epacridaceae (Cairney and Ashford 2002).

In all situations plants with ericoid mycorrhizas are found growing on nutrient-poor soils, suggesting that ericoid mycorrhizal associations confer an important function by increasing the capacity of these hosts to absorb mineral nutrients.

In western North America, the ericaceous species Gaultheria shallon (salal) often proliferates in areas after forests have been cut or burned; the dense growth of salal sometimes results in delaying conifer seedling establishment. In Europe, an array of ericaceous species have colonized vast tracts of heathland.

Few genera in the Ericaceae are considered to be commercially important; however, those that are, contribute significantly to the economies of the areas in which they grow. Shoots of G. shallon,
harvested for floral arrangements in British Columbia, Canada, represent a 45–60 million dollar business every year.

Various species of Vaccinium (the blueberries and cranberries) are
grown for their fruit while the shoots of some Vaccinium species are used in the florist trade. Other ericaceous species, including several in the family Epacridaceae, are grown as ornamentals.

Plant species involved

The order Ericales consists of 25 families with approximately 9,450 species; however, species in only three families (Ericaceae, Epacridaceae and Empetraceae) usually possess typical ericoid mycorrhizas. Although molecular studies have placed the Epacridaceae and Empetraceae in the family Ericaceae (Judd et al. 1999),.

In Hawaii, three species in the genus Vaccinium (Ericaceae) as
well as the species Styphelia tameiameiae (Epacridaceae) have arbuscular mycorrhizas instead of ericoid mycorrhizas (Koske et al. 1990).

The genera Arbutus and Arctostaphylos, both in the Ericaceae, and most members of the family Pyrolaceae have specialized mycorrhizas known
as arbutoid (see Chapter 5); plants in the family Monotropaceae have monotropoid mycorrhizas.

Many ericaceous families including Clethraceae, Grubbiaceae and Cyrillaceae have been poorly studied. Roots of Clethra barbinervis (Clethraceae) collected from the field and roots of seedlings grown in the greenhouse in soil collected near a tree of this species developed the Paris-type arbuscular mycorrhiza (Kubota et al. 2001).

Overall, the mycorrhizal status of many species in the order Ericales remains undescribed. The immense diversity within the order Ericales
and its worldwide distribution makes characterizing plants having ericoid mycorrhizas very difficult.

In general terms, these plants are often perennial shrubs or small trees with sclerophyllous (i.e., highly lignified) leaves. There is a wide range of flower types in this order and many species in the Epacridaceae have highly modified flowers for bird pollination.

Figure 161. Diagrammatic representation of an ericoid species with numerous hair roots (arrow-
heads).
Figure 162. Diagram of a transverse section of a hair root showing enlarged epidermal cells (E)
with a thickened outer tangential wall, one layer of cortex (C), an endodermis (En), a few xylem
tracheary elements (X), and a few phloem cells (P).
Figure 163. Diagram of epidermal cells showing entry of fungal hyphae through the thickened
wall (arrowhead), hyphal complexes (HC), and a narrow hypha (double arrowhead) connecting
adjacent epidermal cells.

Duckett and Read (1995) have shown that isolates of Hymenoscyphus ericae, the most common ericoid fungus, can colonize rhizoids of genera in several leafy liverwort families and, conversely, that cultures of fungi isolated from several leafy liverworts can form typical ericoid mycorrhizas with several ericaceous species. In the liverworts, H. ericae triggers the enlargement of the rhizoid tip within which a hyphal complex develops.

Fungal species involved 

A diverse assemblage of fungi has been isolated from hair roots of a number of genera within the Ericaceae and Epacridaceae. Isolates are generally slow-growing and many produce dark mycelium (dematiaceous mycelium) when grown on various media (Figure 164).

For many years, only one fun- gal species, Hymenoscyphus ericae = Pezizella ericae (Leotiales), identified originally by characteristics of the mycelium in culture and the production of asexual propagules (arthroconidia, Figure 165), had been shown by synthesis experiments to form ericoid mycorrhizas with members of the Ericaceae (Smith and Read 1997).

The characteristic arthroconidia place this fungus in the anamorph genus, Scytalidium. Cultures of the fungus have formed sexual reproductive structures (apothecia with asci and ascospores), and molecular evidence confirms that at least one isolate of
Scytalidium (S. vaccinii) is the anamorph of the teleomorph, H. ericae. Several other isolates are known to form typical ericoid mycorrhizas with a variety of ericaceous hosts (see review by Berch et al. 2002).

Among these, anamorph species of Oidiodendron and their teleomo-rphic states, Myxotrichum and Byssoascus (Hyphomycetes), are especially important. Many unidentified isolates cultured from ericaceous hosts also form typical ericoid mycorrhizas; their taxonomic placement is beginning to be clarified with the help of molecular methods (see Box 13). 

A dark septate fungal endophyte, Phialocephala fortinii, was isolated from the roots of all ericaceous species examined from an alpine heath site and a sand dune site, and from two of five species from a bog site in Alberta, Canada (Hambleton and Currah 1997).

Although this fungal species and other dark septate fungal endophytes do not form typical ericoid mycorrhizas, their prevalence in roots of many species in the Ericaceae (Figures 166, 167) is worth noting and deserves further study (see Chapter 8).

Many fungal isolates have also been obtained from hair roots in the Epacridaceae. The mycelium of many, in culture, is dematiaceous, but isolates can range in colour from white to pink to various dark shades. Molecular studies show genotype similarities between some of the Epacridaceae isolates, Hymenoscyphus, Oidiodendron, and other isolates from the Ericaceae (Berch et al. 2002).

As more host species in both families, Ericaceae and Epacridaceae, are examined with respect to their fungal partners, there is no doubt that the number of identified fungal species will increase. There is some evidence that fungal species belonging to the basidiomycetes may also be involved in forming ericoid mycorrhizas.

Development and structure

The surface of hair root tips is surrounded by mucilage (Figures 168, 169) that potentially harbours microorganisms and in which hyphae of ericoid mycorrhizal fungi may proliferate (Figur 169) before colonizing epidermal cells. The mucilage is secreted by the root cap and differentiating epidermal cells and is composed of sugars, N-acetylgl-ucosamine, polygalacturonic acid andβ–1,4 glucans (Peretto et al. 1990).

It is not known whether this mucilage participates in recognition between fungus and root or whether it simply provides a niche for the growth of fungi and other organisms. Fungal hyphae also secrete a fibrillar sheath containing glucose and mannose sugar residues (Perotto et al. 1995).

This sheath is more pronounced in infective fungal strains compared to non-infective strains, suggesting that components of the sheath are involved in recognition between symbionts. The sheath may also aid in the attachment of hyphae to the surface of the root; once a hypha enters the epidermis, the sheath disappears.

Hyphae of fungal species that can form ericoid mycorrhizas must contact the surface of hair roots and then penetrate the epidermal cell wall (Figures 170, 174) before colonizing epidermal cells and establi-shing hyphal complexes (Figures 170, 171).

Although it has been widely accepted that each epidermal cell is invaded by a separate infection hypha without lateral spread from epidermal cell to epidermal cell, recent evidence in our lab indicates that this may not always be true (Figures 172, 173).

Many species in the Ericaceae and Epacridaceae have very thick epidermal cell walls (Figure 170) through which fungal hyphae must pass in order to enter the root. In the case of Lysinema ciliatum (Epacridaceae), the thickened wall is multi-layered and may function either as a source of nutrients for the enclosed fungus or perhaps to protect the hyphal complex during periods of drought (Ashford et al. 1996).

These authors also suggest that sloughed thick-walled epidermal
cells containing hyphal complexes may act as propagules for the fungus. Observations from field-collected material and from seedlings grown in vitro indicate that the symbiosis between ericoid mycorrhizal fungi and hair roots, regardless of the fungal and plant species involved, share some common characteristics.

The colonization of roots is always restricted to epidermal cells and involves the coiling and branching of a fungal hypha to form a hyphal complex within each cell (Figures 170, 174). Not all epidermal cells of a root become colonized, but in many field collections, colonization levels can be very high (Figure 175).

Fungal hyphae within epidermal cells are surrounded by host plasma membrane (Figure 176). An interfacial matrix is present between the fungal cell wall and host plasma membrane but it is simpler in composition compared to that described for arbuscular mycorrhizas.

Labelling with antibodies specific for host cell wall components has shown that only arabinosyl residues are present (Perotto et al. 1995).
Nevertheless, movement of nutrients between the symbionts must occur across this membrane-interfacial matrix boundary.

Epidermal cells containing hyphal complexes have an enlarged nucleus and are rich in organelles including mitochondria, plastids, and components of the endomembrane system. Fungal hyphae typically have dolipore septa with pronounced associated Woronin bodies (indicative of their ascomycete affiliation), and frequently contain
deposits of glycogen and polyphosphate.

Organelles such as nuclei, mitochondria and ribosomes are present in hyphae. The longevity of any epidermal cell-hyphal complex is variable.
Degradation of the host and fungal cytoplasm occurs in both field-collected material and in in vitro systems.

The breakdown of the symbiotic association may partly depend on the longevity of an individual hair root. It is generally accepted that there is rapid turnover of these fine roots in soil. The extent of development of surface hyphae on hair roots is variable (Figures 171, 177); in some
cases a mantle-like structure forms (Figure 178).

Many of the hyphae on hair root surfaces are runner hyphae from which lateral branches develop that enter epidermal cells (Read 1984).

Figure 164. Culture of Hymenoscyphus ericae.
Figure 165. Arthroconidia (asexual propagules) of Scytalidium vaccinii (the anamorph state of Hymenoscyphus ericae).

Extraradical mycelium

The extent of the extraradical mycelium in ericoid mycorrhizas is very limited and this may be due to the fact that the extensive hair root system plays the major role in soil exploitation (Read 1984).

This author reports that it is difficult to extract mycelium from peat soil and therefore there are currently no good measurements of the extent of the extraradical mycelium from the roots of ericoid plants in such soils.

However, measurements have been obtained in sandy heathland soil; here, extraradical mycelium does not extend beyond 1 cm from the hair root surface and most extraradical hyphae are located on and immediately adjacent to the root surface (Read 1984).

Functions

The prevalence of ericoid mycorrhizas in nutrient-poor heathlands, in both the northern and southern hemispheres, has stimulated research related to their function. Read and colleagues (see Read 1996; Smith and Read 1997) have intensively studied the roles of ericoid mycorrhizas in northern hemisphere heathlands.

Fewer studies have examined the function of epacrid mycorrhizas in the southern hemisphere. Depending on soil conditions, ericoid mycorrhizas can have several roles. It has been demonstrated that they can take up phosphate, an important role of most other mycorrhizas.

Hyphae of Hymenoscyphus ericae, the most prevalent ericoid mycorrhizal fungus, can secrete acid phosphatases onto the surface of the fibrillar sheath surrounding external hyphae that grow close to host
roots. These enzymes enable the host plant to access phosphate from organic and (or) condensed phosphates.

The main role of ericoid mycorrhizas, at least in heathlands, may involve the acquisition of nitrogen which can be limiting to plant growth in these areas . Ericoid fungi are able to obtain nitro- gen from a number of sources. Nitrate and ammonium ions, and free amino acids can be absorbed by hyphae and translocated to the host.

In addition, nitrogen can be obtained from complex organic substrates through the production of proteinases, and from chitin, the major structural component of most fungal cell walls, through the production of chitinases.

These sources of nitrogen would be unavailable to plants without their mycorrhizal associations. Much of the nitrogen in soils supporting ericoid plant species is bound in organic matter; ericoid mycorrhizal associations play a major role in the success of ericoid species in accessing this nitrogen.

In addition, ericoid fungi can degrade pectins and lignins, components of plant cell walls, making carbon compounds available for fungal
growth (and perhaps plant growth) during times of limited photosynthetic activity. The hair roots of ericoid plants occur predominantly in the superficial layers of soil rich in plant litter and microorganisms.

This places them, and their associated mycorrhizal fungi, in a favourable position to scavenge nitrogen, carbon and phosphorus from organic materials. Ericoid mycorrhizal fungi may also have a significant role in terms of mobilization and uptake of iron when it is present at low concentration or low availability.

The production of a hydroxamate siderophore may be involved in the ability of these fungi to access iron under such conditions. In certain environments, ericoid mycorrhizal fungi have the ability to protect their hosts from toxic levels of heavy metals such as copper and zinc, two metals often associated with mine spoils and found at high levels in soils adjacent to smelters (Bradley et al. 1982).

Strains of ericoid mycorrhizal fungi have been isolated from soils contaminated with cadmium and arsenic; these fungi tolerate
higher levels of these metals when grown in vitro compared to strains from non-polluted sites (Perotto et al. 2002).

The slime (mucilage) produced by hyphae of Hymenoscyphus ericae is important in taking up zinc, binding it, and thus decreasing zinc
content in the shoots of host plants (Denny and Ridge 1995). As well, there is the possibilty that hyphae within root epidermal cells may concentrate heavy metals, preventing them from being transported to the shoot where they could interfere with metabolic processes.

The results to date show that ericoid mycorrhizas play a significant role in the success of many members of the Ericales in soils that are nutrient deficient or contaminated with metals.

One concern expressed by Read (1996) is that ericaceous species, adapted to soils with low levels of available nitrogen, may lose this advantage in regions of high deposition of nitrogen from pollution. He suggests that evidence for this already exists in the heathlands of northwest Europe where grasses are replacing ericaceous species.

Figure 166. Dark septate fungal endophytes (arrowheads) in cleared roots of Gaultheria shallon. Figure167. Microsclerotia (arrowheads) of dark septate fungal endophytes in cleared roots of Vaccinium sp. Figure 168. Hair root of Gaultheria shallon stained with toluidine blue O showing the mucilage sheath (*) and trapped sloughed root cap cells (arrowheads). Figure 169. PAS-stained root tip of Gaultheria shallon counterstained with alcian blue showing root cap (rc), mucigel (*) and fungal hyphae of Hymenoscyphus ericae (arrows). Figure 170. Cleared root of Gaultheria shallon showing thickened epidermal cell walls (arrow- heads) and intracellular fungal coils (*). Figure 171. Transverse section of Gaultheria shallon hair root showing a mantle-like structure (m), a hypha penetrating the epidermis (arrow), and hyphal coils (arrowheads).