[su_heading size=”35″]Ectendomycorrhizas[/su_heading]

Definition:

Ectendomycorrhizas are associations formed between a limited number of ascomycete fungi and the conifer genera, Pinus and Larix (Yu et al.2001a). Ectendomycorrhizas have been defined by others in various ways and sometimes have encompassed the arbutoid and monotropoid mycorrhizas.

Structurally, ectendomycorrhizas resemble ectomycorrhizas; they have a mantle and Hartig net but differ in that after Hartig net formation, intracellular hyphae develop in epidermal and cortical cells.

Plant species involved

By our strict definition, only two conifer genera (Pinus and Larix), both in the Pinaceae, form true ectendomycorrhizas. Reports of other genera having this category of mycorrhiza are mainly based
on field-collected samples and these may represent senescent ectomycorrhizas.

Although the occurrence of ectendomycorrhizas is currently restricted to two genera of conifers, this represents a significant distribution in that there are close to 100 species of Pinus and 10 to 12 species of Larix.

However, few species of either genus have been investigated either in the field or under laboratory conditions for the occurrence of ectendomycorrhizas.

Fungal species involved

Initially the fungi forming ectendomycorrhizas were grouped as E-strain fungi mainly because sexual stages were not identified; only general morphological characteristics of hyphae and chlamydospores were used to characterize the isolates (Figures 108, 109).

With the discovery of sexual stages and with the use of molecular methods, a limited number of ascomycetes have been identified as fungal partners in ectendomycorrhizas. Most of the isolates of ectendomycorrhizal fungi belong to two species of Wilcoxina, W.mikolae var. mikolae (Figure 110), W. mikolae var.

tetraspora and W. rehmii in the ascomycete order,Pezizales (Egger 1996). Another member of the Pezizales (Sphaerosporella brunnea) forms ecten domycorrhizas with Pinus contorta. Two genera in the Leotiales (Phialophora finlandia and Chloridium paucisporum) form mycorrhizas resembling ectendomycorrhizas (Yu et al. 2001a).

An interesting feature of several of these fungal species is that they are able to form typical ectomycorrhizas with a number of conifer and angiosperm species (Scales and Peterson 1991a). Therefore, although few fungal species are involved, many tree species could potentially form ectomycorrhizas with them (Yu et al.2001a).

Structural characteristics

In Pinus species, ectendomycorrhizal fungi induce the typical dichotomy of short roots that are also observed with ectomy-corrhizal fungi; as well, clusters of dichotomous short roots often develop.

Few detailed studies of the structure of ectendomycorrhizas have been published, the most thorough being that of Scales and Peterson (1991b) with two species of Wilcoxina in association with Pinus banksiana, grown under laboratory condi tions.

In this and other studies, the main features of ectendomycorrhizas are the colonization of short roots, the development of a thin mantle, a Hartig net, and intracellular hyphae.

The thin mantle may not be apparent when roots are viewed at low magnification, but at higher magnification, mantle features, such as branching of hyphae can be seen . In early stages of mantle formation, hyphae may surround root hairs  while others can be seen embedded in mucilage on the root surface .

Extraradical hyphae are not usually abundant. In sectional view, the mantle appears as a thin layer of hyphae partially embedded in mucilage.The Hartig net develops as a uniseriate layer between epidermal and cortical cells . Frequently, most of the cortex is occupied by Hartig net hyphae .

Intracellular hyphae develop a hyphal complex within epidermal and cortical cells that remains intact when fresh roots are squashed under a cover glass . The branching nature of the hyphal complex is evident in material examined with SEM (Figure 124). The host cell nucleus is usually surrounded by the hyphal complex .

Intracellular hyphae are separated from the host cell cytoplasm by the elaboration of host plasma membrane (Figure 125); these hyphae and Hartig net hyphae have numerous mitochondria as well as small vacuoles with dense deposits, perhaps polyphosphate (Figures 125–128).

Branched Hartig net hyphae (Figure 128) and intracellular hyphae frequently show
Woronin bodies, typical of ascomycetous fungi (Figure 127). Although extraradical hyphae do develop, the extent of their development in the soil has not been assessed and there is no experimental evidence yet that they are involved in transport of nutrients to roots.

 Functions

There is limited information concerning the contribution of the Hartig net and intracellular hyphae to the functioning of ectendomycorrhizas. To date, few definitive studies show benefits to plants that have ectendomycorrhizas. Pinus and Larix seedlings growing in disturbed sites frequently have ectendomycorrhizas and in some cases presumably benefit from the association.

It is known that some fungal species forming ectendomycorrhizas are able to break down complex carbohydrates; perhaps sugars are being transferred to young seedlings before they develop full photosynthetic capacity. Wilcoxina mikolae and Wilcoxina rehmii are able to produce the siderophore, ferricrocin.

Therefore, in mine spoils containing high concentrations of heavy metals such as iron, ectendomycorrhizas formed with Wilcoxina spp. may protect plants from toxicity induced by iron. The high incidence of ectendomycorrhizas reported on Pinus and Larix species in seedling nurseries may be related to the cultural practices.

In one study, a correlation was found between the incidence of ectendomycorrhizas and root rot symptoms on Pinus strobus seedlings, suggesting that ectendomycorrhizas may be indicative of poor seedling health (Ursic et al. 1997). A survey of urban white (Picea glauca) and blue or Colorado (Picea pungens) spruce showed a high occurrence of roots with ectendomycorrhizal associations, particularly on young trees (Danielson and Pruden 1989).

The fungal species involved appeared to be tolerant to the dry, moderately alkaline soil conditions in the urban setting. Much work remains to be done with ecten-
domycorrhizas from a structural and ecological perspective. Ecologically, Wilcoxina species are likely able to form ectomycorrhizas with numerous hosts, and have the potential to link tree species in a fungal network.