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The major types of symbiosis involving fungi

Mycorrhizas

Under agricultural field conditions, plants do

not, strictly speaking, have roots, they have

mycorrhizas. (Stephen Wilhelm)

The term mycorrhizameans, literally, “fungus root.”

It refers to the fact that fungi form many types of sym-

biotic association with the roots or other underground

organs of plants. These symbioses are extremely com-

mon. They are estimated to occur in at least 80% of

all vascular plants, including angiosperms (the flower-

ing plants), gymnosperms (the cone-bearing plants),

many pteridophytes (ferns and their allies), and some

bryophytes (especially liverworts). Many of these asso-

ciations are thought to be mutualistic, because the

fungus typically absorbs mineral nutrients from the

soil and channels these to the plant, while the plant

provides the fungus with sugars. However, there are

several different types of mycorrhiza, with different

properties and features.

We saw in Chapter 1 that fossils from the Rhynie

Chert deposits in Scotland (see Fig. 1.2) contain

fungal structures similar to those of the most common

mycorrhizal fungi today – the arbuscular mycorr-

hizal fungi. So, it seems that some of the earliest land

plants had already established mycorrhizal associa-

tions, and these might even have been a prerequisite

for life on land (Simon et al. 1993).

The various types of mycorrhiza

Table 13.1 lists some of the main types of mycorrhiza

and their main ecological roles. In addition to the com-

mon arbuscular mycorrhizas, there are several other

types that have evolved independently of one another

(Harley & Smith 1983) and that serve different roles.

We will consider the commonest types in terms of

the plant–fungus interaction and their frunctional

significance.

Arbuscular mycorrhizas (AM)

Arbuscular mycorrhizas are the most common type of

mycorrhiza and are found world-wide on crop plants,

wild herbaceous plants, trees, many pteridophytes,

and some bryophytes. Until recently, the AM fungi

were classified as members of the Zygomycota. But

analysis of the genes encoding the small subunit (18S)

ribosomal RNA shows clearly that AM fungi are not

related to Zygomycota and probably share common

ancestry with Ascomycota and Basidiomycota. So they

have been assigned to a new monophyletic group, the

Glomeromycota(Scheussler et al. 2001; see Fig. 2.4).

Seven genera are recognized within this, based primarily

on features of the spores: Acaulospora, Entrophospora,

Archaeospora, Glomus, Paraglomus, Gigaspora, and

Scutellospora. None of these fungi can be grown in

FUNGAL SYMBIOSIS 257

Type of symbiosis

Mycorrhizas

Lichens

Geosiphon pyriforme
Fungal endophytes of grasses
(Chapter 14)
Rumen symbioses (Chapter 8)

Bark beetles and ambrosia fungi

Siricid wood wasps

Fungus gardens of leaf-cutting ants,
termites, and wood-boring
beetles

Other partners

Land plants – bryophytes, pteridophytes,

gymnosperms, angiosperms

Green algae or cyanobacteria

Nostoc(cyanobacterium)

Several grasses in the family Poaceae

Ruminant animals

Beetles (Scolitidae)

Trees

Attine ants, termites, beetles

Fungi

Ascomycota, Basidiomycota, or

Glomeromycota

Ascomycota or sometimes

Basidiomycota

Glomerales-like fungus

Neotyphodium and Clavicipitaceous

fungi

Obligately anaerobic

Chytridiomycota

Various Basidiomycota, Ascomycota,

and mitosporic fungi

Amylostereum areolatum

(Basidiomycota)

Basidiomycota (Termitomyces;

Leucagaricus)