Introduction to Fungi, Third Edition

GLOMALES
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kind of mycorrhiza are obligate mutualistic
symbionts. Glomalean fungi are generally nonspecific
in their host range. The roots of most
groups of vascular land plants are associated with
this type of mycorrhiza, as are the gametophytic
stages of Bryophyta and Pteridophyta.
7.6.2 Taxonomy and evolution of Glomales
Fungi in this group were originally classified in
the Endogonales but are currently placed in a
separate order, the Glomales, with the
Endogonales now reduced to a single genus,
Endogone, with subterranean fleshy sporocarps
which contain zygospores. Each zygospore is
formed after conjugation of two gametangia
(Pegler et al., 1993). One species, E. flammicorona,
forms ectomycorrhizae with some Pinaceae (Fassi
et al., 1969). The fruit bodies of Endogone spp. are
colloquially called ‘pea truffles’ (Plate 3i; Pegler
et al., 1993).
The order Glomales was proposed by Morton
and Benny (1990) to include all soil-borne fungi
which form arbuscules in obligate mutualistic
associations with terrestrial plants. Sexual reproduction
is rare. There are about 150 species and
6 genera in 2 suborders, the Glomineae with
2 families (Glomaceae and Acaulosporaceae),
and the Gigasporineae with a single family
(Gigasporaceae). Members of the Glomineae
(such as Glomus, Acaulospora) form intraradical
vesicles (VAM type), whilst members of the
Gigasporineae have no intraradical vesicles and
are of the AM type. The separation of genera
within the Glomales is based partly on different
patterns of chlamydospore development, and
partly on the structure of the spore wall which
may be complex and multilayered (Hall, 1984;
Morton & Bentivenga, 1994). Schüssler et al.
(2001) have suggested that the Glomales are not
closely related to the Zygomycota and should
be considered as a separate phylum, the
Glomeromycota.
VAM and AM associations are very ancient,
and structures resembling extant arbuscules
have been discovered in the fossilized rhizome
tissues of early vascular plants, including
Devonian psilophytes such as Rhynia (Pirozynski
& Dalpé, 1989; Taylor et al., 1995). Even older are
the fossilized chlamydospores found among
bryophytes of the Ordovician period (some
460 million years old; Redecker et al., 2000a). It
is believed that the origin and evolution of land
plants was dependent on symbiotic associations
of the VAM and AM type (Pirozynski & Malloch,
1975; Malloch, 1987; Simon et al., 1993).
An interesting non-mycorrhizal relative of
the Glomales is the fungus Geosiphon pyriforme,
which is unusual in harbouring a mutualistic
endosymbiont, the cyanobacterium Nostoc. When
the hyphal tip of Geosiphon encounters a suitable
symbiont, this is taken up and the hypha
swells to form a so-called bladder cell. The
Geosiphon Nostoc symbiosis resembles cyanolichens
(see p. 451) in being autotrophic both for
carbon and nitrogen (Schüssler & Kluge, 2001).
Geosiphon reproduces by forming chlamydospores
similar to those of Glomus. Molecular studies
show that Geosiphon is closely related to the
Glomales and may be ancestral to the group
(Gehrig et al., 1996; Redecker et al., 2000b).
7.6.3 Physiological and ecological studies
The immense current interest in AM and VAM
mycorrhiza has its origins in the demonstration
of the improved growth of mycorrhiza-infected
host plants compared to uninfected controls.
Literature on the physiology of this relationship
has been reviewed by Hause and Fester (2005).
The arbuscule is the main interface for nutrient
exchange between the plant and its fungal
partner, although the latter may also be able to
take up nutrients through intercellular hyphae.
The periarbuscular space is a highly acidic
compartment (Guttenberger, 2000) due to the
outward-directed pumping of protons by H þ
ATPases located in the plasma membranes of
both partners. This sets up proton gradients
which may be used for active uptake of sucrose
hydrolysis products (fructose and glucose) by the
fungus, and phosphate and other mineral nutrients
by the plant. Proton-dependent transport
proteins have been localized in both plant and
fungal perihaustorial membranes (see Hause &
Fester, 2005).
The ecology of VAM and AM fungi in crop
plants and natural communities is of particular