Introduction to Fungi, Third Edition

218 ZYGOMYCOTA
mycorrhiza (AM). These fungi are particularly
well-known as mycorrhizal associates of herbaceous
plants, but they may also associate with
trees, especially in the tropics.
Fig 7.4 4 Entomophthora sepulchralis.Three stages in
zygospore formation.Two hyphal bodies conjugate and the
zygospore arises as a bud from the fusion cell (afterThaxter,
1888).
Such symbiotic associations are termed mycorrhiza
(Gr. ‘fungus root’). There are several
different kinds of mycorrhiza, including vesicular
and arbuscular mycorrhiza, ectomycorrhiza
(sheathing mycorrhiza, pp. 21 and 526), ericoid
mycorrhiza (p. 442), and orchid mycorrhiza
(p. 596) (Smith & Read, 1997; Peterson et al.,
2004). It is important to realize that the nature of
the relationships between the fungi and their
host plants in these distinct types of association
is not the same. In this section we shall look
at the Glomales, a group of zygomycetous
fungi causing the development of vesicular
arbuscular mycorrhiza (VAM) and arbuscular
7.6.1 General features of VAM and AM
A coarse, intercellular, aseptate coenocytic mycelium
within the root tissues may develop large,
balloon-shaped intercalary or terminal thickwalled
vesicles (intraradical vesicles) which are
multinucleate and contain large amounts of
lipid (Figs. 7.46c,d). In some plants, e.g. the
roots of Paris, the mycelium emits branches
which penetrate the cortical root cells, forming
extensive intracellular coils. More commonly,
hyphae penetrating host cells fork repeatedly to
form richly branched arbuscules (Fig. 7.46c)
which invaginate the plasmalemma. Plant and
fungal plasma membranes are separated by an
apoplastic compartment, the periarbuscular
space. The arbuscule is therefore a type of
haustorium, and there is an interchange of
nutrients and water across the periarbuscular
space. Arbuscules have a relatively short active
life, lasting only a few days. After this time the
fine tips of the arbuscules are digested by the
host cell so that only irregular clumps of fungal
material remain (Fig. 7.46c).
A coarse, angular and often thick-walled
mycelium extends outwards from infected
roots, sometimes for several cm, and penetrates
into the surrounding soil. It may bear large
(4100 mm dia.) globose multinucleate thickwalled
spores which are sometimes termed
chlamydospores. These spores contain thousands
of nuclei as well as energy reserves including
lipid droplets, glycogen, protein and trehalose.
These spores may be borne singly or in clusters
and are often naked, but in some species,
e.g. Glomus mosseae, they are enveloped in a weft
of hyphae to form a sporocarp (Figs. 7.46a,b).
Chlamydospores are asexual reproductive structures
and are known to survive in dry soil for
many years. For most members of the group only
asexual reproduction is known, but in Gigaspora
decipiens zygospores and azygospores have been
reported in addition to chlamydospores.
This species is heterothallic (Tommerup &