Introduction to Fungi, Third Edition

222 ZYGOMYCOTA
interest (Allen, 1991; Smith & Read, 1997; Leake
et al., 2004). There are numerous reports of
significant improvements in growth rate, dry
weight and mineral content following infection
especially of plants growing on nutrient-deficient
soils. Emphasis has been placed on phosphate
nutrition. The supply of phosphate (as
2
HPO 4 or H 2 PO 4 , depending on soil pH) is often a
limiting factor to plants growing in natural soils.
It is usually present in low concentrations and
diffuses through soil very slowly. Its influx may
increase 3 4-fold in infected plant roots but
there are also significant increases in other
minerals such as Zn, Cu, and ammonium. The
water relations and resistance of infected plants
to infections by pathogens may also be improved.
Increased uptake of minerals is largely due to the
exploration of larger volumes of soil by the
extramatrical hyphae which can extend beyond
the depletion zone surrounding plant roots.
The depletion zone is a region in which minerals
are taken up by plant roots at a rate greater than
can be replenished by diffusion through the soil.
For many plants the depletion zone is only
1 2 mm wide whilst the extramatrical hyphae
may extend for several centimetres and can
penetrate into soil cavities too fine to be
explored by roots. Moreover, phosphate can be
translocated through fungal hyphae towards
the host root at much faster rates than is possible
by diffusion through soil. The improved growth
of host plants associated with increased supply
of minerals obtained through the hyphae of the
mycorrhizal symbiont is achieved at a cost to
the plant, i.e. the drain of photosynthate taken
up by the fungus whose biomass, achieved
largely at the expense of the host, may amount
to 3 20% of the root weight. In experiments in
which 14 CO 2 was supplied to the shoots of young
cucumber plants infected by G. fasciculatum, as
much as 20% of the radioactive carbon fixed by
the plant was used by the fungus (Jakobsen &
Rosendahl, 1990).
In nutrient-deficient soils such as sand dunes,
recently disturbed soil, spoil heaps, areas covered
by volcanic ash, etc., successful colonization by
plants appears to be correlated with root infection
by Glomales (Allen, 1991). In closed vegetation
such as mature grassland which contains a
diversity of plants, spore extraction reveals a
wide diversity of AM and VAM species. The roots
of different plant species making up the community
are in close contact and may also be
connected by a hyphal network (Newman,
1988). There is experimental evidence using
isotopically labelled 15 N, 32 P, and 14 C that there
may be an interchange of mineral nutrients and
carbon between unrelated plant species
mediated by VAM mycelia, but Newman (1988)
has cautioned against the conclusion that any
increases in labelled materials necessarily imply
net gains to receiver plants at the expense of
donors. There is also experimental evidence
using soil microcosms seeded with a mixture of
grassland grasses and dicotyledons and inoculated
with Glomus constrictum that mycorrhizal
infection may increase species diversity by
selectively enhancing the performance of less
dominant dicotyledons. This results largely
from a reduction in relative abundance of
canopy dominants such as Festuca ovina (Grime
et al., 1987).
7.7 Trichomycetes
The Trichomycetes are a group of fungi which
grow commensally in the guts of terrestrial,
freshwater and marine arthropods such as
insects, millipedes and crustaceans. In most
cases there is little evidence that the host is
harmed by their presence, although it has been
shown that some species may extend parasitically
into the ovarian tissue to form chlamydospores
(cysts) in place of eggs. These are deposited
amongst egg masses laid by uninfected females.
McCreadie et al. (2005) have documented an
element of plasticity in the association of a
given trichomycete species, Smittium culisetae,
with its blackfly host which may vary from
commensalistic in well-fed larvae to mutualistic
under starvation conditions to parasitic if the
ovaries of adult females are infected.
More than 50 genera and over 200 species
have been described but doubtless many more
await discovery (Lichtwardt, 1986, 1996; Misra,
1998; Misra & Lichtwardt, 2000). Members of the