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culture, away from their host plants, so they are
considered to be wholly dependent on plants for their
carbon and energy sources.
Roots containing arbuscular mycorrhizal fungi show
no outward signs of infection. Instead, they look like
normal roots, and the extent of colonization by fun-
gal hyphae can only be seen by special techniques. One
method for this is to use differential interference con-
trast microscopy. But the more common method is to
treat roots with strong alkali, which destroys the plant
protoplasm, and then to stain the roots with a fungal
dye such as trypan blue. Fungal hyphae are seen to colo-
nize the roots extensively by growing between the root
cortical cells, often producing large, swollen vesicles,
and by penetrating individual root cortical cells to
form tree-like branching structures termed arbuscules
(Fig. 13.1).
The AM fungi produce large spores, up to 400μm
diameter, which are easily visible to the naked eye. These
spores can be extracted by washing soil through a
series of sieves with successively smaller mesh sizes, then
floating the spores to retrieve them from the sieves.
Single spores can then be used to initiate “pot cultures”
with seedlings in sterilized soil – the standard way of
maintaining different strains. The spores germinate
and infect the roots from an appressorium-like infec-
tion structure on the root surface, similar to the infec-
tion structures of plant pathogens discussed in
Chapter 5. From these initial entry points, the fungi
grow extensively between the cells of the root cortex
(Fig. 13.2), often producing large, swollen vesicles
within the root. These vesicles are thought to have a

storage function. Other hyphae penetrate individual

root cells and branch repeatedly within these to form

dichotomous tree-like structures, termed arbuscules

(Fig. 13.3). These are thought to be the main sites of

nutrient exchange between the fungus and the root cells.

Consistent with this, the invaded cells remain alive

because the plant cell membrane invaginates to sur-

round all the individual branches of the arbuscule – the

membrane itself is never penetrated. The individual

arbuscules live for a relatively short time (2–3 weeks)

and then degenerate, being replaced by new arbuscules

in other parts of the root.

Ecological significance of arbuscular mycorrhizas

Many studies have shown that the principal role of AM

fungi is to provide plants with mineral nutrients, and

especially phosphorusfrom the soil, while the plant

provides the fungus with sugars. In terms of plant nutri-

tion, phosphorus is second only to nitrogen as the major

mineral nutrient that plants require, and yet phosphorus

is a highly immobile element. When added to soil in

the form of soluble phosphate fertilizers, the phosphate

ion readily combines with calcium and other divalent

cations, to form insoluble inorganic phosphates, or it

combines with organic matter to produce insoluble

organic phosphates. The natural rate of release of

phosphate is thus extremely slow and is often the

limiting factor for plant growth. The AM fungi produce

extensive hyphal networks in soil, providing a large

surface area for absorption of phosphorus. These fungi

also release acid phosphatasesto cleave phosphate from

258 CHAPTER 13

Table 13.1The major types of mycorrhiza and their ecological significance.

Mycorrhizal type

Arbucular mycorrhizas

Ectomycorrhizas

Ectendomycorrhizas

Arbutoid mycorrhizas

Monotropoid
mycorrhizas

Ericoid mycorrhizas

Orchid mycorrhizas

Major significance

Phosphorus uptake from

soil

Nitrogen uptake from

soil

Mineral nutrient uptake

from soil

Mineral nutrient uptake

from soil

Plants obtain sugars

from ectomycorrhizal

fungi attached to trees

Nitrogen uptake from

soil

Fungi supply the plant

with sugars

Fungi involved

Glomeromycota

Basidiomycota,

Ascomycota

Ascomycota of the

genus Wilcoxina

Basidiomycota, similar to

ectomycorrhizal fungi

Basidiomycota such as

Boletus edulis

Ascomycota and

mitosporic fungi;

Hymenoscyphus ericae

Rhizoctonia-like fungi

(basidiomycota)

Typical host plants

Many

Forest trees, mainly in

temperate and boreal regions

Mainly pines, spruce, and larch

Arctostaphylos, Arbutus, Pyrola

Nonphotosynthetic plants, e.g.

Monotropa

Heathland plants. Erica,

Calluna, etc.

Orchids