Instant Notes: Plant Biology

which the plant gains at the expense of the fungus. Mycorrhizae resemble

pathogenic fungi in many ways and it can be difficult to distinguish the two

since sites of infectioncan be the same. When working to mutual benefit,

sugars generated by photosynthesis in the plant are transported to the roots

and taken up by the fungi, and nutrients, most importantly nitrogen and

phosphorus, are absorbed by the fungal hyphae in the soil and conducted to

the plant. Many plants with endomycorrhizae can survive without them,

particularly on nutrient-rich soils, but those with ecto- and ectendomycor-

rhizae are normally obligatelyassociated at least at some stage in their life

cycle.

Numerous species of fungi are involved and some are specific to particular

species or genera of plants, whereas others infect many species; most plants can

be infected by many species of fungi. The growth form of the fungi varies

greatly, with some producing a mass of hyphae close to the roots, others

spreading more widely and thinly. Endomycorrhizae grow out from a root to

form an expanding fan and normally do not fuse with other neighboring fungi.

Ectomycorrhizae start growing in a similar way but when they meet hyphae

from other ectomycorrhizae these can fuse, or anastomose, and the mycorrhizae

form a network of interconnecting hyphae in the soil.

The main effect of the fungi is absorbing nutrients from the soil. Roots infected

with endomycorrhizae retain root hairs and are able to absorb nutrients, but

ecto- and ectendomycorrhizal roots generally lose all their root hairs and rely

solely on the fungus. Endomycorrhizal fungi have an extensive hyphal network

in the soil making absorption more efficient than that by roots alone. Ecto- and

ectendomycorrhizae have a similar hyphal network, but are also able to digest

Effects of the
fungi on plants

M1 – Mycorrhiza 203

Sheath of hyphae

Fungal hyphae

Root cortex

10 μm

5 mm

(a)

(b)

Fig. 2. Roots infected with ectomycorrhizae: (a) absorbing root showing branching and

absence of root hairs; (b) dense net of fungal hyphae around root.