Instant Notes: Plant Biology

Section M – Interactions between plants and other organisms

M4 Fungal pathogens and endophytes

Different fungal species possess a wide range of methods capable of degrading

a vast range of materials, including lignin and cellulose. Many fungi are sapro-

phytesand live by degrading material from plants that have already died, and

these are essential in soil formation. Others are pathogensthat cause a wide

range of plant diseases resulting in significant losses in agriculture. Table 1gives

some examples of fungal diseases and the organisms that cause them.

Infection occurs when a sporegerminates on a leaf surface or wound site. The

fungus penetrates, often through stomata, and forms fine threads of cells

(hyphae) which ramify to form a mat (mycelium) throughout the plant. Infection

in cells frequently resembles the arbuscules of endomycorrhizae (Topic M1).

Fungi have complex life cycles, which may involve one or more host plant

(heteroecious life cycle). Wheat rust, Puccinia graminis for instance, a

Basidiomycete, invades two host plants. Commonly, the first host is the

barberry (Berberissp.) from which aeciospores(containing two nuclei, one +

Fungal diseases

Key Notes

Fungi are important saprophytes that recycle dead plant material. They

also cause a wide range of plant disease. Fungal spores germinate on a leaf

surface or wound site and form a mycelium through the plant. Their life

cycles may involve more than one host plant (e.g. wheat rust, which is

heteroecious) or a single host plant (e.g. smut fungi, which are autoecious).

Defenses may be mechanical, preventing spore adherence or penetration,

or chemical, e.g. saponins that disrupt fungal membranes. Other defenses

are induced when a pathogen attacks, genes being activated by an elicitor

binding to a plasma membrane receptor. In a hypersensitive response, cells

fill with phenolic compounds and form a necrotic lesion. A lignin or callose

barrier may form. In many species, antimicrobial phytoalexins are

produced. Systemic acquired resistance occurs when resistance is enhanced

as a result of an infection. Fungi may stimulate changes in metabolism or

phytohormone levels in the plant leading to changes in growth form.

Fungi are often found growing freely within plants which have no, or

minimal symptoms. They can be particularly dangerous to humans as

they can be very toxic. Best known is ergot, a fungus growing in rye that

generates a toxic precursor to LSD. Some may become parasites as the

plant ages.

Related topics Amino acid, lipid, polysaccharide Stress avoidance and adaptation

and secondary product (G5)

metabolism (J5) Mycorrhiza (M1)

Fungal diseases

Plant defenses

against fungal

diseases

Fungal endophytes