Instant Notes: Plant Biology

strain, the other – strain) are produced which infect the wheat. The wheat then

producesuredospores, which spread the infection in the wheat field. The

fungus overwinters as diploid teliospores, produced when the two (+ and –)

nuclei fuse (karyogamy). Meiosis occurs in spring to release basidiospores(+ or

• strain) which then infect the barberry host (Fig. 1).Smut fungihave a simpler
  life cycle involving only one host (autoecious) and produce masses of
  teliospores(the resting stage of the fungus). They are known to infect at least
  4000 species of flowering plants, many of economic importance.

Some defenses are mechanical barriers, for instance the waxy cuticle, which

prevents spores from adhering to the leaf or the fungus from penetrating the

leaf. Pathogenic fungi can often only infect a wound site or by entering the leaf

through stomata. Other defenses are endogenous chemicals, for instance the

saponinsare triterpenes(Topic J5) produced by plants which disrupt the

membranes of fungi. Other defenses against fungi are not induced until a

pathogen attacks. This involves expression of defense geneswhich are activated

by a signaling cascade (Topic F3) within the plant. This is triggered by the

Plant defenses
against fungal
diseases

214 Section M – Interactions between plants and other organisms

Table 1. Examples of fungal diseases and the causal fungi

Species and plant disease Phylum of fungus

Various species (soft rots) Zygomycota

Corn (brown spot) Chytridiomycota

Various (powdery mildew) Ascomycota

Elm (Dutch Elm Disease)

Cereals (smuts, rusts) Basidiomycota

KaryogamyTeliospores

2 n

Aecium

(n+n)

US

M

M

RE

W

h

e

a

t

B

a

rb

e

rr

y

nI

ef

c

ti

o

n

In

fe

c

it

o

n

Meiosis

Basidiospores

n (+ and –)

Spermagonia give

spermatia (n)

that unite with

receptive hyphae

(n)

Telium (n+n)

Mycelium

(n+n)

Uredenium

Uredospores

(n+n)

Aeciospore

(n+n)

Overwinter

SPR

ING

Fig. 1. Life cycle of a rust fungus, black stem rust of wheat.