untitled

are not used commercially because of their nonspecific

toxicity or other undesirable side-effects. Patulinis

one such example (see Fig. 7.13); it was a promising

antibiotic but its development was abandoned be-

cause, among other things, it was found to be a potent

mycotoxin. However, antibiotics are produced naturally

in a wide range of environments and they play signi-

ficant roles in species interactions. Some of the most

common fungi that produce antibiotics in natural and

agricultural environments are species of Ascomycota

and mitosporic fungi, including Penicillium, Aspergillus,

Fusarium, and Trichoderma. The Basidiomycota also

produce several antibiotics, but very few antibiotics have

been recorded from Chytridiomycota, Zygomycota, or

Oomycota, perhaps because these organisms have a

short life cycle and do not need to defend a substrate

against invaders.

Antibiotics in natural environments:

the control of fungi by fluorescent

pseudomonads

The past two decades have brought major advances in
the detection of antibiotics in the root zone (rhizo-
sphere) of crops, helping to explain how fluorescent
pseudomonads can control fungal pathogens of roots.
Fluorescent pseudomonads are found on the roots
of many plants, often at high population levels, and
can be detected easily by plating soil dilutions onto
“King’s B agar.” This is an iron-deficient medium,
so it induces these bacteria to release fluorescent
siderophores (iron-chelating compounds) to capture
iron (Chapter 6). However, only a small subset of
fluorescent pseudomonads (including strains of
Pseudomonas fluorescensand P. aureofaciens) are highly
effective in controlling fungi. These strains produce
specific antifungal antibiotics such as phenazine-1-
carboxylic acid(PCA) and 2,4-diacetylphloroglucinol
(DAPG) (Fig. 12.1).

Such strains were first recognized in field conditions,

where crops such as tobacco or wheat grown in sites

with naturally high populations of PCA- or DAPG-

producers grew better than crops with low population

levels of these bacteria. The terms disease-suppressive

soil, and disease-conducive soilare used to describe

this difference. In experimental studies, disease-

suppressive soils can be converted to disease-conducive

soils by pasteurization (treatment at about 60°C

for 30 minutes). Conversely, the reintroduction of

antibiotic-producing pseudomonads, at sufficiently

high levels, can render the soils suppressive again.

The most detailed studies on soil suppressiveness have

been made for the take-all fungus, Gaeumannomyces

graminis– one of the most important root pathogens

of cereal crops. This fungus infects roots from inoc-

ulum that persists in soil from a previous cereal crop,

and it then grows along the roots as darkly pigmented

“runner hyphae” (see Fig. 9.11). From these, it sends

infection hyphae into the root, destroying the cortical

cells and entering the vascular system, where it destroys

the phloem (sugar-conducting cells) and blocks the

water-conducting xylem vessels with dark vascular

gels. The level of take-all infection increases progres-

sively from one season to the next if cereals are grown

repeatedly in a site. But after 3 or 4 years of cereal mono-

culture the disease reaches a peak and then spontane-

ously declines to a level at which cereals can be grown

continuously without suffering serious yield losses.

This spontaneous decline in the disease level is termed

take-all decline(Fig. 12.2) and it is always strongly cor-

related with a high population of antibiotic-producing

fluorescent pseudomonads, which seem to be favored

by continuous cereal cropping.

A significant advance in understanding this

phenomenon was made in the early 1990s, when

Thomashow et al.(1990) used high performance

liquid chromatography(HPLC) to detect phenazine-

1-carboxylic acid (PCA) in the rhizosphere of young

238 CHAPTER 12

Phenazine-1-carboxylic acid 2,4-diacetyl phloroglucinol

COOH

OH

O C

CH 3

N

N

HO OH

CCH 3

O

Fig. 12.1Two antibiotics from fluorescent pseudomonads
that are widely implicated in control of plant-pathogenic
fungi.

152 etc

Years of cereal cropping

43

Take-all level

Yield

Fig. 12.2Take-all decline when cereals are grown con-

tinuously (year after year) in field conditions.