Growth, Differentiation and Sexuality

204 U. Ugalde
II. Germination
The programmed outcome of every fungal
spore is to land on an appropriate habitat and successfully
establish a new colony. Current evidence
shows that a number of chemosensory signals
are involved in gauging the potential success of
germination at any one circumstance. Many of
these report on the nature of the substratum, be
it as exudates on a host surface or a complex
soil environment where, for example, phenols,
carbohydrates and pH are involved. Additionally,
several examples are known where autoregulatory
signals are operating to prevent the spore from
futile competition with members of its kin which
may have followed a similar dispersion path.
Fig. 11.1. A–J Autoregulatory signals involved in autoinhibition
of germination: A cis-methyl 3,4 dimethoxycinnamate,
B cis-methyl ferulate, C colletofragarone A, D colletofragarone
B, E–G CG1, CG2 and CG3, H quiesone, I
nonanoic acid, and J 1-octen-3-ol
The first demonstration that spores sense and
respondtoanovercrowdedenvironmentthrough
self-produced chemical signals came from studies
with the rust Puccinia graminis (Allen 1955).
These autoregulatory signals have been specifically
termed autoinhibitors or self-inhibitors (Macko and
Staples 1973), and have been reported for more
than 60 fungal species (Allen 1976). A representative
collection is shown in Fig. 11.1.
Germination autoinhibitors are thought to be
producedatthetimeofsporulation,anddeposited
at the outer wall layers of spores, but whether they
are freely deposited or progressively pass from
a bound to an unbound form is still a subject of
speculation (Macko and Staples 1973). Once in contact
with an aqueous medium, they diffuse through
the bulk liquid surrounding the spore but also commonly
partition to the gas phase, enabling the signal
to spread over various diffusion barriers, to
other spores in the vicinity. The signal is extremely
effective, and 50% inhibition has been reported at
nano molar concentrations in those cases in which
it has been measured (Allen 1972).
Germination autoinhibitors of plant rusts
show variability and specificity in mode of
action. In the case of the bean rust Uromyces
phaseoli, the active agent is methyl-cis-3,4dimethoxycinnamate
(Fig. 11.1a) whereas in the
wheatstemrustcounterpartPuccinia graminis,
it is methyl-cis-ferulate (methyl-cis-4-hydroxy-3methoxycinnamate;
Fig. 11.1b). In both cases it
has been shown that the cis isomer is the active
inhibitor, although even low UV radiations result
in a conversion into trans isomers, with both forms
found at equilibrium under natural conditions
(Allen 1972). Other plant pathogens, such as
Colletotrichum fragariae, havebeenreportedto
produce colletofragarone A1 and A2 ((E)- and
(Z)-(3-indoyl)propionic acid; Fig. 11.1c,d) and
colletofragarone B ((2R)-(3-indoyl)propionic acid;
Inoue et al. 1996). The related C. gleosporoides
has been reported to produce CG-SI 1 and 2 ((E)and
(Z)-3-ethylidine-1,3-dihydroindol-2-one;
Fig. 11.1e,f) and CG-SI 3 ((2R)-(3-indoyl)propionic
acid; Fig. 11.1g; Tsurushima et al. 1995). The tobacco
blight fungus Peronospora tabacina,inturn,
uses quiesone (5-isobutyroxy-β-ionone, Fig. 11.1h)
as germination autoinhibitor (Leppik et al. 1972).
In contrast to the chemical specificity displayed
by plant pathogens, saprophytes appear to
use a more generalised signalling cue. Nonanoic
acid (Fig. 11.1i) has been shown to be produced
and sensed by spores of many soil fungi (Garret