74 CHAPTER 4
microcycle sporulation. It occurs naturally in some
fungi, especially if they grow in water films in nutrient-
limited conditions. For example, microcycle sporulation
has been reported for some saprotrophs on leaf surfaces
(e.g. Cladosporium, Alternariaspp., Chapter 11), some
leaf-infecting pathogens (e.g. Septoria nodorum), several
vascular wilt pathogens that colonize xylem vessels (e.g.
Fusarium oxysporum, Chapter 14), and the rhizosphere
fungus Idriella bolleyiwhich is a biological control
agent of root pathogens. All these fungi will germinate
to form normal hyphae in nutrient-rich conditions, so
their microcycling behavior in nutrient-poor conditions
might be a means of spreading to new and potentially
more favorable environments.
Spore germination tropismsA tropismis defined as a directional growth response
of an organism to an external stimulus. The spores of
some fungi show this very markedly, a classic exam-
ple being the yeast-like fungus Geotrichum candidum
which is a common cause of spoilage of dairy prod-
ucts. As shown in Fig. 4.8, the cylindrical spores of this
fungus germinate typically from one or other pole, but
the site of germ-tube emergence is influenced strongly
by the presence of neighboring spores when the
spores are seeded densely on agar and covered with a
coverslip. In these conditions the germ-tubes always
emerge from the end furthest away from a touching
spore – a phenomenon termed negative autotropism.
The causes of this behavior are still unclear. On the one
hand, it has been suggested to involve the release of
auto-inhibitors, which would accumulate maximally in
the zone of contact of two spores but could diffuse away
from the “free” ends, leading to germination there. On
the other hand, oxygen depletion in the zone of spore
contact could be a critical factor for G. candidumbecause the spores always germinate towards an oxy-
gen source (a small hole in a plastic coverslip placed
over the spore layer) and this positive tropism to
oxygen could overcome the negative autotropism of
touching spore pairs.
The spores of Idriella bolleyi (a mitosporic fungus) also
show negative autotropism, but they show an even more
spectacular response when placed in contact with
cereal root hairs (Fig. 4.9). The spores of Idriellaalways
germinate away from living root hairs but towards
dead root hairsand rapidly penetrate them. This
behavior seems to be ecologically relevant because I.
bolleyiis a weak parasite of cereal and grass roots. It
exploits the root cortical cells as they start to senesce
naturally behind the growing root tip, and in doing
so it competes with aggressive root pathogens that
otherwise would use the dead cells as a food base for
infection. Thus, the spore germination tropisms of I.
bolleyihelp to explain its role as a biological control
agent of cereal root pathogens, similar to the role of
nonpathogenic strains of the take-all fungus, dis-
cussed in Chapter 12. The tropic signals for I. bolleyi
spores seem to be quite specific, because G. candidum
and some other fungi tested in the same conditions
showed quite different responses; for example, G. can-
didumgerminated towards both living and dead root
hairs (Allan et al. 1992).
Fungal spores can also show orientation responses
to electrical fields of sufficiently high strength (5–
20 V cm−^1 ). For example, in one study the spores of
Neurospora crassaandMucor mucedowere found to
germinate towards the anode, whereas spores of
Emericella nidulansshowed no significant orientation
response. The somatic (older) hyphae of these and
other fungi showed an array of orientation responses:
Neurospora hyphae grew towards the anode and
formed branches towards the anode; but hyphae of E.
nidulans and M. mucedo grew and branched towards theFig. 4.8Germination behavior of spores of
Geotrichum candidum, when incubated in
a thin water film beneath a coverslip. The
spores always germinate from positions
near their poles. Arrows indicate the posi-
tions of germ-tube outgrowth in different
conditions. (a–d) Negative autotropism of
spores touching in pairs or in groups – the
spores always germinate from a position
furthest from a touching spore. (e) The
presence of oxygen (a small hole in the
coverslip) negates the negative autotropism- the spores germinate from a point clos-
est to the oxygen source. (Redrawn from
Robinson 1973.)