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FUNGAL GROWTH 69

apex. If the tip readjusts to the new osmotic conditions
in time it can grow on, but now from a thinner region
of the apex where the wall has not yet rigidified.
Roughly half of the tips in the original experiment
seemed to be able to do this. However, if the tip can-
not adjust in time then the apex will be sealed off by
rigidification, and growth will only occur when new
tips have been produced – in this case behind the
original apex and by a process that takes several
minutes. This would explain why all the tips stopped
for several minutes when water was replaced by the
isotonic solution, because the tips would need to
make two separate osmotic adjustments and could not
do so before the apex had rigidified.
We shall see later that this explanation was essen-
tially correct. It is now supported by many lines of
evidence from wall enzymology and ultrastructural
studies. But at this stage we should note some further
points. Sometimes the hyphal tips swell and burst in
response to flooding with water, perhaps because the
wall at the extreme apex is too fragile to adjust to
rapid changes in osmotic potential or perhaps because
the wall at the extreme apex is continuously being
degraded by wall-lytic enzymes. Sometimes the tips
grow on as usual after being flooded with water, but a
branch develops later from the position where the
apex had reached at the time of flooding. In any case,
growing hyphal tips are very sensitive to many types
of disturbance and they tend to respond in the same
way – by a “stop–swell–branch” sequence as shown in
Fig. 4.3. This response can be elicited by mild heat or
cold shock, by exposure to an intense light beam, or

even when hyphal tips encounter physical barriers.

Some morphological mutants of Neurospora and

Aspergilluseven show this pattern regularly during

growth on agar plates. In Chapter 5 we shall see that

the stop–swell–branch sequence occurs during the

production of several differentiated structures, includ-

ing the pre-penetration structures of fungal parasites of

plants and animals.

Assembly of the wall at the hyphal apex

Wall synthesis at the hyphal apex is a complex pro-

cess, the details of which are still not fully known, but

from various lines of evidence we can construct a

composite picture of wall growth and subsequent wall

maturation (Figs 4.4, 4.5). Some of the main compon-

ents of this system are discussed below.

Chitin synthase

Chitin synthase catalyses the synthesis of chitin

chains, and is therefore one of the principal enzymes

involved in fungal wall growth. Chitin is known to

be formed in situat the apex, rather than arriving

in membrane-bound vesicles. When hyphal homo-

genates are tested for enzyme activity in vitro, chitin

synthase is found in at least two forms: as an inactive

zymogen in chitosomes (Chapter 3), and as an

integral membrane protein. We saw in Chapter 3 that

chitosomes resemble some of the microvesicles in

the hyphal apex (see Fig. 3.13). However, the “shell”

Fig. 4.3Robertson’s experiments on hyphal
tip growth. See text for details.