FUNGAL GROWTH 73
membrane is stretched. The significance of this is that
the intracellular levels of free calcium are always
tightly regulated – cells maintain a low calcium level
by sequestering excess calcium in intracellular stores
such as the endoplasmic reticulum, mitochondria, and
vacuoles. So any localized ingress of calcium through
the plasma membrane would cause a perturbation,
including an interaction with the cytoskeleton, because
calcium is known to cause the contraction of F-actin.
There is abundant evidence for a role of calcium-
mediated signaling in many fungi and other organisms,
but the details of calcium signaling and how it relates
to tip growth remain unclear. Bartnicki-Garcia (2002)
provides an excellent and thought-provoking review
of this and other outstanding questions in hyphal tip
growth.
Spore germination and the orientation of
hyphal tip growth
Fungi respond to many types of environmental signal,
including signals that trigger spore germination (i.e. the
production of a hyphal tip where none existed before
- see Chapter 10) and signals that change the orien-
tation of hyphal tip growth. Below we consider several
examples of these processes.
Studies on germinating spores
Some fungal spores, such as the uredospores of rust
fungi (Basidiomycota), have a fixed point of germina-
tion termed the germ pore, where the wall is con-
spicuously thinner than elsewhere. Similarly, the
zoospores (motile, flagellate cells) of Chytridiomycota,
Oomycota, and plasmodiophorids have a fixed point
of germination, and they settle and adhere to recept-
ive surfaces so that their future point of germ-tube
outgrowth is located next to that surface (Chapter 10).
However, many spores seem to be able to germinate
from any point on the cell periphery. The germination
process often follows a common pattern (Fig. 4.7).
Initially, the spore swells by hydration, then it swells
further by an active metabolic process and new wall
materials are incorporated over most or all of the cell
surface – the phase termed nonpolar growth. Finally
a germ-tube (a young hypha) emerges from a localized
point on the cell surface, and all subsequent wall
growth is localized to this region. The first sign that
an apex will emerge is the localized development of an
apical vesicle cluster.
In the conidia of Aspergillus nigerthe transition from
nonpolar to polarized growth is temperature-dependent
(Fig. 4.7). At a normal temperature of about 30°C, the
spore initially incorporates new wall material over the
whole surface and then an apex is formed. However
when the spores are incubated at 44°C they continue
to swell for 24 – 48 hours, producing giant rounded cells
up to 20–25μm diameter (a 175-fold increase in cell
volume) with walls up to 2μm thick. At this stage the
cells stop growing. But if these “giant cells” are shifted
down to 30°C before they stop growing they will
respond by producing a hyphal apex, and this behaves
in an unusual way: instead of forming a normal hypha
it produces a small spore-bearing head (Fig. 4.7). These
observations suggest two things. First, that the trans-
ition from nonpolar to polar growth in A. nigeris
temperature-dependent – it is blocked at the restrictive
temperature (44°C). Second, that the fungus can still
“mature” at the restrictive temperature: it reaches a
developmental stage at which it is committed to
sporulate, and it does so as soon as the temperature is
lowered.
The production of spores from germinating spores
with a minimum of intervening growth is termedFig. 4.7Stages in germination of spores
of Aspergillus niger. (a) In normal condi-
tions (e.g. 30°C) the spore swells and
incorporates new wall material over the
whole of the cell surface (shown by stip-
pling), then a germ-tube emerges and
all new wall incorporation is localized
to the hyphal tip. (b) At 44°C the spore
continues to swell and incorporates wall
material in a nonpolar manner, produc-
ing a giant cell with a thick wall. If the
temperature is lowered to 30°C this cell
produces an outgrowth, which immedi-
ately differentiates to produce a spore-
bearing head. (Based on Anderson &
Smith 1971.)