Introduction to Fungi, Third Edition

PHYSIOLOGYOF THE GROWING HYPHA
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(Fig. 1.11), which was at one time thought to be
a causal factor of hyphal tip polarity but is now
regarded as a consequence of it (Harold, 1994).
Proton pumps fuelled by ATP are prominent
also in the vacuolar membrane, the tonoplast
(Fig. 1.11), and their activity acidifies the vacuolar
lumen (Klionsky et al., 1990). The principle of
proton-coupled solute transport is utilized by
the vacuole to fulfil its role as a system for the
storage of nutrients, for example phosphate
(Cramer & Davis, 1984) or amino acids such as
arginine (Keenan & Weiss, 1997), or for the
removal of toxic compounds from the cytoplasm,
e.g. Ca 2þ or heavy metal ions (Cornelius &
Nakashima, 1987).
Fig1.10 Schematic summary of the pathways of membrane
flow in a growing hypha. Secretory proteins (), vacuolar
luminal proteins (), membrane-bound proteins ( ),
endocytosed (g) and autophagocytosed (m) material is
indicated, as are vacuolar degradation products (). Redrawn
and modified from Weber (2002).
to the growth of the hypha at its tip. The spatial
separation of H þ expulsion and re-entry generates
an external electric field carried by protons
1.2.7 Hyphal branching
Assimilative hyphae of most fungi grow monopodially
by a main axis (leading hypha) capable
of potentially unlimited apical growth. Branches
arise at some distance behind the apex, suggesting
some form of apical dominance, i.e. the
presence of a growing apex inhibits the development
of lateral branches close to it. Dichotomous
branching is rare, but does occur in Allomyces
(see Fig. 6.20d) and Galactomyces geotrichum.
In septate fungi, branches are often located
immediately behind a septum. Branches usually
arise singly in vegetative hyphae, although
whorls of branches (i.e. branches arising near a
common point) occur in reproductive structures.
Branching may thus be under genetic or external
control (Burnett, 1976). An even spacing between
vegetative hyphae results from a combination of
chemotropic growth towards a source of diffusible
nutrients, and growth away from staling
products secreted by other hyphae which have
colonized a substratum. The circular appearance
of fungal colonies in Petri dish cultures
arises because certain lateral branches grow out
and fill the space between the leading radial
branches, keeping pace with their rate of growth.
This invasive growth is the most efficient way to
spread throughout a substratum. In nature,
it may be obvious even to the naked eye,
for example, in the shape of fairy rings (see
Figs. 19.18a,b).