tional equivalent of golgi bodies in fungi) further back
in the apical compartment, and to be transported to
the apex where they contribute to apical growth. In sup-
port of this view, cytochemical staining of some of the
vesicles has shown that they contain chitin synthase,
the enzyme responsible for producing the chitin com-
ponent of fungal walls.
The collection of vesicles at the hyphal tip is termed
the apical vesicle cluster (AVC). At high magni-
fication in the true fungi the center of the AVC seems
to be a vesicle-free core consisting of a network of actin
microfilaments, with microtubules running through
this and sometimes extending up to the extreme tip
of the hypha. Early light microscopists had recognized
the existence of the AVC as a phase-dark body in the
hyphal tips of septate fungi, when viewed by phase-
contrast microscopy. It was termed the Spitzenkörper
(“apical body”), and it was noted to disappear when
growth was stopped by applying a mild shock to the
hyphal tip, and to reappear when growth restarted.
Moreover, the Spitzenkörper was seen to shift towards
the side of a hyphal tip when hyphae changed their
growth direction, and more recently the Spitzenkörper
has been shown to split prior to the formation of a
hyphal branch, so that two tips are produced from the
original one. All this evidence points to a central role
of the Spitzenkörper in fungal tip growth – a body
that functions as the equivalent of a “tip-growth
organelle.”
Behind the extreme tip is a short zone with few or
no major organelles. Then typically there is a zone rich
in mitochondria(Figs 3.2, 3.4). Through the actions
of ATPase, this zone almost certainly generates the
proton-motive force (H+gradient across the cell mem-
brane) that drives nutrient uptake at the hyphal tip,
as explained in Chapter 6 (Fig. 6.3). The mitochondria
of fungi have plate-like cisternae similar to those of
animals and plants (see Fig. 3.12), whereas the
mitochondria of Oomycota have tubular cisternae
(Figs 3.2 and 3.3). Further back along the apical com-
partment there is increasing development of a system
of branched tubular vacuoles(Fig. 3.4). These extend50 CHAPTER 3
Fig. 3.2Electron micrograph of the hyphal
tip of Pythium aphanidermatum(Oomycota)
prepared by chemical fixation. The apex
contains a cluster of Golgi-derived vesicles
( V ) of at least two types: large with elec-
tronlucent contents and small with elec-
trondense contents. Mitochondria (M) are
abundant in the subapical zone but are
absent from the extreme tip. Other compon-
ents include: the hyphal wall ( W ), plasma
membrane (P) which has an indented
appearance due to chemical fixation, micro-
tubules (MT ) which are poorly resolved,
and clusters of ribosomes (R). (Courtesy of
C. Bracker; from Grove & Bracker 1970.)