Growth, Differentiation and Sexuality

ingredients such as wall components and synthesising
enzymes but also lytic enzymes presumed
necessary to plastisise the wall and allow insertion
of new wall material, as originally conjectured
by Bartnicki-Garcia (1973). By assuming the VSC
to be stationary, a spherical cell was obtained,
growing only in diameter. By assuming the VSC to
move in one direction, a gradient in wall expansion
was simulated, resembling the outline of a growing
hypha. Qualitatively this can be easily appreciated
by realising that a maximum of vesicles reach the
surface in the direction of movement of the VSC.
Mathematically, the relationship was expressed
as y = x cot xV/N, where x and y are the axes
of the two dimensions, V is the rate of linear
displacement of the VSC, and N is the rate of
increase in area, equivalent to the number of
vesicles released by the VSC per unit of time. The
ratio V/N is the distance, d, between the VSC and
the wall at the extreme tip. When plotting y versus
x, a curve is obtained, called a hyphoid, which
faithfully outlines the shape of median sections
through growing hyphal tips of many fungi, the
emplacement of the VSC closely approximating the
position of the Spitzenkörper. The Spitzenkörper
(apical body) was so named by Brunswick (1924)
who observed it as an iron-haematoxylin positive
area in the cytoplasm of the hyphal tip. A recent
review on the nature and possible roles of the
Spitzenkörper is given by Harris et al. (2005).
Girbardt (1955), using phase contrast microscopy,
observed a Spitzenkörper in growing hyphae
of several fungi. He found that when growth
was arrested, the Spitzenkörper vanished and
reappeared again just before growth resumed.
Electron microscopical observations subsequently
revealed the accumulation of numerous vesicles
at the site where the Spitzenkörper was observed
(Girbardt 1969; Grove and Bracker 1970). A role
in hyphal growth was also evident from Girbardt’s
finding that an off-centre displacement of the
Spitzenkörper preceded a change in growth direction
of the hypha (Girbardt 1957). This observation
was corroborated and extended by observing the
growth direction and shape of fungal hyphae after
experimental displacement of the Spitzenkörper
(Bartnicki-Garcia et al. 1995) or following normal
trajectories of the Spitzenkörper (Riquelme et al.
1998). All these observations were taken as strong
evidence that the Spitzenkörper is the VSC which
collects secretory vesicles from the subapical
cytoplasm and then radiates these vesicles in all
directions. While moving forwards, being pushed
Apical Wall Biogenesis 63
or pulled (Bartnicki-Garcia et al. 1990), it would
create the necessary gradient in vesicles, fusing
with the apical plasma membrane. The VSC model
has been criticised on both cytological grounds
(Heath and Janse van Rensburg 1996) and on the
basis that it is a two-dimensional model which
does not apply to the three-dimensional shape
of the hypha (Koch 1994). The latter critique
was recently addressed by Bartnicki-Garcia
and Gierz (2001), showing that a mathematical
treatment of three dimensions, incorporating
an orthogonal wall expansion (Bartnicki-Garcia
et al. 2000), essentially leads to the same model as
devised for a two-dimensional projection of the
hypha.
The VSC model envisioned by Bartnicki-
Garcia (1990, 2002) incorporates the concept that
the nascent wall is a basically rigid structure and
that the wall vesicles contain plastisising enzymes.
In the steady-state model of apical wall growth
referred to above, lytic enzymes or other plastisising
agents are not deemed necessary, except for
initiation of a new apical growth point. Johnson
et al. (1996) and Bartnicki-Garcia (2002), who
all have advanced the idea of a balance between
lysis and synthesis in apical wall growth, have
argued that the steady-state model of wall growth
would benefit from incorporating the concept
of lysins. This would lead to a reconciliation of
the VSC and steady-state models. On the other
hand, Wessels (1999) has argued that there is no
contradiction between the models if the need
for lytic action is removed from the VSC model.
The VSC model would then address only the
mechanism by which a gradient in wall synthesis
can become established, the essential feature of the
model. As noted by its inventor (Bartnicki-Garcia
2002), the ultimate validity of the VSC hypothesis
depends on the demonstration that the flow of
wall-building vesicles passes through a Spitzenkörper
control gate. Such traffic of vesicles in/out of
the Spitzenkörper is yet to be demonstrated and
measured.
B. The Self-Sustained Gradient Model
As noted above, the wall retains uniform thickness
during apical extension, meaning that the thinning
of the older wall due to expansion must be exactly
compensated by addition of new wall material.
Gooday and Trinci (1980) have indeed shown that
the deposition of chitin at the apex closely parallels