Growth, Differentiation and Sexuality

54 J.H. Sietsma and J.G.H. Wessels
by Reinhardt (1892) in which he described studiesonthewidehyphaeofPeziza
species showing
that symptoms of growth are first manifested at the
apices of hyphae. Many of these observations were
later repeated and extended by Robertson (1958,
1965). With regard to the mechanism involved in
apical wall extension, the conclusions put forward
by Reinhardt are still controversial. He discussed
the then prevailing theory of leading botanists who
regarded enlargement of wall area as a process in
which a plastic wall expands due to turgor pressure
while new wall material is being added by
apposition or intussusception. However, he considered
such a theory inadequate because it would
require an increase in mechanical strength of the
wall, going from the apex to the base of the extension
zone. As he puts it, such an increase in
strength could be achieved by a proportional increase
in wall thickness or a change in the quality
of the molecules making up the wall. He found
no evidence for a change in wall thickness. Later
investigators (Girbardt 1969; Grove and Bracker
1970; Trinci and Collinge 1975) also found that
wall thickness remains uniform in the extension
zone. A change in quality of the wall molecules,
which is precisely the kind of change suggested
by work from our laboratory (Wessels 1986, 1988,
1990, 1993; see also Sect. IV), was considered unlikely
by Reinhardt. In that case, the weakest spot of
the wall would be at the extreme tip, and flooding
with water should result in swelling or bursting at
theverytipandnot,asheobserved,atthebase
of the apex where the cylindrical form is attained.
Swelling and bursting subapically have also been
observed by later investigators (Robertson 1958,
1965; Bartnicki-Garcia and Lippman 1972; Wessels
1988). Reinhardt concluded that the wall must have
uniform strength over the whole apex, and does not
grow by plastic expansion but by intussusception
of wall material maximally at the extreme tip and
declining to zero at the base of the extension zone.
Consequently, turgor was not considered to play
a role in apical wall expansion. However, the orthogonal
trajectories of markers at the extending
tip, also surmised by Reinhardt, have been taken as
evidence by Bartnicki-Garcia et al. (2000) that the
wall expands under uniform pressure which can be
exerted only by turgor.
The development of the technique of microautoradiography
has made it possible to prove that
wall material is indeed maximally deposited at the
extremetipanddeclinestolowvaluesatthebase
of the extension zone (Bartnicki-Garcia and Lipp-
man 1969; Gooday 1971; Katz and Rosenberger
1971a). However, these observations do not clarify
the mechanism of wall growth at the apex.
Assuming that turgor is the driving force of apical
wall expansion, and probably inspired by considerations
of D’Arcy Thompson (1917, see Bonner
1961) on the origin of cellular form, mathematicalmodelshavebeenputforwardtodescribe
apical morphogenesis. They all rely on the concept
that extension at the hyphal apex is due to the
presence of a gradient in the plasticity of the wall,
such that there is a decrease in the tendency of the
wall to yield to turgor pressure from the extreme
tip downwards (da Riva Ricci and Kendrick 1972;
Green 1974; Trinci and Saunders 1977; Saunders
and Trinci 1979; Koch 1982). In all these models,
expansion at any point on the apical dome is determined
by its position, according to a mathematical
function which depends on the shape of the apex.
To put these models through a test, it would
seem necessary to determine the plastic and elastic
properties of the wall at various points over the
growing apex but obviously such measurements
are difficult to make. It has, however, been recognised
that in order to maintain uniform thickness
of the expanding wall, a gradient in wall plasticity
must be paralleled by a gradient in wall deposition.
Gooday and Trinci (1980) have indeed shown that
the deposition of chitin, measured by autoradiography,
closely parallels the derived mathematical
equation for wall plasticity.
If apical wall expansion is driven by turgor,
then why do hyphal tips, when subjected to high
turgor pressure, swell and burst at the base of the
apical dome, rather than at the weakest spot, the
extreme apex? Wessels (1988), inspired by observations
of Picton and Steer (1982) on the growth of
pollen tubes, suggested that cytoskeletal elements
in the hyphal apex could protect the delicate, newly
formed wall from becoming subject to high hydrostatic
pressure. Indeed, evidence was obtained in
both oomycetes and true fungi that actin reinforces
the hyphal apex (see The Mycota, Vol. I, 1st edn.,
Chap. 3, and Heath 2000), and at least the oomycete
Saprolegnia ferax appears to be able to generate
normal hyphal growth in the absence of measurable
turgor pressure (Money and Harold 1993). Although
such observations have still to be reported
for true fungi, this indicates that Reinhardt may
have been correct in believing that the shape of the
apical wall is primarily determined by the cytoplasm,
turgor playing only a secondary role. These
observations are also consistent with his view that