Growth, Differentiation and Sexuality

fungi can be regarded as “tube-dwelling amoebae”,
more recently advocated by Gregory (1984) and
Heath and Steinberg (1999).
However, if the structured apical cytoplasm
prevents turgor pressure from rupturing the most
apical part of the wall (Wessels 1988), then Reinhardt’s
conclusion that the wall must have uniform
strength over the apex, growing by intussusception,
can be dismissed. New models taking into account
the presence of cytoskeletal elements might
assign a fundamental role to the cytoplasm in shaping
the hyphal apex while retaining the concept of
a deformable wall expanding under the influence
of pressure exerted by the cytoplasm (Heath and
Steinberg 1999; Heath 2000). If there is a gradient
in the plasticity of the wall, maximal at the
very apex and declining towards the base of the extension
zone, then how is this gradient generated?
Two possibilities have been considered: either the
wall is originally plastic and expands until it becomes
rigid or the wall is synthesised as a rigid
entity and cannot expand unless it is plastified by
wall-modifying enzymes. Robertson (1958, 1965)
implicitly based his explanation of the behaviour
of living hyphae on a concept of “setting” of the
wall after its formation. Work on wall biogenesis
in Schizophyllum commune has given this concept
a molecular basis and has been named the “steadystate
model of apical wall growth” (Wessels 1986).
Theconceptthatawallmustbecontinuouslyloosened
by lytic enzymes in order to expand is well
established for intercalary wall growth in bacteria
(Koch 1988) and plants (Cleland 1981), and
has also been suggested to occur during growth
of some mushrooms (see The Mycota, Vol. I, 1st
edn., Chap. 22). Johnson (1968), Bartnicki-Garcia
(1973) and Gooday (1978) have advocated that wallloosening
enzymes also play a role in apical extension
of mycelial fungi. However, direct evidence for
this concept, which presumes a “delicate balance
between wall synthesis and wall lysis” (Bartnicki-
Garcia 1973), has not been obtained to date. In
fact, inhibition of chitinases does not affect apical
growth (Gooday et al. 1992). However, the possibility
that other hydrolases or transferases, some with
as yet unknown specificities, play a role in modifying
the apical wall has not been ruled out, and
it may be difficult to do so. For the moment, the
strength of the evidence supports the alternative
concept of an inherently plastic wall which undergoes
hardening with time (Sect. V).
Although the present review concentrates on
the biogenesis of the wall over the apex, it is clear
Apical Wall Biogenesis 55
that hyphal morphogenesis by apical growth involves
activities of the whole hypha (Harold 1990,
1999), with an emphasis on activities involving
the cytoskeleton (see The Mycota, Vol. I, 1st edn.,
Chap. 3; Heath 2000; Steinberg 2000). Molecular genetic
studies are currently identifying the participating
components (McGoldrick et al. 1995; Harris
et al. 1999; Weber et al. 2003). Molecular genetic
studies pertaining to signalling cascades are discussed
below (Sect. VI.B). The advance provided
by these studies is that they guide us to proteins
which contribute to the intricate network of reactions
occurring at the growing tip. Indications
were obtained that microtubules and microfilaments
are involved in the apical organization of
the Spitzenkörper, which is a specialised accumulation
of vesicles implicated in the polar exocytosis of
fungal hyphae. Both kinesins and dynesins motor
molecules are thought to participate in organizing
the Spitzenkörper (Seiler et al. 1997; Riquelme et al.
2000; Weber et al. 2003).
III. Chemical Composition of Fungal
Walls
For detailed information on the chemistry of fungal
walls and its relation to taxonomy, the reader is
referred to a number of reviews (Bartnicki-Garcia
1968; Wessels and Sietsma 1981; Bartnicki-Garcia
and Lippman 1982; Farkas 1985; Ruiz-Herrera 1992;
see The Mycota, Vol. I, 1st edn., Chap. 6, Vol. VIII,
Chap.9,andLatgéandCalderone,Chap.5,thisvolume).
Only general aspects will be mentioned here,
with an emphasis on the cross-linking between different
polymers.
The gross monomeric composition of an
average wall of a species belonging to the Dikaryomycota
(Ascomycetes or Basidiomycetes) shows
a predominance of glucose (about 60%–70%)
and aminosugars (15%–20%), predominantly
N-acetyl-glucosamine. In budding yeasts, mannose
is prominently present as a constituent of
mannoproteins (see The Mycota, Vol. I, 1st edn.,
Chap. 6, and Vol. VIII, Chap. 9). In members of
the Zygomycetes, glucose is usually replaced by
glucuronic acid and the predominant aminosugar
is glucosamine, rather than N-acetylglucosamine
(Datema et al. 1977b).
Roughly half the amount of glucan in hyphal
walls of the Dikaryomycota is alkali-soluble and
represents in most cases a (1-3)-α-linked glucan