302
Chapter 7
Branching
is the mechanism by which new apical compartments are formed, and it occurs at certain
preferred branching points on a hyphal element. In filamentous fungi, it has been suggested that
branching occurs at locations where for one reason or another there is an accumulation of vesicles,
whereas branch formation in filamentous prokaryotes does not result from accumulation of material
(Prosser and Tough, 1991). Since vesicles are synthesized both in the subapical and in the apical
compartment in hyphae of filamentous fungi, it seems reasonable that at positions where the
protoplasmic flow is reduced, e.g., at the position of a septum, there is an accumulation of vesicles.
Branching may therefore be associated with septum formation, and for the filamentous fungus
Geotrichum candidum
more than 70% of the observed branch points in a subapical compartment
are positioned close to the septum separating this compartment from the apical compartment
(Trinci, 1984). For other species of filamentous fungi there is, however, a more equal distribution of
the branch points throughout the subapical compartment. Branching is observed mainly in the
subapical compartment, but in some filamentous species apical branching may occur.
Originally the growth mechanisms of filamentous microorganisms was studied using surface
cultures, but here the morphology is completely different from that found in a submerged culture.
Through the use of automated image analysis it has, however, become possible to analyze a large
number of hyphal elements in submerged cultures, and hereby information on the hyphal
morphology may be obtained (Cox
et a i,
1998). Furthermore, through the use of flow-through cells
that are positioned directly under a microscope equipped with an automated image analysis system
it is possible to follow the outgrowth of single hyphal elements and hereby study the growth
kinetics in great detail (Spohr
et a i
, 1998; Christiansen
et al
., 1999).
Not many models specifically address the growth mechanisms of filamentous microorganisms (for
a recent review see Krabben and Nielsen (1998)) since normally the focus is on the primary
metabolism, where it is not necessary to consider the hyphal structure explicitly; i.e., many of the
intracellularly structured models described in Sections 7.3-7.4 may also be used to describe the
primary metabolism of filamentous microorganisms. To model the formation of secondary
metabolites, which may be determined by the cellular differentiation, it is, however, often necessary
to consider morphological structure. Furthermore, when a description of the morphology of the
hyphal elements is the objective, one must of course include morphological structure in the model.
Megee
et al.
(1970) described the first morphologically structured model for filamentous fungi. The
model was used to describe growth and production formation of
Aspergillus awamori.
Five
separate morphological forms are considered in the model:
ZA
- Apical compartment in actively growing hyphae
ZH
- Subapical compartment in actively growing hyphae
Zc - Conidiophore4 developing hyphae
ZB
- Black spores
ZM
- Matured spores
With these five morphological forms it was possible to describe the complete life cycle of so-called
imperfect fungi
(fungi with no sexual reproduction). The model includes product formation as a
4 Conidiophores are modified hyphae on which the asexual spores are formed.
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