Modeling of Growth Kinetics
301
Figure 7.19 Some typical pictures of hyphal element of filamentous fungi.
A. Single hyphal element of
Aspergillus nidulans
B. Agglomerate of several hyphal elements of
Aspergillus nidulans
In filamentous fungi there is a substantial accumulation of small vesicles at the apex, and the
vesicles are likely to play an important role in the tip extension. They are believed to contain wall
subunits, lytic enzymes, and synthetic enzymes that are transported with the vesicles to specialized
regions of the endomembrane system in the apical and subapical compartments (McIntyre
et al.,
2001). The vesicles, each carrying its load of enzymes and/or wall precursors, are transported by
unknown mechanisms through the cytoplasm to the tip section of the apical cell, normally referred
to as the
extension zone.
When a vesicle comes into contact with the cell membrane at the apex, it
fuses with the membrane, and the vesicle content is released into the wall region. The excreted lytic
enzymes attack the microfibrillar skeleton in the cell wall, resulting a plastification of the wall
structure, which thereby becomes unable to withstand the inner pressure from the cytoplasm. The
microfibrils therefore become stretched, and the surface area of the wall increases. In filamentous
procaryotes (typically
Streptomyces
species) the wall material is soluble in the cytoplasm and is
probably transported to the apex by molecular diffusion. It has been found that the wall section at
the apex is more susceptible to compounds affecting wall synthesis and assembly than other
sections. This indicates that the lytic enzymes in filamentous procaryotes are positioned in the wall
section at the apex (Prosser and Tough, 1991).
When a new tip is formed, it initially grows and its growth corresponds to an increase in the size of
the apical compartment. When the apical compartment has attained a certain size, a septum is
formed behind the tip, and some of the old apical cell mass becomes new subapical cell mass.
Under constant environmental conditions, the size of the apical compartment remains constant, and
the net result of tip extension is therefore formation of subapical cells. The control of septum
formation has been studied in filamentous fungi, and Fiddy and Trinci (1976) introduced the term
duplication cycle
to describe the events that lead to the net formation of a whole new apical
compartment. For
Aspergillus nidulans
the duration of the duplication cycle has been found to be
identical with the doubling time of the biomass.
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