Chapter 4
Fungal growth
The key to the fungal hypha lies in the tip (Noel Robertson)
This chapter is divided into the following major sections:- apical growth of fungal hyphae
- spore germination and the orientation of hyphal tip
growth - the yeast cell cycle
- kinetics of fungal growth
- commercial production of fungal biomass: Quorn™
mycoprotein
In this chapter we focus on the mechanisms of fungal
growth, with special reference to the organization of
growth and wall synthesis at the hyphal tip, which is
central to understanding the biology of fungi. We
also discuss the ways in which hyphal branches arise
and orientate themselves for maximum efficiency of
nutrient capture. And, we consider the kinetics of
fungal growth in batch culture and continuous culture
systems, relating this to industrial-scale processes,
including the commercial production of Quorn™
mycoprotein– a highly successful fermentation sys-
tem for producing “single-cell protein.” Several topics
in this chapter are covered in depth by Gow & Gadd
(1995) and Howard & Gow (2001).Apical growth of fungal hyphae
Apical growth is the hallmark of fungi. Apart from
the fungus-like Oomycota, which have adopted apical
growth by a remarkable degree of convergent evolution
(Latijnhouwers et al. 2003), no other organisms grow
continuously as a tube that extends at the extreme tip
by the localized synthesis of wall components, and,
arguably, no other group shows such extreme plasticity.
The hyphal apex can swell into a balloon-like structure
such as a spore or yeast cell, or it can taper to such a
degree that it can penetrate a layer of inert gold film
or the wall of a host plant by exerting turgor pressure
alone. In other circumstances, the fungal hypha can
give rise to complex tissues and infection structures,
discussed in Chapter 5.
Figure 4.1 illustrates part of this plasticity, when
hyphae of Neurospora crassaare observed by placing a
coverslip over the margin of a colony on an agar plate.
The sequence of nine frames was taken over a 1-hour
period, starting from the time when the coverslip was
added. In the first frame (a) the hyphal tip was grow-
ing normally, and two lateral branches had arisen
behind the growing tip. Soon afterwards (b and c) the
hyphal tips began to swell (a response to disturbance
caused by the coverslip) and then branched repeatedly
from the tips before resuming a more normal pattern
of apical growth.
By taking any convenient reference points, such as
the branching points shown as v1 and v2 in Fig. 4.1,
it is seen that the length of hypha already formed
remains unchanged, and all new growth occurs from
the original hyphal tip or from the branch tips. In
fact, the incorporation of new wall material is mainly
confined to the extreme tip. This is illustrated in
Fig. 4.2, where growing hyphae were exposed to a
short pulse of radiolabeled wall precursors such as(^3) H-N-acetylglucosamine (from which chitin is syn-
thesized) or^3 H-glucose (from which wall glucans are
made) and then autoradiographed. The radiolabel is
incorporated maximally at the hyphal tip, and the rate
of label incorporation falls off sharply over the first few
micrometres – the apical dome of the hypha.
Rapid rates of hyphal tip extension (such as 40μm
min−^1 in N. crassa) can only be possible if the apex
is supplied with vesicles and other cytoplasmic com-
ponents from behind. For this reason, what we term
apical growth is actually apical extension, because the
true rate of growth, defined as increase in biomass per
unit of time, is much slower. The length of hypha
needed to support an extending apex can be estimated
by making a diagonal cut across a colony margin with
a scalpel, so that individual hyphae are severed at dif-
ferent distances from their tips. Hyphae that are cut
too close to the tip die from physical damage. Hyphae
cut further back continue to extend but more slowly
than usual, and eventually a point is reached at which
the cut is so far back that it has no effect on the apical
extension rate. This distance is termed the peripheral
growth zoneof a fungal colony, defined as the length
of hypha needed to maintain the maximum extension
rateof the leading hyphaeat the colony margin; it
varies between fungi, from below 200μm up to several
millimeters for the fastest-extending fungi.