Growth, Differentiation and Sexuality

4 Apical Wall Biogenesis
J.H. Sietsma 1 , J.G.H. Wessels 1
CONTENTS
I. Introduction ......................... 53
II. General Overview of Apical Growth ....... 53
III. Chemical Composition of Fungal Walls .... 55
IV. Biosynthesis of Fungal Walls ............. 56
A. ChitinSynthesis.................... 57
1. Regulation of Chitin Synthase Activity 57
2.ChitinModifications .............. 58
B. β-GlucanSynthesis.................. 59
1. Synthesis of (1-3)-β-Glucan......... 59
2. Modifications of (1-3)-β-Glucan ..... 59
V. Wall Modifications During Wall
Expansion ........................... 60
A. Steady-State Model for Apical Wall Growth 60
B. Determinate Wall-Growth Model
forBuddingYeasts .................. 61
VI. Apical Gradient in Wall Synthesis ......... 62
A. TheVesicleSupplyCentreModel....... 62
B. The Self-Sustained Gradient Model . . . . . 63
VII. Passage of Proteins Through the Hyphal Wall 64
A. TheSecretoryPathway............... 64
B. The Hyphal Wall as a Barrier for Passage
ofProteins ........................ 65
VIII. Conclusion ........................... 66
References ........................... 67
I. Introduction
Polarised growth is a distinctive feature of the fungal
kingdom. Although this process also occurs in
some plant cells, notably pollen tubes and root
hairs, plant cells generally grow by diffuse or intercalary
extension of their walls, a process only
exceptionally seen in fungi, such as in expansion
of mushrooms (see The Mycota, Vol. I, 1st edn.,
Chap. 22). Even growth by budding, as in yeasts,
can be viewed as a form of apical growth, only different
from hyphal growth with respect to the degree
of polarisation of the wall synthetic activities
(Wessels 1990).
The essential features of hyphal tip growth are
polarised synthesis and extensibility of the wall,
1 Department of Plant Biology, University of Groningen, Kerklaan
30, 9751 NN Haren, The Netherlands
and a suitable gradient of rigidification of the wall
to produce the hyphal tube. When the synthetic
activities of the organism exceed the rate at which
individual hyphae can extend, branching occurs,
producing new volume for the increasing cytoplasm
(see The Mycota, Vol. I, 1st edn., Chap. 10).
The branches extend and proliferate, giving rise to
the expanding fungal colony. The adaptive value of
this mode of growth can hardly be doubted: like
mobility in animals, apical growth allows the fungus
to exploit a vast area for nutrients, particularly
because translocation within the hyphal system allows
the fungus to grow for long distances in nonnutritive
areas (see The Mycota, Vol. I, 1st edn.,
Chap. 9, and Vol. VIII, Chap. 6).
Apical extension also allows hyphae to penetrate
solid substrates such as wood and living tissues
of plants and animals. In relation to this invasive
growth, turgor plays an important role (see
The Mycota, Vol. I, 1st edn., Chap. 4, and Vol. VIII,
Chap. 1), as does the excretion of large amounts
of proteins. Such proteins appear excreted primarily
at the growing tips and consist largely of lytic
enzymes which can digest the substrate polymers,
providing nutrients for the fungus and clearing the
path for further penetration. The hyphal wall is an
apparent diffusion barrier for the passage of these
proteins into the milieu. Therefore, specialised regions
in the hyphae or specialised mechanisms
have to be envisaged to let these molecules pass.
This review deals with polarised wall biosynthesis
and with the excretion of proteins through
the nascent wall.
II. General Overview of Apical Growth
Many observations on apical wall growth were already
made in the 19th century and various hypotheses
then put forward to explain apical growth
still survive today. Most notable is a publication
The Mycota I
Growth, Differentation and Sexuality
Kües/Fischer (Eds.)
© Springer-Verlag Berlin Heidelberg 2006