Apical Growth

  • You might like to begin by viewing some movies that show apical growth, hyphal branching and septum formation.
  • To understand the mechanisms involved in apical growth of a hypha we need to look again at the HYPHAL TIP.
  • We already know that the growing hyphal tip is structurally and functionally different from the rest of the hypha - see section on Hyphal Ultrastructure.
  • BUT - the hyphal tip (like the rest of the hypha) is surrounded by a wall - although the wall may be thinner and simpler in structure than the mature lateral wall of the hypha further back - see section on the Fungal Wall.
  • We also know that growth of a hypha is closely linked to the presence of vesicles which form the APICAL VESICULAR CLUSTER (AVC):
    • when a hypha stops growing, these vesicles disappear
    • when growth of the hypha resumes, the vesicles reappear.
  • In addition - the position of the vesicles is linked to the direction of growth of a hypha:
    • when a hypha is growing straight ahead, the vesicles are positioned in the centre of the hyphal tip
    • movement of the vesicles to the left or right side of the hyphal tip is accompanied by a change in direction of growth of the hypha
  • So it's clear that the vesicles play a key role in apical growth.
Vesicles of the AVC contain:
  • wall PRECURSORS - the sub-units or buildng blocks of the wall polymers - e.g. uridine diphosphate N-acetylglucosamine, the sub-unit of chitin
  • wall LYTIC ENZYMES - which help breakdown and separate wall components - e.g. chitinase, glucanase
  • wall SYNTHASE ENZYMES - which help assemble new wall components and so increase the size of the wall - e.g. chitin synthase, glucan synthase.

TWO MODELS have been proposed to explain the mechanisms of apical growth - they differ in whether or not wall lytic enzymes are necessary.

Model 1 - involvement of wall lytic enzymes:
According to this model, if the hypha is going to be able to extend at its tip, there will have to be:
  • some softening (lysis) of the existing wall, and
  • the synthesis and incorporation of new wall material.
But these processes will have to be finely balanced - otherwise, the wall may become too weak or too rigid for further growth
The following series of diagrams helps illustrate what may happen:
Diagram illustrating stage 1 in model 1 for apical growth.
1. Vesicles containing lytic enzymes or wall precursors move through the cytoplasm towards the hyphal tip, where they fuse with the plasma membrane, releasing their contents into the wall.
2. The lytic enzymes released into the wall attack the polymeric fibrils.
Diagram illustrating stage 2 of model 1 for apical growth.
3. The weakened fibrils stretch out and become separated from one another due to the turgor pressure of the protoplasm.
Diagram illustrating stage 3 of model 1 for apical growth.
Diagram illustrating stage 4 of model 1 for apical growth.
4. Synthase enzymes and wall precursors build new fibrils and synthesise additional amorphous components of the wall.
5. The surface area of the hyphal wall increases. Fusion of the vesicles with the plasma membrane ensures that the fomer contribute to the increase in surface area of the latter.
Diagram illustrating stage 5 of model 1 for apical growth.
Model 2 - steady state:
Diagram illustrating the steady state model of apical growth.
According to this model:
  • lytic enzymes are NOT involved in apical growth
  • because the newly formed wall at the extreme tip of the hypha is VISCOELASTIC (essentially fluid)
  • so that as new wall components are added at the tip, the wall flows outwards and backwards (see adjacent diagram)
  • and the wall then RIGIDIFIES progressively behind the tip by the formation of extra chemical bonds.


Empty div