- 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:
- 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.
- 3. The weakened fibrils stretch out and become separated from one another due to the turgor pressure of the protoplasm.
- 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.
- Model 2 - steady state:
- 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.