4.13 Ecological advantage of mycelial growth in colonising solid substrates
The apical growth characteristic of the fungal hypha is the prime attribute of fungi and is, of course, an extreme cellular polarity. Because true extension growth is absolutely limited to the hyphal tip, the whole morphology of the hypha depends on events taking place at its apex. It follows from this that the pattern of hyphae in a mycelium, which is largely a consequence of the distribution of hyphal branches, depends on the pattern of formation of the hyphal tips which initiate those branches.
The dominance of filamentous fungi within the ecosystem is attributed to their lifestyle. By growing in a filamentous fashion, fungi are able to colonise substrates rapidly and grow away from nutrient poor areas. Branching of the filament enables substrates to be efficiently captured for absorption from the environment. Maintaining a high extension rate even under poor nutrient conditions allows fungi to maximise their chances of finding new food sources. The success of this growth habit for exploiting the natural environment can be judged on a number of factors: the extraordinary diversity of fungal species (second only to the insects, but then every insect harbours a few parasitic fungi!), their distribution in virtually every habitat on the planet and the parallel evolution of a similar growth habit by other important soil microorganisms, the prokaryotic streptomycetes and the more fungus-like members of Kingdom Chromista, like the Oomycota. Clearly the ability of a microbe to colonise new substrates rapidly by concentrating extension at the apex of a filament makes it ideally suited for life as a heterotroph in a heterogeneous environment.
Polarised growth of fungal hyphae is achieved by restricting extension to the hyphal apex. The cell wall at the hyphal tip has viscoelastic properties. This means it has some of the characteristics of both a liquid (being able to flow like a viscous fluid) and a solid (resisting and recovering from stretching, compression or distortion). These properties allow the wall at the hyphal apex to yield to the internal turgor pressure within the hypha by extending forward. Further behind the tip the wall is rigidified and resistant to the turgor forces. Turgor pressure generated within the hypha therefore acts as the driving force for hyphal extension.
Hyphal growth at the apex requires synthesis and insertion of new wall material and new membranes in a way that does not weaken the tip. This highly organised process is supported by the flow of vesicles generated within the cytoplasm behind the tip, and is co-ordinated with the growth and replication of all the other cytoplasmic organelles and their migration towards the extending apex. It seems now to be generally accepted that the materials necessary for hyphal extension growth are produced at a constant rate (equal to the specific growth rate) throughout the mycelium and are transported towards the tip of the growing hyphae. Among the materials taking part in this polarised transport are numerous cytoplasmic vesicles, which are thought to contain wall precursors and the enzymes needed for their insertion into the existing wall to extend it. A considerable cytoplasmic architecture is involved in the apical growth of the hypha (Bartnicki-Garcia et al., 1989; Wessels, 1993). We will describe and discuss this in detail in Chapters 5 & 6.
Updated December 16, 2016