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Elongation of Hyphae

Introduction

One key to differentiating fungi from other eukaryotic organisms is the mode of growth and development. In fungi, the principle component of growth and development is the hypha. Hyphae elongate from the apex. In other words, the basis for fungal growth and development is the continued and coordinated growth of a series of hyphal tips, in complex cases, into a 3 dimensional structure. The linear hypha is capable of forming three dimensional organisations by aggregation. In contrast, plants and animals form three dimensional structures through the division of cells in all planes. The elongation of hyphae is critical to understanding the growth strategy of fungi.

Because hyphal elongation is the basis of development, the formation of laterals is critical. The density of branching varies greatly, and the direction the lateral assumes can also vary. By varying both factors, different forms can be developed by fungi.

Elongation

In principle, hyphal elongation is a process where wall materials are delivered in vesicles or vacuoles to the tip by the cytoskeleton, and inserted into the existing wall. The process involves a weakening of the existing wall via enzymic degradation. The internal medium is under pressure largely due to the influx of water because of differences in osmotic potential inside and outside the wall. All parts of the wall are under similar turgor pressure. Any weakening of the hyphal tip results in an extension of the tip, and so maintenance of mechanical strength in the wall at the tip is critically important.

Two models have been developed to explain elongation of hyphae. Both models envisage the fusion of vesicles with the plasmamembrane at the tip. The vesicles would contain enzymes to digest the wall, and wall precursors to construct the new wall. The vesicles would be delivered via the activity of the cytoskeleton.

That hyphae elongate from the apex is supported by experiments utilising labelled precursors. The wall contained labelled materials only at the tip, and not along the hypha. Further, walls at the tip are thinner and less complex. LINK

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Branching

Branching appears to be a function of the total volume of cytoplasm. Presumably, a given volume is necessary to support a given number of hyphal tips. The mechanism determining the precise point of lateral initiation is unclear. The branch angle is initially 90 degrees. However, as laterals become denser, hyphae tend to elongate away from their neighbours (negative autotropism) and grow away from the centre of the colony. This pattern results in a circular colony in axenic culture and the field. LINK

Hyphae also respond to depleted nutrients, secondary metabolites, and staling compounds. These responses result in hyphal tips being located in areas where available nutrients are likely to greatest.

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Changes Over Time

Hyphal morphology may change as the colony ages. Commonly young colonies consist of hyphae of a similar diameter, elongating at a similar rate. Later, the same colony may have coarser hyphae which are growing more rapidly, and a range of hyphal forms in between "mopping up” the remaining nutrients.

Other changes with aging include reduced branch angles, vacuolisation or loss of cytoplasm from some hyphae at the centre of the colony, and changes in the wall structure. Some hyphae in the centre of the colony may become acytoplasmic following digestion from within. In Nitrogen depleted habitats, chitin in older parts of the colony is scavenged, and the Nitrogen removed from chitin is transported to the growing hyphal tip. The result of all the changes is a colony with hyphae that have various forms and functions.

Colonies in axenic culture often appear as concentric rings around the inoculum. The surface hypha appear to have stalled, and the renewed surface growth arises from hyphae within the agar emerging beyond the zone of inhibition. In this case, it appears that the gaseous or surface environment has caused cessation of hyphal elongation at the surface. The cause of the inhibition can be either presence of external inhibitors such as waste products or secondary metabolites, or the absence of exogenous nutrients, or a combination. The observations indicate the importance of the response to environment in determining fungal growth and development.

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Summary

Hyphae elongate apically. Initially, an axenic colony is a circular structure with hyphae evenly spaced over the surface, branching regularly. Over time, hyphae respond differently, in part because the environment has changed. The response is due to staling products and reduction in available nutrients. The colony also commences differentiation. Resources may be removed from central areas, and fresh growth proliferates in areas where available nutrients have been located. Further differentiation may lead to complex structures, dealt with at LINK.

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References

Prosser JI in Gow NAR & Gadd GM 1994 The Growing Fungus. Chapman Hall

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