4.10 Autotropic reactions

The hyphae of many fungi can alter their direction of growth to avoid growing into each other and accentuate exploration of uncolonised regions of the substratum. The avoidance mechanism or negative autotropism (Fig. 16) is particularly evident at low hyphal densities, in regions such as the margin of the growing colony. The ability of hyphae to sense the presence of another hypha is thought to be due either to a localised depletion of oxygen around the target hypha, a higher concentration of carbon dioxide, or the presence of a secreted metabolite.

Autotropisms: diagrammatic illustrations of negative autotropism leading to the hyphal avoidance reaction Fig 16. Autotropisms: diagrammatic illustrations of negative autotropism leading to the hyphal avoidance reaction, which is so crucial to the colonial growth form of filamentous fungi. The drawings show the responses of pairs of hyphae of Aspergillus nidulans, Mucor hiemalis and a wild type strain of Neurospora crassa; the approaching hypha is shown in blue in each case. The point where the avoiding reaction was first observed is indicated by the arrow. The numerals give times in minutes. The medium was overlaid with cellophane membranes prior to inoculation to keep the hyphae on the surface and prevent hyphae diving beneath the surface. Modified and redrawn from Trinci et al., 1979.

The minimum distance of approach of two hyphae before negative autotropism caused one to grow away was 30, 27 and 24 μm respectively in Neurospora crassa, Aspergillus nidulans and Mucor hiemalis (Trinci et al., 1979; see Fig. 16).

In a maturing mycelium, autotropism can be reversed so that young hyphae are attracted (possibly chemotropically?) to an older hypha (called positive autotropism). This can lead to hyphal fusions. In one mechanism the target hypha is induced to branch, and the consequential tip to tip contact is followed by breakdown of the two apices and fusion between the two hyphae (Fig. 17).

Autotropisms: sketch showing positive autotropism leading to a 'tip-to-peg' hyphal fusion reaction
Fig. 17. Autotropisms: sketch showing positive autotropism leading to a ‘tip-to-peg’ hyphal fusion reaction. This is one of the fusion reactions leading to the interconnected mycelial network of the mature colony as illustrated in Fig. 4.6. Other reactions are tip-to-tip and peg-to-peg (see Fig. 2 in Chapter 7). For more detail see Fig. 13 in Chapter 5 and refer to Hickey et al., 2002 and Glass et al., 2004. Particularly fine images can be viewed on the University of Edinburgh’s Fungal Cell Biology Group website at http://129.215.156.68/images_index.html.

This process converts the central regions of a maturing colony into a fully interconnected network through which materials and signals can be communicated efficiently and this enables the vegetative mycelium to make best use of its resources. In the formation of fruit bodies and similar structures, positive autotropism enables many hyphal tips to congregate together to initiate the developing tissue. In these cases hyphal fusions and adhesions may be rare (though they can be used to bind the structure together); instead, developmental regulation organises the concerted contribution of many independent hyphal tips to formation of the tissues and structures of the fruit body (discussed in Chapters 9 & 12).

Updated December 16, 2016