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Formation of Complex Structures

Introduction

All hyphal growth is unidimensional, or apical; hyphae extend from the tip or apex. Branches, once formed, elongate apically. Three dimensional structures consist of hyphae that are glued together by mucilage, and physically bound by specialised hyphae. Specialised tissues within the structure have arisen from hyphae. Filamentous fungi do not form three dimensional tissues through mitotic activity in three dimensions. This unidirectional mode of forming 3 dimensional tissues differentiates fungi from plants and animals.

Development is a dynamic process. The structures we consider below have developed in three dimensions over time. The stages through which these tissue pass are sometimes reversible. Individual hyphae may take different developmental pathways in a tissue. Thus, this description is necessarily simplified.

Hyphae branch and in culture, the resultant mycelia consists of hyphae growing away from other hyphae. It is suggested that this results in a thorough exploration of the substrate maximising utilisation of the nutrients. Complex structures are clearly not exploring the substrate. Rather, horizontal linear structures enable connection from one resource to another, vertical linear structures elevate part of the thallus above the substrate. Three-dimensional organs tend to be structures that provide for or enable continuation of the fungal thallus over space and time.

Linear Organs: Strands, Rhizomorphs and Synnemata

Strands arise when hyphae grow together in parallel. The leading hypha forms a branch at an acute angle, which then commences growth alongside the parent hypha. Both hyphae can continue to branch and aggregate. The central hypha may become slightly coarser, and some hyphae may bind the structure. Essentially, the strand appears to be an informal aggregation of hyphae growing in parallel. However, as hyphae normally grow away from each other in culture, the formation of an aggregate indicates a close genetic control of strand formation.

Rhizomorphs are highly organised strands that superficially resemble roots. LINK The rhizomorph of Armillaria mellea has an apical growing point. Cells of the outer layer of the rhizomorph are thick-walled, tightly packed, and walls may be melanised. Except for symplastic flow in lateral hyphae, water and nutrients do not penetrate through this outer layer because of the presence of hydrophobins and other hydrophobic factors in the outer layers. Cells of the central region are thin walled, and generally separated by air spaces. The hyphae are either fine or coarse, the former being referred to as fibres and the latter as vessels, indicating their possible function. The entire rhizomorph is covered by fine lateral hyphae which radiate around the rhizomorph, and resemble the root hairs of a root. These lateral hyphae may connect to the inner layer of the rhizomorph, enabling cytoplasmic connenction between the outer and inner parts of the rhizomorph.

The function of the rhizomorph is still being clarified. It is clear, however, that rhizomorphs may serve several functions. The air spaces along the inner core may enable transfer of oxygenated air from air pores located at the base of the rhizomorph. An air supply enables rhizomorphs to penetrate anaerobic sites and explore for nutrients. These nutrients may then be translocated back to the rest of the colony. Thus rhizomorphs may be found connecting a colony with its surrounding environment, and in particular, connecting potential substrates.

When strands erect above the substrate, they may then form conidia on the termini. These erect conidiogenous structures are called synnemata. The conidia are commonly present in mucilage. The effect of erect conidiophores is to present spores to passing arthropods which may then effect dispersal of spores. Again, water loss to the surrounding air is reduced because of a layer of hydrophobins over the exposed surface.

Stalks of the fruiting body of agarics consist of parallel hyphae bound by fibre hyphae. Thus they are similar to erect rhizomorphs. The fruiting bodies release spores, so they also have a dispersive function.

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Three Dimensional Structures

Hyphae normally form woven structures which consist of hyphae entwined almost randomly. The weave is open, and particles may be trapped within. The organised structures are called sclerotia. Again, the organised arrrangement of cells in plectenchyma suggest tight genetic control.

Sclerotia are initiated with hyphae aggregated into small knots within the mycelial mass. LINK As the knot increases in size, hyphae in the centre accumulate reserves from the connected mycelium. As the sclerotium increases in size, cells of the outer layer shorten, and may start to resemble connected barrels, the walls become melanised, and thickened. As with rhizomorphs, deposits between cells of the outer layer, and their tight packed structure prevent apoplastic movement of water. The outer cells also become vacuolated, and eventually collapse, providing a protective layer over the surface. Larger sclerotia differentiate further. The giant sclerotia of Polyporus is honeycombed by large cells containing transparent material, thought to be a water and nutrient reserve.

The formation of simple sclerotia is interesting. Hyphae branch dichotomously, repeatedly, and cells remain shortened. The cells inflate and fill with glycogen, polyphosphate, proteins and lipids. These storage cells may be bound by fibrous hyphae. In most cases, a protective layer develops on the surface. The initial formation is remarkably similar to the chains of monilioid cells formed in epidermal cells of roots by Rhizoctonia-like fungi.

The formation of sclerotia is remarkably similar to the early stages of the formation of sporocarps of Glomalean fungi. The outer layer or peridium of these sporocarps is commonly tightly woven, with the spores embedded in a loosely woven matrix. The hyphae of the matrix commonly have a shortened and fattened appearance, and often the hyphae are interwoven in the manner of a mantle of an ectomycorrhiza.

Many sclerotia germinate to form spores. In the most complex cases, the spores are born in basidiocarps or ascocarps. That is, the developmental pathway for the formation and emergence of the fungus from sclerotia has a number of complex processes under genetic control.

The fruit bodies of agarics are differentiated from very early stages. Caps and stem are evident in initials of Coprinus when it is less than 1% of the mature structure. The developmental process through which the fruit body passes is complex and will not be described here, though is well covered in the reference given below.

The process of development of complex structures is governed by genetic controls. Through hormones and other signals, cooperative growth causes differentiation of mycelia into recognisable structures. The pathways through which development passes are similar to other complex organs, but the mechanisms are entirely fungal.

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Summary

Fungi develop a diversity of complex structures. Essentially these fall into two classes, elongated and rounded. The elongated structures connect one location to a second, either providing a conduit for movement of materials and hence the entire thallus from one location to a second, or to place reproductive structures into locations more favourable for dispersal of propagules. The rounded structures provide a potential for survival over time. The fungus survives in the structure, and eventaully uses the reserves within the structure to protect, form and disperse spores.

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References

Elliott CG (1994) Reproduction in Fungi. Chapman Hall, London

Moore D (1998) Fungal Morphogenesis. CUP, Cambridge UK.

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Formation of Caps in Agaricus bisporus

The common mushroom Agaricus bisporus is produced commercially around the world. Approximately 5 x 106 tonnes are produced each year, and the mushrooms come from a limited genetic pool. The formation of the mushroom is critically important to the industry, and as a consequence, we now understand some of the basic parameters of cap formation of this genetic type.

The cap is basically formed from clumps of mycelia aggregating in response to putative chemical messages transmitted laterally and apically through the mycelium. The nature of the messages is still unknown, though given the similarity of responses in plants and animals to developmental chemicals, there is little reason to assume fungi are different. Further, once the developmental pathway is initiated, the body plan is determined at an early stage in the pathway. Caps and stalks can be detected a few days after the bud initial appears.

Similarities with plants and animals include the role of cell death in the development of organs. The first gill spaces form as the result of cell death. Cytological evidence has been used to argue that hyphal compartments are sacrificed to create shape from the undifferentiated cell mass. Cell death appears at specific times and places, indicating tight genetic control, equivalent to programmed cell death in animal and plant development.

Fungi differ from plants and animals in that once a developmental pathway has been established, environmental cues may change the response of the mycelium, even to the extent that a cap may recommence uncontrolled hyphal proliferation, or a second cap may form on top of the first.

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