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RHIZOMORPHS

Introduction

Hyphae grow by apical extension. The fungus proliferates by branching, each tip elongates. Lateral cell division seen in tissues of plants and animals is not found in the fungi. Yet a variety of complex hyphal structures are observed. Lift a layer of moist leaf litter in the forest and a series of strands or cords are evident. These strands have a definite, aggregated structure and appear to link wefts of hyphae ramifying through the degrading litter. Some of the strands resemble shoe-laces indicating complex structure.  Aggregations are probably responsible for the formation of one of the largest organisms known, a fungus which has spread through 45 ha of a forest. What are these aggregations, and how do they function?

Definitions

The coarser strands are called rhizomorphs. They have a definite tubular structure. In transverse section, the outer layer of cells is thick-walled, empty of cytoplasm, and often melanised. The outer layer often contains hydrophobins at the surface of the wall. LINK The inner tissue is comparatively thin walled, and the cytoplasm is active. Often the core of the rhizomorph lacks cells, allowing movement of air, water and solutes to pass along the tube.

The structure of other elongated hyphal aggregations is less clear. The strand may be thinner. The outer layer may contain active cells, and walls may lack melanin. The core is commonly cellular. At it simplest, a strand may consist of a few thin-walled hyphae twined around a single coarse hypha.

The strand also has hyphal connections to the surrounding environment. Where the soil contains nutrient rich materials, the hyphae form a weft through the materials, and a major strand or rhizomorph may develop, establishing a branched structure. Over time, the degree of branching and pattern of rhizomorph distribution through the litter will change, reflecting the changing patterns of distribution of nutrients, and competition with other fungi.

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Function

The function of rhizomorphs is suggested to be related to exploration of the environment. That is, the strands are migratory and exploratory organs, involved with resource capture. As the energy supply of one substrate declines, hyphae grow out. Those hyphae that establish a new resource, thicken, branch and aggregate. The initial aggregation is a strand. Over time, the aggregation becomes thicker. A few fungi in the Basidiomycota have the potential to form rhizomorphs, which may connect resources such as tree stumps, roots or logs, separated by a considerable distance. The fungus which causes brown rot, Serpula lacrimans, is known to form rhizomorphs several metres long. These grow through air vents in the sub-floor cavity, along walls connecting the fungus in soil to timber in buildings.

A hollow rhizomorph has the potential to function as a root or pipe, enabling the movement of considerable quantities of air, water and solutes over significant distances. The rhizomorph connects parts of the fungus providing a conduit for the sharing and transport of resources. Thus that portion of the fungus in air may connect a portion in anaerobic conditions, and the rhizomorph may provide a conduit for exchange of gases. In the case of pathogens, the potential to grow through soil enables the fungus to connect to new resources before the complete degradation of the initial host substrate.

The fungi that form strands may be saprotrophic or pathogenic as indicated above. Some ectomycorrhizal fungi also form strands. Here, the strands are involved with three functions: resource exploration and exploitation, delivery of minerals to the host and formation of fruiting bodies.

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Conclusion

Tubular aggregations of hyphae are found in soil connecting organic resources which are then exploited. Some aggregations may be long. The separation of resources being exploited by one thallus indicates that huge organisations of fungi can form, organisations which are genetically homogeneous. These organisations may become physically separated due to the collapse of interconnections between resource-rich sites by the aggregations. The connections may reestablish because of genetic homogeneity, leading to a dynamic structure over time and space.

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References

Jennings DH & Lysek G 1996 Fungal Biology. Bio Scientific Publishers

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