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Spore ATTACHMENT AND Germination

Introduction

Spores are the fungus in a state of interrupted growth. The spore may be a converted fragment of hypha (asexual and thallic) or a structure formed within a specific process (asexual and blastic, or sexual). The cytoplasm is denser, the walls are thicker, and the metabolic rate is slower than found in hyphae. Before a spore can germinate, it attaches to a surface. The process of attachment is extremely important among plant pathogenic fungi, where the process has been studied in detail. Germination of spores indicates the reactivation of the fungus. Germination may follow a period of dormancy. Germination requires a change in the rate and type of metabolic processes in the spore, followed by the emergence of a hypha from the spore. The germinated spore may go on to develop a mycelium.

ATTACHMENT: Spores adhere to a surface as a consequence of the physico-chemical properties of both. Typically, spores attach to a range of biological and non-biological surfaces. A few fungi attach only to specific plant surfaces where recognition of molecules results in an immediate attachment response.

Non-selective attachment may induce a response from the spore, where the spore actively expresses an adhesive material. Typically, these adhesins are rapidly expressed and may be enhanced as the spore releases an extracellular matrix prior to the emegence of the germ tube. More commonly, spores release an adhesive compound from either compartments or the extracellualr matrix surroundng the cell. The attachment is not dependent on metabolic activity, and it may also be rapid.

Success of passive attachment may be determined by the degree of hydrophobicity of the surface. The presence of hydrophobins on the surface of spores enhances attachment by spores. Additionally, the presence of divalent cations in the solution enhances attachment by bridging the gap, helping to overcome electrostatic charges and reducing the effective distance between surfaces.

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Germination

IMBIBITION: The first stage of germination is metabolic. An event external to the spore triggers a change in metabolic activity in the spore. The event is commonly rehydration. Spores are normally dehydrated, using glycerol as an osmoticum. Spores will swell as they take up water in the presence of free water or high humidity.

MATURATION: In some cases, imbibition will not take place unless the spore has reached physiological maturity. In some spores, water may be imbibed but the metabolic response is muted. The spore remains ungerminated. These spores are said to be dormant.

DORMANCY: Many different processes, affecting the spore wall or the cytoplasm, may break dormancy. Biochemical inhibitors commonly prevent germination immediately following release of spores from the thallus. The inhibitors may be in the substrate (e.g. fungistasis in soil), the cell, or in neighbouring cells.

If dormancy is based on the presence of inhibitors, cycles of rehydration/drying may remove inhibitors held by the spore. Suppression of germination is possible in the presence of high densities of spores possibly due to high concentrations of inhibitors being held in the spore mass. In other fungi, however, high densities of spores increase the rates of germination.

ACTIVATION: Specific activation may also be necessary to initiate germination. Spores of pathogenic fungi commonly require uptake of simple sugars commonly found on leaf or root surfaces to enable germination and subsequent growth. In cases where the interaction between fungus and plant is highly specific, release of specific compounds by the host plant triggers spore germination.

A simlar spore activation is found with many basidiomycetous ectomycorrhizal fungi. Here, the exudates from roots initiate spore germination. The germinated spore is also in a location that may result in the initiation of an ectomycorrhiza.

ENVIRONMENT: The environment is also important. Some fungi must pass through a period of either low or high temperature before they germinate. Spores of the dung fungi Sordaria and Pilobolus require a short period at body temperature before germinating. LINK Germination of these spores is further enhanced if some of the spore wall has been digested away, presumably enabling more rapid entry of water to the spore. Germination of these fungi may also require fatty acids specific to dung.

Environmental conditions such as high temperature may also prevent germination, either by their effect on biochemical processes, or the availability of water. Environmental factors associated with germination are complex.

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Conclusion

It is clear that a variety of factors contribute to the successful germination of a spore. However, enormous resources are wasted because so few spores successfully establish a living and reproducing mycelium.

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References

Jennings DH & Lysek G 1996 Fungal Biology, Bios, pp125 - 129.

Moore D & Novak Frazer LA 2002 Essential Fungal Genetics. Springer.

Tucker SL & Talbot NJ 2001 Surface attachment and prepenetration stage development by plant pathogenic fungi. Annu Rev Phytopathol 39: 385-417.

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