Spore 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. Germination of spores indicates the reactivation of the thallus. 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.
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, and in the presence of free water or high humidity, will swell as they take up water.
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, high densities of spores increase 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 surfaces to enable germination and subsequent growth. Stimulation of the germination of these fungi can be enhanced by adding sugars to the experimental solution in which they are held. In cases where the interaction between fungus and plant is highly specific, the host plant releases specific compounds which trigger 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. These fungi may also require fatty acids specific to dung to germinate.
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.
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.
References
Jennings DH & Lysek G 1996 Fungal Biology, Bios, pp125 - 129.
Moore D & Novak Frazer LA 2002 Essential Fungal Genetics. Springer.
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