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Growth of Hyphae & Development of Fungi

Introduction

Fungi follow a pattern of growth and development which is much less predictable than for plants and animals. If you plate out a hypha from within a compound structure, it is likely that hyphae will emerge and grow on the agar. In other words, fungi do not become fixed into a developmental pathway. All compartments (cells) of a viable hypha may initiate a colony (totipotent). It also means that the lifecycle of a fungus is unpredictable and flexible.

Growth Phases

A typical pattern of growth follows a response to the nutrients in the environment, modified by other environmental factors. Typically, a single unit of fungus will grow rapidly in the beginning. The exponential phase will be followed by a plateau, commonly called the stationary phase, which as the organic nutrients become depleted is followed by a decline phase. These stages are typical of any organism growing in fixed quantities of nutrients, particularly under laboratory conditions.

Environments are typically highly variable. The heterogeneity is caused by variation in deposition of organic nutrients, availability of each nutrient and water and the response of the thallus at each point to environmental factors. Studies on agar indicate that hyphal growth is complex. Initially, hyphal tips elongate exponentially. Subsequently, hypha elongate linearly, and branches grow exponentially. At any one time in a thallus, some hyphae are elongating exponentially and others linearly. LINK Growth in heterogeneous conditions results in the mycelium having different parts in different stages of response. Some parts will be responding to available nutrients with exponential growth, other will be static, and others declining. A single thallus is heterogeneous in nature.

On agar, initial hyphal growth results in undifferentiated mycelium. However, over time, this circular colony becomes differentiated ecause of differences in available food. The hyphae at the margin are exposed to abundant fresh organic nutrients, and the hyphae at the centre to much less.

Further, staling products increase with proximity to the centre. Eventually, hyphae at the centre become a sink for nutrients from the periphery, and have to respond to the less suitable environment. As a consequence, four zones can be differentiated, though note that they intergrade with one another. The outer zone is extending into fresh nutrients. Hyphae adopt a pattern which appears to be very efficient at extracting nutrients, but the mechanism of control is unclear. In some cases, the angle of branches is acute, as if the response is negatively autotrophic.

Behind the leading edge is the productive zone, where the most of the increase in biomass takes place. Hyphal branching is less acute, and hyphae are more likely to be upright or submerged than flat on or embedded in the agar.

Fruiting follows the storage period. Fruiting is commonly a stationary phase, and nutrients are redistributed within the mycelium from storage in the spores.

Finally, the fourth stage is the aged zone. Hyphae are commonly vacuolate or empty, and hyphal walls are degenerating (autolysis). Essentially, fungi recycle the thallus. Ideally, the fungus disappears from the aged zone, but in practice, some evidence of their presence usually remains.

The pattern of hyphal growth and branching presented above assumes little interaction. However, in more complex environments, hyphae will contact genetically compatible hyphae and anastomose with them. LINK As a consequence, the mycelium will become a complex of interlinked groupings of mycelia, with interlinking sections resulting in complex movements of cytoplasm around an ever-changing body. Hyphal anastomosis is an odd phenomenon given that negative autotrophic responses are commonly seen on agar. Hyphae normally avoid or do not react to adjacent hyphae. Yet, anastomosis is a regulated response to the presence of a vegetatively compatible hypha. The physiological and molecular basis of anastomosis requires further study.

Hyphae grow above and below the surface of media. Penetration into media is a major differentiation between the fungi and other saprotrophic microbes. Fungi have the capacity to penetrate complex substrates by either or both enzymatic processes and physical pressure. LINK The result is that hyphae can access nutrients unavailable to other microorganisms. Aerial hyphae also enable contact with substrates that are physically separated from the mycelium. Cytoplasmic continuity to hyphae in aerobic or better nourished conditions enable the penetrating hyphae to be sustained, and a greater diversity of substrates to be utilised.

Further, fungi form a variety of different structures which enable either escape through space such as hyphal strands and rhizomorphs, LINK or survival through time such as sclerotia LINK and spores LINK. While these are all modified hyphae, the various structures increase the diversity of approaches used by fungi to enable continuation in their environments.

Growth of fungi is indeterminate. The diversity of stages of any one mycelium means that fungi, at any one time, may have hyphae in extension, productive, fruiting and senescing phases. The concept of a single response of a body, such as an animal, does not apply to the filamentous fungi. A single fungus may colonise a single forest (up to 50 ha in extent), or turf, and continue over the centuries (AM fungi). While senescence may be found in some fungi, they usually occupy spatially or nutritionally limited habitats, where only limited growth (space or time) is possible.

Fungi have evolved to occupy a wide diversity of habitats. Different organisms have different requirements for growth and development. Apart from energy, almost every factor that can be found to be important for one fungus is likely to be unimportant for another. However, a range of environmental factors influence the response of fungi beyond the overriding nutritional requirements.

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Environmental Conditions

Respiration

Oxygen is used in respiration in most organisms. The fungi include species that are obligately aerobic or obligately anaerobic (eg rumen fungi). LINK However many fungi are in between, with the capacity to function facultatively in aerobic and anaerobic conditions. Oxygen is used for oxidative metabolism, to generate energy. However, it is also essential for biosynthesis of sterols, unsaturated fatty acids and some vitamins. Thus, while many fungi can exist in anaerobic conditions and respire fermentatively, they also have the capacity to transport oxygen or the products of respiration through their cytoplasm, or air in air spaces of the rhizomorph.

The presence of carbon dioxide is also required for some fungi. It is used in heterotrophic carbon fixation, commonly to replace the carbon backbone from the tricarboxylic acid cycle lost to amino acid formation.

Nitrogen and Other Minerals

Nitrogen is required for the formation of amino acids and purines. Many fungi utilise nitrate, but a few require ammonium, or even amides, amino acids or peptides. 

The balance between total carbon and nitrogen uptake is critical for fungal growth and development. Consideration of the carbon/nitrogen ratio is widespread in industry where specific physiological conditions need to be maintained for maximum rates of production. Relatively low nitrogen content will slow the rate of degradation, because of the demand for protein. Conversely, if the organic matter has a high protein content, then the protein may be used as a source of N and organic carbon. Once C and N requirements are met, the rate of growth is determined by availability of other minerals.

Like all eukaryotic organisms, fungi require a diverse array of minerals to sustain growth and development. While we will not go into any detail here, it is well to note that acquisition of minerals such as Fe in competition with other organisms may result from chelating agents holding minerals at the hyphal wall.

Water

Like all microbes, water availability has a major influence on the function of fungi. Most fungi require very high water availability (relative humidity), and rapidly dry out or senesce in dry conditions.

DNA is denatured at a water activity of 0.55 (water activity of pure water is 1). Some spores survive for many years at a water activity just above 0.55. Xerotolerant fungi can grow, albeit slowly, at water activity of 0.64. LINK In these fungi, the metabolic processes take place in the compatible solute glycerol instead of water.

Most organisms are desiccated at a water activity of around 0.996. Even wood rotting fungi require a water activity above 0.97. Thus, food preservation by drying uses the inability of organisms to grow in dry conditions. However, some fungi, especially those of concern to the food industry such as the Trichocomaceae, are able to grow at lower water activities. Note, that your sugary Christmas cake may be subjected to degradation by xerotolerant fungi such as Saccharomyces rouxii , a fungus which is also used to make Soy sauce. LINK

pH

The hydrogen environment of fungi is difficult to study because fungi change the pH as they grow. Some species increase and others decrease pH of their medium. pH of the medium is important for it influences mineral availability, enzyme activity and membrane function. Generally speaking, fungi can tolerate a wide range of pH, though most media used to culture fungi are acidic.

Temperature

Fungi can normally tolerate the range of temperature of the environment from which they are taken. Their response to temperature is quite varied, however. Active growth will usually be associated with a limited range of temperatures. Those fungi that grow between 15 and 35 C are called mesophilic, and above thermophilic. Those that grow at freezing are called psychrophilic. Many fungi remain alive for extended periods at temperatures unsuitable for growth. Those that recover growth after a period at elevated temperatures are called thermostable.

Temperature affects lag time, specific growth rate, and yield in quite different ways for each fungus. High or low temperatures may cause the fungi to enter dormancy, and reversion to original temperatures may be insufficient to restore metabolic activity. LINK

Light

Light has an important influence on fungal growth in specific cases. The effect of UV radiation on spore and sporocarp formation (see below), and phototropic release LINK are clear examples of light being important.  UV radiation also reduces viability of spores especially in air. Overall, light does not play a major part in metabolism and growth of fungi.

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Sporulation

The environment plays a major role in determining whether a fungus forms sexual or asexual spores. Spores are commonly formed as a fungus depletes its energy sources. A variety of environmental triggers may be involved in determining which type of spore is formed.

Apart from any environmental factors, presence of compatible mating types are needed for the formation of sexual structures. LINK The formation of sexual structures follows a pattern of release of hormones from compatible hyphae, LINK through integration of genomes, to formation of the sexual spore LINK.

This brief discussion will concentrate on the environmental aspects. The factors that are important include:

  • Low concentrations of nutrients
  • Light
  • Oxygen
  • Temperature

Response of fungi to nutrients is highly variable. Some fungi are unable to sporulate under conditions of high nutrients. These fungi continue to grow vegetatively in abundant nutrient conditions. For example, many wood rotting fungi require inoculation and growth on wood blocks (of the appropriate type) before basidiomes are formed in the laboratory. For others, the exhaustion of a key nutrient induces sporulation. Low carbon/nitrogen ratios induces formation of hyphal arthrospores, in some fungi, while depletion of carbon regardless of the nitrogen status induces formation of conidia (e.g. Fusarium) in others.

Conditions of high nutrient status have no effect on the sporulation of other fungi. Some “sugar” fungi appear to sporulate immediately after initiation of aerial hyphae, within 24 hours in some cases. These fungi continue to sporulate until organic nutrients are exhausted. LINK

Light is commonly involved in initiation of sexual structures. UV is required to induce formation of ascocarps of Pleospora, a fungus of plant surfaces. Light is needed for basidiocarp initiation in Schizophyllum, a wood degrading fungus. However, day and night are needed to complete the formation of sporangia of Pilobolus, a dung fungus.

Temperature influences formation of sporocarps. Generally speaking, sporulation takes place in a narrower range of temperatures than vegetative growth. Similarly, sporulation appears to require aerobic conditions in most cases.

Sporulation is complex and differs widely among the fungi. No one recipe can be used to induce spores, and triggers for production of sexual and asexual spores are rarely the same, or well understood.

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Summary

Fungi differ from plants and animals in that cells remain totipotent. Fungi differentiate into a variety of structures including spores which enable continuation over space and time. The mechanisms for formation of each type of structure is largely determined by the perception of the environment by the thallus. Water and organic nutrients are particularly important for fungi, thus we would expect explosions of fungi in humid, energy rich habitats. However, a few fungi also tolerate extreme environments, utilising common characteristics of fungi to their extreme potential.

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References

Carlile MJ & Watkinson SC 1994 The Fungi. Academic Press.

Robson GD, van West P & Gadd GM (eds) 2007. Exploitation of Fungi. CUP

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