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TRANSLOCATION AND STORAGE of Organic Carbon in THE FUNGAL Thallus

Introduction

The environment rarely contains an ideal spatial or temporal distribution of nutrients for fungal growth and development. Storage of excess nutrients enables fungi to survive under stressful conditions or periods of low nutrient availability.

Storage Products

Fungi store carbon in various forms. Like all eukaryotes, common products for storing energy in fungi include lipids. Oils drops can be readily seen in most hyphae, especially older compartments, and up to 40% of the cytoplasm may be lipid. As well, organic carbon in the form of glycogen, polyols such as glycerol, and trehalose may be found. LINK The storage of monosaccharides is limited.

Fungi also store a variety of minerals. Nitrogen is commonly abundant at the beginning of a degradative cycle becoming scarce with dissolution. When N is abundant, the storage products will include various complex compounds of N. When N is scarce, fungi commonly store organic molecules. Phosphate is commonly deficient in the environment. Fungi expend energy to take up phosphate. Phosphate is commonly transported and stored in vacuoles as polyphosphate. LINK

The storage products may have multiple functions. Glycerol is common, and may be important in water regulation and metabolic activity in fungi adapted to functioning at low water availability. LINK Trehalose is a transport and storage disaccharide which is uncommon outside the fungi. Thus conversion of energy to trehalose by a fungus makes the energy unavailable to other microbes while retaining an energy concentration gradient that favours the fungus.

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Translocation

Water and solutes travel considerable distances within mycelia. The mechanisms controlling translocation appear to be of two types:

I) Passive, such as bulk flow of nutrients and flow along diffusion gradients

II) Active, vacuolar/vesicular transport under the control of the cytoskeleton. LINK

The flow of small organic molecules is, in principle, similar to that found in all organisms. For instance, glucose molecules are actively taken up at the hyphal tip, where they are immediately converted into trehalose. A relatively high concentration of trehalose develops at the tip, and trehalose then diffuses down the concentration gradient in the hypha away from the tip.

Bulk flow is associated with changes in hydrostatic pressure, with movement of water towards the tip of the hypha as molecules are absorbed. This flow carries other molecules with it. In places where rapid translocation takes place, such as during the formation of a fruiting body, evidence of bulk flow can be seen as exudation of water droplets at the surface.

Lipids and other storage products may be packaged in vesicles or vacuoles, which are moved about the hyphae. LINK While specific details are still unclear, it appears that the vesicles are moved significant distances along hyphae by the action of the cytoskeleton. By encapsulating storage products, the products become unavailable for immediate use. Further, storage products may be moved against diffusion gradients.

Evidence of bidirectional transport comes from studies of mycorrhizal fungi where movement of organic carbon from the host towards the hyphal tip, and of minerals such as phosphate towards the root, is clear.

Translocation is potentially enhanced in the core of strands and rhizomorphs.  In addition, materials may be translocated in different directions in adjacent hyphae using bulk flow. Overall, translocation is protected by the impermeable barrier surrounding the complex lateral organ. LINK

Storage Structures

Storage of requirements for growth takes place in almost all hyphal components. Hyphal fragments can function as stores. These stores require protection.

PROTECTION: The mechanisms used by fungi to protect energy stores include thick walls, protective compounds, hydrophobic surfaces and complex structures. Complex structures are formed under conditions of starvation. The structures commonly called sclerotia, consist of a rind of hyphae with thickened, melanised, hydrophobic walls surrounding hyphae containing high concentrations of lipids and glycogen. LINK

The importance of sclerotia as storage structures is linked to their protective properties. Strains of fungi that form sclerotia survive for much longer periods than strains that do not. Pathogens such as Sclerotinia sclerotiorum are known to re-emerge as problems, because sclerotia may survive more than 20 years in soil. The sclerotia germinate and reinfect susceptible hosts after specific signals from the plant are detected. LINK

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Stores and Parasites

Concentrations of energy are themselves an attractive source of energy. Parasites are used in biocontrol of sclerotia-forming pathogens. The parasites have the capacity to overcome the protective shell of sclerotia, and penetrate to the inner nutritious core. Similarly, many hyperparasites utilise spores as a source of energy.

Conclusion

Fungi store nutrients during periods when the nutrients are available. A variety of organic molecules, including lipids, may be used to store energy. Stores are distributed through the fungal thallus, and concentrated in heavily protected structures. Spores contain an important primary store of organic energy, enabling the germination and establishment of new individuals. Sclerotia enable survival through long periods. The stores of nutrients are also a very attractive source of energy for parasites, and most significant stores are heavily protected.

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References

Cooke RC & Whipps JM 1999 Ecophysiology of Fungi. Blackwell, Ch 6

Jeffries P. & Young T.W.K. 1994 Interfungal Parasitic Relationships. CABI. Ch 7

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