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Colonisation Of Roots by Ectomycorrhizal Fungi

Introduction

Ectomycorrhizas may arise in fresh roots following two processes, extension of hyphae from existing ectomycorrhiza, or initiation from fungal propagules external to the root. In the former process, hyphae emerge from the Hartig net or sheath, and extend through the soil or along the root, and contact another portion of the elongating root. In the latter case, spores and other propagules of ectomycorrhizal fungi may germinate when close to the root, initiate a new colony either in addition to or competition with an existing ectomycorrhiza.

In most annual and perennial plants, root elongation is a seasonal event, either associated with changes in temperature or soil moisture, or both. Fresh roots emerge from the existing system, or seed, and the root tip and elongating hyphae make contact. Initiation of ectomycorrhiza follows recognition of compatibility and competition for sites on the fresh root. Colonisation of the roots of a plant is crucial for the transfer of phosphate to the plant and organic carbon to the fungi. The process of colonisation is not simple. The rate of colonisation is determined by the fungi, host and environment.

Prior to Colonisation

All roots release organic compounds to the rhizosphere. Some compounds stimulate the germination of fungal spores, and attract hyphae to the root surface. At present, the precise function of root exudates is unclear, but the spores of many ectomycorrhizal basidiomycetes will only germinate in the rhizosphere of a compatible host. Considerable speculation surrounds plant hormones and other compounds as indicators of compatibility. However, the specific molecules that determine compatibility remain to be determined in most cases, though they are probably not molecules used in nutrition of the fungi. In addition, helper-bacteria have been shown to modify the initiation of ectomycorrhiza, suggesting that the process is complex and may be staged.

Study of the interaction using molecular techniques is helping to elucidate the various stages of association. Because of the huge range of potentially ectomycorrhizal fungi that any one plant may associate with, the clarification of the process is likely to take some time.

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Colonisation


Atypical ectomycorrhizas can take on different forms. The structure to the right is observed within 5 days of rainfall after a long dry period. The fungi are Ascomycotina, and the appearance is polygonal.

Patch of ectomycorrhiza on the root of Rhodanthe. The form is more labyrinthine.

Thin mantle of ectomycorrhiza formed by Ascomycotina.

Development of ectomycorrhiza differs with plant and fungus. Some details appear to be common. Single spores germinate and when emergent hyphae make contact with a compatible root surface, attachment follows. Hyphae initially grow in the junction between epidermal cells. Laterals project over the surface of individual cells. Hyphae elongate rapidly. The hyphae appear to lose their clamp connections and adopt a pseudoparenchymatous growth pattern. The cells shorten and branching increases. As the root tip becomes ectomycorrhizal, the rate of elongation of the root appears to slow. The root tip then starts to appear fatter, especially as the sheath thickens.

Hyphae proliferate across the surface of the root, sometimes as labyrinthine structures. The anatomy of the inner layers appears to differ from outer layers. Cells of the inner layer tend to have thinner walls and are more closely pressed to the root surface. Hyphae of the outer layer tend to be loosely aggregated and thick-walled. A hydrophobic layer containing melanin is also deposited between cells within the sheath.

The pattern of colonisation initiated from existing ectomycorrhiza differs only in that root elongation commences prior to initial growth of hyphae. In field collected roots, fresh root tips emerge from ectomycorrhiza before hyphal growth commences. When fresh hyphae emerge, they elongate rapidly along root surfaces and rapidly cover the root tips.

In roots from the field, ascomycetous hyphae have been observed to commence growth before the appearance of hyphae of Basidiomycota. The latter soon overgrow the former, and it is difficult to determine whether both types continue. Thus in some instances in Australian conditions, different sources of inoculum and different types of fungi interact in the formation of ectomycorrhiza.

Ultimately, the root tip is covered by hyphae in what is called a sheath or mantle. Hyphae project laterals between the epidermal cells in what is called the Hartig net. The precise anatomy of the ectomycorrhiza depends on both host and fungus. Some associations form mantles that are several layers thick, others are thin. Some Hartig net completely surround the epidermal cells, others barely project into the junction The ectomycorrhiza also has an internal anatomy of interest. Hyphae exude materials that fill the gaps between hyphae. The functional potential of these materials is discussed elsewhere. LINK

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Anatomy of the Ectomycorrhiza

Hyphae do not penetrate the epidermal cells in ectomycorrhiza. However, in ectendomycorrhiza, invagination of epidermal and cortical cells may be observed. The mechanism preventing penetration is unclear. The host clearly has effective responses to pathogens, and these defence responses probably deter fungal penetration. LINK The zone of interaction between symbionts is also complex. The hyphae form fan-like structures as they penetrate between epidermal cells. Epidermal cells elongate as hyphae push them apart. One consequence of the mantle and rearrangement of epidermal cells, is that ectomycorrhiza take on a short, fat appearance, relative to uncolonized root tips.

As hyphal colonisation progresses, in a few cases, the wall of the host becomes invaginated. In Pisonia grandis wall ingrowth results in an increase in the surface area of contact between the symbionts. In all cases, hyphae initially are highly metabolically active. The cytoplasm contains numerous mitochondria and endoplasmic reticulum. The anatomy suggests closely coordinated functional activity, with an initial burst of activity following initiation of colonisation.

Over time, the ectomycorrhiza matures. Hyphae become vacuolate, and presumably take on a storage function. LINK The root also matures with the outer layer of the exodermis becoming suberised. Root laterals are formed, and the process of association is again activated. Hyphae also elongate from the ectomycorrhiza and explore the soil in a characteristic manner away from the root surface.

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References

Smith SE & Read DJ. (2008) Mycorrhizal Symbiosis. Academic Press.

Varma A. & Hock B. (Eds) (1998) Mycorrhiza: Structure, Function, Molecular Biology and Biotechnology. Springer, Berlin.

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