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Other Morphological Types of Arbuscular Mycorrhiza

Introduction

A number of plants form arbuscular mycorrhizas in which the colony has a most unusual morphology. This page examines three different hosts with the goal of elucidating the functional aspects possible in these associations. These “atypical” associations are variations of the more common Paris and Arum types. See AM symbionts for images of Arum and Paris type colonies.

The Gentianaceae

The structure of arbuscular mycorrhizas in the Gentianaceae has been examined on many occasions. In general, the plants form Paris type mycorrhiza. However, in some members of the family, these associations take on variations of the form that indicate supplementary modification to the structures and their function.

Centaurium erythraea is a biennial plant found between herbs of grasslands and fields. The plant is rarely found alone and never in large monospecific clumps.

Laboratory experiments in mixed microcosms have shown that the germinated seed will only grow in the presence of AM fungi. Further, the presence of a “companion” plant with typical mycorrhizas is necessary for the mycorrhizas to spread through the root system of Centaurium. The plant does not appear able to complete its life cycle with fertiliser alone. Thus the host is obligately mycotrophic.

The mycorrhizas have an interesting pattern of development. Within a few days of initial colonisation, cytoplasm is lost from hyphae. The spread of a colony in the root is limited to the colonising ability of the fungal isolate. Even though repeated invasion takes place, the spread of each colony along the root is limited. The mycorrhiza is formed because of repeated invasion of the root by fungi: presumably the fungus gains carbon from "companion" plants.

Because proper controls are not yet possible, the mechanism for plant survival and growth is assumed to be due to increased uptake of minerals associated with the mycorrhizal fungi. However, some members of the family Gentianaceae are nonphotosynthetic mycoparasites, indicating that Centaurium may also remove carbon from the invading fungi.

The degree to which Centaurium and other putatively photosynthetic plants parasitise the mycorrhizal association remains unknown. Many of these putatively parasitic associations have Paris type morphologies. Paris type associations are also found widely in achlorophylous plants. The function of these types of association remains to be expored.

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Thysanotus (Fringed Lily)

Thysanotus is an Australian member of the family Anthericaceae in the Liliflorae. Members of the family, but not Thysanotus typically have arbuscular mycorrhizas of the Arum type.

Thysanotus differs in two respects. Most but not all members of the genus cannot be grown in the absence of mycorrhizas, even in fertile soils, and the morphology of the mycorrhiza is most unusual.

Like Centaurium, Thysanotus requires a living mycorrhizal "companion" plant to enable spread of the mycorrhiza through the root system of Thysanotus. Hyphae rapidly loose cytoplasm following initial colonisation.

The morphology of infection is most unusual. Fungi that otherwise form normal ectomycorrhiza or arbuscular mycorrhiza develop a sheath between the cortex and epidermis.

The root tip is never colonised. The hyphae proliferate covering the surface of all but the passage cells of the exodermis. Over time, hyphae collapse. The layer may be more than one cell thick, but as successive layers are added, the collapsed hyphae leave a structure that is difficult to interpret.

 

peel
Mycorrhiza peeling from a root that has lost its epidermis.

As the root ages, the epidermis is sloughed thereby exposing the sheath. This structure may be misinterpreted as a sheath analogous to an ectomycorrhiza. The sheath is possibly still active, though this is difficult to determine in field-collected material.

The benefit to the host plant was also unusual. Experiments using tuberous species of Thysanotus found that shoot growth was rarely increased following colonisation. However, mycorrhizal Thysanotus linked to companion plants were likely to develop tubers. It seems unlikely that two small leaves would have supplied adequate carbon for the massive tubers that developed. The possible transfer of significant quantities of carbon between plants via the linking mycorrhizal fungi remains to be explored. LINK

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The Proteaceae and Other Nonmycorrhizal Plants

Plants that are normally found without mycorrhizas may become colonised in some circumstances. Placing pre-germinated seeds of most of these species in pot cultures of arbuscular mycorrhizal fungi commonly results in the colonisation of the primary root. The extent and longevity of the colony differs with host and environment. However, the colony rarely contains arbuscules, and the stele may be invaded.

Roots of these species collected from the field are commonly colonised at the tip of the primary root, and in a few rare instances, all root tips. This is particularly the case with seedlings that appear in established and stable vegetation systems. Many colonised seedlings die.

Many non-mycorrhizal plants are weeds of highly disturbed sites. Members of the Brassicaceae can be found on roadsides and in cultivated soils. These sites have reduced quantities of AM fungi in the soil. LINK

Members of Banksia (Proteaceae) are different. Banksia tends to be found in stable, fire prone perennial vegetation. The species that reseed tend to be found in clumps. The plants in the clumps are killed during fire and seed is subsequently released because heat opens the seed capsules. Seedlings germinate in the vicinity of the old clump. The clump tends to "shift" small distances after each fire.  While the reasons for clumping is related to seed fall, the germination and establishment of seedlings into these soils may be related to the effect of fire on the density of AM fungi in the surface layers of the soil. Heat from fire may reduce the fungal population, already lowered by the presence of a non-mycorrhizal host. Thus the plant may maintain a soil environment more suited to its continuation.

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Conclusion

The different types of association indicate complex interactions between mycorrhizal fungi and plants. The possibility of plants parasitising the associated fungi is real in achlorophyllous species. Some plant species in families that contain achlorophyllous members may be partly reliant on the associated fungus for carbon. The issue remains to be explored. Further, the ecology of mycorrhizal fungi and plants that are normally non-mycorrhizal or have atypical associations remains to be teased apart. The precise role of the plant and fungus in determining the distribution of both in the environment is unclear.

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References

McGee PA (1985) Lack of spread of endomycorrhizas of Centaurium . New Phytologist 101, 451-458.

McGee PA (1988) Growth response to and morphology of mycorrhizas of Thysanotus . New Phytologist 109, 459-463.

Tester M, Smith FA & Smith SE (1987) The phenomenon of nonmycorrhizal plants. Canadian Journal of Botany 65, 419-431.

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