Introduction

Arbuscular mycorrhizas (AM) consist of three parts. The fungal component in the root, and soil, and the colonised root itself. Two types of AM are common, Arum and Paris, though some plants form intermediate types or both types in different parts of the root, indicating that the host determines the structure of the intraradical component.

Types of AM

In general terms, the Arum type of AM consist of a hyphal network which extends through the intercellular space of the root cortex. Arbuscules form from lateral projections which invaginate cortical cells. The arbuscule collapses after a 4 to 20 days and leaving a slight unevenness in diamter of the intercellular hyphae. Vesicles tend to be intercellular, and usually located in the cortex.

Paris type of AM are highly variable. They usually have extensive development of intracellular hyphae in the cortex, surrounded by host plasmalemma. Coils are common in a range of hosts: these may be peletons familiar to orchid associations, or more hand-like structures which branch within the cortical cell. Arbuscules may arise as lateral projections from coils. The size and density of coils varies enormously between host species.

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Formation of AM

Colonisation of roots may arise from hyphae that emerge from quiescent or growing hyphae in soil or roots. The process of colonisation has two phases, primary and secondary. Primary colonisation arises from inoculum unattached to that host root.

A hypha may grow more than 15 mm before reaching the root surface, and at a rate of up to 0.5 mm per day. Hyphae do not always grow directly to the root, though they often proliferate in the vicinity of the root surface. After contact, the hyphae may branch on the root surface of compatible plants, forming one or more appressoria. Usually a single appressorium subtends a penetration peg either between cells or through the epidermal cell, and the remaining appressoria senesce. Hyphae may ramify along the outer and inner epidermis without any further penetration.

Subsequent colonisation requires a positive response from the host. The pattern of subsequent penetration of the root is determined by the presence of the hypodermis, especially where the cells of the hypodermis are dimorphic. The fungus does not penetrate the long cells of the hypodermis, probably because the outer surfaces are suberised. Hyphae initiate penetration of the cortex via a passage (short) cell of the hypodermis. This cell subsequently becomes suberised on the outer surface. The fungus may form a loop or coil in this passage cell. In the absence of a hypodermis, the fungus penetrates between cortical cells.

In Arum-type AM, the fungus elongates through the intercellular space, branching repeatedly in a pattern typical for each family. Initially, arbuscules are formed close to the penetration point. As the colony extends within the root, fresh arbuscules are formed in many of the cells. Aged colonies tend to have fewer arbuscules. If the fungus forms intraradical vesicles, these will be appear soon after initiation of the unit. They tend to be formed in specific locations in the root and at specific times of the life cycle indicating that the processes are under host and fungal genetic control.

We have little information on the development of arbuscules in Paris-type AM. However, the main difference is that spread through the cortex is from cell to cell. Arbuscules are formed from laterals in the cell, and several arbuscules may form in any one cell, either together or sequentially. Huge differences between hosts are found in the process of Paris-type colonisation.

Soon after initiation of the colonisation unit, a lateral emerges from external hyphae near the appressorium. The lateral elongates along the root and establishes a second colonisation unit. Secondary colonisation commences 10 to14 days after initiation of primary colonies. AM may spread along a root and through a root system in what is called secondary colonisation or spread. Spread may be up and down a root, and from root to root in soil.

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Colonisation of Soil by AM Fungi

The soil phase of AM is similar in both types of AM and is characteristic of the fungal genus. The fungus spreads along the root surface and out from the root into soil. The mycelium consists of a range of sizes of hyphae, with recently formed hyphae tending to be finer and thinner walled. The coarser hyphae have a general tubular shape, with uneven surfaces due to the senescence of laterals.

Large, rounded blastospores form on or towards the terminus or subterminus of hyphae in Glomus, Paraglomus, Sclerocystis. If the walls of these spores thicken, they may function as resting stages of the fungi. In Acaulospora, Archaeospora and Entrophospora, spores lacking subtending hyphae, but with a similar function, are formed within expanded hyphae, anterior to the terminus. The fungi may also form terminal blastic vesicles which retain thin walls and collapse over time.

In some genera of Glomus, spores may be formed singly or in compound structures called sporocarps. Sporocarps usually contain more than one spore, and in some cases, many thousands of spores, with hyphae surrounding the spores. The sporocarp is similar within a population, though may differ enormously in complexity within a genus. The most complex sporocarps have an outer peridium of hyphae and may be attached to the soil surface by a stipe. The spore aggregations may be organised around sporogenous hyphae, or appear as irregular and open clusters. Sporocarps are more commonly formed by fungi that are found in relatively undisturbed soil. Fungi of agricultural soils tend to only form single spores, though a few species form both. LINK

At the same time as mycorrhizas spread along and within the root system, hyphae elongate into the soil, either initiating colonisation of other roots or ramifying through the soil. In this way, the soil phase is expanded. The density of the hyphae in soil is highest just beneath the soil surface and declines down the profile, even though the density of colonisation within the root system down the soil profile is likely to be similar. This suggests that the fungi grow into soil in direct relation to soil compaction and oxygen status. Hyphae are at low densities in compacted or continually anoxic soil. Spores are formed when adequate reserves of energy have been stored in the hyphae. Spore formation is also usually related to the amount of colonisation associated with the mycelium.

The hyphae also form specific associations within soil fragments. Microscopic examination of microaggregates indicates that hyphae physically bind particles together. LINK The exudates also function, at least temporarily, to glue fragments at this microscopic level. The overall effect is a concentration of hyphae around and within peds, and absence from the air spaces created by aggregation processes. The reorganisation of soil particles is also possible. Peds are probably anaerobic in their centres, thus providing microsites for storage of organic carbon and anaerobic microbes. The outer surfaces are aerobic. Thus the types of mucilage and biochemical activities associated with each type of microbe, will influence the development of the ped and the functional attributes of the soil.

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Survival of AM Fungi in Soil

The mycelium of AM fungi may survive for a considerable period beyond death of the attached host plant. Many fungi have dormancy that enables propagules to remain quiescent under conditions that would otherwise germinate the spores. However, the absence of plants may also enable survival. We have recovered viable propagules from fallow soil that has lacked plants for seven years. Survival is greatest when the soil remains undisturbed and dry.

Depletion of propagules is associated with many factors. Rainfall encourages germination of propagules, which in the absence of a host will cause a depletion of energy reserves. Germinated fungi may remain as activated hyphae in the soil for some months. Fine endophytes (such as Glomus tenue) are among the few fungi that have the potential to regain quiescence. In fungi that do not regain quiescence germinated propagules are lost in the absence of initiation of mycorrhizas.

Other factors reduce the length of viable hyphae. Cultivation and other forms of disturbance shatter the mycelium, directly reducing the density of the fungus. Grazing by mycophagous invertebrates and attack by bacteria and hyperparasitic fungi also reduce the density of the mycelium.

The consequence of these factors is that the population of AM fungi is at a maximum at the point of death of the host.  The fungus survives as mycelium, both hyphae and the blastospores formed asexually by the fungus, in soil and roots.

Sound evidence suggests that many species, especially members of the Acaulosporaceae, have innate dormancy. That is, the fungi, especially the spores pass through a period of maturation for up to 3 months prior to becoming potentially germinable. Some species of Glomus also form dormant propagules, though dormancy in Glomus is not as well documented.

Survival of hyphae in the mineral fraction of soil appears to be shorter than in organic matter. If correct, organic soils may retain colonising potential for longer than mineral ones. The mechanism is unclear, though may be related to the protection from arthropods afforded by the organic cover.

Fungi adapted to cold or hot conditions appear to be able to tolerate extremely low or high soil temperatures respectively, even if the soil remains moist. The fungi remain quiescent under these extreme temperatures. Thus it might be argued that the fungi have an environmentally induced quiescence. Interestingly, plants are also inactive in low and high temperatures. While the mechanism is unclear, quiescence provides protection to the fungi from growing when their host is unavailable.

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Conclusion

Though they colonise a huge array of photobionts, AM fungi form recognisable structures in roots. Further, AM fungi are extremely common in soil, where they play an extremely important role. AM fungi survive for long periods of time, though they are subject to predation and are intolerant of disturbance.

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References

Smith SE & Read DJ 1997 Mycorrhizal Symbiosis. Academic Press.

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