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Symbionts In Abuscular Mycorrhizas

The Fungi

All fungi that form arbuscular mycorrhizas (AM) are members of the Glomeromycota, Order Glomales. The taxon is clearly ancient. Molecular evidence indicates extremely deep and ancient phylogenetic divisions among Glomeromycota. Fossils from the Devonian have a similar morphology to modern day AM suggesting that fungus and plant have had a long period of co-evolution.

AM fungi are aseptate soil-borne species, and with one exception, are not known to pass through a sexual stage. Glomalean fungi are commonly characterised by large spores formed singly in agricultural soils. In less disturbed soils, many species form compound structures called sporocarps which contain many spores. Glomalean fungi are mostly obligate symbionts and must be cultured in association with the host plants, commonly the roots. Spores and sporocarps can be extracted from soil and used to establish pot cultures with suitable hosts in a suitable soil.

Taxonomy

The taxonomy of the Glomeralean fungi is still unclear. Use of molecular data indicates that the division is highly diverse, and many sequences can not be linked to the 200 or so described species. Present hypotheses indicate that Glomeromycota has more than four orders, with up to three families in each. Of the traditional genera, Glomus has been split and placed in at least three different orders, Sclerocystis has been collapsed into Glomus and a single species remains, presumably within the Glomerales. Two members of Acaulospora have been placed in a new order Archaeosporales (Archaeosporaceae) in part because one stage of their life cycle resemble Glomus, and the rest of Acaulospora and Entrophospora remain in the Acaulosporaceae, and placed within the Diversisporales. Gigasporaceae (two genera Gigaspora and Scutellospora) is also placed in the Diversisporales. Paraglomaceae with one genus Paraglomus, is placed in the Paraglomerales. New genera and species are being described regularly.

More than 200 species have been morphologically described, a significant number are members of Glomus. Descriptions based on morphology and developmental processes are difficult because the fungi differ in only minor characters, and considerable variation can be seen in one species (see images below). Molecular data obtained from the ribosomal and other genes, and some biochemical data support the grouping, but also indicate that a huge number of species remain to be delimited. Molecular data also indicate huge variation among isolates of the same species (see images below). Some of that variation is associated with the classical separation of Gondwana from Eurasia, indicating that some species may be of ancient origin. Other species, such as Glomus mosseae, are globally distributed, perhaps because they are associated with agriculture, and have spread with soil and tools of food production.

NBR 2.1
Spore of Glomus mosseae.
Bur 11
Spore of Glomus mosseae.
NBR 4.1
Spore of Glomus mosseae.
NBR 3.1
Spore of Glomus mosseae.

 

NSW 1
Spore of Glomus mosseae.

 

The fungi have been described on the basis of the morphology of the spore. Morphological data does not necessarily match the molecular sequences, the spores of different species are remarkably similar, and some species have multiple types of sequence data within one spore. Members of Glomaceae and Paraglomaceae consist of blastospores with variable number, colour and type of walls, closure of spores, and developmental sequence of wall formation, and formation of spores in sporocarps. Sclerocystis is differentiated from Glomus on the basis of a regular array of spores in a sporocarp. Even Archaeospora has a blastosporic spore type. Ultimately, the use of several gene sequences and morphology and development of spores may be required to clarify identity of fungi.

Members of Gigasporaceae have a bulbous attachment at the base of the blastospore, spores have a varying size, variable wall numbers, types and colours. The species may also be differentiated on the basis of their accessory cells, which are formed in soil. This family does not form compound sporocarps. Gigaspora is simpler than Scutellospora, the latter having more walls and germinates through a germination shield in the spore.

Spores of Acaulosporaceae form within a hypha that subtends a mother cell or saccule. Spores of Entrophospora form in the subtending hypha, and in Acaulospora within a lateral from the subtending hypha of the saccule. As the lateral may be short, or missing altogether, some confusion may arise during identification. Also, the spores are usually formed in soil, though some species can develop spores in roots.

sporelets
Glomus spore with inclusions

Further difficulties arise becaue AM colonies may stain very weakly (Trypan blue is the most commonly used stain, poor staining is found with most stains used on these fungi). Thus their mycorrhizas are difficult to observe in colonised roots.

 

root vesciles
Sporocarp in roots.
vesicles
Thin and thick walled vesicles in the root of onion.

The fungi differ slightly in the morphology of their colonies in roots. LINK Gigasporaceae are not known to form vesicles within the root. Acaulosporaceae form vesicles, but they are always thin-walled. Glomus species may form both thick and thin-walled vesicles in roots. Absence of vesicles is not an important feature, as some sporocarpic isolates of Glomus are not known to form vesicles while others develop sporocarps. Paraglomus may form vesicles and Archaeospora appears to lack vesicles.

Traditional approaches to identification of arbuscular mycorrhizal fungi using morphological data from spores and mycorrhizas is unreliable. Members of the order have been around for a long period. Considerable divergence at the molecular level is not evident necessarily in the morphological features of the fungi.

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The Plants

Approximately 75% of all angiosperm and gymnosperm genera are thought to form AM under appropriate conditions. AM fungi may also be found in some ferns, mosses and liverworts. The host genera include most plants important in agriculture, forestry and horticulture, and include some plants that may, at the same or a different time, have another type of mycorrhiza. Generalisations about types of association are based on examination of a limited number of specimens and species. Care needs to taken when making claims about the ubiquity of AM in particular seasons or conditions, and their functional characteristics.

Colonisation of plants by AM fungi may result in an increase in uptake of minerals by the host, causing increased rates of growth. In some plants species, the increase in growth is the usual response to colonisation. However, some plants are not as responsive and the benefit of AM may be related to specific environmental conditions or other factors. LINK

The differences in growth response following colonisation has led to the concept of dependency of the host. Those plant species that benefit from AM in most situations are called highly dependent. In dependent plants, the uptake of minerals and water is increased by the association in almost all circumstances. Indeed, the presence of AM fungi in these plants downregulates plant P uptake genes, indicating fungal control of this important plant function. Responsive plants tend to be important food crops that are also responsive to P fertiliser.

At the other end of the spectrum are those plants that appear to be occasionally colonised or colonised to a small extent, and whose response is rarely an increase in growth: the plants are termed facultatively mycorrhizal plants. While the fungus may increase host reproduction or survival in drought, or root or seed storage of minerals, the rate of growth of the plant is essentially unaffected by colonisation. The spectrum of plant benefits still remains unclear.

Some plants, especially the nonmycorrhizal species, may be colonised to their detriment. A reduction in rates of growth may make the plants less competitive in situations where mycorrhizal plants are common.

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Nonmycorrhizal Plants

Plant genera that usually lack AM may either form another type of mycorrhiza exclusively, have alternative forms of acquisition of minerals, or occupy sites where stresses other than nutrient deficiency predominate. Many plant species are known to be weedy, that is they have a ruderal strategy. LINK Ruderals rarely depend on mycorrhiza. They occupy disturbed habitats which are unlikely to have high densities of propagules of AM fungi. Ruderal plant species commonly colonise disturbed soil and are then replaced by facultatively mycorrhizal plants. LINK

Individuals in families that are normally nonmycorrhizal may be colonised by AM fungi at different times. The reasons for colonisation are still unclear and the association may not benefit either symbiont. Commonly, the colony in the root will be constrained to the epidermis, or if in the cortex lack arbuscules.

Families that commonly do not form AM include Azoaceae, Brassicaceae, Crassulaceae, Cyperaceae, Juncaceae, Pinaceae, Proteaceae and Restionaceae. Presence of members of these families in high densities have consequences for the establishment and maintenance of populations of AM fungi. High densities of AM fungi are unlikely in these plant patches, and immediate establishment of dependent plant species is unlikely.

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Conclusion

Most plants may become colonised by Glomalean fungi. In some plants, the association will be beneficial for most of the life cycle of the plant, and involve increased uptake of P, among other factors. Other plants, such as the grasses, will benefit for short periods when in pure stands, but may grow slower when in competition with more dependent plant species. The third group of plants are not normally mycorrhizal and they may become colonised under specific conditions. In these conditions, colonisation may kill seedlings and slow the rate of growth of those that survive.

AM fungi are common and widespread. They are placed within a small number of taxa. The taxa have had a long history and slow divergence. Remarkably, only around 200 fungal species have been described, though this may be due to the lack of taxonomists rather than innate differences within the Glomeromycota.

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References

van der Heijden MGA & Sanders IR 2002 (Eds) Mycorrhizal Ecology. Springer.

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

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