What are VAM?

Vesicular-arbuscular mycorrhizas (VAM) and arbuscular mycorrhizas are mutualistic symbioses formed between the roots of most plants and fungi in the order Glomales.

The Glomales are currently classified in the Zygomycetes. However, this is now being questioned. Recent information from sequences of their 18S ribosomal genes indicate that they form an ancient group branching before the Ascomycetes and the Basidiomycetes (Rosendahl and Dodd, 1995).

Image of Gigaspora margarita spores borrowed from La Banque Européenne des Glomales.

Gigaspora spp. and Scutellospora spp., Gigasporaceae, are the fungal partners in arbuscular mycorrhizas.

These are the commonest types of mycorrhiza and can be found in almost all plant communities, natural and agricultural. Vesicular- arbuscular (VA) and arbuscular mycorrhizas are endomycorrhizas formed by Zygomycete like fungi and the roots of most families of Angiosperms as well as Gymnosperms, Pteridophytes and Bryophytes (liverworts). Non-mycotrophy in the Angiosperms appears to be restricted primarily to the families Amaranthaceae, Brassicaceae, Chenopodiaceae and Zygophyllaceae, and many hemiparasitic plants. The mycorrhizal host may be facultatively or obligately dependent on its fungal partner. It appears that these mycorrhizal associations are evolutionary very old and that other types of mycorrhizas and non-mycotrophy evolved more recently. In fact, it has been speculated that VA mycorrhizas may have been involved in the successful invasion of land by vascular plants and played a controlling influence on the evolution of roots. Unlike many of the fungi involved in other types of mycorrhiza, these mycorrhizal fungi cannot be cultured in the absence of plant roots or a root organ culture.

In contrast to most other types of mycorrhiza, the fungi do not substantially change the morphology of host roots. The presence of the fungi has to be determined by clearing, staining and microscopical examination of feeder roots. An experienced worker can readily recognise the coarse fungal hyphae with their distinctive angular morphology. These hyphae are restricted to the cortical region of the roots and never penetrate the stele. All the fungal species form arbuscules - small tree like, hyphal filled, invaginations of the cortical cells - which provide intimate contact between the plasmalemmae of the two symbiotic partners and are, presumably, the point of material exchange between host and fungus. They persist in active form for a very short period, 1 to 15 days. With the exception of species of Gigaspora and Scutellospora, the fungi form vesicles within the roots. These are lipid filled, terminal swellings of hyphae with a storage/perennating function.

That members of two genera do not form vesicles has led to a debate as to whether the name arbuscular mycorrhizas is more appropriate than vesicular-arbuscular mycorrhizas for this relationship or whether the more appropriate of the two terms should be used depending on the fungal species present.

A 5cm segment of living root can be colonised by as many as eight species from three genera of VA mycorrhizal fungi (Tommerup, 1988) and a germinated spore can simultaneously colonise roots of unrelated plants. Tommerup (1988) reported hyphae from a single spore of Glomus caledonium (Nicol. & Gerd.) Trappe & Gerd. colonising roots of lettuce (Lactuca sativa L. - Asteraceae), white clover (Trifolium repens L. - Fabaceae) and rye grass (Lolium perenne L. - Poaceae) with living mycelium interconnecting at least one intraradical colony in each plant.

There is considerable evidence that the mycorrhizal fungi enhance phosphorus uptake of their host plants and that their presence is more prevalent in roots in low P soils. These mycorrhizas also increase the uptake of other mineral nutrients. According to Sieverding (1991), for P, Cu and Zn, the VA fungi may act similarly to fertilization but that this is not the case for Fe.

Mycorrhiza formation has also been shown to confer drought and disease resistance, reduce pest damage and nematode infection, promote seed production, and increase the fitness of plant offspring.

Vesicular-arbuscular and Arbuscular Mycorrhizas

References