1. IDENTIFYING CHARACTERISTICS

Spores

• Produced singly, in aggregates, in an unorganized hyphal matrix, with the structural wall continuous with a wall of the subtending hyphae.
• Form from hyphal swellings up to 150um, several wall layers thick, white-yellow or honey colored-brown, globose.
• Morphological diversification mostly in the number and types of walls formed outside the structural wall.
• Amorphous plug, a septum, inner membranous wall, or thickening of the structural wall used to seal off contents.
• Form flexible inner walls so far limited to those which are membranous.
• Germinate through emergence of germ tube through subtending hypha.

Hyphae

• Relatively straight hyphae ramify along the root cortex (if root anatomy premits), often producing 'H' branches which result in simultaneous growth in two directions.
• Staining of these hyphae is usually relatively dark.
• Oval vesicles, which usually form between root cortex cells, are present in many cases (but not all). These vesicles persist in roots and often develop thickened and/or multilayered walls.

• Division Eumycota

  Subdivision Zygomycotina

  Class Zygomycetes

  Order Glomales

  Genus Glomus

• Carotenoid production induced by trisporic acid (active at 10 -8M).
• Zygospore is only diploid stage.
• Most are heterothallic.
• Sporangium born on Columella of Sporangiophores.
• Aseptate mycelium.
• Cell walls made of N-acetyl-glucosamine (chitin) with §(1-4) glycosidic linkages.
• Obligate symbiotic association with higher and lower plants.
• Hypha penetrate roots and form vesicles and send haustoria into cells.
• Fully terrestrial.

Picture depicts vesicles and arbuscules formed inside the phytobionts cells.

This picture is an artistic rendition of a VA mycorrhizal association.

3. ISOLATION AND ECOLOGY
The establishment and subsequent growth of most plants requires (or is enhanced by) the symbiotic association of fungi known as mycorrhizas. It has been experimentally shown that mycorrhizas can increase the uptake of plant nutrients such as phosphorus and nitrogen. Their role is particularily important when these nutrients are at low concentrations or in insoluble form. A role in the translocation of H2O has also been suggested. About 90% of all vascular plants as well as many non-vascular "lower" plants have VAM symbionts and may have since their induction to land (see Evolution below).

The genus Glomus, togther with the other four genera of Glomales, Acaulospora, Gigaspora, Scutellospora and Sclerocystis, can form mycorrhizal associations with thousands of various plant species. The lack of host specificity contrasts greatly with that of parasitic biotrophs such as rusts and smuts. A possible explanation is that these mutually beneficial associations result in a selection for the ability to form associations and against the development of easily triggered resistance reponses as occurs with parasitic biotrophs (Carlile & Watkinson 1996).

The spores are some of the largest in the fungal kingdom and can be extracted from field soils by a simple wet-sieving procedure. Diluted soil suspensions, broken roots from the field, and transplanted seedlings from the field all work well in laboratory experiments desiring Glomus spp. inoculum.

Parasitism of VAMF spores typically involves members of the Oomycota (water molds) and the Chytridiales which is probably a widespread phenomena and may limit the populations of mycorrhizal fungi in wet soils. Rhizidiomycopsis stomatosa (Hyphochytriales) has been shown to parasitize G. versiforme which is an important mycorrhiza used in pot cultures of crop plants, thus any parasite is of interest to those concerned with the exponentially increasing human consumptive needs. Anguillospora pseudolongissima parasitisim of the spores of G. deserticola is of importance since this species associates with the roots of onion plants (Allium spp.).

Glomus clarum and G. caledonius spores have been shown to contain endogenouse bacteria housed in NT4 spore walls which may play a role in providing metabolites during a stage of rapid multiplication (Germida & Walley. 1995). Others have shown that microorganisms can both stimulate or inhibit or delay VAMF spore germination. Glomus tenuis has been purported to be involved in early successional plant species. Glomus intraradices has been linked to the resistance of the pea pathogen Aphanomyces euteicles by Kjżller, R. and Rosendahl, S. 1996).

4. EVOLUTION

In 1904, Weiss described what appeared to be a mycorrhiza from "lower" coal deposits. Kidstone and Lang (1921) presented a series of plates that show fungal structures closely resembling vesicles and spores in the rhizoidal cortex of Rhynia and Asteroxylon from the early Devonian. Several of these plates suggest that at least some of the structures of VA mycorrhizal associations resemble vesicles and spores in the rhizoidal cortex.

Evidence for the classification of VA mycorrhizal fungi as Zygomycetes: the formation of suspensor cells comparable to Zygomycetes, the formation of zygospores in the genus Endogone (Gerdemann & Trappe, 1974), and chitinous, as opposed to cellulosic, cell walls. Jabaji & Have (1988) suggested that Chytrids also have chitin and that the fatty acid composistion of Glomus intraradices and vesiculiferum and Gigaspora marginata resembles that seen in protoctistan fungi more closely than that of Zygomycetes. Perhaps a reexamination of the taxonomic status of VA fungi is needed. Perhaps these fungi may be a transition between the Chytrids and Zygomycetes.

Allen, M. (1991) The Ecology of Mycorrhizae. Cambridge University Press, New York.

Barea, J.M. (1991). Vesicular-Arbuscular Mycorrhiza as modifiers of soil fertility. Advances in Soil Science 15.

Brundrett M., Bougher N.,Dell B., Grove T., and Malajezuk N. (1996). Working with Mycorhizas in Forestry and Agriculture. ACIAR Monograph 32. 374+xp.

Carlile, M.J. and Watkinson, S.C. (1996) The Fungi. Academic Press. New York.

Germida, J.J. and Walley, F.L. (1995) Failure to decontaminate Glomus clarum NT4 spores is due to spore wall-associated bacteria. Mycologia ? p43-49.

Harley, J.H. and Smith, S.E. (1983) Mycorrhizal Symbiosis. Academic Press, London.

Jeffries, P. and Young, T.W.K. (1994) Interfungal Parasitic Relationships. CAB International. University Press, Cambridge.

Kjżller, R. and Rosendahl, S. (1996) The presence of the arbuscular mycorrhizal fungus Glomus intraradices influences enzymatic activities of the root pathogen Aphanomyces euteiches in pea roots.

Pfleger, F.L. and Linderman, R.G. (1994) Mycorrhizae and Plant Healt. APS press. Minnesota.

Simon, L. (1993) Origin and diversification of endomycorrhizal fungi and coincidence with vascular land plants. Nature 363.

Sylvia, D.M. and Schenck, N.C. (1983) Germination of chlamydospores of three Glomus spp as affected by soil matrix potential and fungal contamination. Mycologia. 74, 30-35.

Walker, J.F. (1996) The Origin and Evolution of Mycorrhizae. Virginia Polytechnic Institute and State University. personal comm.

Webster, J. (1980) Introduction to Fungi. 2nd Edition. Cambridge University Press. NY

The picture to the left shows conetainers of ectomycorrhizal fungi on Douglas Fir seedlings. The tube on the left was not inoculated with the fungus, and the tube on the right was inoculated.