The term "mycorrhiza" implies basically the association of fungi with roots. Indeed, mycorrhiza, not roots are the chief organs of nutrient uptake by the majority of land plants . The different kinds of mycorrhizae have been described on the basis of their fungal associates into those aseptate endophytes in the class Glomeromycetes  and those formed by septate fungi in Ascomycetes and Basidiomycetes. This chapter will consider the association formed by members of the Glomeromycetes, which have been referred to as arbuscular mycorrhiza (AM), and were formally known as vesicular-arbuscular mycorrhiza. The plant hosts of this type of mycorrhiza may belong to Bryophyta, Pteridophyta, Gymnospermae, and the majority of families in the Angiospermae .
Arbuscular mycorrhizae are the most common type of mutualistic symbioses with plants. The association is formed between the roots of an enormously wide variety of host plants, which have true roots, and aseptate, obligate symbiotic fungi. The name of AM is derived from characteristic structures, the arbuscules, formed by the fungi within the cortical cells of the host plants (Figure 12.1a). Arbuscules are formed by all members of the phylum of the Glomeromycetes. In contrast, vesicles (Figure 12.1b), which occur intra- or intercellularly, are formed by all members of the families Glomaceae and Acaulosporaceae, but not by the members of the Gigasporaceae. Spores are considered as structures for reproduction or propagation (Figure 12.1c). An AM has three important components: the root; the fungal structures within the cells of the root; and the external mycelium (Figure 12.1d,e) functioning as a "bridge" between the root and the soil.
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