There are about 150 species of Glomomycetes and Bolomycetes which consists of species that do not produce sporangiospores and zoospores (Table 1.4, Fig. 1.23). The cytoplasm of hyphae contains mitochondria, peroxisomes and unstacked Golgi cisternae. The Glomomycetes (two orders: Endogonales and Glomales) do not have centrioles, and hyphae are not septate and do not produce conidia, aerial spores or stalked sporophores. A sclerotium-like sporocarp contains chlamydospores in the Glomales, or frequent sporocarps form in the Endogonales with zygospores inside. The Bolomycetes have centrioles, septate hyphae and a single large propulsive conidium on unbranched conidiophores. The zygospores are not produced inside a sporocarp. Bolomycetes consist of one family of saprotrophic species.
With the exception of some saprotrophic species in the order Endogonales, the Glomomycetes form close symbiotic associations with plant roots, by extending hyphae between root cells and growing specialized hyphae, the haustoria, into root cells, where they branch profusely. It is this branched arrangement of hyphae that is referred to as the arbuscule inside root cells. Species are obligate symbionts that, strictly defined, depend on their host for organic nutrients. However, this statement has not been completely verified physiologically for all
Fig. 1.23. Glomales (Glomomycetes, Archemycota) growing into a plant root cell (arbuscular mycorrhizal association), and a large chlamydospore in the soil. Not drawn to scale.
species. Most hyphae are intercellular between root cells of the primary cortex and epithelium, that do not penetrate the endodermis, vascular tissues or aerial plant organs. Some hyphae (5-10 ^m diameter) do extend into the soil from the root. Anastomosis of hyphae in the root and substrate occurs. Species are probably asexual and produce large dispersal spores <800 ^m in diameter. The order Endogonales (Endogone and Sclerogone) consists of saprotrophic species or those that form endomycorrhizae. The order Glomales consists of two suborders, Glomineae which form arbuscules and vesicles (Glomus, Sclerocystis, Acaulospora and Entrophospora), and Gigasporineae (Gigaspora and Scutellospora) which form only arbuscules. The latter seem not to have P(1,3)-glucan in the chitin cell wall, unlike other Glomomycetes. Vesicles and spores both store lipids. According to Smith and Read (1997), there is no clear evidence of specificity between the fungus and host root species, so that an association is possible between the fungus and any species capable of forming arbuscular mycorrhizae. However, this is not to say that the extent of root colonization and the effectiveness of the association are equivalent between species and strain combinations. Only a small number of families and genera of terrestrial plants do not form associations with Glomales.
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