Symbiotic fungal associations in 'lower' land plants

An analysis of the current state of knowledge of symbiotic fungal associati ons in 'lower' plants is provided Three fungal phyla, the Zygomycota, Ascom ycota and Basidiomycota, are involved in forming these associations, each p roducing a distinctive suite of structural features in well-defined groups of 'lower' plants. Among the 'lower' plants only mosses and Equisetum appea r to lack one or other of these types of association. The salient features of the symbioses produced by each fungal group are described and the relati onships between these associations and those formed by the same or related fungi in 'higher' plants are discussed. Particular consideration is given t o the question of the extent to which root-fungus associations in 'lower' p lants are analogous to 'mycorrhizas' of 'higher' plants and the need for an alysis of the functional attributes of these symbioses is stressed. Zygomycetous fungi colonize a wide range of extant lower land plants (hornw orts, many hepatics, lycopods, Ophioglossales, Psilotales and Gleicheniacea e), where they often produce structures analogous to those seen in the vesi cular-arbuscular (VA) mycorrhizas of higher plants, which are formed by mem bers of the order Glomales. A preponderance of associations of this kind is in accordance with palaeobotanical and molecular evidence indicating that glomalean fungi produced the archetypal symbioses with the first plants to emerge on to land. It is shown, probably for the first time, that glomalean fungi forming typi cal VA mycorrhiza with a higher plant (Plantago lanceolata) can colonize a thalloid liverwort (Pellia epiphylla), producing arbuscules and vesicles in the hepatic. The extent to which these associations, which are structurall y analogous to mycorrhizas, have similar functions remains to be evaluated. Ascomycetous associations are found in a relatively small number of familie s of leafy liverworts. The structural features of the fungal colonization o f rhizoids and underground axes of these plants are similar to those seen i n mycorrhizal associations of ericaceous plants like Vaccinium. Cross inocu lation experiments have confirmed that a typical mycorrhizal endophyte of e ricaceous plants, Hymenoscyphus ericae, will form associations in liverwort s which are structurally identical to those seen in nature. Again, the func tional significance of these associations remains to be examined. Some members of the Jungermanniales and Metzgeriales form associations with basidiomycetous fungi. These produce intracellular coils of hyphae, which are similar to the pelotons seen in orchid mycorrhizas, which also involve basidiomycetes. The fungal associates of the autotrophic Aneura and of its heterotrophic relative Cryptothallus mirabilis have been isolated. In the l atter case it has been shown that the fungal symbiont is an ectomycorrhizal associate of Betula, suggesting that the apparently obligate nature of the association between the hepatic and Betula in nature is based upon require ment for this particular heterotroph.