The Mysterious Interrelationships Between Fungi and Plants: The Case of Endosymbionts

Abstract

The three major groups of terrestrial fungi (Ascomycetes, Basidiomycetes, and Zygomycetes) are believed to have evolved from a member of the aquatic group Chytridiomycetes that associated with terrestrial plants, either as a saprophyte of dead plant material or as a parasite of living plants (Pirozynski and Malloch, 1975). The earliest fossil evidence of land plants contain fungal remains that may be interpreted as chytridiomycetous and zygomycetous in nature (Kidston and Lang, 1921). These Early Devonian fungal remains have been interpreted as endosymbionts (Pirozynski and Malloch, 1975), but are probably saprophytes due to the degraded condition of host tissues in which mycelium is encountered (Kidston and Lang, 1921). It is evident that sometime after plants colonized land, symbiotic associations began to develop between plants and terrestrial fungal groups. Fossils containing roots from the Triassic were found to contain fungi (Zygomycetes) of the ecologically-important symbiotic association, known as vesicular-arbuscular mycorrhizae (Stubblefield, Taylor, and Trappe, 1987). It is unknown when or from what progenitor the Ascomycetes and Basidiomycetes evolved. It seems probable that the ancestor to these groups was zygomycetous and that their evolution was influenced by their association with land plants since they evolved as mycorrhizae, pathogens, and saprophytes of plant materials. It is within the Clavicipitaceae (Ascomycetes) that another type of fungal endosymbiont evolved. The family Clavicipitaceae contains members that are biotrophic parasites of insects, fungi, and plants. Within the groups that are parasitic on grasses, many are episymbiotic (superficial on host tissues), while some are endosymbiotic (colonize internal tissues of the host). In this chapter we will examine biological aspects of episymbiotic and endosymbiotic members of the Clavicipitaceae.