Fungal-bacterial endosymbiosis

Fungal-bacterial endosymbiosis encompasses the mutualistic relationship between a fungus and intracellular bacteria species residing within the fungus. Many examples of endosymbiotic relationships between bacteria and plants, algae and insects exist and have been well characterized, however fungal-bacteria endosymbiosis has been less well described.

Fungal-bacterial endosymbiosis represents a diverse range of endosymbionts and hosts with respect to the initiation of the association and the benefits provided by and for each partner. Well-studied examples include Burkholderia species (sp.)/Rhizopus microsporus (R. microsporus), Nostoc punctiforme (N. punctiforme)/Geosiphon pyriforme (G. pyriforme) and Candidatus Glomeribacter gigasporarum (Ca. G. sporarum) /Gigaspora margarita (G. margarita) bacteria/fungi associations. What is known on these associations impacts our understanding of the ecological interactions of plants, fungi and bacteria.

The classification of bacterial endosymbionts and their fungal partners occur across a diverse set of phyla. Ca. G. sporarum and Burkholderia sp. have been identified to be β-proteobacteria, a gram-negative class of bacteria, and N. punctiforme is a cyanobacteria . These phyla are not closely related showing that the capability of endosymbiosis with fungi is widely spread. A similar pattern is seen with the fungal partners with examples occurring across broad phyla/divisions such as Glomeromycota, Zygomycota, Ascomycota and Basidiomycota. The common feature of these fungi is that they are often (AM fungi) or ectomycorrhizal fungi and form symbiotic relations with plants as well as with their bacterial endosymbionts. Though commonalities exist, the taxonomic classification does not offer a consistent symbiotic phenotype.

The definition of “endosymbiont” indicates that the bacteria are localized within the cytoplasm of cells or hyphae of the fungi partner. Specifically, the bacteria grow within the membranes of their fungal counterpart, commonly referred to as vacuoles or symbiosomes. This is a feature common in all fungal-bacterial symbiosis suggesting that internalization of the bacteria via phagocytosis is the main method of incorporation.

The bacteria involved may be internalized by the fungi on a cyclic basis or obligatorily living within the fungi. The interaction between N. punctiforme and G. pyriforme is an example of a cyclical association which forms at a certain point in their separate life cycles. N. punctiforme forms masses of filaments which gather in the dimmer underground soil while G. pyriforme grows lateral vegetative hyphae occupying the same area. The endosymbiotic relationship is formed when G. pyriforme engulf and internalize N. punctiforme in their growing hyphae in specialized compartments. Within the fungi, N. punctiforme replicates for the duration of about 6 months, coinciding with the life span of Geosiphon. Ca. G. sporarum, in contrast, is an obligate endosymbiont in the AM fungus G. margarita. They have been observed replicating within vacuoles and have been found in all stages of the life of the fungus including the spores, vegetative hyphae, and plant cell-associated hyphae. It is thought that the bacteria are transmitted vertically from parent to offspring in the fungi as permanent residents. Thus, bacterial endosymbionts are typically incorporated into growing fungi either through phagocytosis during some point in the life cycle of the fungus or passed on vertically forming permanent associations with the fungus.

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