Figure 1.

Schematic representation of endofungal bacteria and their intracellular lifestyle

(A) The Gram-negative bacterium Mycetohabitans rhizoxinica infects its fungal host Rhizopus microsporus, forming an endosymbiotic relationship. Fungal reproduction through spores is dependent on the presence of endosymbionts. Mature sporangia are absent in apo-symbiotic (endosymbiont-free) fungi.

(B) Entrance of M. rhizoxinica into the fungal hyphae is mediated by the secretion of effectors via a type 2 secretion system (T2SS). Once inside, M. rhizoxinica lives in “stealth mode” with the help of a specialized bacterial lipopeptide O-antigen. Although a functional type 3 secretion system (T3SS) is required for the formation of a stable symbiosis, the molecular function of the T3SS-secreted transcription activator-like (TAL) effectors in M. rhizoxinica is currently unknown.