Plant cells engage in mutualistic and parasitic endosymbioses with a wide variety of microorganisms, ranging from Gram-negative (Rhizobium, Nostoc) and Gram-positive bacteria (Frankia), to oomycetes (Phytophthora), Chytridiomycetes, Zygomycetes (arbuscular mycorrhizal fungi) and true fungi (Erysiphe, ascomycete; Puccinia, basidiomycete). Endosymbiosis is characterised by the 'symbiosome', a compartment within host cells in which the symbiotic microorganism is either partially or completely enclosed by a host-derived membrane. The analysis of plant mutants indicates that the genetic requirements for the interaction with rhizobia and arbuscular mycorrhiza fungi are partially overlapping. The extent to which plants use similar or identical developmental programs for the intracellular accommodation of different microorganisms is, however, not clear. For example, plant cells actively weaken their cell wall to facilitate bacterial colonisation, whereas penetration by fungal symbionts appears not to be assisted in this manner. Moreover, different transport requirements are imposed on the symbiotic interface of different interactions indicating that additional system-specific components are likely to exist.