from Penny R Hirsch and Tim H. Mauchline writing in Microbial Ecological Theory: Current Perspectives:
Mutualism is responsible for the genesis of green plants and is implicated in their colonisation of land. Current knowledge of plant-microorganism symbioses includes a range of associations with different degrees of intimacy and mutual dependence but the mutual benefits are not always clear. Complex signalling is involved when the plant immune system recognises beneficial endosymbionts although many have also evolved mechanisms to evade or moderate plant defence pathways. A wide range of bacteria inhabit intercellular spaces but only a few are true endosymbionts able to penetrate living cells whilst remaining membrane-bound, accessing plant carbon compounds in a manner analogous to biotrophic pathogens. Unlike pathogens, they provide nutrients to the plant in exchange. The best-known examples are rhizobia, bacteria that induce root nodules on leguminous plants and fix atmospheric nitrogen; and arbuscular mycorrhizal fungi that sequester phosphate and organic N from soil and provide it to their plant hosts. Both secrete factors prior to contacting plant cells which appear to prepare the hosts for mutual rather than pathogenic interactions and suppress the defence mechanisms. The processes involved in these symbioses are compared to less intimate interactions and the nature of mutualism is discussed.
Further reading: Microbial Ecological Theory: Current Perspectives