Abstract

<jats:p>Many studies of interactions between the organisms which form biotrophic symbioses have concentrated on extracellular events such as signals and responses that involve modifications to cell wall metabolism. It is clear, however, that formation of a biotrophic association must also involve signals and responses at the membrane level and modifications to membrane activity resulting in changes in the transfer of nutrients.
In parasitic biotrophs the modifications to membrane transport result in unidirectional transport (at least in the long term).
In mycorrhizal associations, lichens and N2-fixing symbioses, it appears that well regulated bidirectional transport of nutrients between symbionts must occur, thus allowing a persistent compatible (and mutualistic) symbiosis. Evidence for such membrane modifications comes from changes in ATPase activity. The overall question to be considered in mutualistic symbioses is whether nutrient transfer processes have analogies elsewhere in the physiology of plants or whether 'new' transport events are switched on as a result of interactions between the organisms.</jats:p>