Intercellular Communication and Recognition in the Rhizobium-Legume Symbiosis

Abstract

A complex, multi-step interaction between the soil bacterium Rhizobium and specific leguminous plants results in the induction of nitrogen-fixing root nodules (Rolfe et al 1981). The bacteria respond to plant-secreted compounds which stimulate the expression of the nodulation genes (Innes et al 1985; Mulligan and Long 1985; Rossen et al 1985). The bacteria are thought to respond by releasing compounds which induce marked root hair distortion and by elaborating receptors for the binding of specific plant-derived lectins (Dazzo et al 1985). Only actively growing root hair cells that occur behind the root tip at the time of inoculation, are highly susceptible to infection by compatible Rhizobium strains (Bhuvaneswari et al 1980). The cell wall of the root hair can be penetrated after as little as 24h exposure to the Rhizobium strain (Callaham and Torrey 1981; Ridge and Rolfe 1985). The nucleus of the root hair cell migrates to the site of wall penetration and an infection thread is synthesised by the plant presumably in response to the perturbation of the root hair cell wall (Dart 1974). The bacteria are carried towards the cortex inside the infection thread where they actively divide. Shortly before, or concurrent with the initiation of infection thread synthesis, cortical cells (which are normally non-dividing) are stimulated to divide, probably by diffusable substances released by the bacterium (Bauer et al 1985). The bacteria are packaged into plant-elaborated membranes where, after a period of time, they begin to fix atmospheric nitrogen (Vincent 1980). These steps are summerised in Fig 1.