Endosymbiosis of Rhizobium: Internalization of the “extracellular compartment” and metabolites exchange

Abstract

Following a successful infection, the Rhizobium is released inside the host cell via a process resembling endocytosis. A rapid proloiferation of membrane vesicles and their fusion at the tip of the infection thread allows each bacterium to be released inside a membrane-bound vesicle. During the endosymbiotic state bacteria remain enclosed in a membrane envelope (peribacteroid membrane, PBM) which is constituted by the components of the plasma membrane of the host cell (8). The nature of this “extracellular compartment” is, however, not fully understood. It appears to be unique in that it is able to change its properties depending upon the fate of the process ie. symbiotic vs pathogenic. Physiological “internalization” of this compartment is a key step in acceptance of the foreign organism inside the host cell. Rhizobium enclosed in PBM behaves as an “organelle” having many properties in common with mitochondria. The PBM, while keeping bacteria physiologically “outside” the host cell cytoplasm and avoiding any pathogenic interaction, allows development of a close contact between the plant cell and bacteria for rapid and efficient exchange of metabolites. Recent studies from our laboratory suggestthat many specific host gene products (nodulins) are integrated into the PBM.