Bacteroids in theRhizobium-Legume Symbiosis Inhabit a Plant Internal Lytic Compartment: Implications for other Microbial Endosymbioses

Abstract

All nitrogen-fixing bacteroids within legume root nodule cells are surrounded by a host-derived peribacteroid membrane. Components of this membrane are supplied directly by the ER and Golgi of the host cell. The peribacteroid space lies between the peribacteroid and bacteroid membranes and contains several activities typically found in vacuoles, namely; protease, acid trehalase, alpha-mannosidase isoenzyme II and protein protease inhibitor. Thus bacteroids inhabit an environment which fulfils the definition of a lysosome. Since the endosymbiotic organelles are morphologically different from the lytic compartment normally present in a root cortex cell (the central vacuole), it is proposed that they represent organ-specific modifications of lysosomes, analogous to the protein bodies of seeds. Perisymbiontic membranes are features common to all known plant endosymbioses (involving rhizobia, cyanobacteria, actinomycetes, vesicular-arbuscular mycorrhiza etc.) and the implications of this lead to the hypothesis that in all these cases the endosymbiont is compartmentalized within a specialized host lysosome.