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

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 lysosom

A simple model based on known plant defence reactions is sufficient to explain most aspects of nodulation

We present the following hypothesis; that lipo-oligochitin Nod-factors can act in an elicitor-like fashion inducing, amongst other effects, a plant chitolytic enzyme, capable of hydrolysing the oligochitin chain of the Nod-factor. Decorative groups on the oligochitin chain, e.g. sulphate, may confer partial resistance to hydrolysis upon particular Nod-factors. After entry into the plant, Nod-factor synthesis must be down-regulated in order to avoid further, unwanted, eli-citation and the consequent abortion of the symbiosis. The plant-derived compounds inhibiting the synthesis of bacterial Nod-factors are limiting in root tissue, leading to residual elicitation and the abortion of infection thread formation. Nod-gene anti-induction is, furthermore, inactivated by both light and nitrate, thus contributing to the inhibition of nodulation under these conditions. In nitrogen-fixing nodules, the bacteroids are exposed to both nod-gene inducing and repressing compounds. The slow accumulation of Nod-factors within the peribacteroid space eventually results in the elicitation of phytoalexin synthesis and nodule senescenc