Nodule invasion and symbiosome differentiation during Rhizobium etli Phaseolus vulgaris symbiosis

By means of a detailed ultrastructural analysis of nodules induced by Rhizo bium etli on the roots of Phaseolus vulgaris, we observe that the developme nt of host-invaded cells is not synchronous. An accumulation of mitochondri a was found in freshly invaded host cells, containing only a few symbiosome s (SBs) that are released from highly branched intracellular ramification o f the infection threads. Moreover, besides the fusion between the SE membra ne with host secretory vesicles, we observe also a great number of fusions between the outer leaflets of adjoining SE membranes, thus resulting in str uctures that resemble the tight junction network (zona occludens with a fiv e-layered structure) of epithelian cells. This process was found to be indu ced strongly and earlier both in the invaded host cells of ineffective nodu les (elicited by Fix(-) mutant strains of R. etli) and in the older (senesc ence) invaded cells of effective nodules, whereas bacteroid division is sel dom if ever observed. Our observations strongly suggest that multiple-occup ancy SBs also arise by fusion of single-occupancy SBs and the physiological consequence of this process is discussed.