Journal
MOLECULAR PLANT-MICROBE INTERACTIONS
Volume 13, Issue 7, Pages 733-741Publisher
AMER PHYTOPATHOLOGICAL SOC
DOI: 10.1094/MPMI.2000.13.7.733
Keywords
nifA; nitrogenase activity
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By means of a detailed ultrastructural analysis of nodules induced by Rhizobium etli on the roots of Phaseolus vulgaris, we observe that the development of host-invaded cells is not synchronous. An accumulation of mitochondria was found in freshly invaded host cells, containing only a few symbiosomes (SBs) that are released from highly branched intracellular ramification of the infection threads. Moreover, besides the fusion between the SE membrane with host secretory vesicles, we observe also a great number of fusions between the outer leaflets of adjoining SE membranes, thus resulting in structures that resemble the tight junction network (zona occludens with a five-layered structure) of epithelian cells. This process was found to be induced strongly and earlier both in the invaded host cells of ineffective nodules (elicited by Fix(-) mutant strains of R. etli) and in the older (senescence) invaded cells of effective nodules, whereas bacteroid division is seldom if ever observed. Our observations strongly suggest that multiple-occupancy SBs also arise by fusion of single-occupancy SBs and the physiological consequence of this process is discussed.
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