Nodule Invasion and Symbiosome Differentiation During Rhizobium etli-Phaseolus vulgaris Symbiosis

Abstract

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 SB membrane with host secretory vesicles, we observe also a great number of fusions between the outer leaflets of adjoining SB 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.

A C Subunit of the Plant Nuclear Factor NF-Y Required for Rhizobial Infection and Nodule Development Affects Partner Selection in the Common Bean–Rhizobium etliSymbiosis

“…This phenotype was observed upon inoculation with either R. etli genotype nodC-a or nodC-d. Optical and transmission electron microcopy of nodule sections revealed that GUS RNAi nodules showed a central zone containing numerous infected cells surrounded by the peripheral tissue in which vascular bundles were developed ( Figures 3A and 3C). Infected cells contained small vacuoles and numerous symbiosomes with one or more bacteroids filled with poly-b-hydroxybutyrate granules as previously described for common bean wild-type nodules (Taté et al, 1994;Cermola et al, 2000). On the contrary, the central zone of NF-YC1 RNAi nodules presented small, irregularly shaped, and highly vacuolated cells that were devoid of bacteroids ( Figures 3B and 3D).…”

Key Role of Bacterial NH 4 + Metabolism in Rhizobium-Plant Symbiosis

“…Moreover, some plant and bacterial genes are not expressed simultaneously in all cells of the CT (144,176) (see below and Fig. 2) and signs of nodule senescence, such as the lysis of ICs, initiate their appearance specifically at its center (25). Finally, the heterogeneity of the ICs is confirmed by the observation that in nodules elicited by bacterial mutants having delayed invasion ability, not all the meristematic cells are invaded simultaneously.…”

“…However, more recently it was shown that in developing determinate nodules of P. vulgaris, there is also a spatial component of development. In fact, during nodule organogenesis, it is possible to observe differentiating ICs containing only a few undifferentiated SBs and mature ICs filled with differentiated SBs formed by more than one BT (25). Moreover, some plant and bacterial genes are not expressed simultaneously in all cells of the CT (144,176) (see below and Fig.…”

“…When G. max and P. vulgaris are inoculated with a nifH mutant strain and Vigna unguiculata is nodulated in the absence of N 2 (replaced by argon), the resulting nodules are similar (in shape, dimension, and histology) to normal nodules (8,25,58). This suggests that NH 4 ϩ is not a signal for nodule organogenesis and that younger BTs release NH 4 ϩ by a mechanism not involving nitrogenase action, such as the utilization of amino acids as a C source (see below).…”

A Legume TOR Protein Kinase Regulates Rhizobium Symbiosis and Is Essential for Infection and Nodule Development

“…TORRNAi roots had short RAMs and reduced epidermal (Cermola et al, 2000). D, Infected cells of TOR-RNAi roots show an abundant, clear, multivacuolated cytoplasm.…”