Chapter 22 : Differentiation of Free-Living Rhizobia into Endosymbiotic Bacteroids

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This chapter outlines the complexities of bacterial invasion and bacteroid differentiation. Because the early plant-bacterial signal exchange has been the subject of many excellent recent reviews the chapter emphasizes the later stages about which less is known. The strategies that both bacteria and plant use to maintain the symbiosis and prevent pathogenesis are also discussed. The formation of effective nodules containing differentiated, nitrogen-fixing bacteroids consists of a defined series of stages. In response to chemical signals secreted by plant roots, rhizobia attach to root hairs, which are cells on the root surface that project outward into the soil. Chemotaxis plays an important role in the initial attraction of rhizobia to plant root hairs. The signal transduction pathway leading to the formation of the nodule must have unique characteristics, because nodules are completely different from other normal plant structures. Mutants lacking succinoglycan were first isolated by the inability of colonies to fluoresce on plates containing the laundry whitener Calcofluor. Such mutants still complete Nod factor-dependent events, such as root hair deformation, cortical cell divisions, and infection thread initiation. Immunological studies with monoclonal antibodies have documented changes in lipopolysaccharide (LPS) structure during bacteroid differentiation and when free-living rhizobia are cultured in different media. Knockout strategies are useful because it is easy to map and clone the affected genes and because many functions in bacteroid development are probably not essential for free-living cells.

Citation: Margolin W. 2000. Differentiation of Free-Living Rhizobia into Endosymbiotic Bacteroids, p 441-466. In Brun Y, Shimkets L (ed), Prokaryotic Development. ASM Press, Washington, DC. doi: 10.1128/9781555818166.ch22
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Image of FIGURE 1

Root nodules on alfalfa induced by S. meliloti.

Citation: Margolin W. 2000. Differentiation of Free-Living Rhizobia into Endosymbiotic Bacteroids, p 441-466. In Brun Y, Shimkets L (ed), Prokaryotic Development. ASM Press, Washington, DC. doi: 10.1128/9781555818166.ch22
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Image of FIGURE 2

Scanning electron micrograph of S. meliloti bacteroids in an alfalfa nodule homogenate. The arrows highlight bacteroids, which include a Y-shaped cell in the center of the image and two other elongated cells. The other large structures visible are starch granules.

Citation: Margolin W. 2000. Differentiation of Free-Living Rhizobia into Endosymbiotic Bacteroids, p 441-466. In Brun Y, Shimkets L (ed), Prokaryotic Development. ASM Press, Washington, DC. doi: 10.1128/9781555818166.ch22
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Image of FIGURE 3

Initial interaction between S. meliloti and alfalfa root hairs. S. meliloti cells expressing GFP (appearing bright on the darker background) were photographed by confocal microscopy at different stages, including initial attachment to the root hair (A), entrapment by the curled root hair (B), and migration down the infection thread (C). (Images courtesy of Daniel J. Gage, University of Connecticut.)

Citation: Margolin W. 2000. Differentiation of Free-Living Rhizobia into Endosymbiotic Bacteroids, p 441-466. In Brun Y, Shimkets L (ed), Prokaryotic Development. ASM Press, Washington, DC. doi: 10.1128/9781555818166.ch22
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Image of FIGURE 4

Outline of bacteroid development, from release to differentiation. A 4-week-old indeterminate nodule like that formed by the S. meliloti-alfalfa symbiosis is shown, as adapted from the results of Vasse et al. (1990). The inset at the top depicts bacterial proliferation within the infection thread and subsequent endocytosis into the plant cell and engulfment by the PBM. The cells within the nodule represent a sample of each zone, denoted at the right of the nodule. Bacteroid types and zones are as described in the text and in Vasse et al., 1990.

Citation: Margolin W. 2000. Differentiation of Free-Living Rhizobia into Endosymbiotic Bacteroids, p 441-466. In Brun Y, Shimkets L (ed), Prokaryotic Development. ASM Press, Washington, DC. doi: 10.1128/9781555818166.ch22
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