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. 2021 May 20;203(12):e0053920.
doi: 10.1128/JB.00539-20. Epub 2021 Feb 1.

How Rhizobia Adapt to the Nodule Environment

Raphael Ledermann #  1 , Carolin C M Schulte #  1   2 , Philip S Poole  1
Affiliations

How Rhizobia Adapt to the Nodule Environment

Raphael Ledermann et al. J Bacteriol. .

Abstract

Rhizobia are a phylogenetically diverse group of soil bacteria that engage in mutualistic interactions with legume plants. Although specifics of the symbioses differ between strains and plants, all symbioses ultimately result in the formation of specialized root nodule organs that host the nitrogen-fixing microsymbionts called bacteroids. Inside nodules, bacteroids encounter unique conditions that necessitate the global reprogramming of physiological processes and the rerouting of their metabolism. Decades of research have addressed these questions using genetics, omics approaches, and, more recently, computational modeling. Here, we discuss the common adaptations of rhizobia to the nodule environment that define the core principles of bacteroid functioning. All bacteroids are growth arrested and perform energy-intensive nitrogen fixation fueled by plant-provided C4-dicarboxylates at nanomolar oxygen levels. At the same time, bacteroids are subject to host control and sanctioning that ultimately determine their fitness and have fundamental importance for the evolution of a stable mutualistic relationship.

Keywords: growth arrest; host sanctioning; metabolism; microaerobiosis; modeling; nitrogen fixation; rhizobium-legume symbiosis; root nodule.

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Figures

FIG 1
FIG 1
Central features of bacteroids determined from genome scale data sets. A meta-analysis of 18 transcriptome and 1 proteome data sets for bacteroids of various rhizobial strains was performed. The bar graph shows the number of data sets in which a COG category contained more upregulated than downregulated (black) or more downregulated than upregulated (white) genes/proteins. Where the total number is <19, some data sets contained either no differentially expressed genes/proteins or equal numbers of up- and downregulated genes/proteins in this category. Arrows indicate the 3 categories that were most often significantly enriched within upregulated (black) or downregulated (white) genes/proteins, with numbers in parentheses indicating the number of data sets for which significant up/downregulation was observed. P values were determined with a hypergeometric test followed by Bonferroni multiple-test correction, and a P value of <0.05 was considered significant. For the same analysis separated according to terminally differentiated and not terminally differentiated bacteroids, see Fig. S1 in the supplemental material.
FIG 2
FIG 2
Central metabolic and transport reactions in bacteroids. Shown is a schematic map of major metabolic pathways and transport reactions in bacteroids as well as important flows of electrons and ATP. This figure was created with BioRender.
See this image and copyright information in PMC

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