Rhizobium bacteroid development

Lead Research Organisation: John Innes Centre
Department Name: Contracts Office


The infection of legume hosts by rhizobia is typically initiated by rhizobia attaching to root hairs. This is followed by a complex developmental pathway that results in the formation of root nodules. The differentiated form of rhizobia present in root nodules (bacteroids), obtain dicarboxylic acids (succinate, fumarate and malate) as a carbon and energy source from the plant. It has always been assumed that these dicarboxylic acids are oxidised by the TCA-cycle to provide electrons and ATP for N2-reduction to ammonium and the bacteroids simply secrete the ammonium to the plant. A major reassessment of this was caused by our demonstration that both ammonium and alanine are secreted by bacteroids, which is supported by our recent work showing that bacteroids can completely stop all assimilation of ammonium. However, we also demonstrated that an even more complex exchange is required with an obligate requirement for amino acid uptake by nodule bacteria via the ABC transporters Aap and Bra. This phenomenon was named symbiotic auxotrophy because R. leguminosarum only becomes auxotrophic when in symbiosis with the plant and is caused by the shut-down of amino acid synthesis by bacteroids. We demonstrated that this developmental pathway is regulated by a number of factors so we dissected the transcriptional changes that occur over time as bacteroids develop. A major breakthrough in this has been to recognise that many of the early transcriptional changes in developing bacteroids (~50%) also occur in free-living rhizosphere bacteria. Once these shared transcriptional changes are removed the changes specific to developing bacteroids are revealed. For the first time this has enabled us to initiate a project to specifically examine the early development genes in bacteroid formation. We are now investigating the regulatory network that governs bacteroid development using transcriptional regulator mutants, microarrays, Chip-seq, and biochemical analysis.


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