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 nitrogen reduction to ammonium and that the bacteroids simply secrete ammonium to the plant. A major reassessment of this was caused by our demonstration that both ammonium and alanine are secreted by bacteroids (1), 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 (2). This phenomenon was named symbiotic auxotrophy because R. leguminosarum only becomes auxotrophic when in symbiosis with the plant with a shut-down of amino acid synthesis by bacteroids. It has led us to propose that bacteroids could be considered as organelles (3). We are now investigating the regulatory network that governs bacteroid development using transcriptional regulator mutants, microarrays, ChIP-Seq and biochemical analysis.
(1) Lodwig, E.M., Hosie, A.H.F., Bourdes, A., Findlay, K., Karunakaran, R., Downie, J.A.& Poole, P.S. (2003) Amino-acid cycling drives nitrogen fixation in the legume-Rhizobium symbiosis. Nature 422: 722-726.
(2) Prell, J., White, J.P., Bourdès, A., Bunnewell, S., Bongaerts, R.J. & Poole, P.S. (2009) Legumes regulate Rhizobium bacteroid development and persistence by the supply of branched-chain amino acids. Proceedings of the National Academy of Sciences USA 106, 12477-12482.
(3) Prell, J., Bourdès, A., Kumar, S., Lodwig, E., Hosie, A., Kinghorn, S., White, J. & Poole, P.S. (2010) Role of symbiotic auxotrophy in the Rhizobium-legume symbioses. PLoS One 5(11):e13933