Project description:The bacterium, Sinorhizobium meliloti, interacts symbiotically with leguminous plants such as Medicago truncatula to form nitrogen-fixing root nodules. During symbiosis, plant and bacterial cells differentiate in a coordinated manner, resulting in specialized plant cells that contain nitrogen-fixing bacteroids. Medicago nodules are organized in structurally distinct tissue zones, representing different stages of bacterial and plant differentiation. We used laser-capture microdissection (LCM) to analyze bacterial and plant gene expression in four root nodule regions. In parallel, we analyzed gene expression in nodules formed by wild type bacteria on six plant mutants with nitrogen fixation deficiencies (dnf). We found that bacteroid metabolism is drastically remodeled during bacteroid differentiation. Many processes required for bacterial growth are down-regulated in the nitrogen fixation zone. The overall transcriptional changes are similar to those occurring during nutrient limitation by the stringent response. We also observed differential expression of bacterial genes involved in nitrogen fixation, cell envelope homeostasis, cell division, stress response and polyamine biosynthesis at distinct stages of nodule development. In M. truncatula we observed the differential regulation of several host processes that may trigger bacteroid differentiation and control bacterial infection. We analyzed plant and bacterial gene expression simultaneously, which allowed us to correlate processes in both organisms.
Project description:The transformation of Sinorhizobium bacteria into nitrogen-fixing bacteroids located in the root-nodules of the Medicago plants is controlled by specific peptides released by the plants. In our study root nodules, and differently purified bacteroid mixtures were studied. Detailed description about sample preparation and data interpretation can be found in "Identification of Nodule-Specific Cysteine-Rich Plant Peptides in Endosymbiotic Bacteria" by H. Durgo et al., Proteomics (2015). CID spectra supporting all NCR identifications can be viewed using MS-Viewer on the public Protein Prospector website (prospector.ucsf.edu), search key: h7swi0wcik.
Project description:Legumes grow specialized root nodules that are distinct from lateral roots in morphology and function, with nodules intracellularly hosting beneficial nitrogen-fixing bacteria that provide the plant with nitrogen. We have previously shown that a lateral root-like program underpins nodule initiation, but there must be additional developmental programs that confer nodule identity. Here, we show that two members of the LIGHT SENSITIVE SHORT HYPOCOTYL (LSH) transcription factor family, known to define organ boundaries and meristem complexity in the shoot, function as regulators of nodule organ identity. LSH1/LSH2 function upstream of and together with the known nodule regulators Nuclear Factor Y A1 and NODULE ROOT1/2. The principal outcome of LSH1/LSH2 function is the production of cells able to accommodate nitrogen-fixing bacteria, a unique nodule feature. We conclude that the coordinate recruitment of a pre-existing shoot developmental program, in parallel to a root program, underpins the divergence between lateral roots and nodules.
