Plants normally acquire nitrogen from the soil, but leguminous plants are almost independent of soil N due to a beneficial association with rhizobia bacteria that induce the formation of N fixing root nodules. During symbiosis bacteria invade developing nodules and within a group of invaded plant cells, differentiate into intracellular bacteroids. A single or few bacteroids surrounded by a plant membrane form organelle-like structures, the symbiosome, which convert the unusable N2 of the air to NH4+ while receiving energy from the host plant. Fixed N is then assimilated and transported to the rest of the plant. In this context, our efforts are focused on unravelling the complex N fluxes occurring in the symbiosis. Several bacterial and plant genes involved in N metabolism were cloned and characterised, allowing us to clarify the regulatory circuitry responsible for the down-regulation of bacterial genes involved in NH4+ transport and assimilation, occurring early during nodulation, i.e. before the activation of the genes involved in N2 fixation. This process is a key step for both differentiation of N2-fixing symbiosomes and development of nodules (see attached file).
In addition to clarifying the molecular underpinnings of the rhizobia-legume symbiosis, our findings are shedding light on plant development. In fact, nodule formation implies a progression of temporally and spatially regulated events of cell dedifferentiation/differentiation. The use of experimental approaches such as histochemical localisation of reporter genes, in situ hybridization, immunolocalisation, etc. allowed us to gain knowledge about the morphogenetic events leading to the formation of a nodule including: root hair deformation; development and growth of infection threads; induction of a nodule primordium; induction, activity and persistence of a nodular meristem(s); intracellular invasion and differentiation of symbiosome. We showed that bacterial and plant mutants are powerful tools to correlate a developmental event with: the degree of nodule invasion; the regulatory signals involved (e.g. hormones); the metabolites (e.g. amino acids, sugars, etc.) exchanged; and the genes expressed (specific cell lines, particular developmental stages). On the other hand, we established that the symbiotic alterations induced by the treatment with hormones or metabolic intermediates are helpful strategies for some of the established correlations. Thus, our research is focused on the impact of some bacterial and plant genes, including those coding for NH4+ transporters, amino acids and plant hormones (such as auxins) biosynthesis/degardation, on the regulation of nodulation (nodule number) and on nodule development, activity, maintenance, and senescence (final stage of development).
The overall innovation of our studies arises from a new approach to the development of N-efficient legumes as well as on new concepts that allow the optimisation of biological N fixation, based on a full exploitation of NH4+ transport systems (uptake and/or efflux systems) from both partners.
Because of the importance of legumes in agriculture and forestry our work also have practical implications reducing the need for costly and polluting synthetic fertilizers. For instance, our understanding of bacterial characteristics that contribute to nodulation may lead to the development of more efficient bacterial inoculants (e. g. able to compete successfully with indigenous strains).
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