In a world context of overconsumption of nitrogen fertilizers leading to environmental damages, the study of nitrogen-fixing symbioses is a major issue, in both non-legumes and legumes. Nitrogen is a key element in plants, and in spite of huge amounts of nitrogen in the atmosphere (78% of dinitrogen), its availability is one of the major limiting factor for plant growth, specially in several Southern countries.
The research project of the team focusses on Casuarina glauca, a tropical tree growing in semi-arid and arid areas, that has the ability to develop nitrogen fixing nodules (also referred as « actinorhiza ») following a symbiotic interaction with the soil actinobacteria Frankia. Studies aim to decipher the molecular mechanisms underlying the actinorhizal symbiosis, with a major focus on the early stages of the dialogue between the host plant and the microorganism.
To reach this objective, two major issues are under investigation:
· The first concerns the symbiotic signals secreted by the microorganism Frankia that are necessary for the symbiotic interaction. Recent data suggest that the actinobacterial molecules are different from the Nod factors synthetized by most rhizobial strains and necessary for the infection and the nodulation process in legumes.
· The second issue is to isolate key plant genes involved in the perception and transduction of the symbiotic signaling molecules emitted by the microorganism Frankia. Our data have established that some of these symbiotic genes are shared by the three main root endosymbioses: Frankia-actinorhizal plants, legume-Rhizobium, and endomycorrhizal symbiosis.
In the long term, these findings which revealed a "common symbiotic pathway", will allow to develop strategies to transfer nitrogen-fixing symbiotic ability to cereals such as rice that lack the ability to develop nitrogen fixing nodules.
Casuarina glauca as an actinorhizal model plant
The Rhizogenesis team’s research project is centered on the tropical actinorhizal tree C. glauca. This species was chosen because molecular tools and genetic transformation procedures can be used for the functional characterization of key genes involved in the symbiotic interaction and because of its environmental value in Southern countriessuc as Senegal, Egypt, India or China.
In addition to their ability to enrich soils in nitrogen and organic matter, like most plants, casuarina trees can also develop a symbiotic association with soil mycorrhizal fungi. This interaction improves mineral nutrition, in particular phosphorous nutrition and improves water uptake. In addition, in conditions of iron and/or phosphorous deficiency, some casuarina species - including C. glauca - produce closely spaced short lateral roots called proteoid roots to increase the uptake of some minerals such as iron and phosphorous, which are poorly soluble in the soil.
Understanding the molecular basis for Casuarina root plasticity and its ability to thrive on poor soils is a major research topic in agronomy.