Symbiotic Exchange of Signals: Analysis of Molecular Mechanisms in Nod-independent nitrogen fixing symbioses – SESAM

Symbiotic nitrogen fixation plays a crucial role in natural ecosystems and sustainable agriculture. Rhizobial/legumes and actinorhizal symbioses represent the two most important nitrogen-fixing symbioses between plants and microbes. Legumes include important agricultural crops such as soybean, alfalfa and pea. Actinorhizal plants symbioses with actinobacteria from the genus Frankia are widespread, ecologically important, and, in many environments crucial for soil stabilization and recovery. Actinorhiza include 8 plant families belonging to three Rosid orders (Fagales, Curcubitales and Rosales). The most studied actinorhiza are Alder and Casuarina.
A major advance in our understanding of nitrogen-fixing bacteria-plant interactions was the finding, in the early 1990s, that plant-derived flavonoids lead to the production of lipo-chitooligosaccharide signal molecules called Nod factors (Nfs) by the rhizobia. This led to major breakthroughs in our knowledge of the molecular dialog between rhizobia and legume, including the key role of Nfs in the control of host specificity. The ubiquitous presence of nod genes and Nfs in all rhizobia led to the development of a universal model for the specific recognition of nitrogen-fixing bacteria by host plants. However, the universality of this paradigm has recently been reassessed by the findings that the common nod genes are absent in Frankia (Partner 2) and in photosynthetic bradyrhizobia (Partner 3) indicating that these bacteria use alternative pathways to initiate nitrogen-fixing symbioses.
To understand the complexity and unique features of nod-independent nitrogen-fixing root nodule symbioses, an ANR Programme blanc NewNod: “Identification of new molecular determinants involved in the early steps of plant-bacteria interactions” involving partners 1, 2 and 3 has been initiated in 2006. Studies of Partners 1 and 2 on actinorhizal symbiosis have led to the emergence of a vision of a partly conserved physiological response with some conserved determinants on the plant side and a markedly different physiological response with no detectable conserved determinants on the microbe side. In addition, Partner 3 has provided experimental evidence that cytokinins are likely to play a major role during the photosynthetic Bradyrhizobium/Aeschynomene nod-independent process.
The major challenge of this SESAM project is to characterize the novel bacterial signaling molecules involved in nod-independent nitrogen-fixing symbioses and to understand how these signals activate host plants for nodulation.
Our objectives are:
1) To characterize bacterial signals and corresponding genes involved in host recognition
2) To characterize the plant nod-independent signaling pathway via comparison to transduction cascades in others endosymbioses
3) To identify early plant symbiotic key genes specific to Nod-independent processes
4) To integrate and develop bioinformatics tools to handle data obtained for Nod-independent symbioses (NewNod and SESAM project) and to set up a Web platform to analyse and freely distribute worldwide Nod-independent genomic resources.

The overall results from this work will deepen our understanding of Nod-independent symbioses and is expected to solve the enigma of how Frankia and photosynthetic bradyrhizobia “talk” with their host plants. Results obtained in this project will lead to the identification of new signaling pathways governing interactions between plant and bacteria and will contribute to a general understanding of the evolutionary origin of root symbioses.