Résumé
After water and light, nitrogen and phosphorus are the major elements limiting plant production worldwide. The use of fertilizers to offset these deficiencies is now actively disapproved because of their high price, their effect on climate change and their negative environmental impact. One solution could be to exploit plants that have acquired the ability to adapt to deficient environments. One example is given by plants that develop symbiotic associations with nitrogen-fixing bacteria in order to benefit from the large reservoir of atmospheric nitrogen. Two groups of plants are known to form nitrogen-fixing root nodules: legumes that associate with rhizobia and plants belonging to eight angiosperm families, called actinorhizal plants, that associate with the actinomycete Frankia. These plants can thrive on nitrogen-poor soil and have long been used to increase soil fertility. Among them, Casuarina a tropical tree originating from Australia, presents a very important ecological asset for Southern countries due to its high ability to colonize deficient soils. Our team has focused on the cellular and molecular studies of the plant genes involved at different steps of the interaction between Frankia and Casuarina glauca. Several candidate genes from Casuarina have been characterized, including cg12, a subtilase gene expressed during early infection events, CgMT1 a metallothionein gene involved in stress responses and CgSymRK, a gene from the signalling pathway involved at the beginning of the symbiosis. More recently, a genomic approach has been initiated in order to sequence the ESTs from roots and nodules of Casuarina. Comparison between our data and legumes EST databases should reveal molecular mechanisms that are common and unique to the two endophytic root nodule symbioses and bring new information to further our understanding of the evolution of plant endosymbiosis across the plant kingdom.