The native region of #Casuarinaceae$ family (actinorhizal plants) is Oceania (Australia and Indo-Pacific islands). In New Caledonia, in spite of the reduced area, a great number of endemic #Casuarinaceae$ are present. In fact, 8 species of #Gymnostoma$ out of the 18 which make up this genus and 1 species of #Casuarina$ are originated from New Caledonia. The two genera, except 1 species, occur in soils derivated from ultramafic rock and are adapted to poor and toxic soils. Furthermore, actinorhizal plants can establish nitrogen-fixing symbioses with the slow growing actinomycete #Frankia$. Because of their ecological characteristics, actinorhizal plants frequently occur as pioneer vegetation and are used in the rehabilitation of degraded sites. This work was a first study of the ecology of the symbiosis #Casuarinaceae$-#Frankia$ in New Caledonia. It was focused on two major topics. The first part will be concerned with the symbiosis, particularly with cross-inoculation trials using crushed nodule suspension as inoculum. The biodiversity of the microsymbiot #Frankia$, in relation to plants species and soil, will be analysed in the second part. The results obtained show that this symbiosis is characterized by a great biodiversity. Inside the #Gymnostoma$ group, no host specificity was observed. These results were obtained by cross-inoculation tests and molecular characterization of #Frankia$. Furthermore, no correlation was shown between site or soil and the diversity of #Frankia$ strains. (Résumé d'auteur)
