Diversity of Frankia strains from Casuarinaceae

The 90 species which composes the family Casuarinaceae are originated from Asia and Oceania and were grouped into four genera: Casuarina, Allocasuarina, Gymnostoma and Ceuthostoma. Actinorhizal plants (i.e. plants which are involved in a nitrogen-fixing symbiosis with the actinomycete Frankia) can be found in each of these genera. The aim of this work was to develop a range of techniques to study the diversity of the microsymbionts associated with plants belonging to the genera Casuarina, Allocasuarina and Gymnostoma. This was done by comparing phenotypic and genotypic characters of Frankia strains cultured in vitro or directly from crushed nodules. This included tests for determining host compatibility and molecular experiments based on sequence variations in 16S ribosomal and nif genes. Results clearly indicate that Casuarinaceae compatible-Frankia strains belong to two phenotypically and phylogenetically distinct groups, one being composed of strains infective on Casuarina and Allocasuarina, the second including strains which can nodulate both Gymnostoma and plants belonging to the Elaeagnaceae family. Moreover, subgrouping can be related to the host plant species in the genera Casuarina and Allocasuarina while no such correlation was observed among strains nodulating Gymnostoma species. (Texte intégral