Isolation, characterisation (PCR-RFLP) and specificity of Frankia from eight Gymnostoma species endemic to New Caledonia

Abstract Eight Frankia-Gymnostoma nitrogen-fixing symbioses endemic to New Caledonia were studied using a range of techniques including molecular typing of Frankia , infectivity/effectivity and host spectrum of the isolates. The study was conducted on 128 Frankia isolates from the eight Gymnostoma species. The RFLP analysis of the rRNA 16S-23S ITS allowed us to cluster the strains in four groups. Symbiotic characterization of the four groups failed to reveal any host specificity both in term of infectivity or effectivity. The characteristics of the strains were those of Eleagnaceae -infective Frankia strains. Co-inoculation of two Gymnostoma species with mixtures of strains evidenced 11 % of mixed RFLP profiles. In such controlled conditions, this was interpreted as co-infections. This work evidenced original features of these Gymnostoma isolates: (i) they were closer to Eleagnaceae -infective Frankia than Casuarina -infective Frankia ; (ii) closer to atypical Frankia from Casuarina ; (iii) poorly host-specific; and (iv) they had a tendency of co-infection. They could represent a primitive group of Frankia able to survive in a wide range of habitats.