Host range of Frankia strains isolated from actinorhizal plants growing in Japan and their relatedness based on 16S rDNA

The host range of seven Frankia strains isolated from the root nodules of five actinorhizal plants in four genera growing in Japan and their relatedness based on 16S rDNA were examined. The Ceq1 strain isolated from Casuarina equisetifolia formed many nodules with high nitrogen fixation activity on the source host, C. equisetifolia. The Ceq1 strain formed nodules on Alnus sieboldiana, Alnus hirsta and Myrica rubra, but their nodulation ratio and nitrogen fixation activity were very low. The Asi1 strain isolated from A. sieboldiana, the Ahi1 strain isolated from A. hirsta, and the Mru1, Mru2 and Mru8 strains isolated from M. rubra formed many nodules on both Alnus species and M. rubra. The Ema2 strain isolated from Elaeagnus macrophylla formed nodules on both Elaeagnus species and M. rubra. Based on the aspect of nodulation and nitrogen fixation activity, Ceq1 strain is an effective strain only for Casuarina, five strains of Asi1, Ahi1, Mru1, Mru2 and Mru8 are effective strains for both Alnus and Myrica, and Ema2 strain is an effective strain for both Elaeagnus and Myrica. The results of phylogenetic analyses based on 16S rDNA sequences showed that seven strains were separated into two major clusters. Cluster 1 was further grouped into two subclusters, 1a and 1b. The Asi1, Ahi1, Mru1, Mru2 and Mru8 strains belonged to 1a and the Ceq1 strain belonged to cluster 1b. The Ema2 strain was classified into cluster 2. This phylogenetic classification of Frankia based on 16S rDNA sequences appears to be consistent with the host range determined using an inoculation test.