Filtering force for Frankia assembly on host root-nodules
The Frankia composition in the root-nodule community was significantly different from that in the rhizosphere community (PERMANOVA: P < 0.001). Community variance ofFrankia in rhizosphere soils was greater than that in host root–nodules (PERMDISP: P < 0.001; Fig. 2a). The communities of Frankia in host root nodules consisted of a small proportion of the overall members of the communities in the rhizosphere soils (8/16 OTUs were detected from both nodules and rhizosphere soils). Furthermore, rank-abundance dominance plots showed that three OTUs (OTU01, 02, and 03) were dominant and abundance decay was very steep in root nodules (Fig. S4). However, dominance was not obvious and abundance decay was gradual in rhizosphere communities (Fig. S4). These results show that a host plant establishes a symbiotic interaction with limited members from a diverse resource of symbionts in soil.
The number of Frankia OTUs from root nodules was significantly lower than the expected OTU richness by random sampling of the rhizosphere composition (Fig. 2b). Moreover, significantly differentFrankia composition was found in the root nodules in the null model (PERMANOVA: P < 0.001). These results provide evidence that filtering force works to shape symbiont communities in the host plant with strains in narrower ranges of Frankia strains than expected from Frankia pools in soil.