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.