Rhizosphere Bacterial Communities Alter in Process to Mycorrhizal Developments of a Mixotrophic Pyrola japonica

Rhizosphere bacteria work in synergy with mycorrhizal fungi to promote plant growth. The community structure of rhizosphere bacteria may be influenced by continuous changes in fungal associations with host plants. Asiatic herbaceous plant Pyrola japonica (Ericaceae) forms arbutoid mycorrhizas without fungal mantles, with its mycorrhizal development being visually distinguishable at the cellular level. This study aimed to investigate roles of rhizosphere bacteria and their community shifts along with mycorrhizal developments. We examined bacterial communities at three different developmental stages of mycorrhizal roots-limited, full, and digested-via a partial 16S rRNA amplicon sequencing. Both α- and β-diversities in the full condition were significantly lower than those in the limited and digested conditions. Significant clusters of bacterial compositions were found among all treatments. In terms of ecological processes of community assembly, communities in limited conditions and bulk soil were influenced by both deterministic and stochastic processes, whereas those in full and digested conditions were regulated only by stochastic ways. Furthermore, the order Rhizobiales and Actinomycetales known as mycorrhizal helper bacteria were characterized in the full and digested conditions through phylogenetic analysis and detection of indicator taxa. These results suggest that mycorrhizal fungi may play ecologically important roles not only as temporal drivers initiating the formation rhizosphere bacterial communities but also as key founders exerting continuous influences to establish priority effects. Moreover, the rhizosphere bacterial community remains after mycorrhizal degeneration and their historical continuity may contribute to maintaining plant-mycorrhizal fungi-bacterial associations.