Rhizosphere effects on the microbial community: Specificity and conservatism across geographically disjunct Panax species

A deeper understanding of plant control over the rhizosphere, especially from the evolutionary perspective, is needed for environmentally friendly breeding to take advantage of soil microbes. In the present study, paired bulk and rhizosphere soil samples from 810 plants of three Panax species (P. ginseng, PG; P. quinquefolium, PQ; P. notoginseng, PN) were used to assess rhizosphere effects on microbial diversity, phylogenetic structure, and community composition. The rhizosphere effects were generally stronger in tetraploid PG/PQ than in diploid PN, and the edaphic factors capable of explaining within-species effect variations differed among the three species. However, these species were consistent in terms of the greater biodiversity and stronger phylogenetic clustering in the rhizosphere than in bulk soil, as well as the upward trend of effect strength with plant age. The rhizosphere-enriched taxa of the three species showed strong phylogenetic clustering, as indicated by phylogenetic sampling theory (z score = -11.33) and UniFrac distances, and the coenrichment was mainly reflected in specific orders of Alpha- and Gammaproteobacteria. The potential ecological similarity of rhizosphere-enriched taxa might represent the evolutionarily conserved part of genetic traits involved in rhizosphere recruitment and microbial function requirements in the three species. In summary, this study reveals the specificity and conservatism of rhizosphere effect evolution in these related but Holarctic disjunct Panax species, which advances our understanding of plant-microbe coevolution in the rhizosphere.