Developmental stage-specific triterpenoid saponin accumulations in <i>Ardisia crenata</i> rhizosphere and its influence on rhizosphere microbial communities

Plant-specialized metabolites (PSMs) act as candidate drivers of rhizosphere microbiome assembly by recruiting specific microbial taxa. The resulting PSM-microbe interactions influence the host-plant fitness and population dynamics, ultimately impacting the aboveground biodiversity. Although saponins are widely distributed PSMs in the angiosperms, their dynamics and impact on soil microbiomes in a natural ecosystem remain unclear. Here, we investigated the ecological role of a triterpenoid saponin, ardisiacrispin, synthesized by the shade-tolerant shrub <i>Ardisia crenata</i> (Primulaceae), in a forest ecosystem. First, we quantified the saponin concentrations in both the roots and rhizosphere soils of <i>A. crenata</i> at two different developmental stages (i.e., seedling and adult). Next, we assessed how saponin treatment alters the microbial communities in forest soil. Finally, we integrated 16S rRNA and the internal transcribed spacer region sequencing data from the field-collected <i>A. crenata</i> rhizosphere with the results from <i>in vitro</i> saponin-treatment experiments to determine whether saponins selectively enrich or deplete specific microbial taxa. We found that the rhizosphere saponin content primarily varies with the developmental stages of <i>A. crenata</i>, with higher saponin concentrations in adults than in seedlings. The saponin-treatment experiments revealed that ardisiacrispins modify the soil microbial diversity and community structure in accordance with their concentration. Moreover, several microbial taxa were consistently enriched or depleted in the saponin-treated soil, which mirrors the shifts observed from seedling to adult rhizospheres. Thus, ardisiacrispin can mediate rhizosphere microbial community assembly in a natural ecosystem. Our findings highlight the importance of the developmental stage-specific accumulation of saponins in the rhizosphere for plant-microbe interactions.