Triterpenoid and Steroidal Saponins Differentially Influence Soil Bacterial Genera

Abstract

Plant specialized metabolites (PSMs) are secreted into the rhizosphere, i.e., the soil zone surrounding the roots of plants. They are often involved in root-associated microbiome assembly, but the association between PSMs and microbiota is not well characterized. Saponins are a group of PSMs widely distributed in angiosperms. In this study, we compared the bacterial communities in field soils treated with the pure compounds of four different saponins. All saponin treatments decreased bacterial α-diversity and caused significant differences in β-diversity when compared with the control. The bacterial taxa depleted by saponin treatments were higher than the ones enriched; two families, Burkholderiaceae and Methylophilaceae, were enriched, while eighteen families were depleted with all saponin treatments. Sphingomonadaceae, which is abundant in the rhizosphere of saponin-producing plants (tomato and soybean), was enriched in soil treated with α-solanine, dioscin, and soyasaponins. α-Solanine and dioscin had a steroid-type aglycone that was found to specifically enrich Geobacteraceae, Lachnospiraceae, and Moraxellaceae, while soyasaponins and glycyrrhizin with an oleanane-type aglycone did not specifically enrich any of the bacterial families. At the bacterial genus level, the steroidal-type and oleanane-type saponins differentially influenced the soil bacterial taxa. Together, these results indicate that there is a relationship between the identities of saponins and their effects on soil bacterial communities.

Disentangling plant- and environment-mediated drivers of active rhizosphere bacterial community dynamics during short-term drought

“…Actinobacteria are known to degrade saponins in the rhizosphere. They are also known as assimilators of saponins, which can explain the cooccurrence of saponins with Actinobacterial OTUs in our study [63]. In addition, the increased accumulations of diterpenoids were also observed in the planted rhizosphere soils stressed by drought.…”

“…Saponins are a group of specialized plant metabolites that can exhibit biological activities such as antibacterial, antifungal, and cytotoxic properties [9,62]. Saponins are secreted from plant roots into the rhizosphere and can alter soil bacterial communities [63]. Saponins also have a role in detoxifying free radicals [64] thus helping combat the oxidative stress imposed by drought.…”

Primary and Secondary Metabolites in Lotus japonicus

“…Nakayasu et al treated field soil with α-solanine, dioscin, a soyasaponin mixture from soybeans, and glycyrrhizin. 128 This led to the depletion of several families of soil bacteria and an enrichment of a few bacterial families. However, it remains yet unknown which effect saponins have on soil microbial communities in context with plants and complex root exudate cocktails.…”

“…Soyasaponins in root exudates may play an important role in shaping the composition of rhizosphere microbial communities. Nakayasu et al treated field soil with α-solanine, dioscin, a soyasaponin mixture from soybeans, and glycyrrhizin . This led to the depletion of several families of soil bacteria and an enrichment of a few bacterial families.…”

α‐Tomatine gradient across artificial roots recreates the recruitment of tomato root‐associated Sphingobium

“…The application of daidzein to field soil altered bacterial communities to resemble those in the soybean rhizosphere, typically by enriching Comamonadaceae (Okutani et al, 2020 ). Other PSMs, such as saponins and alkaloids, also modify the soil microbiota when added to the soil (Fujimatsu et al, 2020 ; Maver et al, 2021 ; Nakayasu, Yamazaki, et al, 2021 ; Shimasaki et al, 2021 ; Sugiyama, 2021 ).…”