Flavonoids and saponins in plant rhizospheres: roles, dynamics, and the potential for agriculture

Abstract

Plants are in constant interaction with a myriad of soil microorganisms in the rhizosphere, an area of soil in close contact with plant roots. Recent research has highlighted the importance of plant specialized metabolites (PSMs) in shaping and modulating the rhizosphere microbiota; however, the molecular mechanisms underlying the establishment and function of the microbiota mostly remain unaddressed. Flavonoids and saponins are a group of PSMs whose biosynthetic pathways have largely been revealed. Although these PSMs are abundantly secreted into the rhizosphere and exert various functions, the secretion mechanisms have not been clarified. This review summarizes the roles of flavonoids and saponins in the rhizosphere with a special focus on interactions between plants and the rhizosphere microbiota. Furthermore, this review introduces recent advancements in the dynamics of these metabolites in the rhizosphere and indicates potential applications of PSMs for crop production and discusses perspectives in this emerging research field.

Fe-Based Nanomaterial-Induced Root Nodulation Is Modulated by Flavonoids to Improve Soybean (Glycine max) Growth and Quality

“…It is worth noting that DEGs of soybean roots after treatment with Fe-based NMs were mainly enriched in the pathway involved in “phenylpropanoid biosynthesis”, “flavonoid biosynthesis”, and “ATP-binding cassette (ABC) transporters” (Figure S10c-d). ABC transporters mediate the secretion of flavonoids into the rhizosphere . These altered pathways could increase flavonoid contents in roots and release them to the rhizosphere.…”

“…ABC transporters mediate the secretion of flavonoids into the rhizosphere. 56 These altered pathways could increase flavonoid contents in roots and release them to the rhizosphere. Besides, "carbohydrate metabolism" was observed in roots after Fe-based NMs treatment, which might be due to the improved photosynthesis (chlorophyll content and Pn) by Fe-based NMs (Figure S10c,d, Figure S1b, Figure 1b), sustainably supplying the carbohydrate and energy for symbiosis and nodule formation.…”

Triterpenoid and Steroidal Saponins Differentially Influence Soil Bacterial Genera

“…Phenylobacterium , Amycolatopsis , Sediminibacterium , and Ochrobactrum were the predominant taxa correlated with tea saponin degradation in the gut of the camellia weevil [ 67 ]. However, bacteria capable of metabolizing aglycones were not isolated from soils, and the saponin catabolic pathway in bacteria remains unclear, especially in the rhizosphere [ 34 ]. To elucidate the saponin metabolism and its significance in microbiome formation, it is necessary to isolate bacterial strains belonging to taxa enriched by saponin treatments and to investigate their abilities to assimilate saponins and colonize the rhizosphere of saponin-producing plants.…”

“…Recent studies have shown that saponins are secreted from plant roots into the rhizosphere [ 31 , 32 , 33 , 34 ]. Previously, we demonstrated that soyasaponins and α-tomatine were secreted from soybean and tomato roots in both hydroponic and field conditions [ 35 , 36 , 37 ].…”

Flavonoids Are Intra- and Inter-Kingdom Modulator Signals

“…A deeper understanding of the mechanisms involved in beneficial plant-bacteria interactions mediated by flavonoids may be crucial in implementing sustainable strategies aimed at increasing plant growth and productivity [36], accelerating phyto-rhizoremediation [218], improving plant protection from pathogens and, more generally, the holobiont resistance to stresses [219].…”