Enhanced dandelion phytoremediation of Cd-contaminated soil assisted by tea saponin and plant growth-promoting rhizobacteria

Purpose Phytoremediation is a cost-effective and environment-friendly method to remove heavy metals from soils, but there is a bottleneck with the low solubility and bioavailability of metals. To overcome this barrier, tea saponin (Ts) and plant growth-promoting rhizobacteria (PGPR) were used to enhance the extraction efficiency of hyperaccumulator dandelion from Cd-contaminated soils.Material and methods Cd-tolerant PGPR were isolated and inoculated in soils cultivated with dandelion, while Ts was spiked at optimal dosage to assess the potential of dandelion-PGPR-Ts combined remediation of Cd-contaminated soils. The controlled experiment was divided into five treatments, including CK (soil), D (dandelion + soil), TD (Ts + dandelion + soil), PD (PGPR + dandelion + soil), and PTD (PGPR + Ts + dandelion + soil) treatments. Plant growth and physiological properties, and Cd accumulation were determined. The speciation of soil Cd were extracted by BCR. Additionally, rhizosphere soil physicochemical properties and enzyme activities were measured, and a high-throughput sequencing technique was used to analyze microbial community structure.Results and discussion The results showed that the combination of Ts and PGPR significantly increased the seed germination rate and biomass of dandelion by improving nutrient supply, soil enzyme activities, and plant antioxidant enzyme activities, with 425.74% and 430.01% increase in shoot and root biomass, respectively. The PGPR and Ts increased dandelion's phytoextraction of Cd up to 11.3-fold, and the effect of Ts on the activation dynamics of Cd speciation was observed with a 10.94% increase in the weak acid soluble fraction and a 21.98% decrease in the residual fraction of Cd. Moreover, the PGPR inoculation significantly increased the abundance of Actinobacteria and Gemmatimonadota that were considered to be metal-resistant bacteria.Conclusion Planting dandelion with the application of Ts and PGPR not only increased the biomass of dandelion by improving rhizosphere soil nutrition and microecology but also promoted the mobility and bioavailability of Cd, which enhanced the extraction efficiency of hyperaccumulators. These results reveal a cutting-edge application of phytoremediation assisted by biosurfactants and microbes for Cd-contaminated soil.