Novel plant growth-promoting endophytic bacteria, Stenotrophomonas maltophilia SaRB5, facilitate phytoremediation by plant growth and cadmium absorption in Salix suchowensis

Seven plant growth-promoting bacteria (PGPB) were isolated from extracts of surface-sterilized Sedum alfredii Hance. Among the seven isolates, the strain SaRB5 identified as Stenotrophomonas maltophilia through 16S rDNA sequence analysis, exhibited highest levels of heavy metal resistance and plant growth-promoting traits. SaRB5 tolerated high concentrations of cadmium (Cd) (1.5 mg·kg^-1) and demonstrated production of indole-3-acetic acid and siderophores when grown with or without Cd. Whole genome sequence alignment and comparative genomic analysis revealed thirty genes associated with plant growth and Cd tolerance metabolism in strain SaRB5, including trpA, trpB, czcB, and others. Pot experiments indicated that SaRB5-treated willow biomass increased by 71.7 % and extracted 129.2 % more Cd. Meanwhile,the inoculation of SaRB5 enhanced the release of organic acids, amino acids, and salicylic acid from plants into the soil, consequently facilitating the recruitment of specific beneficial microbiota at the rhizoplane (including Bradyrhizobium, Mesorhizobium, Romboutsia, and Turicibacter). The recruitment of Bradyrhizobium and Mesorhizobium and the strong upregulation of the nifH gene suggest that SaRB5 inoculation favored efficient nitrogen-fixing microbes at the rhizoplane. Romboutsia and Turicibacter had an impact on the mobilization of iron-manganese oxide-bound Cd and its uptake. This study suggests that SaRB5 enhances phytoremediation not only through the direct effects of the inoculum itself but also by impacting the indigenous rhizoplane bacterial community.