The plant-growth promoting bacteria promote cadmium uptake by inducing a hormonal crosstalk and lateral root formation in a hyperaccumulator plant Sedum alfredii

Plant growth-promoting bacteria (PGPB) that inhabit hyperaccumulating plants assist cadmium (Cd) absorption, but the underlying mechanism has not been comprehensively studied. For this reason, we combined the fluorescence imaging, and transcriptomic and metabolomic methods in a Cd hyperaccumulator, Sedum alfredii, inoculated or not with PGPB Pseudomonas fluorescens. The results showed that the newly emerged lateral roots, that were heavily colonized by P. fluorescens, are the main entry for Cd influx in S. alfredii. Inoculation with P. fluorescens promoted a lateral root formation of its host plant, leading to a higher Cd phytoremediation efficiency. Furthermore, the plant transcriptome revealed that 146 plant hormone related genes were significantly up-regulated by the bacterial inoculation, with 119 of them showing a complex interaction, which suggests that a hormonal crosstalk participated root development. The targeted metabolomics analysis showed that P. fluorescens inoculation significantly increased indole acetic acid concentration and significantly decreased concentrations of abscisic acid, brassinolide, trans-zeatin, ethylene and jasmonic acid in S. alfredii roots, thereby inducing lateral root emergence. Altogether, our results highlight the importance of PGPB-induced lateral root formation for the increased Cd uptake in a hyperaccumulating plant.