Sequential soil application of bacterial siderophores and biosurfactants enhances Cu, Ni, Pb, and Zn accumulation and physiological performance in Brassica juncea L. Czern

Achieving efficient plant uptake of potentially toxic elements (PTEs) for phytoremediation and agromining remains difficult due to limited metal bioavailability and metal-induced metabolic inhibition. In this proof-of-concept study, the uptake of PTEs by Brassica juncea (L.) Czern. was enhanced through sequential soil application of bacterial siderophores and biosurfactants synthesized by Pseudomonas sp. ANT_H12B and Bacillus sp. ANT_WA51, respectively. Siderophores were first applied to chelate and solubilize metals, followed by biosurfactants to disrupt their soil binding, thus improving PTE mobility and availability to plants. Experiments were conducted using soils with and without artificial contamination by copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn). The combined treatment with both metabolites significantly increased metal bioavailability - particularly for Pb (up to 321%) and Zn (up to 618%) - promoting plant growth, chlorophyll synthesis, metal accumulation and translocation. PTE uptake efficiency was markedly improved: in shoots, siderophores and biosurfactant-based treatments increased the concentration of Cu by 59%, Ni by 75%, Pb by 272%, Zn by 89%, compared to the control (metabolite-untreated plants); in roots, the concentration of Cu increased by 73%, Ni by 105%, Pb by 101% and Zn by 101%. Moreover, in the metals-untreated soil, plants supplemented with both metabolites exhibited reduced oxidative stress and improved chloroplast ultrastructure, further supporting the biostimulant properties of metabolites. These findings demonstrate the potential of microbially derived metabolites as safe, sustainable agents that enhance PTE uptake while supporting soil and plant health, enabling ecotoxicologically sound phytoremediation and agromining. • Siderophores and biosurfactants improved Cu, Ni, Pb, and Zn bioavailability in soil. • Metabolites enhanced growth and physiological performance of Brassica juncea . • Combined soil treatment with both metabolites enhanced metal uptake of plants. • Oxidative stress markers in plants were reduced following metabolite application. • Biosurfactants mitigated siderophore binding to the soil matrix, improving their efficiency.