Rhizobial symbiosis modulates mercury accumulation and metabolic adaptation under hydrological extremes

Phytoremediation offers a sustainable strategy for mitigating mercury (Hg) contamination, yet its efficacy under variable water availability remains poorly understood. Robinia pseudoacacia, a leguminous tree with notable phytoremediation potential, was investigated under combined Hg exposure and water stress-drought (HgD) or flooding (HgF)-with or without rhizobia inoculation. In a controlled greenhouse study, HgD exposure enhanced root dry biomass, increased nodule nitrogenase activity, and promoted root Hg accumulation, indicating a detoxification mechanism via root retention. In contrast, HgF suppressed plant growth and nitrogen fixation, reduced total Hg uptake, and increased Hg translocation to shoots, suggesting redistribution to protect root function. Multi-omics analyses revealed that both HgD and HgF induced genes involved in cysteine and methionine metabolism (e.g., GSS, GCLC, ACS), enhancing thiol-mediated Hg detoxification and altering sulfur allocation. L-serine biosynthesis was consistently downregulated. Hormonal responses diverged: HgD suppressed jasmonic acid biosynthesis (downregulation of AOS, AOC) and reduced 12-OPDA levels, whereas HgF activated α-linolenic acid oxidation, elevating 12-OPDA and its derivatives (e.g., colneleic acid). Rhizobial inoculation further improved root Hg retention, upregulated antioxidant enzymes (SOD, POD), and maintained membrane integrity. Under HgD, inoculation enhanced phenylpropanoid metabolism (upregulation of PAL, CCR, CAD), promoting lignification. Under HgF, it stimulated the pentose phosphate pathway (via PFK induction), optimizing carbon flux for stress resilience. These findings demonstrate that Robinia-rhizobia symbiosis mediates distinct physiological and metabolic reprogramming under drought and flooding, enabling context-specific Hg detoxification. This highlights Robinia's potential as a robust phytoremediator in Hg-contaminated environments with fluctuating water regimes.