Genomic and functional characterization of Rhizobium metallidurans NS41, a heavy-metal-resistant, non-legume-associated endophyte that reduces oxidative stress in metal-exposed maize.

Heavy metal (HM) contamination threatens plant health and poses significant challenges to agriculture and ecosystem functioning. Plant-associated microorganisms from HM-contaminated environments play a vital role in enhancing host stress tolerance and are promising candidates for sustainable, microbe-based strategies to support plant fitness. Non-legume-associated rhizobia, however, remain poorly described in this context. In this study, we characterized Rhizobium metallidurans strain NS41, an endophyte of goldenrod (Solidago canadensis) growing at a highly metal-contaminated site. To our knowledge, this is the first report of this species associated with a non-leguminous host, thereby expanding current knowledge about R. metallidurans. Genome analysis revealed determinants of HM resistance, oxidative and osmotic stress mitigation, and metabolism of plant-derived compounds, but no canonical nodulation genes, indicating a facultative endophytic, rather than nodulation-dependent, lifestyle adapted to metalliferous environments. In vitro, NS41 demonstrated tolerance to high Zn, Cd, and As(III) concentrations (MIC 20 mM, 2.5 mM, 5 mM, respectively), produced indole-3-acetic acid and ammonia, and solubilized phosphates. In a maize pot experiment in HM-contaminated soil, seed inoculation with NS41 resulted in lower hydrogen peroxide and anthocyanin levels and higher chlorophyll content, without significantly altering biomass or Cd accumulation. This identifies NS41 as a stress-mitigating endophyte rather than a strong direct growth promoter under the tested conditions. Together, these genomic features and stress-mitigating phenotype support further investigation of NS41 as a plant-protective bioinoculant for improving plant performance on HM-contaminated soils.