Erwinia sp. PSI-03 Promotes Plant Growth and Detoxifies Selenite Through Selenium Nanoparticles Biosynthesis

The mechanisms of selenium (Se) oxyanion transformation in endophytic bacteria remain poorly understood, which limits their application in biofortification and phytoremediation. Here, we investigated these mechanisms using the plant-growth-promoting (PGP) endophyte Erwinia sp. PSI-03. Under 2 mM selenite stress, the strain intracellularly and extracellularly produced spherical selenium nanoparticles (SeNPs; ab57 nm average diameter). Multi-omics analyses revealed that these SeNPs were formed through parallel enzymatic (mediated by sulfite reductase, cysI) and non-enzymatic (via glutathione and l-cysteine) reduction pathways. Additionally, γ-glutamyl-Se-methylselenocysteine was identified as a key organo-selenium metabolite. Selenite exposure induced extensive reprogramming of the metabolome and transcriptome, highlighting key roles for glutathione metabolism and stress response systems related to cell wall/membrane maintenance, oxidative phosphorylation, two-component signaling systems, and DNA repair. Intriguingly, selenite stress concurrently stimulated bacterial synthesis of PGP compounds, including the auxin precursor indole-3-pyruvate, the defense hormone salicylic acid, and acetate. Consistent with this, under selenite-free and high-selenite (12 mg kg^-1 Se) conditions, inoculation with Erwinia sp. PSI-03 significantly promoted tea plant growth. Compared to uninoculated controls, the leaf biomass increased by 52.8% and 51.7%, and the total biomass by 82.9% and 49.6%, respectively. These findings establish a paradigm where endophytic bacteria simultaneously detoxify Se and promote plant health, offering a robust strategy for agricultural and environmental Se management.