γ-Polyglutamic acid alleviates chromium toxicity in tobacco plants: physiological regulation and microbial mechanism

Chronic chromium (Cr) contamination in agricultural soils causes pronounced physiological perturbations in Nicotiana tabacum L. cv. Honghuadajinyuan, urging eco-friendly remediation strategies. Notably, the multifunctional biopolymer γ-polyglutamic acid (γ-PGA) holds great promise for heavy metal detoxification and soil remediation, but its role in alleviating Cr-induced stress in plants remains unclear. A pot experiment with Cr-contaminated soil (201.64 mg kg⁻¹) systematically assessed graded γ-PGA (0, 5, 10, 20 mg kg⁻¹) impacts on soil properties, microbial communities, and tobacco growth, physiology, metabolites, and Cr translocation. The results showed that application of 5 mg kg⁻¹ γ-PGA significantly improved tobacco agronomic traits and biomass (root: +117.94 %; shoot: 395.21 %), reduced Cr content by 18.1 % (root) and 13.0 % (shoot), and enhanced stress tolerance through synergistic optimization of soil-plant-microbial interactions. Specifically, γ-PGA increased soil available nitrogen (AN), phosphorus (AP), and potassium (AK) contents as well as activities of urease, sucrase, and catalase (CAT), effectively optimizing soil fertility and biological functionality. Furthermore, high-throughput sequencing revealed γ-PGA-driven changes in soil microbial communities. Beneficial bacterial genera Brevibacillus (+74.79 %) and Candidatus_Chloroploca (+35.04 %) were significantly enriched, whereas Cr-tolerant genera Croceicoccus (-66.58 %) and Sphingobium (-43.03 %) showed marked reduction. In parallel, fungal communities exhibited increased abundances of Mycothermus (+40.17 %), Olpidium (+117.74 %), and Pseudallescheria (+87.96 %). At the plant level, γ-PGA concurrently promoted leaf redox homeostasis, elevated carbon/nitrogen metabolism enzyme activities (e.g., SPS, GS/GOGAT), and activated key pathways governing carbon/nitrogen metabolism, energy transfer, and antioxidative defense. Collectively, these findings indicate that 5 mg kg⁻¹ γ-PGA effectively mitigates Cr phytotoxicity in tobacco cultivation systems via Cr chelation, rhizosphere microenvironment optimization, and plant metabolic resilience enhancement. Notably, its remediation efficacy exhibits a dose-dependent pattern, with low concentrations achieving optimal effects, highlighting the importance of targeted dosage regulation.