Mechanistic aspect of humic acid and Pseudomonas nitroreducens strain UF50 to alleviate nickel toxicity in soybean

Nickel (Ni): A dual-role element in plants and soil is beneficial in small doses and harmful in excess. Phytodetoxification of plants or phytoremediation of soil utilizing biological tools could be a sustainable approach in alleviating Ni-toxicity. This study explored the effect of humic acid (HA) and rhizobacteria to phytodetoxify Ni in polluted soils using soybean. The trial involved use of 1 % HA, 2.8 × 106 CFU/mL strain UF50 culture and 5 mM NiCl₂·6 H₂O in vitro. Ni-stress (1188.5 mg/kg Ni^2 + L⁻¹) was induced within 10 days of stress (DOS) and 4 days of recovery. Precisely, treatment with HA+UF50 enhanced antioxidant activities, soybean tolerance, growth, and biomass accumulation after 10 DOS. Untreated plants were stunted, chlorotic with increased content of superoxide anions (87 %), malondialdehyde (99.06 %), and hydrogen peroxide (1.4201-fold). Under same condition, abscisic acid (ABA) content and inducible expression rose by 1.02 and 2-fold, respectively. Particularly, treatment with HA+UF50 elevated salicylic acid and elemental concentrations of P, K⁺, Ca²⁺, and Mg²⁺ by 96.27, 9.56, 31.48 percent and 1.2313-fold, respectively. Inoculation of UF50 alone or in combination with HA reduced root Ni content by (30 %), and increased shoot contents by (81 %), as depicted by molecular expression patterns. Treatment upregulated expression of metal transporters GmHMA13, GmHMA14, and stress regulator transcripts GmWRKY172, GmbZIP152 coupled with down-regulation of GmAB15–1 under Ni-stress. Co-treated plants reduced ABA, enhanced SA levels and expression of GmPR2, augmenting resilience to Ni-toxicity. This result affirms the importance of neutralizing Ni-toxicity in soil and plants using HA+UF50 composite as sustainable eco-friendly alternative for phytodetoxification/phytoremediation of Ni-contaminated agroecosystem.