Rhizosphere-competent actinobacteria with ACC deaminase activity mitigate heavy metal stress in corn (Zea mays)

Plant growth-promoting actinobacteria can enhance plant resilience to heavy metal contamination. This study evaluated the effects of 1-aminocyclopropane-1-carboxylate deaminase (ACCD)-producing rhizosphere-competent (RC) and non-rhizosphere-competent (NRC) actinobacterial strains on corn growth and lead (Pb) tolerance. A greenhouse experiment was conducted under Pb stress [1000 mg/kg soil as Pb(NO₃)₂] to compare the two isolates. Inoculated plants performed better than uninoculated controls, particularly in non-contaminated soils. Under Pb stress, the isolates #22 (NRC) and #30 (RC) significantly enhanced plant height, dry weight, and chlorophyll content, with RC isolate #30 exhibiting the most pronounced effects. Identified as Streptomyces mirabilis UAE2, isolate #30 reduced electrolyte leakage by 26.7% and H₂O₂ content by 23.7%, while boosting antioxidant enzyme activities compared to plants grown in Pb-contaminated soil. In addition, isolate #30 stimulated the production of key plant growth regulators, including auxins and polyamines, further improving stress tolerance. A central mechanism of isolate #30 involved lowering endogenous ACC levels in plant tissues by 67.2–72.8% under Pb stress, thereby mitigating ethylene-induced growth inhibition. Notably, isolate #30 decreased Pb accumulation in shoots and roots by 51.1% and 63.0%, respectively. The same isolate also significantly increased microbial activity in the soil, highlighting its potential to improve phytoremediation and soil health. The RC strain S. mirabilis UAE2 demonstrated superior Pb tolerance and regulation of Pb bioavailability, making it a promising candidate for sustainable bioremediation strategies.