Strain-specific endophytes modulate physiological and metabolic responses of Perilla frutescens to cadmium stress

Perilla frutescens is a medicinal plant that bioactive compounds such as perillaldehyde and luteolin, but cadmium (Cd) contamination threatens its safe cultivation and medicinal quality. While dark septate endophytes (DSEs) are known to increase plant stress tolerance, their strain-specific roles in regulating Cd uptake and secondary metabolite biosynthesis remain poorly understood. This study aimed to investigate three DSE strains, DSE6 ( Paraphoma radicina ), DSE7 ( P. chlamydocopiosa ), and MD3-1 ( Acrocalymma aquatica ), to determine their effects on growth, physiological traits, Cd accumulation, and key metabolite production in P. frutescens under 0, 1, 5, and 10 mg/kg Cd stress. DSE6 and DSE7 significantly reduced root Cd content and improved photosynthetic pigments, antioxidant activity, and biomass under moderate Cd levels. Notably, MD3-1 did not reduce Cd uptake but increased luteolin content by 22.67 % under high Cd stress, suggesting a decoupling between Cd accumulation and metabolite production. All three strains increased perillaldehyde content, with DSE7 showing the greatest increase (262 %) at 5 mg/kg Cd. These findings demonstrate that DSEs regulate Cd tolerance and medicinal quality via distinct mechanisms, and that internal metabolic tolerance may sustain bioactive compound synthesis under heavy metal stress. This highlights their potential as microbial agents for cultivating medicinal plants in contaminated soils.