Ectomycorrhizal and dark septate endophytic fungi synergistically enhance salt tolerance of Pinus tabulaeformis via antioxidant defense and ion homeostasis

AIMS: Plant root symbiotic fungi, ectomycorrhizal fungi (ECMFs) and dark septate endophytes (DSEs), increase host salt tolerance, but their combined effects remain unclear. This study aimed to evaluate the effects of these fungi isolated from Pinus tabulaeformis on seedling growth and physiology under NaCl stress, and clarify the pathways of their synergistic improvement of P. tabulaeformis salt tolerance. METHODS AND RESULTS: Two experiments were performed: (1) An in vitro assay tested the salt tolerance of two ectomycorrhizal fungi (ECMF: Suillus granulatus, Pisolithus tinctorius) and two dark septate endophytes (DSEs: Pseudopyrenochaeta sp., Pleotrichocladium opacum) at NaCl concentrations of 0, 0.1, 0.2, 0.4, and 0.6 mol·L-1; (2) A pot experiment evaluated P. tabulaeformis seedlings inoculated with single/mixed fungi under soil NaCl stress (0, 1, 2, 3 g·kg-1). Fungal biomass and most antioxidant/osmoregulatory traits peaked at moderate NaCl levels in vitro, except for Pleotrichocladium opacum (Po), where superoxide dismutase (SOD), peroxidase (POD), and soluble protein contents increased with rising NaCl concentration. In the pot experiment, inoculated seedlings exhibited improved plant height, biomass, root development, antioxidant enzyme activities, and osmolyte accumulation under salt stress, along with reduced malondialdehyde (MDA) content, Na+ accumulation, and Na+/K+ ratios, compared with uninoculated controls. Mixed inoculation of ECMF and DSEs showed synergistic effects on most growth and stress resistance indicators relative to single inoculation. CONCLUSIONS: The results highlight the potential of ECMF and DSE to increase P. tabulaeformis salt tolerance via growth promotion, antioxidant defense, and ion homeostasis, with combined inoculation offering synergistic benefits for saline soil restoration.