Cadmium (Cd), as a widespread metal pollutant, readily accumulates in the food chain due to its easy absorption and high mobility. The endophytic fungi are cosmopolitan microorganisms that occur widely in association with plants in a heavy metal stress environment. A current pot experiment was conducted to evaluate the influence of Piriformospora indica, as a root endosymbiotic fungus, on the biomass and biochemical responses of sunflower cv. Zaria under excessive Cd concentrations (0, 40, 80 and 120 mg Cd/kg) in the soil, in a 2 × 4 factorial randomized block design in five replicates. In response to increasing Cd levels in soil, root colonization, growth parameters, and total chlorophyll and carotenoids contents were significantly reduced (P < 0.05), whereas root and leaf Cd accumulation, malondialdehyde (MDA) amount and the antioxidant enzymes activities of catalase (CAT), guaiacol peroxidase (POD), ascorbate peroxidase (APX) and superoxide dismutase (SOD) were increased (P < 0.05). Under different levels of Cd in soil, presence of P. indica (in inoculated sunflowers) had a significant increase (P < 0.05) on growth rate, photosynthesis pigments content, root Cd accumulation, and activities of CAT, POD, APX and SOD in compare to absence of P. indica (in non-inoculated ones). Also, P. indica-inoculated plants showed a reduced MDA concentration and leaf Cd accumulation than un-inoculated sunflowers. The results indicated that P. indica, as an appropriate fungal association, can improve tolerance of sunflower to Cd toxicity through increased levels of photosynthesis pigments and antioxidants, and reduced Cd accumulation and MDA content of the leaf. Therefore, this root endophyte can be modulated cadmium toxicity in Helianthus annuus under excessive cadmium in soil, and recommended as a complement crop-growing strategy in the fields under Cd-contaminated soils, for other studies
