Overexpression of MdbHLH104 results in Fe overaccumulation in transgenic apple plantlets. A, Phenotypes of the MdbHLH104 transgenic apple lines and the wild-type controls grown in soil under conditions of sufficient Fe. B, Magnified image of the mature leaves near the basal stem, indicated by arrowheads. C, Perls stain for Fe signals in the mature leaves. D, Trypan blue staining for cell death in the mature leaves. E, Histochemical detection of O2·− and H2O2 in the mature leaves. Leaves were infiltrated with 0.5 mg/mL NBT for 1 h for O2·− staining and 1 mg/mL DAB for 8 h for H2O2 staining in the dark. F, O2·− accumulation and H2O2 contents in the mature leaves.