The effect of L-histidine on nickel translocation and the activities of antioxidant enzymes in hyperaccumulator (Odontarrhena inflata) and non-accumulator (Aurinia saxatilis) plants

Background and Aims: The role of L-histidine (L-His) in nickel (Ni) hyperaccumulation is not well known. The present study aimed to understand the impact of L-His on Ni translocation and Ni tolerance in Odontarrhena inflata and Aurinia saxatilis. Methods: To determine the impact of L-His and L-alanine (L-Ala) on the shoot Ni accumulation, we quantified the Ni concentration in plants pretreated with amino acids for 4 h and then exposed to Ni for 8 h. Hydrogen peroxide (H₂O₂) and antioxidant enzymes activities were determined after 4 h of pretreatments and 48 h of Ni treatments. Results: L-histidine increased Ni translocation to shoots in O. inflata and A. saxatilis. Ni increased the activity of POD, APX, and CAT in both species, but the activities of APX and CAT were higher in O. inflata. Ni exposure increased the H₂O₂ concentration in A. saxatilis. L-His pretreatment did not decrease the H₂O₂ concentration in Ni-treated plants, but decreased the activity of POD, APX, and CAT only at 300 µM Ni in O. inflata. In A. saxatilis, L-His pretreatment, decreased the activity of CAT, but increased POD activity at 150 and 300 µM Ni. Pretreatment with L-Ala decreased POD and APX activity but had no significant impact on H₂O₂ concentration and CAT activity. Conclusion: L-Histidine promoted root-to-shoot Ni translocation and alleviated Ni toxicity, possibly through inducing the activities of antioxidant enzymes, in hyperaccumulator and non-accumulator plants. Histidine might not only facilitate the Ni translocation, but also contribute to Ni tolerance, at least in A. saxatlilis.