Foliar application of zinc promotes cadmium absorption by increasing expression of cadmium transporter genes and activities of antioxidative enzymes in winter wheat

Abstract

To elucidate the mechanisms of zinc (Zn) application alleviating cadmium (Cd) toxicity in wheat, we characterized plant growth, antioxidant system, leaf cell ultrastructure and Cd transporter gene expression in a hydroponic experiment. Results showed that addition of Zn (Zn+Cd) or pretreatment with foliarly applied 2 g L -1 ZnSO 4 ·7H 2 O (pre-Zn+Cd) significantly exacerbated Cd-induced growth inhibition and diminished root morphological parameters, root cell viability, and chlorophyll content. In addition, the Cd concentrations were increased in roots and shoots in the Zn+Cd and pre-Zn+Cd treatments compared to Cd treatment, but the Cd translocation factor was decreased. After Zn application, the Cd concentrations increased in the root cell wall fraction but decreased in the soluble fraction. Activity of antioxidative enzymes (SOD; superoxide dismutase, POD; peroxidase, CAT; catalase) and content of non-enzymatic antioxidants (MDA; malondialdehyde, ASA; ascorbic acid, GSH; glutathione) were increased after Cd exposure, particularly in the foliar Zn treatment. Gene expression analysis showed that five genes ( TaHMA2 , TRIAE5370 , TCONS1113 , TRIAE5770 , TRIAE1060, and TCONS5200 ) participated in the root absorption of Cd, whereas TCONS5200 and TRIAE5660 contributed to Cd transfer to shoots. Application of Zn increased number of chloroplasts, but the chloroplast structure was destroyed under Zn treatment during Cd exposure. These results indicate that 2 g L -1 ZnSO 4 ·7H 2 O increased toxicity of 50 μM Cd. Increased Cd uptake due to the upregulated expression of Cd transporter genes and increased ROS accumulation may be the reasons for Zn application aggravating Cd toxicity.