Foliar Application of Zinc Promotes Cadmium Absorption by Increasing Expression of Cadmium Transporter Genes and Activities of Antioxidative Enzymes in Winter Wheat

Cadmium (Cd) is a deleterious non-essential metal in plants. To elucidate the mechanisms by which zinc (Zn) application alleviates cadmium (Cd) toxicity in wheat, we characterized plant growth, antioxidant system, leaf cell ultrastructure, and Cd transporter gene expression in winter wheat under Cd exposure (50 µmol L−1 Cd) with foliar Zn application in a hydroponic experiment. Results showed that Zn addition (Zn + Cd) or pretreatment (pre-Zn + Cd) at 2 g L−1 as ZnSO4·7H2O significantly exacerbated Cd-induced growth inhibition and diminished root morphological parameters, root cell viability, and chlorophyll content. In addition, the Cd content increased in roots and shoots in the Zn + Cd and pre-Zn + Cd treatments, but the Cd translocation factor decreased, when compared to the treatment without Zn application. After Zn application, the Cd content increased in the root cell wall fraction but decreased in the soluble fraction. The activities of antioxidative enzymes (superoxide dismutase, peroxidase, and catalase) and the contents of non-enzymatic antioxidants (malondialdehyde, ascorbic acid, and glutathione) significantly increased in the roots and shoots of wheat after Cd exposure, particularly in the Zn + Cd and pre-Zn + Cd treatments. Gene expression analysis showed that five genes (TaHMA2, TRIAE5370, TCONS1113, TRIAE5770, TRIAE1060, and TCONS5200) participated in root absorption of Cd, whereas TCONS5200 and TRIAE5660 contributed to Cd transfer to shoots. Foliar application of Zn increased the number of chloroplasts, but the chloroplast structure was destroyed in the Zn + Cd treatment. These results indicated that 2 g L−1 ZnSO4·7H2O increased the toxicity of 50 µmol L−1 Cd. Increased Cd uptake due to the upregulated expression of Cd transporter genes and increased reactive oxygen species accumulation may be the reasons why Zn application aggravated Cd toxicity.