Copper but not manganese application reduces cadmium uptake and accumulation in bread wheat

Wheat can readily accumulate the toxic metal cadmium (Cd) in its grains, which potentially poses a risk to human health via food chain. Applications of essential metals are effective strategies for limiting grain Cd accumulation. However, the effects of copper (Cu) or manganese (Mn) application on Cd uptake and accumulation in wheat are not well known. Here, we performed hydroponic experiments using two bread wheat (Triticum aestivum L., AABBDD) varieties. Solution application of Cu (S_Cu) and Mn (S_Mn) alone or foliar application of Cu (F_Cu) and Mn (F_Mn) alone did not alter the dry weights of roots and shoots when compared with control under Cd stress. Mineral analysis found that S_Cu reduced Cd concentrations of roots, shoots and whole plant by 64.02-70.38 %, 10.21-11.93 % and 46.48-48.29 %, respectively; while, it promoted root-to-shoot Cd translocation by 143.55-209.12 %. Although F_Cu did not reduce Cd concentration of roots, it caused decreases in Cd concentrations in shoots (21.96-24.73 %) and whole plants (15.81-17.14 %), as well as root-to-shoot Cd translocation (21.27-31.32 %). Subcellular distribution analysis showed that both S_Cu and F_Cu limited Cd distribution in the root cell wall and soluble fractions. They also decreased the cellulose concentration. On contrary, both S_Mn and F_Mn increased Cd uptake and accumulation in both varieties. RNA-seq analysis revealed that differentially expressed genes caused by Cu or Mn application were mainly involved in cell wall organization, and hemicellulose and cellulose metabolic processes. Cu application might upregulate the expression of TaYSL6 and downregulate the expression of TaIRT1 to limit Cd uptake and accumulation.