MdWRKY11 improves copper tolerance by directly promoting the expression of the copper transporter gene MdHMA5

Overuse of fungicides and fertilizers has resulted in copper (Cu) contamination of soils and toxic levels of Cu in apple fruits. To breed Cu-resistant apple (Malus domestica) cultivars, the underlying molecular mechanisms and key genes involved in Cu resistance must be identified. Here, we show that MdWRKY11 increases Cu tolerance by directly promoting the transcription of MdHMA5. MdHMA5 is a Cu transporter that may function in the storage of excess Cu in root cell walls and stems for Cu tolerance in apple. The transcription factor MdWRKY11 is highly induced by excess Cu. MdWRKY11 overexpression in transgenic apple enhanced Cu tolerance and decreased Cu accumulation. Apple calli transformed with an MdWRKY11-RNAi construct exhibited the opposite phenotype. Both an in vivo chromatin immunoprecipitation assay and an in vitro electrophoretic mobility shift assay indicated that MdWRKY11 binds to the promoter of MdHMA5. Furthermore, MdWRKY11 promoted MdHMA5 expression in transgenic apple plants, as revealed by quantitative PCR. Moreover, inhibition of MdWRKY11 expression by RNA interference led to a significant decrease in MdHMA5 transcription. Thus, MdWRKY11 directly regulates MdHMA5 transcription. Our work resulted in the identification of a novel MdWRKY11-MdHMA5 pathway that mediates Cu resistance in apple.