MdWRKY39 negatively regulates apple phosphorus-deficiency tolerance by inhibiting MdPHT1;7 expression

WRKY transcription factors have been reported to participate extensively in plant responses to abiotic stress. Phosphorus deficiency can limit apple plant growth and bud differentiation resulting in reduced yield; however, the response of WRKY transcription factors to phosphorus deficiency in apple plants has rarely been studied. In this study, we identified a MdWRKY39 gene in apple and investigated its role in phosphorus-deficiency tolerance. The MdWRKY39 protein fused to green fluorescent protein was localized to the nucleus and its expression was repressed by low phosphorus supply. Overexpression of MdWRKY39 reduced apple plant phosphorus content and root hair density. In vitro and in vivo experiments indicated that MdWRKY39 down-regulates MdPHT1;7 tran-scription by directly binding to its W-box cis-elements. More specifically, MdWRKY39 binds to the TGAC motif of the W-box element in the MdPHT1;7 promoter. Our results showed that MdWRKY39 directly targets MdPHT1;7, thereby negatively regulating phosphorus-deficiency tolerance in apple plants.