Melatonin promotes potassium deficiency tolerance by regulating HAK1 transporter and its upstream transcription factor NAC71 in wheat

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Abstract
Melatonin (MT) is involved in various physiological processes and stress responses in animals and plants. However, little is known about the molecular mechanisms by which MT regulates potassium deficiency (DK) tolerance in crops. In this study, an appropriate concentration (50 μmol/L) was found to enhance the tolerance of wheat plants against DK. RNA‐seq analysis showed that a total of 6253 and 5873 differentially expressed genes (DEGs) were separately identified in root and leaf tissues of the DK + MT‐treated wheat plants. They functionally involved biological processes of secondary metabolite, signal transduction, and transport or catabolism. Of these, an upregulated high‐affinity K transporter 1 (TaHAK1) gene was next characterized. TaHAK1 overexpression markedly enhanced the K absorption, while its transient silencing exhibited the opposite effect, suggesting its important role in MT‐mediated DK tolerance. Moreover, yeast one‐hybrid (Y1H) was used to screen the upstream regulators of TaHAK1 gene and the transcription factor TaNAC71 was identified. The binding between TaNAC71 and TaHAK1 promoter was evidenced by using Y1H, LUC, and EMSA assays. Transient overexpression of TaNAC71 in wheat protoplasts activated the TaHAK1 expression, whereas its transient silencing inhibited the TaHAK1 expression and aggravated the sensitivity to DK. Exogenous MT application greatly upregulated the expression of TaHAK1 in both transient overexpression and silencing systems. Our findings revealed some molecular mechanisms underlying MT‐mediated DK tolerance and helped broaden its practical application in agriculture.

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