Regulation of AtKUP2 Expression by bHLH and WRKY Transcription Factors Helps to Confer Increased Salt Tolerance to Arabidopsis thaliana Plants

Potassium transporters play an essential role in maintaining cellular ion homeostasis, turgor pressure, and pH, which are critical for adaptation under salt stress. We identified a salt responsiveAvicennia officinalisKUP/HAK/KT transporter family gene,AoKUP2, which has high sequence similarity to itsArabidopsisorthologAtKUP2. These genes were functionally characterized in mutant yeast cells andArabidopsisplants. BothAoKUP2andAtKUP2were induced by salt stress, andAtKUP2was primarily induced in roots. Subcellular localization revealed that AoKUP2 and AtKUP2 are localized to the plasma membrane and mitochondria. Expression ofAtKUP2andAoKUP2inSaccharomyces cerevisiaemutant strain (BY4741trk1 Delta::loxP trk2 Delta::loxP) helped to rescue the growth defect of the mutant under different NaCl and K(+)concentrations. Furthermore, constitutive expression ofAoKUP2andAtKUP2conferred enhanced salt tolerance inArabidopsisindicated by higher germination rate, better survival, and increased root and shoot length compared to the untreated controls. Analysis of Na+ and K+ contents in the shoots and roots showed that ectopic expression lines accumulated less Na+ and more K+ than the WT. Two stress-responsive transcription factors, bHLH122 and WRKY33, were identified as direct regulators ofAtKUP2expression. Our results suggest that AtKUP2 plays a key role in enhancing salt stress tolerance by maintaining cellular ion homeostasis.