Foxtail millet SiHAK1 excites extreme high-affinity K+ uptake to maintain K+ homeostasis under low K+ or salt stress.

KEY MESSAGE: This is the first evidence that SiHAK1 acts as a K+ transporter and is modulated by internal and external K+, which expands our understanding of the significant physiological roles of large HAK/KUP/KT transporters in crops. Crop genomes have shown the richness of K+ transporters in HAK/KUP/KT (High Affinity K+/K+ Uptake Proteins/K+ Transporter) family, and much progress have been achieved toward understanding the diverse roles of K+ uptake and translocation, and abiotic stresses resistance in this family. The HAK/KUP/KT family has increasingly been recognized to be at a pivotal status in the mediation of K+ translocation and long-term transport; however, our understanding of the molecular mechanisms remains limited. Foxtail millet is an ideal plant for studying long-distance potassium (K) transport because of its small diploid genome and better adaptability to arid lands. Here, we identified 29 putative HAK/KUP/KT proteins from the Setaria italica genome database. These genes were distributed in seven chromosomes of foxtail millet and divided into five clusters. SiHAK1 exhibited widespread expression in various tissues and significant up-regulation in the shoots under low K condition. SiHAK1 was localized in the cell membrane and low K elicited SiHAK1-meidated high-affinity K+ uptake activity in Cy162 yeast cells and Arabidopsis athak5 mutants. The transport activity of SiHAK1 was coordinately modulated by external K+ supply and internal K+ content in the cell under low K and high salt environment. Our findings reveal the K uptake mechanisms of SiHAK1 and indicated that it may be involved in the mediation of K homeostasis in S. italica under K+-deficiency and salt stress.