Identification of TtHAKs in Tetragonia tetragonoides and preliminary functional characterization of their involvement in the adaptation to extreme abiotic stresses.

The plant K+ uptake permease/high-affinity K+ transporter/K+ transporter family (HAK) is the largest potassium transporter family and plays a crucial role in abiotic stress responses and biological processes. Salt/alkali and high osmotic stress have become global problems due to their effects on plant growth and development, causing high accumulation of Na+ and OH−, related imbalance of ionic homeostasis, and nutritional deficiency. HAKs are pivotal channel proteins for both Na+ and K+ that are involved in the molecular mechanism of salt tolerance and translocation and accumulation of salt ions in the representative euhalophyte plant Tetragonia tetragonoides (Pall.) Kuntze. In this study, 22 TtHAKs were identified from the T. tetragonoides genome. Phylogenetic analysis classified these TtHAKs into five distinct clusters containing 4, 2, 2, 2, and 12 members, with relatively conserved protein structural features. Additionally, evolution analysis showed that these salt ion transporter genes were expanded in the genome, while their gene structure and conserved protein motifs were relatively conserved. Many cis-acting elements related to stress and hormones were found in the promoter regions of TtHAKs. Gene expression profiles determined using RNA sequencing (RNA-seq) and weighted gene co-expression network analysis showed that these TtHAKs exhibited distinct expression patterns in different tissues and in response to salt, drought, and potassium deficiency treatments. The expression levels of several TtHAKs, determined by quantitative real-time PCR, showed a significantly positive correlation with RNA-seq data. Three TtHAKs were cloned and functionally confirmed with a yeast heterologous expression system. In conclusion, our study revealed the evolutionary and functional characterization of the TtHAK family in T. tetragonioides.