Genome-wide identification of the HKT transcription factor family and their response to salt stress in foxtail millet (Setaria italica)

High-affinity K+ transporters (HKT) play an essential role in plant growth, development and response to abiotic stresses. Their main functions are to maintain the balance between Na+ and K+, and prevent sodium toxicity in root and leaf. Foxtail millet is a high stress-tolerant crop. It could reveal their potential functions in salt tolerance to investigation of the HKT genes. In this study, we identified 7 SiHKT genes in foxtail millet. These genes were classified into two subfamilies by phylogenetic analysis, prediction of conserved motif, and gene structure analysis. Comparative modeling and evaluation of HKT-transporters by AlphaFold2 informed further studies of the protein structure of SiHKTs. The gene expression of SiHKTs under salt stress showed that there were significant differences in SiHKTs in different tissues. At the early stage of salt stress, SiHKTs play a major role in transferring K+ into aboveground; and under salt stress condition, SiHKTs play an important part in preventing sodium toxicity in root. It implies that SiHKTs members might perform multiple functions. In addition, we predicted the transcription factors that regulate the gene expression of SiHKTs using WGCNA. This study provides the systematic description of the HKT gene family, which provides a framework for further functional analysis of SiHKTs, and is valuable for the development and utilization of the germplasm source of salt tolerance in foxtail millet.