Identification of the HAK/KUP/KT Potassium Transporter Gene Family in Sweet Potato and Functional Characterization of IbHAK5A.

Potassium (K+) is an essential mineral element for plant growth and development. Members of the HAK/KUP/KT (HAK) gene family serve pivotal roles in K+ uptake, translocation and homeostasis. Although numerous HAK genes have been extensively identified across diverse plant species, a comprehensive genomic and functional analysis of this family in sweet potato (Ipomoea batatas L.) remains lacking. In this study, 22 putative IbHAK genes were identified and classified into four distinct clades (I-IV). A systematic characterization was performed for each IbHAK gene, including protein physicochemical properties, chromosome distribution, gene structure, synteny, and promoter cis-elements. Notably, five IbHAK5 genes (IbHAK5A-IbHAK5E) clustered on the HAK gene tree with AtHAK5, OsHAK5, and ZmHAK5. This suggests that small-scale duplication events likely drove the expansion of HAK5 in sweet potato. Among them, IbHAK5A, a gene with broad expression across tissues and strong transcriptional induction under low-K+ (LK) stress, was cloned. The function was then characterized in a K transporter-deficient yeast mutant and an Arabidopsis hak5 mutant. Transcription factor IbPTL1 directly binds the IbHAK5A promoter, upregulates its expression, and integrates into the K+ signaling. In this work, we provide foundational insights into the underlying molecular mechanisms governing K+ acquisition in sweet potato.