Functional Characterization of IbHK1a Reveals Its Role in Enhancing Drought and Salt Tolerance Through Reactive Oxygen Species Regulation and Two-Component System Signaling in Sweet Potato (Ipomoea batatas L.)

Drought and salinity are major abiotic stresses that severely constrain plant growth and agricultural productivity. Histidine kinases (HKs), as key components of the plant two-component system (TCS), play crucial roles in environmental signal perception and adaptive responses. In this study, we functionally characterized a sweet potato (Ipomoea batatas L.) HK gene, IbHK1a, and investigated its role in drought and salt stress tolerance. Expression analysis revealed that IbHK1a is predominantly expressed in root tissues, particularly in storage and fibrous roots, indicating its potential involvement in stress sensing and adaptation. Subcellular localization demonstrated that the IbHK1a protein is localized to the plasma membrane, suggesting a role in external signal perception. To elucidate its biological function, IbHK1a was heterologously overexpressed in Arabidopsis thaliana. Transgenic plants exhibited significantly enhanced tolerance to drought and salt stress, as evidenced by higher seed germination rates, improved primary root growth, reduced leaf wilting, and increased survival rates compared with wild-type (WT) plants. Physiological analyses showed that IbHK1a overexpression led to increased activities of antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), accompanied by reduced accumulation of reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) and malondialdehyde (MDA). Consistently, leaf histochemical staining confirmed lower ROS accumulation in transgenic plants under stress conditions. In sweet potato, overexpression of IbHK1a in transgenic hairy roots enhanced tolerance to drought and salinity, whereas RNA interference lines displayed increased sensitivity, further confirming its positive regulatory role. Additionally, protein interaction analysis indicated that IbHK1a interacts with Arabidopsis histidine phosphotransferase proteins (AHPs), suggesting its involvement in conserved TCS-mediated phosphorelay signaling pathways. Functional complementation analysis demonstrated that IbHK1a partially rescues the stress-sensitive phenotype of the AHK1 mutant, indicating functional conservation with Arabidopsis AHK1. Collectively, these findings demonstrate that IbHK1a positively regulates drought and salt stress tolerance by enhancing antioxidant defense and ROS homeostasis. Its interaction with AHPs and partial complementation of the ahk1 mutant further support its involvement in the conserved TCS phosphorelay pathway. These results establish IbHK1a as an important component of abiotic stress responses and a potential genetic target for improving drought and salinity tolerance in sweet potato.