Melatonin alleviates Li toxicity in wheat via regulating redox and K⁺ homeostasis

Lithium (Li) pollution in agricultural soils poses a significant environmental hazard, impairing crop growth by inducing oxidative stress and disrupting nutrient homeostasis. Not only does Li⁺ compete with K⁺ for uptake, but Li-induced reactive oxygen species (ROS) accumulation may potentially trigger massive K⁺ loss via ROS-sensitive K⁺-efflux channels, leading to major disruption in the cell's metabolic activity and programmed cell death. Melatonin (Mel) has emerged as a potential mitigator of abiotic stresses, but its role in regulating ROS levels and K⁺ transport under Li toxicity remains unexplored. Here, we employed an integrated approach, combining non-invasive microelectrode ion flux estimation, confocal imaging, and qPCR analysis, to demonstrate that Mel alleviates Li-induced K⁺ loss in wheat via several concurrent mechanisms: (i) suppressing NADPH oxidase (RBOH)-derived ROS production while enhancing antioxidant activities; (2) modulating Ca²⁺ signalling by regulating Ca²⁺-ATPases (ACA/CAX) and CBL/CIPK; (3) downregulating the K⁺-efflux channel GORK and upregulating influx transporters (HAK5, AKT1) to restore K⁺ retention; and (4) regulating plasma membrane H⁺-ATPase activity. Our results show that Mel simultaneously regulates ROS and K⁺ homeostasis to confer Li tolerance, providing critical insights for sustainable phytoremediation and safe crop production in Li-affected soils.