Exogenous Calcium Enhances Castor Tolerance to Saline–Alkaline Stress by Regulating Antioxidant Enzyme Activity and Activating Ca2+ and ROS Signaling Crosstalk

Saline-alkaline stress is a major factor limiting agricultural development, with calcium (Ca²⁺) playing a role in regulating plant tolerance through multiple signaling pathways. However, the specific mechanisms by which Ca²⁺ mediates saline-alkaline stress tolerance at the molecular level remain incompletely understood. This study investigates the effects of exogenous Ca²⁺ application on enhancing plant tolerance to saline-alkaline stress, focusing on its impact on the antioxidant system and Ca²⁺ and reactive oxygen species (ROS) signaling pathways. Through physiological assays and transcriptomic analyses, we evaluated oxidative damage markers, antioxidant enzyme activities, and the expression of key Ca²⁺ and ROS signaling genes. The results showed that saline-alkaline stress significantly elevated ROS levels, which led to increased membrane lipid peroxidation and induced upregulation of antioxidant response elements in castor roots. Exogenous calcium treatment reduced ROS accumulation by increasing superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities and decreasing malondialdehyde (MDA) levels, demonstrating a marked improvement in the antioxidant system. Transcriptomic analysis identified <i>CAT2</i> (LOC107261240) as the primary target gene associated with increased CAT activity in response to exogenous calcium. Additionally, the upregulation of specific Ca²⁺ channels, Ca²⁺ sensors, ROS receptors, and antioxidant-related genes with calcium treatment highlights the critical role of Ca²⁺-ROS signaling crosstalk in enhancing stress tolerance. Protein-protein interaction analysis identified <i>APX3</i> and other hub genes involved in Ca²⁺-ROS signaling transduction and the regulation of antioxidant activity. These findings enhance our understanding of calcium's complex regulatory roles in plant abiotic stress responses, offering new theoretical insights for improving crop resilience in agriculture.