Calcium Channel Blockers Modulate Calcium Homeostasis and Signaling Gene Expression in Sophora tonkinensis

Calcium signaling plays a critical role in plant growth, development, and stress adaptation. However, the tissue-specific mechanisms governing calcium homeostasis in medicinal plants adapted to high-calcium karst environments remain poorly understood. This study investigated the functional divergence and potential molecular basis of calcium homeostasis in leaves and root tips of Sophora tonkinensis under pharmacological perturbation.

Treatments included lanthanum chloride (LaCl₃) (a plasma membrane Ca²⁺ channel blocker), sodium orthovanadate (Na₃VO₄) (a Ca²⁺-ATPase inhibitor), and ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) (an extracellular Ca²⁺ chelator). Subcellular calcium distribution and expression of 14 calcium signaling-related genes were analyzed at 20, 40, and 60 days post-treatment using potassium pyroantimonate precipitation in conjunction with transmission electron microscopy and quantitative real-time PCR.

LaCl₃ treatment initially induced intracellular calcium accumulation, followed by the engagement of alternative efflux pathways in leaves and increased extracellular deposition in root tips. Na₃VO₄ treatment exacerbated intracellular calcium overload, resulting in sustained calcium toxicity in leaves, whereas root tips exhibited coordinated calcium redistribution. EGTA treatment dampened overall calcium signaling. However, root tips maintained functionality by mobilizing internal calcium stores. Gene expression analysis revealed a significant upregulation of Sophora tonkinensis calmodulin-like protein 9 (StCML9) in roots (up to 11.7-fold relative to the control), with expression closely synchronized with Ca²⁺ fluctuations, suggesting a potential role for StCML9 in calcium sensing and adaptive responses. In contrast, Sophora tonkinensis calcium-dependent protein kinases (StCDPKs) were generally suppressed in leaves.

These findings suggest that S. tonkinensis may employ organ-specific calcium distribution and transcriptional reprogramming to maintain calcium homeostasis under signaling perturbation. This study provides correlative evidence that contributes to mechanistic insights into the species' adaptation to high-calcium karst environments and offers candidate genes for future functional validation.