Tissue-specific Ca²⁺ profiles associated with differential Ca²⁺ signalling and salinity stress tolerance between quinoa and spinach

Background and aims: Soil salinity leads to oxidative stress by increasing production of reactive oxygen species (ROS). At the same time, stress-induced ROS signalling may play an important adaptive role, alongside cytosolic calcium ([Ca²⁺]_cyt) signalling. However, little is known about the interaction between ROS and [Ca²⁺]_cyt signalling as a determinant of differential salinity stress tolerance between halophytes and glycophytes. Methods: The spatiotemporal dynamics of Ca²⁺ signalling in the elongation and mature zones of quinoa (halophyte) and spinach (glycophyte) roots in response to NaCl and H₂O₂ stress was examined using fluorescence Ca²⁺ imaging, non-invasive microelectrode Ca²⁺ flux measurements, and expression analysis of Ca²⁺-related genes. Results: Quinoa maintained higher root cell viability and was less affected by both NaCl and H₂O₂ stresses. This different response was achieved by several complementary mechanisms associated with Ca²⁺ signalling, including (i) tissue-specific Ca²⁺ flux patterns in quinoa, followed by (ii) differential induction of Ca²⁺ transporters (e.g. CAX and ACA) and Ca²⁺ sensors (CBLs; CIPKs) for regulating cytosolic Ca²⁺ concentration. Also contributing were more efficient upregulation of the SOS1-mediated Na⁺ exclusion system and higher ROS scavenging in quinoa. Conclusions: The quinoa's superior salinity tolerance is conferred by its ability to orchestrate precise spatiotemporal control of the ROS-Ca²⁺ hub, leading to efficient stress signaling and mitigation. Future work should focus on the functional validation of these candidate genes (e.g., ANNEXIN1, CBL-CIPK networks) in model and crop plants under field conditions.