Dissecting the ROS-signalling component of salinity tolerance: Tissue-specific K+/Na+ homeostasis in quinoa and spinach roots.

This study combines electrophysiological, imaging, and molecular techniques to compare reactive oxygen species (ROS)-mediated K⁺/Na⁺ regulation in root elongation (EZ) and mature zones (MZ) of halophytic quinoa (Chenopodium quinoa) and glycophytic spinach (Spinacia oleracea). Under salinity stress, quinoa exhibited transient ROS (H2O2) accumulation followed by rapid recovery, whereas spinach showed prolonged oxidative stress and severe ionic imbalance in roots. Quinoa plants avoided cytosolic Na+ toxicity by excluding Na⁺ via the upregulation of salt overly sensitive (SOS1) genes and enhanced vacuolar sequestration via NHX. Quinoa maintained K⁺ homeostasis under ROS through biphasic regulation linked to tissue-specific expression of K+ transporter genes GORK, AKT1, HAK5, and KEA, while spinach possessed a sustained K⁺ loss. Transcriptomic analysis revealed quinoa's robust induction of MAPK signalling and ethylene-related genes, contrasting with spinach's reliance on ABA and delayed antioxidant responses. Overall, the differential sensitivity of root zones was attributed to quinoa's spatially restricted ROS signalling, which fine-tunes ion transporter activity, while spinach showed excessive ROS production and K+ loss. These results demonstrate that quinoa's oxidative tolerance arises from coordinated ROS-hormone-transporter interactions in a highly tissue-specific manner, providing a mechanistic framework for improving crop resilience.