Nitric Oxide-induced Na+/H+ exchange activity confers salt tolerance in pea (Pisum sativum L.) mesophyll cells.

Salinity stress severely hinders global agricultural productivity, and the issue will only increase under current climate scenarios. Due to complexity of salinity tolerance traits, crop breeding for salt tolerance remains a highly challenging task. The exogenous application of growth regulators, such as nitric oxide (NO), is considered a viable practical alternative to boost crop yield and quality under conditions of soil salinity. Numerous papers reported beneficial role of exogenous NO application on plant growth under salt stress, but very few explored the mechanistic basis of this process. Here, we investigated the effects of NO (generated by 0.1mM NO donor sodium nitroprusside) on ionic homeostasis in pea mesophyll cells in response to 100mM NaCl and 10mM H₂O₂ treatments. Membrane potential (MP) and fluxes of Na⁺, K⁺, and Ca²⁺ were measured using the Microelectrode Ion Flux Estimation (MIFE) technique. Application of NO reduced Na⁺ accumulation and salt-induced K⁺ loss from leaf mesophyll, thus improving cell viability and leaf photochemistry (SPAD and Fv/Fm characteristics). These ameliorating effects were attributed to NO's ability to restore (otherwise depolarized) MP, enhance Na⁺ efflux from the cytosol, and alter sensitivity of reactive oxygen species (ROS)-inducible Ca²⁺- and K⁺-permeable ion channels. Pharmacological experiments indicated that the Na⁺ efflux was attributed to Na⁺/H⁺ exchanger activity. Altogether, this study demonstrated, for the first time, a direct control of plasma membrane ion transporters in leaf mesophyll cells by NO, thereby affecting NaCl-induced Ca²⁺ signaling and intracellular Na⁺ and K⁺ homeostasis, thus conferring salt tolerance in pea mesophyll cells. These findings expanded our understanding of the role of NO in enhancing salinity stress tolerance in plants.