Both NaCl and HO Long-Term Stresses Affect Basal Cytosolic Ca Levels but Only NaCl Alters Cytosolic Ca Signatures in .

Salinity is one of the formidable environmental factors that affect plant growth and development and constrain agricultural productivity. Experimentally imposed short-term NaCl treatment triggers a transient increase in cytosolic free Ca concentration ([Ca]) via Ca influx across the plasma membrane. Salinity stress, as well as other stresses, induces the production of reactive oxygen species (ROS), such as HO. It is well established that short-term HO treatment also triggers a transient increase in [Ca]. However, whether and how long-term NaCl and HO treatments affect the basal levels of [Ca] as well as plant responses to additional NaCl and HO stresses remain poorly understood. Using an aequorin-based Ca imaging assay, we found that the long-term treatment of seedlings with both moderate NaCl and HO in the growth media reduced the basal [Ca] levels. Interestingly, we found that the long-term treatment with NaCl, but not HO affected the responses of plants to additional NaCl stress, and remarkably the roots displayed enhanced responses while the leaves showed reduced responses. These findings suggest that plants adapt to the long-term NaCl stress, while HO might be an integrator of many stresses.