Relationship between NaCl- and H2O2-Induced Cytosolic Ca2+ Increases in Response to Stress in Arabidopsis

Salinity is among the environmental factors that affect plant growth and development and constrain agricultural productivity. Salinity stress triggers increases in cytosolic free Ca²⁺ concentration (Ca²⁺]_i) via Ca²⁺ influx across the plasma membrane. Salinity stress, as well as other stresses, induces the production of reactive oxygen species (ROS). It is well established that ROS also triggers increases in Ca²⁺]_i. However, the relationship and interaction between salinity stress-induced Ca²⁺]_i increases and ROS-induced Ca²⁺]_i increases remain poorly understood. Using an aequorin-based Ca²⁺ imaging assay we have analyzed Ca²⁺]_i changes in response to NaCl and H₂O₂ treatments in Arabidopsis thaliana. We found that NaCl and H₂O₂ together induced larger increases in Ca²⁺]_i in Arabidopsis seedlings than either NaCl or H₂O₂ alone, suggesting an additive effect on Ca²⁺]_i increases. Following a pre-treatment with either NaCl or H₂O₂, the subsequent elevation of Ca²⁺]_i in response to a second treatment with either NaCl or H₂O₂ was significantly reduced. Furthermore, the NaCl pre-treatment suppressed the elevation of Ca²⁺]_i seen with a second NaCl treatment more than that seen with a second treatment of H₂O₂. A similar response was seen when the initial treatment was with H₂O₂; subsequent addition of H₂O₂ led to less of an increase in Ca²⁺]_i than did addition of NaCl. These results imply that NaCl-gated Ca²⁺ channels and H₂O₂-gated Ca²⁺ channels may differ, and also suggest that NaCl- and H₂O₂-evoked Ca²⁺]_i may reduce the potency of both NaCl and H₂O₂ in triggering Ca²⁺]_i increases, highlighting a feedback mechanism. Alternatively, NaCl and H₂O₂ may activate the same Ca²⁺ permeable channel, which is expressed in different types of cells and/or activated via different signaling pathways.