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 pretreatment 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.