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 Ca 2+ concentration ([Ca 2+ ] i ) via Ca 2+ 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 [Ca 2+ ] i . However, the relationship and interaction between salinity stress-induced [Ca 2+ ] i increases and ROS-induced [Ca 2+ ] i increases remain poorly understood. Using an aequorin-based Ca 2+ imaging assay we have analyzed [Ca 2+ ] i changes in response to NaCl and H 2 O 2 treatments in Arabidopsis thaliana . We found that NaCl and H 2 O 2 together induced larger increases in [Ca 2+ ] i in Arabidopsis seedlings than either NaCl or H 2 O 2 alone, suggesting an additive effect on [Ca 2+ ] i increases. Following a pre-treatment with either NaCl or H 2 O 2 , the subsequent elevation of [Ca 2+ ] i in response to a second treatment with either NaCl or H 2 O 2 was significantly reduced. Furthermore, the NaCl pre-treatment suppressed the elevation of [Ca 2+ ] i seen with a second NaCl treatment more than that seen with a second treatment of H 2 O 2 . A similar response was seen when the initial treatment was with H 2 O 2 ; subsequent addition of H 2 O 2 led to less of an increase in [Ca 2+ ] i than did addition of NaCl. These results imply that NaCl-gated Ca 2+ channels and H 2 O 2 -gated Ca 2+ channels may differ, and also suggest that NaCl- and H 2 O 2 -evoked [Ca 2+ ] i may reduce the potency of both NaCl and H 2 O 2 in triggering [Ca 2+ ] i increases, highlighting a feedback mechanism. Alternatively, NaCl and H 2 O 2 may activate the same Ca 2+ permeable channel, which is expressed in different types of cells and/or activated via different signaling pathways.