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Changes in Stomatal Behavior and Guard Cell Cytosolic Free Calcium in Response to Oxidative Stress
Authors:McAinsh M R  Clayton H  Mansfield T A  Hetherington A M
Affiliation:Institute of Environmental and Biological Sciences, Division of Biological Sciences, Lancaster University, Lancaster, United Kingdom LA1 4YQ.
Abstract:We have investigated the cellular basis for the effects of oxidative stress on stomatal behavior using stomatal bioassay and ratio photometric techniques. Two oxidative treatments were employed in this study: (a) methyl viologen, which generates superoxide radicals, and (b) H2O2. Both methyl viologen and H2O2 inhibited stomatal opening and promoted stomatal closure. At concentrations less than or equal to]10-5 M, the effects of methyl viologen and H2O2 on stomatal behavior were reversible and were abolished by 2 mM EGTA or 10 mu]M verapamil. In addition, at 10-5 M, i.e. the maximum concentration at which the effects of the treatments were prevented by EGTA or verapamil, methyl viologen and H2O2 caused an increase in guard cell cytosolic free Ca2+ (Ca2+]i), which was abolished in the presence of EGTA. Therefore, at low concentrations of methyl viologen and H2O2, removal of extracellular Ca2+ prevented both the oxidative stress-induced changes in stomatal aperture and the associated increases in Ca2+]i. This suggests that in this concentration range the effects of the treatments are Ca2+-dependent and are mediated by changes in Ca2+]i. In contrast, at concentrations of methyl viologan and H2O2 > 10-5 M, EGTA and verapamil had no effect. However, in this concentration range the effects of the treatments were irreversible and correlated with a marked reduction in membrane integrity and guard cell viability. This suggests that at high concentrations the effects of methyl viologen and H2O2 may be due to changes in membrane integrity. The implications of oxidative stress-induced increases in Ca2+]i and the possible disruption of guard-cell Ca2+ homeostasis are discussed in relation to the processes of Ca2+-based signal transduction in stomatal guard cells and the control of stomatal aperture.
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