Spatial variation in H2O2 response of Arabidopsis thaliana root epidermal Ca2+ flux and plasma membrane Ca2+ channels

Hydrogen peroxide is an important regulatory agent in plants. This study demonstrates that exogenous H2O2 application to Arabidopsis thaliana root epidermis results in dose-dependent transient increases in net Ca2þ influx. The magnitude and duration of the transients were greater in the elongation zone than in the mature epidermis. In both regions, treatment with the cation channel blocker Gd3þ prevented H2O2-induced net Ca2þ influx, consistent with application of exogenous H2O2 resulting in the activation of plasma membrane Gd3þsensitive Ca2þ-influx pathways. Application of 10 mM H2O2 to the external plasma membrane face of elongation zone epidermal protoplasts resulted in the appearance of a hyperpolarization-activated Ca2þpermeable conductance. This conductance differed from that previously characterized as being responsive to extracellular hydroxyl radicals. In contrast, in mature epidermal protoplasts a plasma membrane hyperpolarization- activated Ca2þ-permeable channel was activated only when H2O2 was present at the intracellular membrane face. Channel open probability increased with intracellular [H2O2] and at hyperpolarized voltages. Unitary conductance decreased thus: Ba2þ > Ca2þ (14.5 pS) > Mg2þ > Zn2þ (20 mM external cation, 1 mM H2O2). Lanthanides and Zn2þ (but not TEAþ) suppressed the open probability without affecting current amplitude. The results suggest spatial heterogeneity and differential sensitivity of Ca2þ channel activation by reactive oxygen species in the root that could underpin signalling