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 Ca2+ 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 Gd³⁺ prevented H₂O₂-induced net Ca²⁺ influx, consistent with application of exogenous H₂O₂ resulting in the activation of plasma membrane Gd³⁺-sensitive Ca²⁺-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 Ca²⁺-permeable 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 Ca2+-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: Ba²⁺ > Ca²⁺ (14.5 pS) > Mg²⁺ > Zn²⁺ (20 mm external cation, 1 mm H₂O₂). Lanthanides and Zn²⁺ (but not TEA⁺) suppressed the open probability without affecting current amplitude. The results suggest spatial heterogeneity and differential sensitivity of Ca²⁺ channel activation by reactive oxygen species in the root that could underpin signalling.