Ozone detoxification in the mesophyll cell wall during a simulated oxidative burst

Area of Science:

• Plant Physiology
• Biochemistry
• Environmental Science

Background:

• Ozone (O3) is a phytotoxic air pollutant causing oxidative stress in plants.
• Plant cell walls possess antioxidant systems to detoxify reactive oxygen species (ROS).
• Oxidative bursts can generate superoxide radicals (O2*-) and hydrogen peroxide (H2O2).

Purpose of the Study:

• To assess the role of superoxide radical (O2*-) in ozone (O3) reduction within mesophyll cell walls.
• To compare O3 reduction pathways involving O2*- with those involving hydrogen peroxide (H2O2) and ascorbate.
• To evaluate the effectiveness of O2*- generation during oxidative bursts in detoxifying O3.

Main Methods:

• Computational modeling of O3 reduction reactions in plant cell walls.
• Comparison of O3 flow through O2*- dependent and H2O2-forming reaction sequences.
• Inclusion of direct O3 reaction with ascorbate and O2*- disproportionation/reduction.
• Simulations based on experimental exposure of Phaseolus vulgaris leaves to O3.

Main Results:

• O3 reduction via reaction with O2*- accounted for only 25-44% of O3 detoxified by direct ascorbate reaction.
• Over 99% of O2*- was converted to H2O2 through spontaneous disproportionation (pH 5) or ascorbate reduction (pH 7).
• Calculated steady-state H2O2 concentrations (40-80 microM) were primarily scavenged by peroxidase.
• Postulated H2O2 generation rates were excessively high for significant O3 reduction.

Conclusions:

• Superoxide radical (O2*-) generation during oxidative bursts is inefficient for detoxifying ozone (O3) in plant cell walls.
• The calculated H2O2 generation rates were too high to be effectively utilized for O3 reduction.
• Superoxide dismutase induction in the cell wall under O3 exposure warrants further investigation.