Increased anion channel activity is an unavoidable event in ozone-induced programmed cell death

Background: Ozone is a major secondary air pollutant often reaching high concentrations in urban areas under strong daylight, high temperature and stagnant high-pressure systems. Ozone in the troposphere is a pollutant that is harmful to the plant. Principal Findings: By exposing cells to a strong pulse of ozonized air, an acute cell death was observed in suspension cells of Arabidopsis thaliana used as a model. We demonstrated that O₃ treatment induced the activation of a plasma membrane anion channel that is an early prerequisite of O₃-induced cell death in A. thaliana. Our data further suggest interplay of anion channel activation with well known plant responses to O₃, Ca²⁺ influx and NADPH-oxidase generated reactive oxygen species (ROS) in mediating the oxidative cell death. This interplay might be fuelled by several mechanisms in addition to the direct ROS generation by O₃; namely, H₂O₂ generation by salicylic and abscisic acids. Anion channel activation was also shown to promote the accumulation of transcripts encoding vacuolar processing enzymes, a family of proteases previously reported to contribute to the disruption of vacuole integrity observed during programmed cell death. Significance: Collectively, our data indicate that anion efflux is an early key component of morphological and biochemical events leading to O₃-induced programmed cell death. Because ion channels and more specifically anion channels assume a crucial position in cells, an understanding about the underlying role(s) for ion channels in the signalling pathway leading to programmed cell death is a subject that warrants future investigation.