Isoprene synthase expression and protein levels are reduced under elevated O3 but not under elevated CO2 (FACE) in field-grown aspen trees

Emission of hydrocarbons by trees has a crucial role in the oxidizing potential of the atmosphere. In particular, isoprene oxidation leads to the formation of tropospheric ozone and other secondary pollutants. It is expected that changes in the composition of the atmosphere will influence the emission rate of isoprene, which may in turn feedback on the accumulation of pollutants and greenhouse gases. We investigated the isoprene synthase (ISPS) gene expression and the ISPS protein levels in aspen trees exposed to elevated ozone (O-3) and/or elevated carbon dioxide (CO2) in field-grown trees at the Aspen Free-Air Carbon Dioxide Enrichment (FACE) experimental site. Elevated O-3 reduced ISPS mRNA and the amount of ISPS protein in aspen leaves, whereas elevated CO2 had no significant effect. Aspen clones with different O-3 sensitivity showed different levels of inhibition under elevated O-3 conditions. The drop in ISPS protein levels induced a drop in the isoprene emission rate under elevated O-3. However, the data indicated that other mechanisms also contributed to the observed strong inhibition of isoprene emission under elevated O-3.