Atmospheric CO₂ Alters Resistance of Arabidopsis to Pseudomonas syringae by Affecting Abscisic Acid Accumulation and Stomatal Responsiveness to Coronatine

Atmospheric CO₂ influences plant growth and stomatal aperture. Effects of high or low CO₂ levels on plant disease resistance are less well understood. Here, resistance of Arabidopsis thaliana against the foliar pathogen Pseudomonas syringae pv. tomato DC3000 (Pst) was investigated at three different CO₂ levels: high (800 ppm), ambient (450 ppm), and low (150 ppm). Under all conditions tested, infection by Pst resulted in stomatal closure within 1 h after inoculation. However, subsequent stomatal reopening at 4 h, triggered by the virulence factor coronatine (COR), occurred only at ambient and high CO₂, but not at low CO₂. Moreover, infection by Pst was reduced at low CO₂ to the same extent as infection by mutant Pst cor^-. Under all CO₂ conditions, the ABA mutants aba2-1 and abi1-1 were as resistant to Pst as wild-type plants under low CO₂, which contained less ABA. Moreover, stomatal reopening mediated by COR was dependent on ABA. Our results suggest that reduced ABA levels at low CO₂ contribute to the observed enhanced resistance to Pst by deregulation of virulence responses. This implies that enhanced ABA levels at increasing CO₂ levels may have a role in weakening plant defense.