Different signaling and cell death roles of heterotrimeric G protein α and β subunits in the arabidopsis oxidative stress response to ozone

Arabidopsis thaliana plants with null mutations in the genes encoding the alpha and beta subunits of the single heterotrimeric G protein are less and more sensitive, respectively, to O-3 damage than wild-type Columbia-0 plants. The first peak of the bimodal oxidative burst elicited by O-3 in wild-type plants is almost entirely missing in both mutants. The late peak is normal in plants lacking the G beta protein but missing in plants lacking the G alpha protein. Endogenous reactive oxygen species (ROS) are first detectable in chloroplasts of leaf epidermal guard cells. ROS production in adjacent cells is triggered by extracellular ROS signals produced by guard cell membrane-associated NADPH oxidases encoded by the AtrbohD and AtrbohF genes. The late, tissue damage-associated component of the oxidative burst requires only the Ga. protein and arises from multiple cellular sources. The early component of the oxidative burst, arising primarily from chloroplasts, requires signaling through the heterotrimer (or the G beta gamma complex) and is separable from G alpha-mediated activation of membrane-bound NADPH oxidases necessary for both intercellular signaling and cell death.