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

Arabidopsis thaliana plants with null mutations in the genes encoding the α and β subunits of the single heterotrimeric G protein are less and more sensitive, respectively, to O₃ damage than wild-type Columbia-0 plants. The first peak of the bimodal oxidative burst elicited by O₃ in wild-type plants is almost entirely missing in both mutants. The late peak is normal in plants lacking the Gβ protein but missing in plants lacking the Gα 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 Gβγ complex) and is separable from Gα-mediated activation of membrane-bound NADPH oxidases necessary for both intercellular signaling and cell death.