Autophagy controls reactive oxygen species homeostasis in guard cells that is essential for stomatal opening.

Reactive oxygen species (ROS) function as key signaling molecules to inhibit stomatal opening and promote stomatal closure in response to diverse environmental stresses. However, how guard cells maintain basal intracellular ROS levels is not yet known. This study aimed to determine the role of autophagy in the maintenance of basal ROS levels in guard cells. We isolated the () mutant, which is impaired in stomatal opening in response to light and low CO concentrations. Disruption of other autophagy genes, including , , , and , also caused similar stomatal defects. The mutants constitutively accumulated high levels of ROS in guard cells, and antioxidants such as ascorbate and glutathione rescued ROS accumulation and stomatal opening. Furthermore, the mutations increased the number and aggregation of peroxisomes in guard cells, and these peroxisomes exhibited reduced activity of the ROS scavenger catalase and elevated hydrogen peroxide (HO) as visualized using the peroxisome-targeted HO sensor HyPer. Moreover, such ROS accumulation decreased by the application of 2-hydroxy-3-butynoate, an inhibitor of peroxisomal HO-producing glycolate oxidase. Our results showed that autophagy controls guard cell ROS homeostasis by eliminating oxidized peroxisomes, thereby allowing stomatal opening.