The plant hormone abscisic acid (ABA) triggers production of reactive oxygen species (ROS) in guard cells via the AtrbohD and AtrbohF NADPH oxidases, leading to stomatal closure. The ABA-activated SnRK2 protein kinase open stomata 1 (OST1) (SRK2E/SnRK2.6) acts upstream of ROS in guard cell ABA signaling. Here, we report that OST1 phosphorylates Ser13 and Ser174 on AtrbohF. In addition, substitution of Ser174 to Ala results in a ∼40% reduction in the phosphorylation of AtrbohF by OST1. We also show that OST1 physically interacts with AtrbohF. These results provide biochemical evidence suggesting that OST1 regulates AtrbohF activity. MINT- 7260179, MINT- 7260147, MINT- 7260165: OST1 (uniprotkb: Q940H6) phosphorylates (MI: 0217) ATRBOHF (uniprotkb: O48538) by protein kinase assay (MI: 0424) MINT- 7260208: OST1 (uniprotkb: Q940H6) and ATRBOHF (uniprotkb: O48538) physically interact (MI: 0915) by bimolecular fluorescence complementation (MI: 0809)