Open stomata 1 ( OST 1) kinase controls R –type anion channel QUAC 1 in A rabidopsis guard cells

Under drought stress, the stress hormone ABA addresses the S n R kinase OST 1 via its cytosolic receptor and the protein phosphatase ABI 1. Upon activation, OST 1 phosphorylates the guard cell S –type anion channel SLAC 1. Arabidopsis ABI 1 and OST 1 loss‐of‐function mutants are characterized by an extreme wilting 'open stomata′ phenotype. Given the fact that guard cells express both SLAC ‐ and R–/ QUAC ‐type anion channels, we questioned whether OST 1, besides SLAC 1, also controls the QUAC 1 channel. In other words, are ABI 1/ OST 1 defects preventing both of the guard cell anion channel types from operating properly in terms of stomatal closure? The activation of the R –/ QUAC ‐type anion channel by ABA signaling kinase OST 1 and phosphatase ABI 1 was analyzed in two experimental systems: Arabidopsis guard cells and the plant cell‐free background of Xenopus oocytes . Patch‐clamp studies on guard cells show that ABA activates R–/ QUAC ‐type currents of wild‐type plants, but to a much lesser extent in those of abi1–1 and ost1–2 mutants. In the oocyte system the co‐expression of QUAC 1 and OST 1 resulted in a pronounced activation of the R–type anion channel. These studies indicate that OST 1 is addressing both S–/ SLAC ‐ and R–/ QUAC ‐type guard cell anion channels, and explain why the ost1–2 mutant is much more sensitive to drought than single slac1 or quac1 mutants.