Plant Shaker channels form the major K+ conductance of the plasma membrane, thereby mediating large K+ fluxes required for plant growth and development. Also, owing to the role of K+ ion in control of the cell osmotic potential, they are involved in the modulation of stomatal movements, which is necessary for allowing CO2 uptake while limiting transpirational water loss. Thus, they contribute to plant adaptation to changes in water availability and air humidity. Shaker channels are homo- or heterotetramers associating 4 Shaker subunits. In the model plant Arabidopsis, the Shaker subunit AtKC1 is not able to form a functional homotetrameric channel at the plasma membrane. Instead, it is retained in the endoplasmic reticulum (ER). However, when co-expressed with other Shaker subunits, it forms heteromeric channels targeted to the plasma membrane and endowed with new functional properties. Thus, AtKC1 behaves as a silent regulatory subunit. The atkc1 mutant displays a remarkable and pleiotropic phenotype, including reduced growth and impaired stomatal control leading to increased transpirational water loss. The project proposes an integrated approach associating molecular, biochemical and biophysical approaches, along with cell biology and whole plant physiology, in order to unravel the role that AtKC1 plays in stomatal regulation and adaptation to water stress. Different aspects will be investigated: the molecular determinants of AtKC1 activity (for instance, regulatory partner proteins), and physiological impacts of AtKC1 activity on whole plant and guard cell physiology under different environmental and hormonal conditions. Physiological aspects will be studied in wild type and atkc1 KO mutant plants, using the Phenopsis phenotyping platform for whole plant transpiration measurements and electrophysiology techniques for measuring K+ channel activity and membrane potential changes in epidermal and guard cells.