Significance Stomatal openings in plant leaves admit CO ₂ and release water vapor between plant and atmosphere. Environmental stimuli such as darkness or dryness lead to closure, whereas light or low CO ₂ stimulate opening. Each stoma is defined by a pair of guard cells, and its pore aperture is controlled by ion channels that regulate the turgor pressure in these cells. SLAC1 anion channels are of central importance here, and environmental factors affect channel activity by controlling SLAC1 phosphorylation. Our studies define multiple sites of regulatory phosphorylation in the context of an atomic structure of the SLAC1 channel, providing a mechanistic understanding of the fine-tuning of channel activity and thereby of stomatal apertures in response to the environment.