2.A.16.5.1
The plant guard cell S (Slow)-type anion channel, SLAC1 (based on activation kinetics of anion channel currents in response to voltage changes); functions in stomatal signalling, controls turgor pressure, and regulates the exchange of water and CO₂ (Chen et al. 2010). Also called carbon dioxide insensitive (CDI3) and ozone sensitive (OZS1) (Kollist et al., 2011). Heterotrimeric G proteins regulate guard cell ion channels (Zhang, 2011). Evolutionary studies have been reported (Dreyer et al. 2012). The transmembrane region of guard cell SLAC1 channels detect CO₂ signals via an abscisic acid (ABA)-independent pathway (Yamamoto et al. 2016).  SLAC1 is activated by the protein kinase OST1 (OPEN STOMATA 1), the Ca²⁺-dependent protein kinases (CPKs), the GHR1 (GUARD CELL HYDROGEN PEROXIDE-RESISTANT 1) transmembrane receptor-like protein (TC# 1.A.87.2.8), or the PYL5 abscisic acid (ABA) receptor (Q9FLB1)  (Wang et al. 2017). The structure of SLAC1 in an inactivated, closed state has been determined (Li et al. 2022). The cytosolic N-terminus and C-terminus are partially resolved and form a plug-like structure which packs against the TM domain. Breaking the interactions between the cytosolic plug and the TMD triggers channel activation. An inhibition-release model is proposed for SLAC1 activation by phosphorylation, that the cytosolic plug dissociates from the TMD upon phosphorylation, and induces conformational changes to open the pore. These findings facilitate an understanding of the regulation of SLAC1 activity and stomatal aperture in plants (Li et al. 2022).