Structure and Activity of SLAC1 Channels for Stomatal Signaling in Leaves
Ya-nan Deng, Hamdy Kashtoh, Quan Wang, Guang-xiao Zhen, Qi-yu Li, Ling-hui Tang,Hai-long Gao, Chun-rui Zhang, Li Qin, Min Su, Fei Li, Xia-he Huang, Ying-chun Wang, Qi Xie,Oliver B. Clarke, Wayne A. Hendrickson, and Yu-hang Chen
Stomata in leaves regulate gas exchange between the plant and its atmosphere. Various environmental stimuli elicit abscisic acid (ABA); ABA leads to phosphoactivation of slow anion channel 1 (SLAC1); SLAC1 activity reduces turgor pressure in aperture-defining guard cells; and stomatal closure ensues. We used electrophysiology for functional characterizations of Arabidopsis thaliana SLAC1 (AtSLAC1) and cryoelectron microscopy (cryo-EM) for structural analysis of Brachypodium distachyon SLAC1 (BdSLAC1), at 2.97-？ resolution. We identified 14 phosphorylation sites in AtSLAC1 and showed nearly 330-fold channel-activity enhancement with 4 to 6 of these phosphorylated. Seven SLAC1-conserved arginines are poised in BdSLAC1 for regulatory interaction with the N-terminal extension. This BdSLAC1 structure has its pores closed, in a basal state, spring loaded by phenylalanyl residues in high-energy conformations. SLAC1 phosphorylation fine-tunes an equilibrium between basal and activated SLAC1 trimers, thereby controlling the degree of stomatal opening.