Arabidopsis calcineurin B-like-interacting protein kinase 8 and its functional homolog in tomato negatively regulates ABA-mediated stomatal movement and drought tolerance.

Stomatal movement is critical for water transpiration, gas exchange, and responses to biotic stresses. Abscisic acid (ABA) induces stomatal closure to prevent water loss during drought. We report that Arabidopsis CIPK8 negatively regulates ABA-mediated stomatal closure and drought tolerance. CIPK8 is highly enriched in guard cells and transcriptionally induced by ABA. Functional loss of CIPK8 results in hypersensitive stomatal closure to ABA and enhanced drought tolerance. Guard cell-specific downregulation of CIPK8 mimics the phenotype of cipk8 whereas guard cell-specific expression of a constitutive active CIPK8 (CIPK8CA) has an opposite effect, suggesting a cell autonomous activity of CIPK8. CIPK8 physically interacts with CBL1 and CBL9. Functional loss of CBL1 and CBL9 mimics ABA-hypersensitive stomatal closure of cipk8 whereas abolishes the effect of CIPK8CA, indicating that CIPK8 and CBL1/CBL9 form a genetic module in ABA-responsive stomatal movement. SlCIPK7, the functional homolog of CIPK8 in tomato (Solanum lycopersicum), plays a similar role in ABA-responsive stomatal movement. Genomic editing of SlCIPK7 results in more drought-tolerant tomato, making it a good candidate for germplasm improvement.

Arabidopsis protein S-acyl transferases positively mediate BR signaling through S-acylation of BSK1.

Protein S-acyl transferases (PATs) catalyze S-acylation, a reversible post-translational modification critical for membrane association, trafficking, and stability of substrate proteins. Many plant proteins are potentially S-acylated but few have corresponding PATs identified. By using genomic editing, confocal imaging, pharmacological, genetic, and biochemical assays, we demonstrate that three Arabidopsis class C PATs positively regulate BR signaling through S-acylation of BRASSINOSTEROID-SIGNALING KINASE1 (BSK1). PAT19, PAT20, and PAT22 associate with the plasma membrane (PM) and the trans-Golgi network/early endosome (TGN/EE). Functional loss of all three genes results in a plethora of defects, indicative of reduced BR signaling and rescued by enhanced BR signaling. PAT19, PAT20, and PAT22 interact with BSK1 and are critical for the S-acylation of BSK1, and for BR signaling. The PM abundance of BSK1 was reduced by functional loss of PAT19, PAT20, and PAT22 whereas abolished by its S-acylation-deficient point mutations, suggesting a key role of S-acylation in its PM targeting. Finally, an active BR analog induces vacuolar trafficking and degradation of PAT19, PAT20, or PAT22, suggesting that the S-acylation of BSK1 by the three PATs serves as a negative feedback module in BR signaling.