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By modulating stomatal opening and closure, plants control gas exchange, water loss, and photosynthesis in response to various environmental signals. During light-induced stomatal opening, the transport of ions and solutes across the plasma membrane (PM) of the surrounding guard cells results in an increase in turgor pressure, leading to cell swelling. Simultaneously, vesicles for exocytosis are delivered via membrane trafficking to compensate for the enlarged cell surface area and maintain an appropriate ion-channel density in the PM. In eukaryotic cells, soluble N -ethylmaleimide-sensitive factor adaptor protein receptors (SNAREs) mediate membrane fusion between vesicles and target compartments by pairing the cognate glutamine (Q)- and arginine (R)-SNAREs to form a core SNARE complex. Syntaxin of plants 121 (SYP121) is a known Q-SNARE involved in stomatal movement, which not only facilitates the recycling of K+ channels to the PM but also binds to the channels to regulate their activity. In this study, we found that the expression of a receptor-like cytoplasmic kinase, low-K+ sensitive 4/schengen 1 (LKS4/SGN1), was induced by light; it directly interacted with SYP121 and phosphorylated T270 within the SNARE motif. Further investigation revealed that LKS4-dependent phosphorylation of SYP121 facilitated the interaction between SYP121 and R-SNARE vesicle-associated membrane protein 722 (VAMP722), promoting the assembly of the SNARE complex. Our findings demonstrate that the phosphorylation of SNARE proteins is an important strategy adopted by plants to regulate the SNARE complex assembly as well as membrane fusion. Additionally, we discovered the function of LKS4/SGN1 in light-induced stomatal opening via the phosphorylation of SYP121. [Display omitted] • RLCK LKS4/SGN1 is involved in light-induced stomatal opening • LKS4 directly binds and phosphorylates SYP121 during light-induced stomatal opening • LKS4-mediated phosphorylation of SYP121 facilitates the formation of SNARE complex • LKS4 participates in the redistribution of KAT1 to the plasma membrane Ding et al. observe light-triggered phosphorylation of SYP121 and show its importance in light-induced stomatal opening in Arabidopsis. They identify LKS4 as the kinase responsible for phosphorylation of SYP121, which facilitates the assembly of the SYP121-containing SNARE complex and redistribution of the K+ channel KAT1 to the plasma membrane. [ABSTRACT FROM AUTHOR]