Fig. 4. (A) SphK and S1P signaling in responses of A. thaliana guard cells to abscisic acid (ABA). ABA stimulates SphK and formation of S1P and/or phyto-S1P. Phosphorylated sphingoid bases, acting through the heterotrimeric G protein GPA1 by a mechanism that is still not known, inhibit inwardly rectifying K⁺ channels and activate slow anion channels, thereby inhibiting stomatal opening and promoting stomatal closure. S1P and/or phyto-S1P can also stimulate cell proliferation in a GPA1-dependent manner. (B) Structures of phytosphingosine, dihydrosphingosine, and sphingosine.