In mammalian cells sphingosine-1-phosphate (S1P) is a well-established messenger molecule that participates
in a wide range of signalling pathways. The objective of the work reported here was to investigate the extent to
which phosphorylated long-chain sphingoid bases, such as sphingosine-1-phosphate and phytosphingosine-1-
phosphate (phytoS1P) are used in plant cell signalling. To do this, we manipulated Arabidopsis genes capable of metabolizing these messenger molecules. We show that Sphingosine kinase1 (SPHK1) encodes an enzyme that phosphorylates sphingosine, phytosphingosine and other sphingoid long-chain bases. The stomata of SPHK1-KD Arabidopsis plants were less sensitive, whereas the stomata of SPHK1-OE plants were more sensitive, than wild type to ABA. The rate of germination of SPHK1-KD was enhanced, whereas the converse was true for SPHK1-OE seed. Reducing expression of either the putative Arabidopsis S1P phosphatase (SPPASE) or the DPL1 gene, which encodes an enzyme with S1P lyase activity, individually, had no effect on guard-cell ABA signalling; however, stomatal responses to ABA in SPPASE DPL1 RNAi plants were compromised. Reducing the expression of DPL1 had no effect on germination; however, germination of SPPASE RNAi seeds was more sensitive to applied ABA. We also found evidence that expression of SPHK1 and SPPASE were coordinately regulated, and discuss how this might contribute to robustness in guard-cell
signalling. In summary, our data establish SPHK1 as a component in two separate plant signalling systems, opening the possibility that phosphorylated long-chain sphingoid bases such as S1P and phytoS1P are ubiquitous messengers in plants.