Involved in the production of sphingolipid metabolites. Phosphorylates sphingosine and various l sphingoid long-chain base (LCB) products, such as phytosphingosine (PHS, 4-hydroxysphinganine), 4-hydroxy-8-sphingenine, 4,8-sphingadienine and D-erythro-dihydrosphingosine, but has a very few activity toward D,L-threo- dihydrosphingosine. Is required for abscisic acid (ABA) signaling that mediates stomatal closure, inhibition of seed germination and root elongation. May function upstream of PLDALPHA1 and phosphatidic acid (PA) in an amplification response to ABA that mediates stomatal closure. a sphingoid base + ATP = a sphingoid 1-phosphate + ADP + H(+) Activated by phosphatidic acid (PA). Binding with PA stimulates the activity by promoting the binding of substrate to the catalytic site. Associated with the tonoplast. Highly expressed in flowers and siliques and at lower levels in roots, leaves and stems. No visible phenotype under normal growth conditions. The predicted gene At4g21540 has been split into 2 genes: At4g21534 and At4g21540. The predicted gene At4g21540 has been split into 2 genes: At4g21534 and At4g21540.