Phosphatidic acid (PA) and phytosphingosine-1-phosphate (phyto-S1P) have both been identified as lipid messengers mediating plant response to abscisic acid (ABA). To determine the relationship of these messengers, we investigated the direct interaction of PA with Arabidopsis sphingosine kinases (SPHKs) that phosphorylate phytosphingosine to generate phyto-S1P. Two unique SPHK cDNAs were cloned from the annotated At4g21540 locus of Arabidopsis, and the two transcripts are differentially expressed in Arabidopsis tissues. Both SPHKs are catalytically active, phosphorylating various longchain sphingoid bases (LCBs) and are associated with the tonoplast. They both interact with PA as demonstrated by lipid-filter binding, liposome binding, and surface plasmon resonance (SPR). SPHK1 and SPHK2 exhibited strong binding to 18:1/ 18:1, 16:0/18:1, and 16:0/18:2 PA, but poor binding to 16:0/16:0, 8:0/8:0, 18:0/18:0, and 18:2/18:2 PA. Surface dilution kinetics analysis indicates that PA stimulates SPHK activity by increasing the specificity constant through decreasing Km B. The results show that the annotated At4g21540 locus is actually comprised of two separate SPHK genes. PA binds to both SPHKs, and the interaction promotes lipid substrate binding to the catalytic site of the enzyme. The PA-SPHK interaction depends on the PA molecular species. The data suggest that these two Arabidopsis SPHKs are molecular targets of PA, and the PA stimulation of SPHK is part of the signaling networks in Arabidopsis.