Sphingolipids are critical structural components of all eukaryotic cell membranes. Although sphingolipids had long been regarded as metabolically inert structural components of the membranes, several studies now show that sphingolipid long chain bases (LCBs) and their phosphorylated derivatives (LCB-Ps) act as important cellular regulatory molecules. In yeast, LCB-Ps play an important role in regulation of cell growth, stress response and survival. In addition, it was recently suggested that sphingosine-1-phosphate might play a similar role as an intercellular messenger in plants mediating ABA-regulated stomatal closure. Despite their importance as signaling molecules, the synthesis and metabolism of the LCB-Ps in plants and the mechanisms by which the LCB-Ps regulate several cellular processes in any organism is largely unknown.. The enzymes involved in LCB-P metabolism have been highly conserved through evolution, and thus homology searches identify the genes predicted to encode the Arabidopsis orthologs.  A useful strategy for ascertaining the function of a putative Arabidopsis ortholog has relied upon heterologous expression in yeast mutants that are optimized for characterizing the activity of the protein encoded by the Arabidopsis gene. Using this strategy, we have identified several Arabidopsis genes that are involved in LCB-P metabolism using heterologous expression in yeast. The enzymatic characteristics and the substrate specificities of these proteins were determined either by expressing the proteins in yeast or using purified proteins that were expressed in E. coli. The cellular localization of these enzymes was also determined. The function of LCB-Ps in plants was studied using Arabidopsis knockout plants for these genes