Plants form ethanolamine (Etn) moieties by decarboxylating serine or phosphatidylserine (PtdSer), and use them to make phosphatidylethanolamine, phosphatidylcholine, choline, and glycine betaine. Serine decarboxylation is mediated by a serine decarboxylase (SDC) that is unique to plants and has a characteristic N-terminal extension. This extension was shown to have little influence on function of the enzyme in vitro. To explore the importance of SDC and its extension in vivo, native or truncated versions of the Arabidopsis enzyme were expressed in tobacco. Transgene expression increased SDC activity by up to 10-fold and free Etn level up to 6-fold, but did not change levels of serine, choline, phosphocholine, or phosphatidyl bases. The truncated enzyme gave significantly higher Etn levels. These results show that SDC activity exerts substantial control over flux to Etn, and suggest that the enzyme's N-terminus may have a regulatory role. In complementary studies with Arabidopsis, we showed that a mutant with 9-fold elevated mitochondrial PtdSer decarboxylase activity had normal pools of serine, Etn, and Etn metabolites. Taken together, these data indicate that serine decarboxylation is the main source of Etn moieties in plants. The ability to enhance serine --> Etn flux should advance engineering of choline and glycine betaine accumulation.