Catalyzes the biosynthesis of ethanolamine from serine. Decarboxylation of free serine is the major source of ethanolamine production in plants and ethanolamine metabolism is crucial for the synthesis of choline, phosphatidylethanolamine (PE) and phosphatidylcholine (PC), and thus for plant growth (By similarity). H(+) + L-serine = CO2 + ethanolamine Belongs to the group II decarboxylase family.