Catalyzes N-methylation of phosphoethanolamine, phosphomonomethylethanolamine and phosphodimethylethanolamine, the three methylation steps required to convert phosphoethanolamine to phosphocholine. Required for root system development and epidermal cell integrity through its role in choline and phospholipid metabolism. phosphoethanolamine + S-adenosyl-L-methionine = H(+) + N-methylethanolamine phosphate + S-adenosyl-L-homocysteine Phospholipid metabolism; phosphatidylcholine biosynthesis; phosphocholine from phosphoethanolamine: step 1/1. Altered root development due to defect in primary root elongation and root epidermal cell development. Belongs to the class I-like SAM-binding methyltransferase superfamily. PEAMT family.