This is a key enzyme of plant metabolism catalyzing the first reaction in the biosynthesis from L-phenylalanine of a wide variety of natural products based on the phenylpropane skeleton. L-phenylalanine = (E)-cinnamate + NH4(+) Phenylpropanoid metabolism; trans-cinnamate biosynthesis; trans-cinnamate from L-phenylalanine: step 1/1. Homotetramer. Expressed mainly in roots. Contains an active site 4-methylidene-imidazol-5-one (MIO), which is formed autocatalytically by cyclization and dehydration of residues Ala-Ser-Gly. Belongs to the PAL/histidase family.