Biotransformation of 1 mM (E)-cinnamic acid by thirty-day-old whole ribworts (Plantago lanceolala) was investigated. After addition of this caffeic acid precursor in medium culture, the root accumulation of the two main caifeic acid glycoside esters (CGEs), plantamoside and verbascoside was compared with in vitro ribwort control. No major modification of these two CGEs storage was highlighted, p-Coumaroyl-glucose and feruloyl-glucose were neo-synthesized and accumulated in the roots of seedlings fed with cinnamic acid. Maximum level of p-coumaroyl-glucose was higher than those produced by seventy-five-day-old ribworts previously investigated. Hence, the complete structure of the latter glucosylated acid was determined by both H-1 and C-13 NMR spectroscopy as (E)-p-coumaroyl-1-O-beta-D-glucopyranoside.