Efforts to directly test the physiological relevance of AmISY were not successful because silencing systems in A. majus using virus-induced gene silencing are highly inefficient (19, 20). However, our hypothesis that AmISY is the physiologically relevant iridoid synthase in A. majus is corroborated by the steady-state kinetic parameters (Fig. 2c; kcat = 0.72 ± 0.02 s−1 and Km = 1.1 ± 0.1 μm), which are similar to those measured for CrISY, for which the physiological role has been confirmed by gene silencing (11). qRT-PCR of AmISY (Fig. 2d) with cDNA from A majus root, leaves, and flowers further indicated that AmISY is highly expressed in leaves and not expressed in roots and flowers. Although antirrhinoside is found in all A. majus tissues, the compound could be synthesized exclusively in leaves and then distributed throughout the plant. Phloem mobility of antirrhinoside has been demonstrated (2).