The
localization of acteoside and its precursors in different organs
of sesame plant (Sesamum indicum L.),
including roots, stems (epidermis, cortex, and stele), leaves (blades
and petioles), flower petals, and seeds (immature and mature) was
investigated. The highest acteoside content was found in leaf blades
(12.3% dry weight), followed by in petioles (3.1%) and petals (2.7%)
and at a trace levels or not detected in other organs. In contrast,
the known precursors of acteoside, cinnamate, p-coumarate,
caffeate, tyramine, tyrosol, salidroside, l-3,4-dihydroxyphenylalanine,
dopamine, and hydroxytyrosol were not detected in any organs, even
by multiple-reaction monitoring analysis by liquid chromatography–electrospray
ionization tandem mass spectrometry, despite acteoside being present
at high levels in the leaves. The leaves stained with diphenylborinic
acid 2-aminoethyl ester (DPBA), producing the acteoside–DPBA
complex that was detected by fluorescence at an excitation wavelength
of 365 nm, showed accumulation of acteoside at the veins and glandular
trichomes. For the first time, the leaf blade organ was shown to contain
a large amount of acteoside in the sesame plant, with acteoside synthesis
in the leaf petioles and/or blades and accumulation in the glandular
trichomes with transport through the veins. However, tyrosine and
phenylalanine contents were very low with no downstream metabolites
or known acteoside precursors detected in any organs.