Accumulation and Subcellular Localization of Acteoside in Sesame Plants (Sesamum indicum L.)

Abstract

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.