Seeds of grapes of Vitis vinifera var alphonse lavallee (Royal): a possible model tissue for studying tocotrienol biosynthesis

Tocopherols and tocotrienols are phytochemicals generally known as vitamin E. Structurally they are very similar sharing an identical aromatic moiety (chromanol head) and an isoprenoid side chain that only differs in the degree of saturation. Tocopherols are widely distributed throughout the plant kingdom, found in practically all plant tissues and located in the inner envelope membrane of plastids. In contrast to these well characterized tocopherols little is known on the presence, location and synthesis of tocotrienols. Studies so far have been hampered by the lack of a suitable model tissue in which high tocotrienol synthesis rates could be demonstrated. Here it is shown irrevocably that tocotrienols are present in seeds of mature grapes at a relatively high concentration of (33,2 +/- 6,0) μg tocotrienols / g dry weight. Showing a distribution pattern between the α- and γ- isomers of respectively 40 and 60 %. These data are in strong contrast with the general idea that tocotrienols are only present in monocotyledonous plants. In leaves or flowers of the grape plants on the other hand, no measurable tocotrienol concentrations were detected. Tocopherols were present in all tested tissues with highest concentrations in green leaves (30 μg tocopherols / g dry weight) and lowest in flesh of grapes (5 μg tocotrienols / g dry weight). Oil extracted from the grapes was 25-fold enriched in tocotrienols. To study the biosynthesis of tocotrienols a new procedure was developed mainly differing from original methods described in Methods of enzymology by threlfall et al. who worked on complete plants, and by Soll working on chloroplasts extracted from green tissues, none of the plant tissues Threlfall and Soll discribe produce tocotrienols. With the use of radioactive labeled precursors de novo synthesis of tocotrienols in dissected endosperm of developing grape seeds (45 DAF) was demonstrated. The use of endosperm of developing grapes as a model tissue to study tocotrienol location, biosynthesis and function is discussed.