Anthocyanins protect light-sensitive thiarubrine phototoxins
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Thiarubrines are phototoxic plant pigments that decompose to thiophenes when exposed to sunlight. We investigated the mechanism of thiarubrine photoprotection in Ambrosia chamissonis (Less.) Greene (Asteraceae), which contains high amounts of these chemicals in its stems and leaf petioles. Thiarubrines are compartmentalized in laticifers that are surrounded by anthocyanin-containing cells. When this light-screening sheath was removed and laticifers exposed to light, rapid bleaching of the thiarubrine contents occurred. The leaves and stems of A. chamissonis seedlings were found to contain 10.5±6.8 µg/g total anthocyanins, predominantly cyanidin 3-O-(6′-O-malonylglucoside) and cyanidin 3-O-glucoside, while none was detected in roots. To correlate anthocyanin distribution with thiarubrine photoprotection, changes in thiarubrine A and thiophene A levels were measured in seedlings exposed to light. In roots, thiarubrine A levels decreased by 94% after 30 min of irradiation, and thiarubrines were completely absent after 4 h. A concomitant 3-fold increase in thiophene A levels in roots occurred during light exposure. In leaves and stems, thiarubrine A levels did not change appreciably during light exposure, with a nominal increase from 102.8±33.1 µg/g FW to 108.4±20.7 µg/g FW after 4 h. To confirm their photoprotective function, solutions of cyanidin 3-O-glucoside were used to filter visible light incident on a solution of thiarubrine A. Anthocyanin solutions of greater than 0.1 mM completely prevented thiarubrine photoconversion. This is the first report that anthocyanins function to photoprotect light-sensitive defensive chemicals in plants.
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