Differential localization of flavonoid glucosides in an aquatic plant implicates different functions under abiotic stress

Flavonoids may mediate UV protection in plants either by screening of harmful radiation or by minimizing the resulting oxidative stress. To help distinguish between these alternatives, more precise knowledge of flavonoid distribution is needed. We used confocal laser scanning microscopy (cLSM) with the "emission fingerprinting" feature to study the cellular and subcellular distribution of flavonoid glucosides in the giant duckweed (Spirodela polyrhiza), and investigated the fitness effects of these compounds under natural UV radiation and copper sulphate addition (oxidative stress) using common garden experiments indoors and outdoors. cLSM "emission fingerprinting" allowed us to individually visualize the major dihydroxylated B-ring-substituted flavonoids, luteolin 7-O-glucoside and luteolin 8-C-glucoside, in cross-sections of the photosynthetic organs. While luteolin 8-C-glucoside accumulated mostly in the vacuoles and chloroplasts of mesophyll cells, luteolin 7-O-glucoside was predominantly found in the vacuoles of epidermal cells. In congruence with its cellular distribution, the mesophyll-associated luteolin 8-C-glucoside increased plant fitness under copper sulphate addition but not under natural UV light treatment, whereas the epidermis-associated luteolin 7-O-glucoside tended to increase fitness under both stresses across chemically diverse genotypes. Taken together, we demonstrate that individual flavonoid glucosides have distinct cellular and subcellular locations and promote duckweed fitness under different abiotic stresses.