Dynamic changes in plant secondary metabolites during UV acclimation in Arabidopsis thaliana

Plants respond to environmental stress by synthesizing a range of secondary metabolites for defense purposes. Here we report on the effect of chronic ultraviolet (UV) radiation on the accumulation of plant secondary metabolites in Arabidopsis thaliana leaves. In the natural environment, UV is a highly dynamic environmental parameter and therefore we hypothesized that plants are continuously readjusting levels of secondary metabolites. Our data show distinct kinetic profiles for accumulation of tocopherols, polyamines and flavonoids upon UV acclimation. The lipid-soluble antioxidant α-tocopherol accumulated fast and remained elevated. Polyamines accumulated fast and transiently. This fast response implies a role for α-tocopherol and polyamines in short-term UV response. In contrast, an additional sustained accumulation of flavonols took place. The distinct accumulation patterns of these secondary metabolites confirm that the UV acclimation process is a dynamic process, and indicates that commonly used single time-point analyses do not reveal the full extent of UV acclimation. We demonstrate that UV stimulates the accumulation of specific flavonol glycosides, i.e. kaempferol and (to a lesser extent) quercetin di- and triglycosides, all specifically rhamnosylated at position seven. All metabolites were identified by Ultra Performance Liquid Chromatography (UPLC)-coupled tandem mass spectrometry. Some of these flavonol glycosides reached steady-state levels in 3-4 days, while concentrations of others are still increasing after 12  days of UV exposure. A biochemical pathway for these glycosides is postulated involving 7-O-rhamnosylation for the synthesis of all eight metabolites identified. We postulate that this 7-O-rhamnosylation has an important function in UV acclimation.