Underlying mechanism in rapid changes in UV screening and antioxidant activity by flavonoids
Final Report Abstract
The accumulation of UV-absorbing compounds (i.e. flavonoids) and the associated decrease in epidermal UV transmittance is one response of plants to UV exposure. However, the degree and rapidity to which plants can modulate UV shielding and antioxidant activity over the course of a day is poorly understood but might affect food quality. There are strong interlinks and concomitant changes in okra UV transmittance, flavonoids, UV-absorbance and antioxidant activity in response to fluctuations in solar UV. If okra plants were transferred from the -UV environment to the +UV environment the plants start to increase their flavonoid content after a response time of 2 hours. This response was to a greater extent when the plants were transferred at 9 am than at 1 pm. A correlation of UVB radiation and flavonoids reveal highest values 30 minutes to 1 hour prior the flavonoid measurements. This underlines the importance of the harvest-time point for health-promoting compounds in fruits and vegetables. Besides a high number of quercetin glycosides e.g. the main compound quercetin-3-O-glucoside-xyloside okra also has high concentrations of hydroxycinnamic acids such as sinapoyl glucoside. Results show that sun adapted tomato plants did not show a significant diurnal change over the course of a day on a sunny day in their flavonoid content. Nevertheless, chlorophyll fluorescence measurements reveal higher plant stress especially in photosystem II at noon that does not seem to be interlinked to flavonoids. Plants transferred into a +UV environment still show slight but not significant diurnal changes. However, plants transferred to a -UV environment dropped their flavonoid content slightly and showed a steady-state flavonoid content over 5 days. Tomatoes contain naringenin chalcone and quercetin glycosides normally linked to a UV response in plants. It is of great interest if the shift of certain compounds is related to UV absorption or antioxidant activity. The UVR8 photoreceptor in "Arabidopsis thaliana" is specific to ultraviolet-B (UV-B; 280-315 nm) radiation and its activation leads to a number of UV-B acclimation responses, including the accumulation of flavonoids. UVR8 is involved in a signaling cascade that includes COP1, HYH5 and HYH such that the lack of any one of these components leads to a reduction in a plant’s ability to accumulate flavonoids in response to UV; high drop-outs are evident in cop1 mutants and very low concentrations of flavonoids occur in hy5-kss50hyh double mutants. The predominant phenolics in Arabidopsis thaliana are sinapic acid derivatives as well as non-aclyated quercetin and kaempferol di- and tri-glycosides containing glucose and rhamnose as glycosylated sugar moieties. How this flavonoid profile in Arabidopsis thaliana is influenced by UV, how quickly these changes occur when UV conditions change, and what components of the UV-B signalling pathway are involved in rapid acclimation responses to UV is poorly understood. In the present study, we explored these questions by characterizing the flavonoid profiles of Arabidopsis thaliana signaling mutants and wildtypes grown under different intensities of constant UV-B+PAR ratios and then transferring a subset of plants to alternate UV conditions. Results indicate that flavonoid accumulation in Arabidopsis thaliana is triggered by UV and this response is amplified by higher levels of UV but not to the same degree by all compounds. The catechol structure in quercetin seems to be less important than the glycosylation pattern, e.g. having 2 rhamnose moieties in determining responsivity. At low UV+PAR intensities the introduction of UV leads to an initial increase of flavonoids in the wild-types that was detected after 3 days. It took 7 days for these changes to be detected in plants grown under high UV+PAR intensities suggesting a priming of PAR. Thus, the flavonoid profile in Arabidopsis thaliana is altered over time following exposure to UV and PAR, but the functional significance of these changes is unclear at present.
Publications
- The interlink of UV transmittance and flavonoids in okra driven by diurnal changes, 12th World Congress on Polyphenol Applications, Bonn, Germany, 25.09.-28.09.2018
S. Neugart, M. Tobler, P. Barnes
- The interlink of UV transmittance and flavonoids in okra driven by diurnal changes, 29th International Conference on Polyphenols, Madison, USA, 16.07.-20.07.2018
S. Neugart, M. Tobler, P. Barnes
- The interlink of UV transmittance and flavonoids in okra driven by diurnal changes, 2nd UV4plants Network Meeting, Bled, Slowenia, 15.04.-18.04.2018
S. Neugart, M. Tobler, P. Barnes
(See online at https://doi.org/10.19232/uv4pa.2018.1) - Different irradiances of UV and PAR in the same ratios alter the flavonoid profiles of Arabidopsis thaliana wild-types and UV-signalling pathway mutants, Photochemical and Photobiological Sciences, 2019, Advance Article.
S. Neugart, M. Tobler, P. Barnes
(See online at https://doi.org/10.1039/c8pp00496j)
