Ultraviolet-B radiation impacts light-mediated turnover of the photosystemII reaction center heterodimer in arabidopsis mutants altered in phenolic metabolism

Ultraviolet-B (UV-B) radiation can have a negative impact on the growth and development of plants. Plants tolerant to UV-B alleviate these effects usi ng UV-screening Figments that reduce the penetration of UV-B into mesophyll tissue. Little is known about the relative contribution of specific phenol ic compounds to the screening capacity of leaves. The D1 and D2 proteins co nstituting the photosystem (PS) II reaction center heterodimer are targets of UV-B radiation and can be used as an in situ sensor for UV penetration i nto photosynthetic tissue. Degradation of these proteins occurs under very low fluences of UV-B, and is strongly accelerated in the presence of visibl e light. Using the D1-D2 degradation assay, we characterized UV-B sensitivi ty of Arabidopsis mutants (tt4, tt5, and fah1) that are genetically altered in their composition of phenolic compounds. We found that changes in pheno l metabolism result in altered rates of PSII reaction center heterodimer de gradation under mixtures of photusynthetically active radiation and UV-B. A comparison of D2 degradation kinetics showed increased UV sensitivity of t he Landsberg (Landsberg erecta) tt5 mutant relative to the Landsberg tl4 mu tant and the Landsberg wild type. Despite a lack of flavonoid accumulation, the tt4 mutant is not particularly UV sensitive. However, the tolerance of this mutant to UV-B may reflect the increased accumulation of sinapate est ers that strongly absorb in the UV range, and may thus protect the plant ag ainst environmentally relevant UV-B radiation. This sinapate-mediated prote ction is less obvious for the tt4 mutant of Columbia ecotype, indicating th at the relative contribution of particular phenolics to the total screening capacity varies with the genetic background. The role of sinapate esters i n UV screening is further substantiated by the results with the fah1 mutant where absence of most of the sinapate eaters results in a significantly ac celerated degradation of D2 under mixed light conditions. Because the latte r mutant is not expected to be deficient in flavonoids, the relative contri bution of flavonoids as protectants of PSII reaction center heterodimer aga inst UV-B damage in Arabidopsis needs to be re-evaluated vis-a-vis screenin g by simple phenolics like sinapate esters.