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 using UV-screening pigments that reduce the penetration of UV-B into mesophyll tissue. Little is known about the relative contribution of specific phenolic compounds to the screening capacity of leaves. The D1 and D2 proteins constituting 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 into photosynthetic tissue. Degradation of these proteins occurs under very low fluences of UV-B, and is strongly accelerated in the presence of visible light. Using the D1-D2 degradation assay, we characterized UV-B sensitivity of Arabidopsis mutants (tt4, tt5, and fah1) that are genetically altered in their composition of phenolic compounds. We found that changes in phenol metabolism result in altered rates of PSII reaction center heterodimer degradation under mixtures of photosynthetically active radiation and UV-B. A comparison of D2 degradation kinetics showed increased UV sensitivity of the Landsberg (Landsberg erecta) tt5 mutant relative to the Landsberg tt4 mutant 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 esters that strongly absorb in the UV range, and may thus protect the plant against environmentally relevant UV-B radiation. This sinapate-mediated protection is less obvious for the tt4 mutant of Columbia ecotype, indicating that the relative contribution of particular phenolics to the total screening capacity varies with the genetic background. The role of sinapate esters in UV screening is further substantiated by the results with the fah1 mutant where absence of most of the sinapate esters results in a significantly accelerated degradation of D2 under mixed light conditions. Because the latter mutant is not expected to be deficient in flavonoids, the relative contribution of flavonoids as protectants of PSII reaction center heterodimer against UV-B damage in Arabidopsis needs to be re-evaluated vis-a-vis screening by simple phenolics like sinapate esters.

The light dependence of D1 phosphorylation is unique to higher plants, being constitutive in cyanobacteria and algae. In a photoautotrophic higher plant, Spirodela oligorrhiza, grown in greenhouse conditions under natural diurnal cycles of solar irradiation, the ratio of phosphorylated versus total D1 protein (D1-P index: [D1-P]/[D1] ϩ [D1-P]) of photosystem II is shown to undergo reproducible diurnal oscillation. These oscillations were clearly out of phase with the period of maximum in light intensity. The timing of the D1-P index maximum was not affected by changes in temperature, the amount of D1 kinase activity present in the thylakoid membranes, the rate of D1 protein synthesis, or photoinhibition. However, when the dark period in a normal diurnal cycle was cut short artificially by transferring plants to continuous light conditions, the D1-P index timing shifted and reached a maximum within 4 to 5 h of light illumination. The resultant diurnal oscillation persisted for at least two cycles in continuous light, suggesting that the rhythm is endogenous (circadian) and is entrained by an external signal.Photosynthetic oxygen evolution involves a supramolecular protein-pigment complex, PSII (Ort and Yocum, 1996;Mattoo et al., 1999). The PSII reaction center, which includes the D1 and D2 protein heterodimer, binds most of the nonprotein components of the PSII electron transport chain (Nanba and Satoh, 1987;Michel and Deisenhofer, 1988;Mattoo et al., 1989; Hankamer et al., 1997). Light is central to the metabolism of the D1 protein, regulating its synthesis (Mattoo et al., 1984), intramembrane translocation (Mattoo and Edelman, 1987; Callahan et al., 1990), posttranslational phosphorylation Elich et al., 1992) and acylation (Mattoo and Edelman, 1987;Mattoo et al., 1993), and its rate of degradation (Mattoo et al., 1984; Greenberg et al., 1987; Aro et al., 1993). The posttranslational phosphorylation of D1 occurs at its N-terminal Thr residue, catalyzed by a light-dependent redox-regulated kinase Elich et al., 1992).Protein phosphorylation is a mechanism used by eukaryotes to regulate cellular activity (Stone and Walker, 1995). In plants, protein phosphorylation is a key response to environmental signals such as wounding (Usami et al., 1995) and light (Allen, 1992). The greatest concentration of phosphoproteins in plants is found in the chloroplast membranes (Bennett, 1991). Phosphorylation and dephosphorylation of the D1 protein are strictly light dependent (Elich et al., 1993(Elich et al., , 1997. Reversible, redox-sensitive phosphorylation of the light-harvesting chlorophyll apoprotein is thought to be a mechanism maximizing quantum yield by equalizing electron flow through PSII and PSI (Allen, 1992); however, the role of phosphorylation of D1 or other PSII proteins is largely unknown. It has variously been suggested that phosphorylation regulates D1 degradation, maintaining it as a storage form prior to its replacement (Rintamäki et al., 1995a), or that it regulates dimerization of the reaction center...