(Dedicated to Professor Klaus Hahlbrock on the occasion of his 60th birthday) SUMM.^RY Epidermal tissue was isolated from Scots pine {Pinus sylvestris L.) needles by enzymatic digestion in order to study tissue distribution of u.v,-B-screening pigments. Up to 90 "o ofthe needle content of a group of diacylated flavonoi glycosides tbat were structurally closely related was found in the epidermal layer. Among these metabolites. 3",6"-di-para-coumaroyl-isoquerfitrin and 3",6"-di-para-coum3roy]-astragal in were the main u.v.-B-induced compounds in cot\ ledons and primary needles, respectively. However, catechin and astragalin (kaempferol 3-glucoside), two non-acylated fla\'onoid metabolites, were only observed in total needle extracts, and at levels independent of u.v.-B treatment. According to this metabolite distribution, tbe mRNA of chalcone syntbase, the key enzyme to flavonoids, was found in epidermal and mesophyll as well as vascular tissues. The major alkaliextractable wall-hound phenolic metabolites, astragalin, 4-coumaric acid, and ferulic acid, a minor component of tbe cell wall, were also found exclusively in the epidermal layer. These compounds were not stimulated by u.v.-B irradiation within the experimental period. Staining of needle cross sections and epidermal layer preparations with NaturstofTreagenz A confirmed the specific localization of wall-bound astragalin in the outer wall of the epidermal layer. Model calculations of u.v.-B absorptions at 300 nm of soluble and cell-wall-bound metabolites of the epidermal layer revealed an almost complete shielding of the mesophyll tissue from u.v.-B radiation.

No abstract

No abstract

No abstract

UV-B-induced perturbations of cell cycle progression in asynchronous human keratinocytes were analysed during two cell cycles with respect to their cell cycle stage at the time of irradiation using BrdUrd/Hoechst flow cytometry. Exponentially growing SCL-2-keratinocytes exposed to UV-B radiation showed a short delay in G1-phase exit and were blocked in the S and G2/M phases of the first cell cycle. UV-A wavelengths did not show any detectable effect on cell cycle progression. In contrast, 137Cs-irradiation of these cells induced a temporary G2 block only. Micronucleus frequency increased in gamma-irradiated cells as soon as the cells started to divide and reached a plateau when most of the cells had divided. Continuous treatment with caffeine starting immediately after 137Cs gamma-irradiation prevented accumulation of cells in G2 phase, but did not influence the frequency of micronuclei. In UV-B-irradiated keratinocytes, however, the damage-induced cell cycle perturbations were merely reduced by caffeine, but not eliminated. Compared with gamma-irradiation a moderate induction of micronuclei was observed in UV-B-irradiated cells. Caffeine, however, potentiated the induction of micronuclei by UV-B. These different effects on cell cycle kinetics and micronucleus induction indicate different mechanisms of DNA damage caused by UV-B- and gamma-irradiation that may be repaired through different pathways.