ULTRAVIOLET-B-RADIATION-INDUCED CHANGES IN NICOTINAMIDE AND GLUTATHIONE METABOLISM AND GENE-EXPRESSION IN PLANTS

Pea (Pisum sativum L. cv. Greenfeast) plants were exposed to supplemen tary ultraviolet-B (UV-B) radiation (biologically effective dose rates normalised to 300 nm UV-B-BE,B-300: 0.18, 0.32 or 1.4 W m(-2)). Leaf nicotinamide, trigonelline, GSH(tot) (total glutathione) and (GSSG (ox idised glutathione) levels remained unchanged after exposure to the lo west dose rates. 1.4 W m(-2) UV-B-BE,B-300 gave rise to 60-fold and 4. 5-fold increases in GSSG and GSH(tot), respectively. 3.5-fold and 9.5- fold increases were found in nicotinamide and trigonelline, respective ly. cab (Chlorophyll-a/b-binding protein) transcript levels decreased and CHS (chalcone synthase) and PAL (phenylalanine ammonia-lyase) mRNA increased after shorter UV-B exposures (hours) to the higher dose rat e of UV-B and after exposure to the intermediate dose rate. CHS and PA L mRNAs also increased after prolonged exposure to the lowest dose rat e. cab transcripts completely disappeared. whereas CHS and PAL mRNA le vels rose by 60-fold and 17-fold, respectively, after 12 h exposure at the highest dose rate and 12 h of development. Our results indicate t hat nicotinamide or trigonelline do not function as signalling compoun ds for CHS and PAL gene expression. Elevated nicotinamide and trigonel line levels occur in response to UV-B, but only at UV-B doses high eno ugh to cause oxidative stress.