Nicotinamide and trigoneUine contents increased in

Pea (Pisum sativum L. cv. Greenfeast) plants were exposed to supplementary ultraviolet‐B (UV‐B) radiation (biologically effective dose rates normalised to 300 nm, UV‐BBE300: 0.18, 0.32 or 1.4 W m−2). Leaf nicotinamide, trigonelline, GSHtot (total glutathione) and GSSG (oxidised glutathione) levels remained unchanged after exposure to the lowest dose rates. 1.4 W m−2 UV‐BBE,300: gave rise to 60‐fold and 4.5‐fold increases in GSSG and GSHtot, respectively. 3.5‐fold and 9.5‐fold increases were found in nicotinamide and trigonelline, respectively, 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 rate of UV‐B, and after exposure to the intermediate dose rate. CHS and PAL mRNAs also increased after prolonged exposure to the lowest dose rate. cah transcripts completely disappeared, whereas CHS and PAL mRNA levels 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 that nicotinamide or trigonelline do not function as signalling compounds for CHS and PAL gene expression. Elevated nicotinamide and trigonelline levels occur in response to UV‐B, but only at UV‐B doses high enough to cause oxidative stress.

The aim of this study was to investigate the regulatory effect of ultraviolet-B (UV-B) radiation on a number of key stress response genes found in the epidermis and mesophyll of Pisum sativum L., Argenteum mutant. This mutant was chosen for the ease with which the entire epidermis can be removed from the mesophyll tissue. An additional goal was to explore the potential modifying effect of pre-acclimation of plants to UV-B radiation prior to exposure by UV-B during treatment. Results showed that mRNA accumulation was similar during acute short-term UV-B exposure for chalcone synthase ( Chs ) and short-chain alcohol dehydrogenase ( SadA ) in both epidermis and mesophyll. In contrast, the mRNA levels differed considerably between tissues for phenylalanine ammonia lyase, chalcone isomerase and lipid transfer protein. After 24 h incubation in visible light after cessation of UV-B exposure, the regulation of mRNA levels also differed between Chs and SadA , the former showing no expression in the epidermis and the latter none in the mesophyll. Acclimation to low UV-B levels before acute exposures resulted in delayed induction of Chs and SadA . Measurements of UV-Binduced cyclobutane pyrimidine dimers (CPDs) showed a greater formation in epidermis than in mesophyll. In addition, acclimation at low UV-B levels resulted in significantly higher basal levels of CPDs than in non-acclimated plants in both mesophyll and epidermis and also in increased damage in concomitant acute exposures. The lack of correlation between the number of CPDs and levels of transcripts for defence genes, indicates that DNA damage does not control transcription of these genes.

Fluence-response curves were obtained at nine wavelengths in the interval 280–360 nm for mRNA transcripts of four molecular markers induced by ultraviolet-B (UV-B) radiation in Arabidopsis thaliana (L.) Heynh.: CHS (encoding chalcone synthase), PDX1.3 (encoding an enzyme involved in formation of pyridoxine), MEB5.2 (encoding a protein with unknown function but which is strongly upregulated by UV-B), and LHCB1*3 (encoding a chlorophyll a/b binding protein). Intact Arabidopsis plants were irradiated for 3 h using a high intensity deuterium radiation source and narrow bandwith filters without supplementary PAR. The results obtained suggest the existence of two distinct UV-B signal responses: one sensitive between 300 and 310 nm and the other sensitive around 280–290 nm. Among the investigated molecular markers, CHS and PDX1.3 were regulated through the chromophore absorbing around 300 nm, whereas MEB5.2 and LHCB1*3 were regulated through the chromophore absorbing at 280–290 nm. The results obtained show that at least two signal transduction pathways exist that regulate gene expression as a result of absorption of UV-B radiation in plants.

Bioassays are valuable for studying environmental transformation processes like this, where chemical analysis and subsequent toxicological evaluation requires available standard compounds and information on toxicological potency. The use of bioassays allows a rapid evaluation of toxicological relevance.

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