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 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.