THE EFFECT OF ULTRAVIOLET-B RADIATION ON GENE-EXPRESSION AND PIGMENT COMPOSITION IN ETIOLATED AND GREEN PEA LEAF TISSUE - UV-B-INDUCED CHANGES ARE GENE-SPECIFIC AND DEPENDENT UPON THE DEVELOPMENTAL STAGE

The effect of ultraviolet-B radiation (UV-B: 280-320 nm) on gene expre ssion and pigment composition has been investigated in pea tissue at d ifferent stages of development. Pea (Pisum sativum L., cv. Feltham Fir st) seedlings were grown for 17d and then exposed to supplementary UV- B radiation. Chlorophyll a per unit fresh weight decreased by more tha n 20% compared with control levels after exposure to UV-B radiation fo r 7d. In contrast, chlorophyll b content remained the same or increase d slightly. Leaf protein biosynthesis, as determined by S-35-methionin e incorporation, was rapidly inhibited by UV-B radiation, although the steady-state levels of proteins were either unchanged or only slightl y altered. RNA transcripts for the chlorophyll a/b binding protein (ca b) were also rapidly reduced to low or even undetectable levels in the expanded third leaf or younger leaf bud tissue after exposure to UV-B radiation. In contrast, cab RNA transcripts were either low or undete ctable in etiolated pea tissue, but increased substantially in light a nd during exposure to UV-B radiation. The cab RNA transcripts were sti ll present at control levels in pea plants after 7d of greening under supplementary UV-B radiation or W-B alone. The protein composition cha nged significantly over the 7d of greening, but no differences could b e detected between the light treatments. The increase in chlorophyll c ontent was slightly greater during de-etiolation under supplementary U V-B radiation than under control irradiance. Under UV-B radiation alon e, chlorophyll was synthesized at a greatly reduced rate. Changes in p rotective pigments were also determined. Anthocyanins did not change i n either etiolated or green tissue exposed to UV-B radiation. However, other flavonoids increased substantially in either tissue during expo sure to light and UV-B radiation. The RNA levels for chalcone synthase were measured in green and etiolated tissue exposed to UV-B radiation . The chs RNA transcripts were present in low or undetectable amounts in control tissues. In green leaf tissue exposed to supplementary UV-B , a transient increase was detected. The transcripts for chs reached a maximum level after approximately 8 h UV-B exposure, and then decline d to lower levels over subsequent days of diurnal photoperiods. Howeve r, a constant increase in chs was found after continuous exposure to U V-B for up to 30h. In etiolated tissue, either white-light, supplement ary UV-B or UV-B alone gave small increases in chs, only 8 h of UV-B r adiation alone gave any substantial increase in chs expression. Overal l, these results clearly demonstrate that the response to increased le vels of UV-B radiation is dependent upon the developmental stage of th e tissue and involves complex changes in gene expression.