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

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Abstract
The effect of ultraviolet‐B radiation (UV‐B: 280–320nm) on gene expression and pigment composition has been investigated in pea tissue at different stages of development. Pea (Pisum sativum L., cv. Feltham First) seedlings were grown for 17d and then exposed to supplementary UV‐B radiation. Chlorophyll a per unit fresh weight decreased by more than 20% compared with control levels after exposure to UV‐B radiation for 7d. In contrast, chlorophyll b content remained the same or increased slightly. Leaf protein biosynthesis, as determined by 35S‐methionine incorporation, was rapidly inhibited by UV‐B radiation, although the steady‐state levels of proteins were either unchanged or only slightly altered. RNA transcripts for the chlorophyll a/b binding protein (cab) 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 undetectable in etiolated pea tissue, but increased substantially in light and during exposure to UV‐B radiation. The cab RNA transcripts were still present at control levels in pea plants after 7d of greening under supplementary UV‐B radiation or UV‐B alone. The protein composition changed significantly over the 7d of greening, but no differences could be detected between the light treatments. The increase in chlorophyll content was slightly greater during de‐etiolation under supplementary UV‐B radiation than under control irradiance. Under UV‐B radiation alone, chlorophyll was synthesized at a greatly reduced rate. Changes in protective pigments were also determined. Anthocyanins did not change in either etiolated or green tissue exposed to UV‐B radiation. However, other flavonoids increased substantially in either tissue during exposure 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 declined to lower levels over subsequent days of diurnal photoperiods. However, a constant increase in chs was found after continuous exposure to UV‐B for up to 30 h. In etiolated tissue, either white‐light, supplementary UV‐B or UV‐B alone gave small increases in chs, only 8 h of UV‐B radiation alone gave any substantial increase in chs expression. Overall, these results clearly demonstrate that the response to increased levels of UV‐B radiation is dependent upon the developmental stage of the tissue and involves complex changes in gene expression.