The effect of supplementary ultraviolet-B radiation on mRNA transcripts, translation and stability of chloroplast proteins and pigment formation in Pisum sativum L.

The effect of supplemental UV-B radiation on gene expression for three photosynthetic proteins (chlorophyll a/b-binding protein, D1 polypeptide of PS II and RUBISCO) and on flavonoid composition has been investigated in expanded leaves and in apical buds of pea seedlings. In the expanded third leaves, UV-B caused a decrease in nuclear-encoded Lhcb and RbcS transcript levels within 2 d of UV-B treatment and a later decline in the SSU and LHCB polypeptides. The effect of UV-B on the chloroplast-encoded rbcL and psbA genes was more complex. Levels of LSU and D1 initially appeared to be regulated at the translational and/or post-translational level and only later in the UV-B treatment, at the level of mRNA abundance. UV-B also accelerated the degradation of the D1 polypeptide. For all genes studied the UV-B-induced inhibition of transcripts was greater in leaves than in the apical buds, indicating that the bud is less sensitive to UV-B damage. The mRNA transcript levels for chalcone synthase (chs) were also measured. In leaves, chs RNA transcripts were initially present in low amounts and exposure to supplemental UV-B resulted in a transient increase in the level of these transcripts. In contrast, chs transcripts in the apical buds were initially present at high levels, with UV-B exposure resulting in a slow progressive decline in transcript levels. These results indicate that the response to UV-B is complex and is dependent on the organ studied.