Treatment with supplementary UV‐B resulted in decreases in transcripts of the photosynthetic genes Lhcb and psbA and concomitant increase in transcripts of two pathogen‐related genes, PR‐1 and PDF1·2, in Arabidopsis thaliana. UV‐B exposure caused increases in jasmonic acid (JA) levels and ethylene production. UV‐B treatment of jar1 and etr1‐1 mutants, which are insensitive to JA and ethylene, respectively, showed that the increase in PR‐1 transcripts was dependent on ethylene and PDF1·2 transcripts on both JA and ethylene. In contrast, the down‐regulation of photosynthetic transcripts was independent of both compounds. Previous studies have indicated a role for reactive oxygen species (ROS) in the UV‐B‐induced down‐regulation of the photosynthetic genes and up‐regulation of PR‐1 genes. Here we have shown that ROS are also required for the UV‐B‐induced up‐regulation of PDF1·2 genes. The results indicate that the effects of UV‐B on the three sets of genes are mediated through three distinct signal transduction pathways which are similar, but not identical, to pathways initiated in response to pathogen infection. In addition, the increased sensitivity of both jar1 and etr1‐1 mutants to UV‐B radiation, as compared with wild‐type plants, indicated that intact JA and ethylene signal pathways are required for defence against UV‐B damage.

Supplementary UV-B was shown to lead to a decrease in transcripts encoding the photosynthetic genes Lhcb and psbA and a concomitant increase in transcripts encoding three acid-type pathogenesis-related proteins, PR-1, PR-2 and PR-5, in Arabidopsis thaliana. UV-B radiation has been reported to lead to the generation of reactive oxygen species (ROS). Here we report that ROS are required for UV-B-induced down-regulation of the photosynthetic genes and up-regulation of PR genes, as the addition of antioxidants before UV-B treatment resulted in a marked reduction in the effect of UV-B on both sets of genes. Rises in ROS are frequently accompanied by increases in salicylic acid (SA) accumulation. UV-B treatment of transgenic NahG Arabidopsis plants, which are unable to accumulate SA, showed that the increase in PR transcripts, but not the decrease in photosynthetic transcripts, was dependent on the increase in SA. In addition, a 3 d exposure to UV-B radiation resulted in a 7-fold increase in SA levels. Oxidant treatment of NahG plants indicated that ROS could not up-regulate PR genes in the absence of SA accumulation; however, the down-regulation of photosynthetic transcripts was unchanged from that in wild-type plants. The results indicate that the effects of UV-B on the two sets of genes are mediated through two distinct signal tranduction pathways. One pathway is ROS-dependent but SA-independent and mediates the down-regulation of photosynthetic genes. The other is SA-and ROS-dependent and mediates the up-regulation of the acidic-type PR genes.

No abstract

The relationship between UV-B-induced changes in gene expression and carbohydrate levels in pea seedlings has been hivestigated. The effect of supplementary UV-B radiation on the transcript abundance for two photosynthetic genes, photosynthesis, respiration and the levels of carbohydrates was determined in fully expanded leaves of 17-dold pea seedlings nnder high (HL: 350 ^/mol m"^ s~') and low (LL: 150//mol m"^ s"') light. Supplementary UV-B eaused downregulation of the photosynthetic genes in green leaves under LL and to a lesser extent under HL. In contrast to previous studies, UV-B radiation resulted in a decrease in glucose levels rather than an increase under LL. Sucrose and starch levels were not affected until longer exposure. Effects of UV-B on carbohydrate levels were, however, minimal under HL. The effects on transcript levels were most marked under LL and therefore could not he attributed to elevated carhohydrate levels. Comparison of UV-B effects on carbohydrates in source (leal) and sink (green huds) organs indicated that changes in carbohydrates in response to UV-B are probably indirect and arise from effects of UV-B on photosynthesis in source organs.

Abstract— Supplementary UVB radiation caused the down‐regulation of transcripts of the photosynthetic genes Lhcb and psbA in fully expanded pea leaves. This response was less marked in apical buds and was ameliorated by high light. The effect of UVB on leaves was mimicked by free radical generators and prevented by antioxidant feeding. Under UVB, active oxygen species levels, determined by level of lipid peroxides, was highest in leaves under low light and lower under high light and in buds. Activities of ascorbate peroxidase, glutathione reductase and superoxide dismutase were increased in response to UVB irradiation and were higher under high light and in buds. These results indicate that active oxygen species are involved in the UVB signal pathway leading to the down‐regulation of photosynthetic genes. Furthermore, they suggest that the antioxidant capacity of plant tissue determines the susceptibility of these transcripts to UVB radiation.