Ultraviolet‐B‐induced responses in Arabidopsis thaliana: role of salicylic acid and reactive oxygen species in the regulation of transcripts encoding photosynthetic and acidic pathogenesis‐related proteins

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Abstract
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.