Upstream Open Reading Frame-mediated Upregulation of ANAC082 Expression in Response to Nucleolar Stress in Arabidopsis

Perturbations in ribosome biogenesis cause a type of cellular stress called nucleolar or ribosomal stress, which triggers adaptive responses in both animal and plant cells. The Arabidopsis ANAC082 transcription factor has been identified as a key mediator of the plant nucleolar stress response. The 5'-untranslated region (5'-UTR) of mRNA contains an upstream ORF (uORF) encoding an evolutionarily conserved amino acid sequence. Here, we report that this uORF mediates the upregulation of ANAC082 expression in response to nucleolar stress. When transgenic Arabidopsis plants containing a luciferase reporter gene under the control of the promoter and 5'-UTR were treated with reagents that induced nucleolar stress, expression of the reporter gene was enhanced in a uORF sequence-dependent manner. Additionally, we examined the effect of an endoplasmic reticulum (ER) stress-inducing reagent on reporter gene expression because the closest homolog of in Arabidopsis, , is involved in the ER stress response. However, the uORF did not respond to ER stress. Interestingly, although has a uORF with an amino acid sequence similar to that of the uORF, the C-terminal sequence critical for regulation is not well conserved among homologs in Brassicaceae. Transient expression assays revealed that unlike the uORF, the uORF does not exert a sequence-dependent repressive effect. Altogether, our findings suggest that the uORF is important for the nucleolar stress response but not for the ER stress response, and that for this reason, the uORF sequence-dependent regulation was lost in during evolution.