Arabidopsis mutants representing each of the four Mediator modules reveal unique functions in the transcriptional response to salt stress

Changes in growth environment trigger stress responses in most organisms. The mechanisms mediating these responses are only partly understood and involve signaling pathways and transcription factors. Mediator is a conserved co-regulator complex required for transcriptional regulation of all eukaryotic protein-encoding genes. However, its function in abiotic stress responses is elusive. We here describe global gene expression changes triggered by salt stress in Arabidopsis. To explore the involvement of Mediator in salt stress response we characterized med9, med16, med18, and cdk8 mutants representing each of the four modules of Mediator. Our transcriptome data revealed enrichment of shared and specific cis-elements corresponding to unique transcription factors in promoters of mis-regulated genes for each mutant. We show that individual Mediator subunits interact with specific transcription factors to generate a transcriptional stress response and that the mutant phenotypes support the transcriptome data. med16 and med18, and to some extent cdk8, display defects in abscisic acid and anthocyanin metabolism and we identify signal molecules, transcription factors and target genes involved in these pathways as dysregulated in the Mediator mutants. Our results reveal how signals from different stress response pathways are dependent on and integrated by Mediator subunits to coordinate a functional response to salt stress.