Arabidopsis mutants for Mediator Head, Middle, Tail, and Kinase modules reveal distinct roles in regulating the transcriptional response to salt stress

• med9, med16, med18 and cdk8 Mediator mutants are sensitive than to salt stress • med9, med16, med18 and cdk8 display defects in ion leakage, leaf greening and root growth • Promoters of med9, med16, med18 and cdk8 DEGs show enrichment for specific TF sites • med16, med18 , and cdk8 , exhibit defects in ABA and anthocyanin metabolism Environmental changes trigger stress responses in living organisms. Although the underlying mechanisms are only partly understood, they involve intricate signaling pathways and transcription factors (TFs). 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. Here, we describe global gene expression changes induced by salt stress in Arabidopsis thaliana . To investigate the involvement of Mediator, we analyzed med9, med16, med18 , and cdk8 mutants, each representing one of the four Mediator modules. Our results demonstrate that promoters of differentially expressed genes (DEGs) for each mutant are enriched for binding sites of specific TFs. Phenotypic analyses further support the transcriptomic data: med16 and med18 , and to a lesser extent cdk8 , exhibit defects typical to mutations that affect abscisic acid and anthocyanin metabolism and we identify dysregulated signaling molecules, TFs, and target genes in these pathways. 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.