MED8 keeps H₂O₂-dependent gene expression under control

He et al. demonstrate that Mediator complex subunits have functions beyond their known roles in transcription. Plant Cell https://doi.org/10.1093/plcell/koab079

By Huaming He, Patrick Willems, Frank Van Breusegem, and Amna Mhamdi

Background: Plants often face challenging environments to which they respond by adjusting gene transcription or protein activity. Reactive oxygen species (ROS) are metabolites spontaneously produced by plants that reflect physiological changes elicited by various developmental and stress contexts. ROS, such as hydrogen peroxide (H₂O₂), act together with different metabolic pathways involving additional signals such as the phytohormones salicylic acid and jasmonic acid. Changes in gene expression driven by H₂O₂ are well characterized; however, the components that regulate H₂O₂ -dependent gene expression remain largely enigmatic. To address this issue, we performed a genetic screen using Arabidopsis (Arabidopsis thaliana) plants expressing a highly and rapidly inducible H₂O₂ promoter line.

Question: How does MED8 affect H₂O₂-dependent gene expression and stress responses?

Findings: We show that a mutation in the Mediator subunit MED8 confers tolerance to oxidative stress triggered by genetic and pharmacological means. We demonstrate that MED8 mainly functions as a repressor of H₂O₂-induced gene expression, negatively regulating pathways that are dependent on salicylic acid and jasmonic acid. Our results support a scenario wherein MED8 achieves its role by interacting with other mediator subunits and also with components that are not required for the known roles of the Mediator complex in transcription, such as NOT2, a key regulator of microRNA biogenesis.

Next steps: One of our immediate prospects is to identify the components implicated in the regulatory node by which MED8 achieves tight control over H₂O₂-dependent gene expression. Next, we will define the specificities of the different Mediator subunits in regulating oxidative stress. For a future challenge, we aim for a comprehensive overview of the Mediator function beyond transcription.

Huaming He, Jordi Denecker, Katrien Van Der Kelen, Patrick Willems, Robin Pottie, Su Yin Phua, Matthew A. Hannah, Didier Vertommen, Frank Van Breusegem, and Amna Mhamdi. (2021). The Arabidopsis Mediator Complex Subunit 8 Regulates Oxidative Stress Responses. Plant Cell. https://doi.org/10.1093/plcell/koab079