Oxidative stress is H₂O₂ under the bridge without MED8

In plants challenged by episodic stresses, the expression of genes required for stress tolerance can impinge on growth and development. In favorable conditions, plants are better off repressing these genes. This regulatory role has now been assigned to MEDIATOR SUBUNIT 8 (MED8), a component of the Mediator complex that acts as a molecular bridge between transcription regulators and RNA polymerase. MED8 is required to inhibit hydrogen peroxide (H₂O₂)-related gene expression.

In addition to being damaging, reactive oxygen species (ROS) can act as signaling molecules. The involvement of ROS in processes like stress responses and gene expression modulation has been extensively documented (Waszczak et al., 2018). Remarkably, accumulation of different types of ROS can trigger transcriptional changes specific to each distinct ROS. For instance, perturbing the levels of H₂O₂, one of the most abundant ROS in cells, affects pathways involving various phytohormones such as the defense-related salicylic acid (SA) and jasmonic acid (JA). However, the precise mechanisms by which H₂O₂ influences transcriptional activation of defense pathways remain unclear.

In this issue, He et al. (2021) used a genetic screen to identify oxidative stress response regulators. The authors present the identification of one mutant that causes constitutive and enhanced expression of EAL4, a highly and rapidly H₂O₂-inducible gene. They reasoned that this mutant was likely to be impaired in a negative regulator of oxidative stress-related gene expression. With whole-genome sequencing, the authors mapped the mutation in this line to a pre-mature stop codon in MED8, leading to a truncated MED8 protein (MED8ΔQ) lacking the last 147 C-terminal amino acids.

MED8 is a component of the multi-subunit Mediator complex, a conserved eukaryotic transcriptional regulator (Bourbon, 2008). The Mediator complex interacts with a multitude of transcription factors and RNA polymerase, conveying signals from specific transcription factors to the polymerase. In this way, external and internal cues are decoded into physiological responses. To grasp the physiological function of MED8 with regards to H₂O₂, the authors conducted transcriptomic analyses comparing med8 mutants (i.e. homozygous MED8ΔQ plants) and wild-type plants. MED8ΔQ did not have an extensive genome-wide impact on gene expression in mock conditions. On the other hand, in oxidative stress conditions, MED8ΔQ led to a dramatically altered transcriptional response compared to wild-type plants. The transcriptional signature of med8 upon oxidative stress mainly dwells in the activation of defense and hormone pathways like SA and JA. In other words, MED8 inhibits these defense pathways in wild-type plants. In fact, genetic evidence showed that med8 mutants are more tolerant to oxidative stress than wild-type plants and that SA biosynthesis and signaling are required for this phenotype.

As the Mediator complex acts as hub in protein interaction networks, the authors hypothesized that the phenotype of med8 originates from an impairment in MED8ΔQ interactome. Consequently, protein–protein interaction analyses were carried out to generate a protein interactome network map (see Figure 1). This approach aimed to shed light on novel roles for the Mediator complex and more precisely on how MED8 inhibits H₂O₂ tolerance and SA-related defense pathways. More than a hundred MED8 interactors were identified, including two known negative regulators of SA-related defense NFXL1 and CDK8. Interestingly, the interactomes of the full-length MED8 and MED8ΔQ largely overlapped (84%), suggesting that MED8ΔQ can still integrate the Mediator complex. The med8 phenotype may therefore result from one or a few specific impaired interaction(s). Interestingly, a transcription regulator involved in miRNA biogenesis named NOT2 displayed a weaker interaction with MED8ΔQ compared to full-length MED8. Remarkably, the similarity of not2 and med8 phenotypes suggests that MED8 may function at least partially via its interaction with NOT2.

Figure.

MED8 and the Mediator complex have a multitude of interacting partners involved in numerous biological and cellular processes. Interactome of the Mediator complex detected by IP–MS experiments with MED8 (He et al., 2021; gray), reported in plants (String database; blue) and detected by Maji et al. (2019; red) (from He et al. [2021], Figure 8).

He et al. (2021) demonstrate here that MED8 acts as a negative regulator of H₂O₂ tolerance and largely operates via the repression of the SA pathway. Nevertheless, given the diversity of its interactors (see Figure), MED8 is surely involved in the regulation of numerous other pathways, some being touched upon here like miRNA biogenesis. This pleiotropy will undoubtedly constitute an exciting challenge for future studies aiming to uncouple these MED8-regulated pathways.