Functional partitioning of transcriptional regulators by patterned charge blocks.

Components of transcriptional machinery are selectively partitioned into specific condensates, often mediated by protein disorder, yet we know little about how this specificity is achieved. Here, we show that condensates composed of the intrinsically disordered region (IDR) of MED1 selectively partition RNA polymerase II together with its positive allosteric regulators while excluding negative regulators. This selective compartmentalization is sufficient to activate transcription and is required for gene activation during a cell-state transition. The IDRs of partitioned proteins are necessary and sufficient for selective compartmentalization and require alternating blocks of charged amino acids. Disrupting this charge pattern prevents partitioning, whereas adding the pattern to proteins promotes partitioning with functional consequences for gene activation. IDRs with similar patterned charge blocks show similar partitioning and function. These findings demonstrate that disorder-mediated interactions can selectively compartmentalize specific functionally related proteins from a complex mixture of biomolecules, leading to regulation of a biochemical pathway.

Papillae formation on Arabidopsis leaf trichomes requires the function of Mediator tail subunits 2, 14, 15a, 16, and 25.

MAIN CONCLUSION: Arabidopsis Mediator subunits 2, 14, 15a, 16, and 25 are required for papillae development on the trichome cell wall surface. Arabidopsis leaf hairs exhibit raised protrusions, termed papillae, on their cell wall surfaces. Here, we show that the glassy hair mutant, glh2, exhibits trichomes with an approximate 11-fold decrease in papillae density on their surfaces in comparison to wild type. This phenotype was found to be the result of mutations in Arabidopsis Mediator subunit 16. MED16 is localized to the nucleus of trichomes, consistent with Mediator's role in transcription. The expression patterns of the trichome development reporters, ETR2pro::GUS and GL2pro::GUS, as well as GL2 transcript levels were not altered in the glh2 mutant. Screening of available T-DNA insertion lines in other subunits of the Mediator tail module revealed glassy trichome phenotypes in med2, med14, and med15a mutants. The data suggest that the Mediator complex is required for expression of genes involved in trichome papillae development.

Papillae formation on trichome cell walls requires the function of the mediator complex subunit Med25.

KEY MESSAGE: Glassy Hair 1 (GLH1) gene that promotes papillae formation on trichome cell walls was identified as a subunit of the transcriptional mediator complex MED25. The MED25 gene is shown to be expressed in trichomes. The expression of the trichome development marker genes GLABRA2 (GL2) and Ethylene Receptor2 (ETR2) is not affected in the glh1 mutant. Presented data suggest that Arabidopsis MED25 mediator component is likely involved in the transcription of genes promoting papillae deposition in trichomes. The plant cell wall plays an important role in communication, defense, organization and support. The importance of each of these functions varies by cell type. Specialized cells, such as Arabidopsis trichomes, exhibit distinct cell wall characteristics including papillae. To better understand the molecular processes important for papillae deposition on the cell wall surface, we identified the GLASSY HAIR 1 (GLH1) gene, which is necessary for papillae formation. We found that a splice-site mutation in the component of the transcriptional mediator complex MED25 gene is responsible for the near papillae-less phenotype of the glh1 mutant. The MED25 gene is expressed in trichomes. Reporters for trichome developmental marker genes GLABRA2 (GL2) and Ethylene Receptor2 (ETR2) were not affected in the glh1 mutant. Collectively, the presented results show that MED25 is necessary for papillae formation on the cell wall surface of leaf trichomes and suggest that the Arabidopsis MED25 mediator component is likely involved in the transcription of a subset of genes that promote papillae deposition in trichomes.