Genome-wide occupancy of histone H3K27 methyltransferases CURLY LEAF and SWINGER in Arabidopsis seedlings

Late Embryogenesis Abundant Protein-Client Protein Interactions

The intrinsically disordered proteins belonging to the LATE EMBRYOGENESIS ABUNDANT protein (LEAP) family have been ascribed a protective function over an array of intracellular components. We focus on how LEAPs may protect a stress-susceptible proteome. These examples include instances of LEAPs providing a shield molecule function, possibly by instigating liquid-liquid phase separations. Some LEAPs bind directly to their client proteins, exerting a holdase-type chaperonin function. Finally, instances of LEAP-client protein interactions have been documented, where the LEAP modulates (interferes with) the function of the client protein, acting as a surreptitious rheostat of cellular homeostasis. From the examples identified to date, it is apparent that client protein modulation also serves to mitigate stress. While some LEAPs can physically bind and protect client proteins, some apparently bind to assist the degradation of the client proteins with which they associate. Documented instances of LEAP-client protein binding, even in the absence of stress, brings to the fore the necessity of identifying how the LEAPs are degraded post-stress to render them innocuous, a first step in understanding how the cell regulates their abundance.

Active and repressed biosynthetic gene clusters have spatially distinct chromosome states

While colocalization within a bacterial operon enables coexpression of the constituent genes, the mechanistic logic of clustering of nonhomologous monocistronic genes in eukaryotes is not immediately obvious. Biosynthetic gene clusters that encode pathways for specialized metabolites are an exception to the classical eukaryote rule of random gene location and provide paradigmatic exemplars with which to understand eukaryotic cluster dynamics and regulation. Here, using 3C, Hi-C, and Capture Hi-C (CHi-C) organ-specific chromosome conformation capture techniques along with high-resolution microscopy, we investigate how chromosome topology relates to transcriptional activity of clustered biosynthetic pathway genes in Arabidopsis thaliana Our analyses reveal that biosynthetic gene clusters are embedded in local hot spots of 3D contacts that segregate cluster regions from the surrounding chromosome environment. The spatial conformation of these cluster-associated domains differs between transcriptionally active and silenced clusters. We further show that silenced clusters associate with heterochromatic chromosomal domains toward the periphery of the nucleus, while transcriptionally active clusters relocate away from the nuclear periphery. Examination of chromosome structure at unrelated clusters in maize, rice, and tomato indicates that integration of clustered pathway genes into distinct topological domains is a common feature in plant genomes. Our results shed light on the potential mechanisms that constrain coexpression within clusters of nonhomologous eukaryotic genes and suggest that gene clustering in the one-dimensional chromosome is accompanied by compartmentalization of the 3D chromosome.

RNA polymerase II-independent recruitment of SPT6L at transcription start sites in Arabidopsis

SPT6 is a conserved elongation factor that is associated with phosphorylated RNA polymerase II (RNAPII) during transcription. Recent transcriptome analysis in yeast mutants revealed its potential role in the control of transcription initiation at genic promoters. However, the mechanism by which this is achieved and how this is linked to elongation remains to be elucidated. Here, we present the genome-wide occupancy of Arabidopsis SPT6-like (SPT6L) and demonstrate its conserved role in facilitating RNAPII occupancy across transcribed genes. We also further demonstrate that SPT6L enrichment is unexpectedly shifted, from gene body to transcription start site (TSS), when its association with RNAPII is disrupted. Protein domains, required for proper function and enrichment of SPT6L on chromatin, are subsequently identified. Finally, our results suggest that recruitment of SPT6L at TSS is indispensable for its spreading along the gene body during transcription. These findings provide new insights into the mechanisms underlying SPT6L recruitment in transcription and shed light on the coordination between transcription initiation and elongation.

Inhibition of Polycomb Repressive Complex 2 activity reduces trimethylation of H3K27 and affects development in Arabidopsis seedlings

BACKGROUND

Polycomb repressive complex 2 (PRC2) is an epigenetic transcriptional repression system, whose catalytic subunit (ENHANCER OF ZESTE HOMOLOG 2, EZH2 in animals) is responsible for trimethylating histone H3 at lysine 27 (H3K27me3). In mammals, gain-of-function mutations as well as overexpression of EZH2 have been associated with several tumors, therefore making this subunit a suitable target for the development of selective inhibitors. Indeed, highly specific small-molecule inhibitors of EZH2 have been reported. In plants, mutations in some PRC2 components lead to embryonic lethality, but no trial with any inhibitor has ever been reported.

RESULTS

We show here that the 1,5-bis (3-bromo-4-methoxyphenyl)penta-1,4-dien-3-one compound (RDS 3434), previously reported as an EZH2 inhibitor in human leukemia cells, is active on the Arabidopsis catalytic subunit of PRC2, since treatment with the drug reduces the total amount of H3K27me3 in a dose-dependent fashion. Consistently, we show that the expression level of two PRC2 targets is significantly increased following treatment with the RDS 3434 compound. Finally, we show that impairment of H3K27 trimethylation in Arabidopsis seeds and seedlings affects both seed germination and root growth.

CONCLUSIONS

Our results provide a useful tool for the plant community in investigating how PRC2 affects transcriptional control in plant development.

The Chromatin-Remodeling Factor PICKLE Antagonizes Polycomb Repression of FT to Promote Flowering

Changing daylength (or photoperiod) is a seasonal cue used by many plants to adjust the timing of their floral transition to ensure reproductive success. An inductive long-day photoperiod triggers the expression of FLOWERING LOCUS T (FT), which promotes flowering. FT, encoding a major component of florigen, is induced in leaf veins specifically at dusk through the photoperiod pathway; however, the modulation of FT expression in response to photoperiod cues remains poorly understood. Here, we report that the balance between Polycomb group (PcG) and Trithorax group (TrxG) proteins sets appropriate FT expression in long days in Arabidopsis (Arabidopsis thaliana). In PcG mutant lines, FT was highly derepressed, but FT expression was decreased to an almost wild-type level and pattern upon the additional disruption of chromatin-remodeling factors PICKLE (PKL) and ARABIDOPSIS HOMOLOG OF TRITHORAX1 (ATX1), but not by disruption of photoperiod pathway components. PKL interacts with ATX1 to mediate trimethylation of histone H3 on lysine-4 at the FT locus, leading to antagonistic effects of PKL and ATX1 on PcG proteins in the regulation of FT expression. Therefore, the TrxG-like protein PKL prevents PcG-mediated silencing to ensure specific and appropriate expression of FT, thereby determining the proper flowering response.

Arabidopsis EMSY-like (EML) histone readers are necessary for post-fertilization seed development, but prevent fertilization-independent seed formation

Seed development is a complex regulatory process that includes a transition from gametophytic to sporophytic program. The synchronized development of different seed compartments (seed coat, endosperm and embryo) depends on a balance in parental genome contributions. In the most severe cases, the disruption of the balance leads to seed abortion. This represents one of the main obstacles for the engineering of asexual reproduction through seeds (apomixis), and for generating new interspecies hybrids. The repression of auxin synthesis by the Polycomb Repressive Complex 2 (PRC2) is a major mechanism contributing to sensing genome balance. However, current efforts focusing on decreasing PRC2 or elevating auxin levels have not yet resulted in the production of apomictic seed. Here, we show that EMSY-Like Tudor/Agenet H3K36me3 histone readers EML1 and EML3 are necessary for early stages of seed development to proceed at a normal rate in Arabidopsis. We further demonstrate that both EML1 and EML3 are required to prevent the initiation of seed development in the absence of fertilization. Based on the whole genome expression analysis, we identify auxin transport and signaling genes as the most enriched downstream targets of EML1 and EML3. We hypothesize that EML1 and EML3 function to repress and soften paternal gene expression by fine-tuning auxin responses. Discovery of this pathway may contribute to the engineering of apomixis and interspecies hybrids.