Arabidopsis MSI1 connects LHP1 to PRC2 complexes

Polycomb group (PcG) proteins form essential epigenetic memory systems for controlling gene expression during development in plants and animals. However, the mechanism of plant PcG protein functions remains poorly understood. Here, we probed the composition and function of plant Polycomb repressive complex 2 (PRC2). This work established the fact that all known plant PRC2 complexes contain MSI1, a homologue of Drosophila p55. While p55 is not essential for the in vitro enzymatic activity of PRC2, plant MSI1 was required for the functions of the EMBRYONIC FLOWER and the VERNALIZATION PRC2 complexes including trimethylation of histone H3 Lys27 (H3K27) at the target chromatin, as well as gene repression and establishment of competence to flower. We found that MSI1 serves to link PRC2 to LIKE HETEROCHROMATIN PROTEIN 1 (LHP1), a protein that binds H3K27me3 in vitro and in vivo and is required for a functional plant PcG system. The LHP1–MSI1 interaction forms a positive feedback loop to recruit PRC2 to chromatin that carries H3K27me3. Consequently, this can provide a mechanism for the faithful inheritance of local epigenetic information through replication.     

EMF genes regulate Arabidopsis inflorescence development.

Mutations in EMBRYONIC FLOWER (EMF) genes EMF1 and EMF2 abolish rosette development, and the mutants produce either a much reduced inflorescence or a transformed flower. These mutant characteristics suggest a repressive effect of EMF activities on reproductive development. To investigate the role of EMF genes in regulating reproductive development, we studied the relationship between EMF genes and the genes regulating inflorescence and flower development. We found that APETALA1 and AGAMOUS promoters were activated in germinating emf seedlings, suggesting that these genes may normally be suppressed in wild-type seedlings in which EMF activities are high. The phenotype of double mutants combining emf1-2 and apetala1, apetala2, leafy1, apetala1 cauliflower, and terminal flower1 showed that emf1-2 is epistatic in all cases, suggesting that EMF genes act downstream from these genes in mediating the inflorescence-to-flower transition. Constitutive expression of LEAFY in weak emf1, but not emf2, mutants increased the severity of the emf phenotype, indicating an inhibition of EMF activity by LEAFY, as was deduced from double mutant analysis. These results suggest that a mechanism involving a reciprocal negative regulation between the EMF genes and the floral genes regulates Arabidopsis inflorescence development.     

The BAM1/BAM2 receptor-like kinases are important regulators of Arabidopsis early anther development

Anther development involves the formation of several adjacent cell types required for normal male fertility. Only a few genes are known to be involved in early anther development, particularly in the establishment of these different cell layers. Arabidopsis thaliana BAM1 (for BARELY ANY MERISTEM) and BAM2 encode CLAVATA1-related Leu-rich repeat receptor-like kinases that appear to have redundant or overlapping functions. We characterized anther development in the bam1 bam2 flowers and found that bam1 bam2 anthers appear to be abnormal at a very early stage and lack the endothecium, middle, and tapetum layers. Analyses using molecular markers and cytological techniques of bam1 bam2 anthers revealed that cells interior to the epidermis acquire some characteristics of pollen mother cells (PMCs), suggesting defects in cell fate specification. The pollen mother-like cells degenerate before the completion of meiosis, suggesting that these cells are defective. In addition, the BAM1 and BAM2 expression pattern supports both an early role in promoting somatic cell fates and a subsequent function in the PMCs. Therefore, analysis of BAM1 and BAM2 revealed a cell-cell communication process important for early anther development, including aspects of cell division and differentiation. This finding may have implications for the evolution of multiple signaling pathways in specifying cell types for microsporogenesis.     

The Polycomb group protein MEDEA and the DNA methyltransferase MET1 interact to repress autonomous endosperm development in Arabidopsis

In flowering plants, double fertilization of the female gametes, the egg and the central cell, initiates seed development to give rise to a diploid embryo and the triploid endosperm. In the absence of fertilization, the FERTILIZATION‐INDEPENDENT SEED Polycomb Repressive Complex 2 (FIS‐PRC2) represses this developmental process by histone methylation of certain target genes. The FERTILIZATION‐INDEPENDENT SEED (FIS) class genes MEDEA (MEA) and FERTILIZATION‐INDEPENDENT ENDOSPERM (FIE) encode two of the core components of this complex. In addition, DNA methylation establishes and maintains the repression of gene activity, for instance via DNA METHYLTRANSFERASE1 (MET1), which maintains methylation of symmetric CpG residues. Here, we demonstrate that Arabidopsis MET1 interacts with MEA in vitro and in a yeast two‐hybrid assay, similar to the previously identified interaction of the mammalian homologues DNMT1 and EZH2. MET1 and MEA share overlapping expression patterns in reproductive tissues before and after fertilization, a prerequisite for an interaction in vivo. Importantly, a much higher percentage of central cells initiate endosperm development in the absence of fertilization in mea‐1/MEA; met1‐3/MET1 as compared to mea‐1/MEA mutant plants. In addition, DNA methylation at the PHERES1 and MEA loci, imprinted target genes of the FIS‐PRC2, was affected in the mea‐1 mutant compared with wild‐type embryos. In conclusion, our data suggest a mechanistic link between two major epigenetic pathways involved in histone and DNA methylation in plants by physical interaction of MET1 with the FIS‐PRC2 core component MEA. This concerted action is relevant for the repression of seed development in the absence of fertilization.     

Condensin I and condensin II proteins form a LINE-1 dependent super condensin complex and cooperate to repress LINE-1

Condensin I and condensin II are multi-subunit complexes that are known for their individual roles in genome organization and preventing genomic instability. However, interactions between condensin I and condensin II subunits and cooperative roles for condensin I and condensin II, outside of their genome organizing functions, have not been reported. We previously discovered that condensin II cooperates with Gamma Interferon Activated Inhibitor of Translation (GAIT) proteins to associate with Long INterspersed Element-1 (LINE-1 or L1) RNA and repress L1 protein expression and the retrotransposition of engineered L1 retrotransposition in cultured human cells. Here, we report that the L1 3′UTR is required for condensin II and GAIT association with L1 RNA, and deletion of the L1 RNA 3′UTR results in increased L1 protein expression and retrotransposition. Interestingly, like condensin II, we report that condensin I also binds GAIT proteins, associates with the L1 RNA 3′UTR, and represses L1 retrotransposition. We provide evidence that the condensin I protein, NCAPD2, is required for condensin II and GAIT protein association with L1 RNA. Furthermore, condensin I and condensin II subunits interact to form a L1-dependent super condensin complex (SCC) which is located primarily within the cytoplasm of both transformed and primary epithelial cells. These data suggest that increases in L1 expression in epithelial cells promote cytoplasmic condensin protein associations that facilitate a feedback loop in which condensins may cooperate to mediate L1 repression.     

Interdependence of PRC1 and PRC2 for recruitment to Polycomb Response Elements

Polycomb Group (PcG) proteins are epigenetic repressors essential for control of development and cell differentiation. They form multiple complexes of which PRC1 and PRC2 are evolutionary conserved and obligatory for repression. The targeting of PRC1 and PRC2 is poorly understood and was proposed to be hierarchical and involve tri-methylation of histone H3 (H3K27me3) and/or monoubiquitylation of histone H2A (H2AK118ub). Here, we present a strict test of this hypothesis using the Drosophila model. We discover that neither H3K27me3 nor H2AK118ub is required for targeting PRC complexes to Polycomb Response Elements (PREs). We find that PRC1 can bind PREs in the absence of PRC2 but at many PREs PRC2 requires PRC1 to be targeted. We show that one role of H3K27me3 is to allow PcG complexes anchored at PREs to interact with surrounding chromatin. In contrast, the bulk of H2AK118ub is unrelated to PcG repression. These findings radically change our view of how PcG repression is targeted and suggest that PRC1 and PRC2 can communicate independently of histone modifications.     

Emx1 and Emx2 cooperate in initial phase of archipallium development

Emx1 and Emx2 are mouse cognates of the Drosophila head gap gene, ems. Previously we have reported that the dentate gyrus is affected in Emx2 single mutants, and defects are subtle in Emx1 single mutants. In most of the cortical region Emx1 and Emx2 functions would be redundant. To test this assumption here we examined the Emx1 and Emx2 double mutant phenotype. In the double mutants the archipallium was transformed into the roof without establishing the signaling center at the cortical hem and without developing the choroid plexus. We propose that Emx1 and Emx2 cooperate in generation of the boundary between the roof and archipallium; these genes develop the archipallium against the roof. This process probably occurs immediately after the neural tube closure concomitant with the Emx1 expression.