Whole-genome analysis of histone H3 lysine 27 trimethylation in Arabidopsis

Trimethylation of histone H3 lysine 27 (H3K27me3) plays critical roles in regulating animal development, and in several cases, H3K27me3 is also required for the proper expression of developmentally important genes in plants. However, the extent to which H3K27me3 regulates plant genes on a genome-wide scale remains unknown. In addition, it is not clear whether the establishment and spreading of H3K27me3 occur through the same mechanisms in plants and animals. We identified regions containing H3K27me3 in the genome of the flowering plant Arabidopsis thaliana using a high-density whole-genome tiling microarray. The results suggest that H3K27me3 is a major silencing mechanism in plants that regulates an unexpectedly large number of genes in Arabidopsis (~4,400), and that the maintenance of H3K27me3 is largely independent of other epigenetic pathways, such as DNA methylation or RNA interference. Unlike in animals, where H3K27m3 occupies large genomic regions, in Arabidopsis, we found that H3K27m3 domains were largely restricted to the transcribed regions of single genes. Furthermore, unlike in animals systems, H3K27m3 domains were not preferentially associated with low-nucleosome density regions. The results suggest that different mechanisms may underlie the establishment and spreading of H3K27me3 in plants and animals

Electron Energy Partition across Interplanetary Shocks. III. Analysis

An analysis of model fit results of 15,210 electron velocity distribution functions (VDFs), observed within 2 hr of 52 interplanetary (IP) shocks by the Wind spacecraft near 1 au, is presented as the third and final part on electron VDFs near IP shocks. The core electrons and protons dominate in the magnitude and change in the partial-to-total thermal pressure ratio, with the core electrons often gaining as much or more than the protons. Only a moderate positive correlation is observed between the electron temperature and the kinetic energy change across the shock, while weaker, if any, correlations were found with any other macroscopic shock parameter. No VDF parameter correlated with the shock normal angle. The electron VDF evolves from a narrowly peaked core with flaring suprathermal tails in the upstream to either a slightly hotter core with steeper tails or much hotter flattop core with even steeper tails downstream of the weaker and strongest shocks, respectively. Both quasi-static and fluctuating fields are examined as possible mechanisms modifying the VDF, but neither is sufficient alone. For instance, flattop VDFs can be generated by nonlinear ion acoustic wave stochastic acceleration (i.e., inelastic collisions), while other work suggested they result from the combination of quasi-static and fluctuating fields. This three-part study shows that not only are these systems not thermodynamic in nature; even kinetic models may require modification to include things like inelastic collision operators to properly model electron VDF evolution across shocks or in the solar wind.Peer reviewe

Electron Energy Partition across Interplanetary Shocks. II. Statistics

A statistical analysis of 15,210 electron velocity distribution function (VDF) fits, observed within +/- 2 hr of 52 interplanetary (IP) shocks by the Wind spacecraft near 1 au, is presented. This is the second in a three-part series on electron VDFs near IP shocks. The electron velocity moment statistics for the dense, low-energy core, tenuous, hot halo, and field-aligned beam/strahl are a statistically significant list of values illustrated with both histograms and tabular lists for reference and baselines in future work. Given the large statistics in this investigation, the beam/strahl fit results in the upstream are now the most comprehensive attempt to parameterize the beam/strahl electron velocity moments in the ambient solar wind. The median density, temperature, beta, and temperature anisotropy values for the core(halo)[beam/strahl] components, with subscripts ec(eh)[eb], of all fit results, respectively, are n(ec(h)[b]) similar to 11.3(0.36)[0.17] cm(-3), T-ec(h)[b],T-tot similar to 14.6(48.4)[40.2] eV, beta(ec(h)[b],tot) similar to 0.93(0.11)[0.05], and Alpha(ec(h)[b]) similar to 0.98(1.03)[0.93]. This work will also serve as a 1 au baseline and reference for missions like Parker Solar Probe and Solar Orbiter.Peer reviewe

Convergent evolution of water conducting cells in Marchantia recruited the ZHOUPI gene promoting cell wall reinforcement and programmed cell death

A key adaptation of plants to life on land is the formation of water conducting cells (WCC) for efficient long-distance water transport. Based on morphological analyses it is thought that WCC have evolved independently on multiple occasions. For example, WCC have been lost in all but a few lineages of bryophytes but strikingly, within the liverworts a derived group, the complex thalloids, has evolved a novel externalised water conducting tissue composed of reinforced, hollow cells termed pegged rhizoids. Here we show that pegged rhizoid differentiation in Marchantia polymorpha is controlled by orthologues of the ZHOUPI and ICE bHLH transcription factors required for endosperm cell death in Arabidopsis seeds. By contrast, pegged rhizoid development was not affected by disruption of MpNAC5, the Marchantia orthologue of the VND genes that control WCC formation in flowering plants. We characterize the rapid, genetically controlled programmed cell death process that pegged rhizoids undergo to terminate cellular differentiation, and identify a corresponding upregulation of conserved putative plant cell death effector genes. Lastly, we show that ectopic expression of MpZOU1 increases production of pegged rhizoids and enhances drought tolerance. Our results support that pegged rhizoids having evolved independently of other WCC. We suggest that elements of the genetic control of developmental cell death are conserved throughout land plants and that the ZHOUPI/ICE regulatory module has been independently recruited to promote cell wall modification and programmed cell death in liverwort rhizoids and in the endosperm of flowering plant seed

The domesticated transposase ALP2 mediates formation of a novel Polycomb protein complex by direct interaction with MSI1, a core subunit of Polycomb Repressive Complex 2 (PRC2)

A large fraction of plant genomes is composed of transposable elements (TE), which provide a potential source of novel genes through "domestication"-the process whereby the proteins encoded by TE diverge in sequence, lose their ability to catalyse transposition and instead acquire novel functions for their hosts. In Arabidopsis, ANTAGONIST OF LIKE HETEROCHROMATIN PROTEIN 1 (ALP1) arose by domestication of the nuclease component of Harbinger class TE and acquired a new function as a component of POLYCOMB REPRESSIVE COMPLEX 2 (PRC2), a histone H3K27me3 methyltransferase involved in regulation of host genes and in some cases TE. It was not clear how ALP1 associated with PRC2, nor what the functional consequence was. Here, we identify ALP2 genetically as a suppressor of Polycomb-group (PcG) mutant phenotypes and show that it arose from the second, DNA binding component of Harbinger transposases. Molecular analysis of PcG compromised backgrounds reveals that ALP genes oppose silencing and H3K27me3 deposition at key PcG target genes. Proteomic analysis reveals that ALP1 and ALP2 are components of a variant PRC2 complex that contains the four core components but lacks plant-specific accessory components such as the H3K27me3 reader LIKE HETEROCHROMATION PROTEIN 1 (LHP1). We show that the N-terminus of ALP2 interacts directly with ALP1, whereas the C-terminus of ALP2 interacts with MULTICOPY SUPPRESSOR OF IRA1 (MSI1), a core component of PRC2. Proteomic analysis reveals that in alp2 mutant backgrounds ALP1 protein no longer associates with PRC2, consistent with a role for ALP2 in recruitment of ALP1. We suggest that the propensity of Harbinger TE to insert in gene-rich regions of the genome, together with the modular two component nature of their transposases, has predisposed them for domestication and incorporation into chromatin modifying complexes

The responsibility to protect and the use of force: remaking the procrustean bed?

The emergence of the Responsibility to Protect (R2P) owed much to the need to enhance the UN’s ability to act forcibly in the face of the most extreme cases of gross human suffering. Too often in the past such responses were emasculated or thwarted by the necessity to successfully navigate the UN Charter’s prescriptions over the use of force, by the unwillingness of member states to provide military forces, or by a combination of the two. In accepting that certain types of inhuman activity can lead to the legitimate use of force within the UN Charter framework, the adoption of R2P appeared to resolve at least some of these problems, and as such it offered hope to those wishing to see the UN adopt a more assertive response to the grossest of human rights abuses. But, using stalemate over Syria as its backdrop, this article demonstrates the dubiousness of the claim that such a normative development can ever trump the hard edged political and strategic factors which determine when states will accept and/or participate in the use of force, and it suggests a radical solution to the dangers inherent in R2P’s intimate association with military intervention