During sexual reproduction, one-half of the genetic material is deposited in gametes, and a complete set of chromosomes is restored upon fertilization. Reduction of the genetic information before gametogenesis occurs in meiosis, when cross-overs (COs) between homologous chromosomes secure an exchange of their genetic information. COs are not evenly distributed along chromosomes and are suppressed in chromosomal regions encompassing compact, hypermethylated centromeric and pericentromeric DNA. Therefore, it was postulated that DNA hypermethylation is inhibitory to COs. Here, when analyzing meiotic recombination in mutant plants with hypomethylated DNA, we observed unexpected and counterintuitive effects of DNA methylation losses on CO distribution. Recombination was further promoted in the hypomethylated chromosome arms while it was inhibited in heterochromatic regions encompassing pericentromeric DNA. Importantly, the total number of COs was not affected, implying that loss of DNA methylation led to a global redistribution of COs along chromosomes. To determine by which mechanisms altered levels of DNA methylation influence recombination-whether directly in cis or indirectly in trans by changing expression of genes encoding recombination components-we analyzed CO distribution in wild-type lines with randomly scattered and wellmapped hypomethylated chromosomal segments. The results of these experiments, supported by expression profiling data, suggest that DNA methylation affects meiotic recombination in cis. Because DNA methylation exhibits significant variation even within a single species, our results imply that it may influence the evolution of plant genomes through the control of meiotic recombination.epigenetic | chromatin | epigenetic recombinant inbred lines | met1-3 R egulation of meiotic recombination, as with other essential chromosomal activities like transcription and replication, depends on both DNA sequence and chromatin properties (1, 2). Although regulatory aspects of meiotic recombination have been studied in great detail, it is still not well understood how chromatin structure influences the frequency and distribution of recombination events, as reflected by the final number and distribution of cross-overs (COs) along chromosomes. Biased chromosomal positioning of COs has been recognized for many years; indeed, COs are most likely to occur in euchromatic chromosomal arms, distal to the recombinationally suppressed pericentromeric heterochromatin. These two chromatin compartments are characterized by differences in the abundance of genes and transposable elements (TEs). TEs accumulate in pericentromeric regions, whereas genes are enriched in distal euchromatin. Because suppressive epigenetic marks are primarily directed at silencing TEs, these two chromatin types also differ in their epigenetic signatures. Pericentromeric chromatin is enriched in the methylation of histone H3 at lysine 9 (H3K9me) and encompasses hypermethylated DNA. In contrast, distal chromatin exhibits active marks such as acet...

It is commonly observed that onset or release of transcriptional gene silencing (TGS) correlates with alteration of repressive epigenetic marks. The TGS regulator MOM1 in Arabidopsis is exceptional since it regulates transcription in intermediate heterochromatin with only minor changes in epigenetic marks. We have isolated an enhancer of the mom1 mutation that points towards regulatory interplay between MOM1 and RNA polymerase-V (Pol-V). Pol-V transcribes heterochromatic loci, which seems to be required for maintenance of their silencing; however, it is still not clear how Pol-V is targeted to heterochromatin. We now provide evidence that Pol-V is required for MOM1-mediated suppression of transcription at a subset of its chromosomal targets. Thus, Pol-V genetically interacts with MOM1 in the control of gene silencing. Interestingly, functional cooperation of MOM1 and Pol-V not only broadens the range of the controlled loci in comparison to each individual factor, but also determines the degree of TGS.

SignificancePlants contain a unique family of membrane receptors, which are different from the ones found in bacteria and animals. These proteins are able to sense very different signals, such as steroid molecules, peptides, and proteins at the cell surface using a spiral-shaped ligand binding domain. Ligand binding allows the receptor to engage with a smaller coreceptor kinase, which is shared among different receptors. Here it is analyzed how one coreceptor protein can contribute to the sensing of two different ligands involved in plant growth and organ abscission and to activation of their cognate receptors.

The leucine-rich repeat receptor kinase (LRR-RK) BRASSINOSTEROID INSENSITIVE 1 (BRI1) requires a shape-complementary SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) co-receptor for brassinosteroid sensing and receptor activation. Interface mutations that weaken the interaction between receptor and co-receptor in vitro reduce brassinosteroid signalling responses. The SERK3 elongated (elg) allele maps to the complex interface and shows enhanced brassinosteroid signalling, but surprisingly no tighter binding to the BRI1 ectodomain in vitro. Here, we report that rather than promoting the interaction with BRI1, the elg mutation disrupts the ability of the co-receptor to interact with the ectodomains of BRI1-ASSOCIATED-KINASE1 INTERACTING KINASE (BIR) receptor pseudokinases, negative regulators of LRR-RK signalling. A conserved lateral surface patch in BIR LRR domains is required for targeting SERK co-receptors and the elg allele maps to the core of the complex interface in a 1.25 Å BIR3-SERK1 structure. Collectively, our structural, quantitative biochemical and genetic analyses suggest that brassinosteroid signalling complex formation is negatively regulated by BIR receptor ectodomains.

Receptor kinases with extracellular leucine-rich repeat domains (LRR-RKs) form the largest group of membrane signaling proteins in plants. LRR-RKs can sense small molecule, peptide, or protein ligands, and may be activated by ligand-induced interaction with a shape complementary SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) co-receptor kinase. We have previously shown that SERKs can also form constitutive, ligand-independent complexes with the LRR ectodomains of BAK1-interacting receptor-like kinase 3 (BIR3) receptor pseudokinases, negative regulators of LRR-RK signaling. Here we report that receptor chimera in which the extracellular LRR domain of BIR3 is fused to the cytoplasmic kinase domains of the SERK-dependent LRR-RKs BRASSINOSTEROID INSENSITIVE1, HAESA and ERECTA form tight complexes with endogenous SERK co-receptors in the absence of ligand stimulus. Expression of these chimeras under the control of the endogenous promoter of the respective LRR-RK leads to strong gain-of-function brassinosteroid, floral abscission, and stomatal patterning phenotypes, respectively. Importantly, a BIR3-GSO1/SGN3 chimera can partially complement sgn3 Casparian strip formation phenotypes, suggesting that SERK proteins also mediate GSO1/SGN3 receptor activation. Collectively, our protein engineering approach may be used to elucidate the physiological functions of orphan LRR-RKs and to identify their receptor activation mechanism in single transgenic lines.

Retrotransposons are ubiquitous mobile genetic elements constituting a major part of eukaryotic genomes. Yet, monitoring retrotransposition and subsequent copy number increases in multicellular eukaryotes is intrinsically difficult. By following the transgenerational accumulation of a newly activated retrotransposon EVADE (EVD) in Arabidopsis, we noticed fast expansion of activated elements transmitted through the paternal germ line but suppression when EVD-active copies are maternally inherited. This parent-of-origin effect on EVD proliferation was still observed when gametophytes carried mutations for key epigenetic regulators previously shown to restrict EVD mobility. Therefore, the main mechanism preventing active EVD proliferation seems to act through epigenetic control in sporophytic tissues in the mother plant. In consequence, once activated, this retrotransposon proliferates in plant populations owing to suppressed epigenetic control during paternal transmission. This parental gateway might contribute to the occasional bursts of retrotransposon mobilization deduced from the genome sequences of many plant species.

194 words) Receptor kinases with extracellular leucine-rich repeat domains (LRR-RKs) form the largest group of membrane signaling proteins in plants. LRR-RKs can sense small molecule, peptide or protein ligands, and may be activated by ligand-induced interaction with a shape complementary SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) coreceptor kinase. We have previously shown that SERKs can also form constitutive, ligandindependent complexes with the LRR ectodomains of BAK1-interacting receptor-like kinase 3 (BIR3) receptor pseudokinases, negative regulators of LRR-RK signaling. Here we report that receptor chimaera in which the extracellular LRR domain of BIR3 is fused to the cytoplasmic kinase domains of the SERK-dependent LRR-RKs BRASSINOSTEROID INSENSITIVE1, HAESA and ERECTA form tight complexes with endogenous SERK co-receptors in the absence of ligand stimulus. Expression of these chimaera under the control of the endogenous promoter of the respective LRR-RK leads to strong gain-of-function brassinosteroid, floral abscission and stomatal patterning phenotypes, respectively. Importantly, a BIR3-GSO1/SGN3 chimera can partially complement sgn3 Casparian strip formation phenotypes, suggesting that GSO1/SGN3 receptor activation is also mediated by SERK proteins. Collectively, our protein engineering approach may be used to elucidate the physiological functions of orphan LRR-RKs and to identify their receptor activation mechanism in single transgenic lines. 2 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 MAP kinase signaling pathway (Meng et al., 2015) with diverse roles in plant development involvesthe LRR-RK ERECTA (ER) and its paralogs ERECTA-LIKE1 (ERL1) and ERL2 (Torii et al., 1996;Shpak, 2013). ER, ERL1 and ERL2 together control stomata development and their correct spacing on the leaf surface (Shpak et al., 2005). Cysteine-rich EPIDERMAL PATTERNING FACTOR peptides (EPFs) bind to the ectodomains of ER, ERL1 and ERL2 which form constitutive complexes with the ectodomain of the receptor-like protein (RLP) TOO MANY MOUTH (TMM) Vl and elution at 0.75 ml min -1 was monitored by ultraviolet absorbance at λ = 280 nm. To chimera; # numbers indicate independent lines.(C) Anti-GFP western blot together with the Ponceau-stained membrane as loading control.

The precise execution of various cellular functions relies on the maintenance of signaling specificity from input detection to cellular outputs. However, diverse signaling pathways share similar or identical intermediate components. A well‐conserved intermediate, the Mitogen‐Activated Protein Kinase (MAPK) cascade, participates in a myriad of signaling pathways, regulating signal transduction from input to output. This typifies the “hourglass conundrum”, where a multitude of inputs and outputs all operate through a limited number of common intermediates. Therefore, understanding how MAPK cascades regulate a variety of outputs with specificity is a fundamental question in biology. This review highlights four major insulating mechanisms that improve signaling specificity: selective activation, compartmentalization, combinatorial signaling, and cross‐pathway inhibition. We focus on plant pathways that share MAPK cascade components and compare mechanisms with those of animals and yeast. We hope this conceptual overview will aid future studies to better understand plant signaling specificity.