Constitutive Activation of Leucine-Rich Repeat Receptor Kinase Signaling Pathways by BAK1-INTERACTING RECEPTOR-LIKE KINASE3 Chimera.

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, ligand-independent complexes with the LRR ectodomains of BAK1-INTERACTING RECEPTOR-LIKE KINASE3 (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 coreceptors 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-GASSHO1 (GSO1)/SCHENGEN3 (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.

Mechanistic basis for the activation of plant membrane receptor kinases by SERK-family coreceptors.

Plant-unique membrane receptor kinases with leucine-rich repeat ectodomains (LRR-RKs) can sense small molecule, peptide, and protein ligands. Many LRR-RKs require SERK-family coreceptor kinases for high-affinity ligand binding and receptor activation. How one coreceptor can contribute to the specific binding of distinct ligands and activation of different LRR-RKs is poorly understood. Here we quantitatively analyze the contribution of SERK3 to ligand binding and activation of the brassinosteroid receptor BRI1 and the peptide hormone receptor HAESA. We show that while the isolated receptors sense their respective ligands with drastically different binding affinities, the SERK3 ectodomain binds the ligand-associated receptors with very similar binding kinetics. We identify residues in the SERK3 N-terminal capping domain, which allow for selective steroid and peptide hormone recognition. In contrast, residues in the SERK3 LRR core form a second, constitutive receptor-coreceptor interface. Genetic analyses of protein chimera between BRI1 and SERK3 define that signaling-competent complexes are formed by receptor-coreceptor heteromerization in planta. A functional BRI1-HAESA chimera suggests that the receptor activation mechanism is conserved among different LRR-RKs, and that their signaling specificity is encoded in the kinase domain of the receptor. Our work pinpoints the relative contributions of receptor, ligand, and coreceptor to the formation and activation of SERK-dependent LRR-RK signaling complexes regulating plant growth and development.

Parent-of-origin control of transgenerational retrotransposon proliferation in Arabidopsis.

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.

Loss of DNA methylation affects the recombination landscape in Arabidopsis.

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 well-mapped 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.

MOM1 and Pol-IV/V interactions regulate the intensity and specificity of transcriptional gene silencing.

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.

Publisher Correction: The SERK3 elongated allele defines a role for BIR ectodomains in brassinosteroid signalling.

In the version of this Letter originally published, there were errors in the x axis labels of Figs 1, 2 and 4: in Fig. 1b, the label Col-0 should not have been included on the axis; in Fig. 2b BIR should have read BIR2, and DN221 should have read D122N; in Fig. 4f, pSEK3 should have read pSERK3. These figures have now been amended in all versions of the Letter.

The SERK3 elongated allele defines a role for BIR ectodomains in brassinosteroid signalling.

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 activation1. Interface mutations that weaken the interaction between receptor and co-receptor in vitro reduce brassinosteroid signalling responses2. The SERK3 elongated (elg) allele3-5 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 signalling6. 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.