Publications by authors named "Reidunn B Aalen"

40 Publications

The sequenced genomes of nonflowering land plants reveal the innovative evolutionary history of peptide signaling.

Plant Cell 2021 09;33(9):2915-2934

Section for Evolutionary Biology and Genetics, Department of Biosciences, University of Oslo, 0316 Oslo, Norway.

An understanding of land plant evolution is a prerequisite for in-depth knowledge of plant biology. Here we extract and explore information hidden in the increasing number of sequenced plant genomes, from bryophytes to angiosperms, to elucidate a specific biological question-how peptide signaling evolved. To conquer land and cope with changing environmental conditions, plants have gone through transformations that must have required innovations in cell-to-cell communication. We discuss peptides mediating endogenous and exogenous changes by interaction with receptors activating intracellular molecular signaling. Signaling peptides were discovered in angiosperms and operate in tissues and organs such as flowers, seeds, vasculature, and 3D meristems that are not universally conserved across land plants. Nevertheless, orthologs of angiosperm peptides and receptors have been identified in nonangiosperms. These discoveries provoke questions regarding coevolution of ligands and their receptors, and whether de novo interactions in peptide signaling pathways may have contributed to generate novel traits in land plants. The answers to such questions will have profound implications for the understanding of the evolution of cell-to-cell communication and the wealth of diversified terrestrial plants. Under this perspective, we have generated, analyzed, and reviewed phylogenetic, genomic, structural, and functional data to elucidate the evolution of peptide signaling.
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http://dx.doi.org/10.1093/plcell/koab173DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8462819PMC
September 2021

The Arabidopsis (ASHH2) CW domain binds monomethylated K4 of the histone H3 tail through conformational selection.

FEBS J 2020 10 24;287(20):4458-4480. Epub 2020 Mar 24.

Department of Biological Sciences, University of Bergen, Norway, Bergen.

Chromatin post-translational modifications are thought to be important for epigenetic effects on gene expression. Methylation of histone N-terminal tail lysine residues constitutes one of many such modifications, executed by families of histone lysine methyltransferase (HKMTase). One such protein is ASHH2 from the flowering plant Arabidopsis thaliana, equipped with the interaction domain, CW, and the HKMTase domain, SET. The CW domain of ASHH2 is a selective binder of monomethylation at lysine 4 on histone H3 (H3K4me1) and likely helps the enzyme dock correctly onto chromatin sites. The study of CW and related interaction domains has so far been emphasizing lock-key models, missing important aspects of histone-tail CW interactions. We here present an analysis of the ASHH2 CW-H3K4me1 complex using NMR and molecular dynamics, as well as mutation and affinity studies of flexible coils. β-augmentation and rearrangement of coils coincide with changes in the flexibility of the complex, in particular the η1, η3 and C-terminal coils, but also in the β1 and β2 strands and the C-terminal part of the ligand. Furthermore, we show that mutating residues with outlier dynamic behaviour affect the complex binding affinity despite these not being in direct contact with the ligand. Overall, the binding process is consistent with conformational selection. We propose that this binding mechanism presents an advantage when searching for the correct post-translational modification state among the highly modified and flexible histone tails, and also that the binding shifts the catalytic SET domain towards the nucleosome. DATABASES: Structural data are available in the PDB database under the accession code 6QXZ. Resonance assignments for CW42 in its apo- and holo-forms are available in the BMRB database under the accession code 27251.
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http://dx.doi.org/10.1111/febs.15256DOI Listing
October 2020

Control of Organ Abscission and Other Cell Separation Processes by Evolutionary Conserved Peptide Signaling.

Plants (Basel) 2019 Jul 15;8(7). Epub 2019 Jul 15.

Section for Genetics and Evolutionary Biology, Department of Biosciences, University of Oslo, N-0316 Oslo, Norway.

Plants both generate and shed organs throughout their lifetime. Cell separation is in function during opening of anthers to release pollen; floral organs are detached after pollination when they have served their purpose; unfertilized flowers are shed; fruits and seeds are abscised from the mother plant to secure the propagation of new generations. Organ abscission takes place in specialized abscission zone (AZ) cells where the middle lamella between adjacent cell files is broken down. The plant hormone ethylene has a well-documented promoting effect on abscission, but mutation in ethylene receptor genes in only delays the abscission process. Microarray and RNA sequencing have identified a large number of genes differentially expressed in the AZs, especially genes encoding enzymes involved in cell wall remodelling and disassembly. Mutations in such genes rarely give a phenotype, most likely due to functional redundancy. In contrast, mutation in the () blocks floral organ abscission in Arabidopsis. encodes a small peptide that signals through the leucine-rich repeat receptor-like kinases HAESA (HAE) and HAE-LIKE2 (HSL2) to control floral organ abscission and facilitate lateral root emergence. Untimely abscission is a severe problem in many crops, and in a more applied perspective, it is of interest to investigate whether IDA-HAE/HSL2 is involved in other cell separation processes and other species. Genes encoding IDA and HSL2 orthologues have been identified in all orders of flowering plants. Angiosperms have had enormous success, with species adapted to all kinds of environments, adaptations which include variation with respect to which organs they shed. Here we review, from an evolutionary perspective, the properties of the IDA-HAE/HSL2 signaling module and the evidence for its hypothesized involvement in various cell separation processes in angiosperms.
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http://dx.doi.org/10.3390/plants8070225DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6681299PMC
July 2019

The dynamics of root cap sloughing in Arabidopsis is regulated by peptide signalling.

Nat Plants 2018 08 30;4(8):596-604. Epub 2018 Jul 30.

Section of Genetics and Evolutionary Biology, Department of Biosciences, University of Oslo, Oslo, Norway.

The root cap protects the stem cell niche of angiosperm roots from damage. In Arabidopsis, lateral root cap (LRC) cells covering the meristematic zone are regularly lost through programmed cell death, while the outermost layer of the root cap covering the tip is repeatedly sloughed. Efficient coordination with stem cells producing new layers is needed to maintain a constant size of the cap. We present a signalling pair, the peptide IDA-LIKE1 (IDL1) and its receptor HAESA-LIKE2 (HSL2), mediating such communication. Live imaging over several days characterized this process from initial fractures in LRC cell files to full separation of a layer. Enhanced expression of IDL1 in the separating root cap layers resulted in increased frequency of sloughing, balanced with generation of new layers in a HSL2-dependent manner. Transcriptome analyses linked IDL1-HSL2 signalling to the transcription factors BEARSKIN1/2 and genes associated with programmed cell death. Mutations in either IDL1 or HSL2 slowed down cell division, maturation and separation. Thus, IDL1-HSL2 signalling potentiates dynamic regulation of the homeostatic balance between stem cell division and sloughing activity.
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http://dx.doi.org/10.1038/s41477-018-0212-zDOI Listing
August 2018

Conservation of the abscission signaling peptide IDA during Angiosperm evolution: withstanding genome duplications and gain and loss of the receptors HAE/HSL2.

Front Plant Sci 2015 30;6:931. Epub 2015 Oct 30.

Section for Genetics and Evolutionary Biology, Department of Biosciences, University of Oslo Oslo, Norway.

The peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), which signals through the leucine-rich repeat receptor-like kinases HAESA (HAE) and HAESA-LIKE2 (HSL2), controls different cell separation events in Arabidopsis thaliana. We hypothesize the involvement of this signaling module in abscission processes in other plant species even though they may shed other organs than A. thaliana. As the first step toward testing this hypothesis from an evolutionarily perspective we have identified genes encoding putative orthologs of IDA and its receptors by BLAST searches of publically available protein, nucleotide and genome databases for angiosperms. Genes encoding IDA or IDA-LIKE (IDL) peptides and HSL proteins were found in all investigated species, which were selected as to represent each angiosperm order with available genomic sequences. The 12 amino acids representing the bioactive peptide in A. thaliana have virtually been unchanged throughout the evolution of the angiosperms; however, the number of IDL and HSL genes varies between different orders and species. The phylogenetic analyses suggest that IDA, HSL2, and the related HSL1 gene, were present in the species that gave rise to the angiosperms. HAE has arisen from HSL1 after a genome duplication that took place after the monocot-eudicots split. HSL1 has also independently been duplicated in the monocots, while HSL2 has been lost in gingers (Zingiberales) and grasses (Poales). IDA has been duplicated in eudicots to give rise to functionally divergent IDL peptides. We postulate that the high number of IDL homologs present in the core eudicots is a result of multiple whole genome duplications (WGD). We substantiate the involvement of IDA and HAE/HSL2 homologs in abscission by providing gene expression data of different organ separation events from various species.
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http://dx.doi.org/10.3389/fpls.2015.00931DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4627355PMC
November 2015

Antagonistic peptide technology for functional dissection of CLE peptides revisited.

J Exp Bot 2015 Aug 30;66(17):5367-74. Epub 2015 Jun 30.

Division of Plant and Crop Sciences, School of Biosciences, University of Nottingham, Loughborough LE12 5RD, UK Centre for Plant Integrative Biology, University of Nottingham, Loughborough LE12 5RD, UK Department of Plant Systems Biology, VIB, B-9052 Ghent, Belgium Department of Plant Biotechnology and Genetics, Ghent University, B-9052 Ghent, Belgium

In the Arabidopsis thaliana genome, over 1000 putative genes encoding small, presumably secreted, signalling peptides can be recognized. However, a major obstacle in identifying the function of genes encoding small signalling peptides is the limited number of available loss-of-function mutants. To overcome this, a promising new tool, antagonistic peptide technology, was recently developed. Here, this antagonistic peptide technology was tested on selected CLE peptides and the related IDA peptide and its usefulness in the context of studies of peptide function discussed. Based on the analyses, it was concluded that the antagonistic peptide approach is not the ultimate means to overcome redundancy or lack of loss-of-function lines. However, information collected using antagonistic peptide approaches (in the broad sense) can be very useful, but these approaches do not work in all cases and require a deep insight on the interaction between the ligand and its receptor to be successful. This, as well as peptide ligand structure considerations, should be taken into account before ordering a wide range of synthetic peptide variants and/or generating transgenic plants.
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http://dx.doi.org/10.1093/jxb/erv284DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4526918PMC
August 2015

The IDA/IDA-LIKE and PIP/PIP-LIKE gene families in Arabidopsis: phylogenetic relationship, expression patterns, and transcriptional effect of the PIPL3 peptide.

J Exp Bot 2015 Aug 10;66(17):5351-65. Epub 2015 Jun 10.

Department of Biology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway

Peptide ligands play crucial roles in the life cycle of plants by modulating the innate immunity against pathogens and regulating growth and developmental processes. One well-studied example is INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), which controls floral organ abscission and lateral root emergence in Arabidopsis thaliana. IDA belongs to a family of five additional IDA-LIKE (IDL) members that have all been suggested to be involved in regulation of Arabidopsis development. Here we present three novel members of the IDL subfamily and show that two of them are strongly and rapidly induced by different biotic and abiotic stresses. Furthermore, we provide data that the recently identified PAMP-INDUCED SECRETED PEPTIDE (PIP) and PIP-LIKE (PIPL) peptides, which show similarity to the IDL and C-TERMINALLY ENCODED PEPTIDE (CEP) peptides, are not only involved in innate immune response in Arabidopsis but are also induced by abiotic stress. Expression patterns of the IDA/IDL and PIP/PIPL genes were analysed using in silico data, qRT-PCR and GUS promoter lines. Transcriptomic responses to PIPL3 peptide treatment suggested a role in regulation of biotic stress responses and cell wall modification.
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http://dx.doi.org/10.1093/jxb/erv285DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4526919PMC
August 2015

The ASH1-RELATED3 SET-domain protein controls cell division competence of the meristem and the quiescent center of the Arabidopsis primary root.

Plant Physiol 2014 Oct 17;166(2):632-43. Epub 2014 Jul 17.

Department of Biosciences, University of Oslo, N-0316 Oslo, Norway (R.K., T.T., M.A.R., H.Z.N., U.H., S.V.V., G.E., R.B.A.);Department of Plant Systems Biology, Vlaams Instituut voor Biotechnologie and Department of Plant Biotechnology and Bioinformatics, Ghent University, B-9052 Ghent, Belgium (J.H., T.L., L.D.V.); andCentre for Plant Integrative Biology, University of Nottingham, Sutton Bonington Campus, Sutton Bonington, Leicestershire LE12 5RD, United Kingdom (R.S., M.J.B.)

The stem cell niche of the Arabidopsis (Arabidopsis thaliana) primary root apical meristem is composed of the quiescent (or organizing) center surrounded by stem (initial) cells for the different tissues. Initial cells generate a population of transit-amplifying cells that undergo a limited number of cell divisions before elongating and differentiating. It is unclear whether these divisions occur stochastically or in an orderly manner. Using the thymidine analog 5-ethynyl-2'-deoxyuridine to monitor DNA replication of cells of Arabidopsis root meristems, we identified a pattern of two, four, and eight neighboring cells with synchronized replication along the cortical, epidermal, and endodermal cell files, suggested to be daughters, granddaughters, and great-granddaughters of the direct progeny of each stem cell. Markers of mitosis and cytokinesis were not present in the region closest to the transition zone where the cells start to elongate, suggesting that great-granddaughter cells switch synchronously from the mitotic cell cycle to endoreduplication. Mutations in the stem cell niche-expressed ASH1-RELATED3 (ASHR3) gene, encoding a SET-domain protein conferring histone H3 lysine-36 methylation, disrupted this pattern of coordinated DNA replication and cell division and increased the cell division rate in the quiescent center. E2Fa/E2Fb transcription factors controlling the G1-to-S-phase transition regulate ASHR3 expression and bind to the ASHR3 promoter, substantiating a role for ASHR3 in cell division control. The reduced length of the root apical meristem and primary root of the mutant ashr3-1 indicate that synchronization of replication and cell divisions is required for normal root growth and development.
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http://dx.doi.org/10.1104/pp.114.244798DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4213094PMC
October 2014

Tools and Strategies to Match Peptide-Ligand Receptor Pairs.

Plant Cell 2014 May 7;26(5):1838-1847. Epub 2014 May 7.

Zentrum für Molekularbiologie der Pflanzen, University Tübingen, 72076 Tübingen, Germany.

Peptide signals have emerged as an important class of regulators in cell-to-cell communication in plants. Several families of small, secreted proteins with a conserved C-terminal Pro-rich motif have been identified as functional peptide signals in Arabidopsis thaliana. These proteins are presumed to be trimmed proteolytically and undergo posttranslational modifications, such as hydroxylation of Pro residues and glycosylation, to form mature, bioactive signals. Identification and matching of such ligands with their respective receptors remains a major challenge since the genes encoding them often show redundancy and low expression restricted to a few cells or particular developmental stages. To overcome these difficulties, we propose the use of ectopic expression of receptor genes in suitable plant cells like Nicotiana benthamiana for testing ligand candidates in receptor output assays and in binding studies. As an example, we used the IDA peptide HAE/HSL2 receptor signaling system known to regulate floral organ abscission. We demonstrate that the oxidative burst response can be employed as readout for receptor activation by synthetic peptides and that a new, highly sensitive, nonradioactive labeling approach can be used to reveal a direct correlation between peptide activity and receptor affinity. We suggest that these approaches will be of broad value for the field of ligand-receptor studies in plants.
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http://dx.doi.org/10.1105/tpc.113.120071DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4079353PMC
May 2014

The arabidopsis histone methyltransferase SUVR4 binds ubiquitin via a domain with a four-helix bundle structure.

Biochemistry 2014 Apr 25;53(13):2091-100. Epub 2014 Mar 25.

Department of Biosciences, University of Oslo , N-0316 Oslo, Norway.

In eukaryotes, different chromatin states facilitate or repress gene expression and restrict the activity of transposable elements. Post-translational modifications (PTMs) of amino acid residues on the N-terminal tails of histones are suggested to define such states. The histone lysine methyltransferase (HKMTase) SU(VAR)3-9 RELATED4 (SUVR4) of Arabidopsis thaliana functions as a repressor of transposon activity. Binding of ubiquitin by the WIYLD domain facilitates the addition of two methyl groups to monomethylated lysine 9 of histone H3. By using nuclear magnetic resonance (NMR) spectroscopy, we identified SUVR4 WIYLD (S4WIYLD) as a domain with a four-helix bundle structure, in contrast to three-helix bundles of other ubiquitin binding domains. NMR titration analyses showed that residues of helix α1 (Q38, L39, and D40) and helix α4 (N68, T70, A71, V73, D74, I76, S78, and E82) of S4WIYLD and residues between the first and second β-strands (T9 and G10) and on β-strands 3 (R42, G47, K48, and Q49) and 4 (H68, R72, and L73) undergo significant chemical shift changes when the two proteins interact. A model of the complex, generated using HADDOCK, suggests that the N-terminal and C-terminal parts of S4WIYLD constitute a surface that interacts with charged residues close to the hydrophobic patch of ubiquitin. The WIYLD domains of the closely related SUVR1 and SUVR2 Arabidopsis proteins also bind ubiquitin, indicating that this is a general feature of this domain. The question of whether SUVR proteins act as both readers of monoubiquitinated H2B and writers of histone PTMs is discussed.
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http://dx.doi.org/10.1021/bi401436hDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4254770PMC
April 2014

IDA: a peptide ligand regulating cell separation processes in Arabidopsis.

J Exp Bot 2013 Dec 22;64(17):5253-61. Epub 2013 Oct 22.

Department of Biosciences, University of Oslo, N-0316 Oslo, Norway.

In contrast to animals, plants continuously produce new organs, such as leaves, flowers, and lateral roots (LRs), and may shed organs that have served their purpose. In the model plant Arabidopsis thaliana the peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) signals through the leucine-rich repeat-receptor-like kinases (LRR-RLKs) HAESA (HAE), and HAESA-LIKE2 (HSL2) to control the abscission of floral organs after pollination. Recent work from other plant species indicates that this signalling system is conserved and could regulate leaf abscission in soybean and tomato. Abscission is a cell separation process involving the breakdown of cell walls between adjacent files of abscission zone (AZ) cells at the base of organs to be shed. The emergence of new lateral root primordia (LRP), initiated deep inside the root under the influence of the phytohormone auxin, is similarly dependent on cell wall dissolution to separate cells in the overlying tissues. It has been shown that this process also requires IDA, HAE, and HSL2. The receptors are redundant in function during floral organ abscission, but during lateral root emergence (LRE) they are differentially involved in regulating cell wall remodelling (CWR) genes. An overview is given here of the similarities and differences of IDA signalling during floral organ abscission and LRE.
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http://dx.doi.org/10.1093/jxb/ert338DOI Listing
December 2013

NEVERSHED and INFLORESCENCE DEFICIENT IN ABSCISSION are differentially required for cell expansion and cell separation during floral organ abscission in Arabidopsis thaliana.

J Exp Bot 2013 Dec 20;64(17):5345-57. Epub 2013 Aug 20.

Department of Biology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway.

Floral organ shedding is a cell separation event preceded by cell-wall loosening and generally accompanied by cell expansion. Mutations in NEVERSHED (NEV) or INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) block floral organ abscission in Arabidopsis thaliana. NEV encodes an ADP-ribosylation factor GTPase-activating protein, and cells of nev mutant flowers display membrane-trafficking defects. IDA encodes a secreted peptide that signals through the receptor-like kinases HAESA (HAE) and HAESA-LIKE2 (HSL2). Analyses of single and double mutants revealed unique features of the nev and ida phenotypes. Cell-wall loosening was delayed in ida flowers. In contrast, nev and nev ida mutants displayed ectopic enlargement of abscission zone (AZ) cells, indicating that cell expansion alone is not sufficient to trigger organ loss. These results suggest that NEV initially prevents precocious cell expansion but is later integral for cell separation. IDA is involved primarily in the final cell separation step. A mutation in KNOTTED-LIKE FROM ARABIDOPSIS THALIANA1 (KNAT1), a suppressor of the ida mutant, could not rescue the abscission defects of nev mutant flowers, indicating that NEV-dependent activity downstream of KNAT1 is required. Transcriptional profiling of mutant AZs identified gene clusters regulated by IDA-HAE/HSL2. Several genes were more strongly downregulated in nev-7 compared with ida and hae hsl2 mutants, consistent with the rapid inhibition of organ loosening in nev mutants, and the overlapping roles of NEV and IDA in cell separation. A model of the crosstalk between the IDA signalling pathway and NEV-mediated membrane traffic during floral organ abscission is presented.
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http://dx.doi.org/10.1093/jxb/ert232DOI Listing
December 2013

Floral organ abscission peptide IDA and its HAE/HSL2 receptors control cell separation during lateral root emergence.

Proc Natl Acad Sci U S A 2013 Mar 11;110(13):5235-40. Epub 2013 Mar 11.

Department of Biosciences, University of Oslo, Blindern, 0316 Oslo, Norway.

Throughout their life cycle, plants produce new organs, such as leaves, flowers, and lateral roots. Organs that have served their purpose may be shed after breakdown of primary cell walls between adjacent cell files at the site of detachment. In Arabidopsis, floral organs abscise after pollination, and this cell separation event is controlled by the peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), which signals through the leucine-rich repeat receptor-like kinases HAESA (HAE) and HAESA-LIKE2 (HSL2). Emergence of new lateral root primordia, initiated deep inside the root under the influence of auxin, is similarly dependent on cell wall dissolution between cells in the overlaying endodermal, cortical, and epidermal tissues. Here we show that this process requires IDA, HAE, and HSL2. Mutation in these genes constrains the passage of the growing lateral root primordia through the overlaying layers, resulting in altered shapes of the lateral root primordia and of the overlaying cells. The HAE and HSL2 receptors are redundant in function during floral organ abscission, but during lateral root emergence they are differentially involved in regulating cell wall remodeling genes. In the root, IDA is strongly auxin-inducible and dependent on key regulators of lateral root emergence--the auxin influx carrier LIKE AUX1-3 and AUXIN RESPONSE FACTOR7. The expression levels of the receptor genes are only transiently induced by auxin, suggesting they are limiting factors for cell separation. We conclude that elements of the same cell separation signaling module have been adapted to function in different developmental programs.
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http://dx.doi.org/10.1073/pnas.1210835110DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3612645PMC
March 2013

Tackling drought stress: receptor-like kinases present new approaches.

Plant Cell 2012 Jun 12;24(6):2262-78. Epub 2012 Jun 12.

Division of Agricultural and Environmental Sciences, School of Biosciences, University of Nottingham, Loughborough, Leicestershire LE12 5RD, UK.

Global climate change and a growing population require tackling the reduction in arable land and improving biomass production and seed yield per area under varying conditions. One of these conditions is suboptimal water availability. Here, we review some of the classical approaches to dealing with plant response to drought stress and we evaluate how research on RECEPTOR-LIKE KINASES (RLKs) can contribute to improving plant performance under drought stress. RLKs are considered as key regulators of plant architecture and growth behavior, but they also function in defense and stress responses. The available literature and analyses of available transcript profiling data indeed suggest that RLKs can play an important role in optimizing plant responses to drought stress. In addition, RLK pathways are ideal targets for nontransgenic approaches, such as synthetic molecules, providing a novel strategy to manipulate their activity and supporting translational studies from model species, such as Arabidopsis thaliana, to economically useful crops.
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http://dx.doi.org/10.1105/tpc.112.096677DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3406892PMC
June 2012

KNAT1, KNAT2 and KNAT6 act downstream in the IDA-HAE/HSL2 signaling pathway to regulate floral organ abscission.

Plant Signal Behav 2012 Jan;7(1):135-8

Department of Molecular Biosciences, University of Oslo, Oslo, Norway.

Cell separation processes, such as abscission, are critical for plant development and play key roles from sculpting the form of the plant to scattering seeds. It is however essential that such processes are under tight temporal and spatial regulation. Floral organ abscission in Arabidopsis thaliana is regulated by a ligand-receptor module consisting of the signaling peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) and the two receptor-like kinases HAESA (HAE) and HAESA-LIKE 2 (HSL2), and it is the restricted expression pattern of IDA that hinders cell separation from occurring in the abscission zones (AZs) of other organs where HAE and HSL2 are present. In the July issue of The Plant Cell we report on the identification of additional components acting downstream in the IDA signaling pathway. Through a screen for mutations that restore floral organ abscission in ida mutants, we identified two new alleles of the KNOTTED-LIKE HOMEOBOX gene BREVIPEDICELLUS (BP)/KNOTTED-LIKE FROM ARABIDOPSIS THALIANA1 (KNAT1) and show that BP/KNAT1 is important in regulating the timing of floral abscission by controlling AZ cell size and by regulating KNAT2 and KNAT6.
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http://dx.doi.org/10.4161/psb.7.1.18379DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3357354PMC
January 2012

Arabidopsis class I KNOTTED-like homeobox proteins act downstream in the IDA-HAE/HSL2 floral abscission signaling pathway.

Plant Cell 2011 Jul 8;23(7):2553-67. Epub 2011 Jul 8.

Department of Molecular Biosciences, University of Oslo, N-0316 Oslo, Norway.

Floral organ abscission in Arabidopsis thaliana is regulated by the putative ligand-receptor system comprising the signaling peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) and the two receptor-like kinases HAESA and HAESA-LIKE2. The IDA signaling pathway presumably activates a MITOGEN-ACTIVATED PROTEIN KINASE (MAPK) cascade to induce separation between abscission zone (AZ) cells. Misexpression of IDA effectuates precocious floral abscission and ectopic cell separation in latent AZ cell regions, which suggests that negative regulators are in place to prevent unrestricted and untimely AZ cell separation. Through a screen for mutations that restore floral organ abscission in ida mutants, we identified three new mutant alleles of the KNOTTED-LIKE HOMEOBOX gene BREVIPEDICELLUS (BP)/KNOTTED-LIKE FROM ARABIDOPSIS THALIANA1 (KNAT1). Here, we show that bp mutants, in addition to shedding their floral organs prematurely, have phenotypic commonalities with plants misexpressing IDA, such as enlarged AZ cells. We propose that BP/KNAT1 inhibits floral organ cell separation by restricting AZ cell size and number and put forward a model whereby IDA signaling suppresses BP/KNAT1, which in turn allows KNAT2 and KNAT6 to induce floral organ abscission.
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http://dx.doi.org/10.1105/tpc.111.084608DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3226213PMC
July 2011

The CW domain, a new histone recognition module in chromatin proteins.

EMBO J 2011 May 26;30(10):1939-52. Epub 2011 Apr 26.

Department of Molecular Biology, University of Bergen, Bergen, Norway.

Post-translational modifications of the N-terminal histone tails, including lysine methylation, have key roles in regulation of chromatin and gene expression. A number of protein modules have been identified that recognize differentially modified histone tails and provide their proteins with the capacity to sense such modifications. Here, we identify the CW domain of plant and animal chromatin-related proteins as a novel module that recognizes different methylated states of lysine 4 on histone H3 (H3K4me). The solution structure of the CW domain of the Arabidopsis ASH1 HOMOLOG2 (ASHH2) histone methyltransferase provides insight into how different CW domains can distinguish different methylated histone tails. We provide evidence that ASHH2 is acting on H3K4me-marked genes, allowing for ASHH2-dependent H3K36 tri-methylation, which contributes to sustained expression of tissue-specific and developmentally regulated genes. This suggests that ASHH2 is a combined 'reader' and 'writer' of the histone code. We propose that different CW domains, dependent on their specificity for different H3K4 methylations, are important for epigenetic memory or participate in switching between permissive and repressive chromatin states.
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http://dx.doi.org/10.1038/emboj.2011.108DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3098480PMC
May 2011

The SUVR4 histone lysine methyltransferase binds ubiquitin and converts H3K9me1 to H3K9me3 on transposon chromatin in Arabidopsis.

PLoS Genet 2011 Mar 10;7(3):e1001325. Epub 2011 Mar 10.

Department of Molecular Biosciences, University of Oslo, Oslo, Norway.

Chromatin structure and gene expression are regulated by posttranslational modifications (PTMs) on the N-terminal tails of histones. Mono-, di-, or trimethylation of lysine residues by histone lysine methyltransferases (HKMTases) can have activating or repressive functions depending on the position and context of the modified lysine. In Arabidopsis, trimethylation of lysine 9 on histone H3 (H3K9me3) is mainly associated with euchromatin and transcribed genes, although low levels of this mark are also detected at transposons and repeat sequences. Besides the evolutionarily conserved SET domain which is responsible for enzyme activity, most HKMTases also contain additional domains which enable them to respond to other PTMs or cellular signals. Here we show that the N-terminal WIYLD domain of the Arabidopsis SUVR4 HKMTase binds ubiquitin and that the SUVR4 product specificity shifts from di- to trimethylation in the presence of free ubiquitin, enabling conversion of H3K9me1 to H3K9me3 in vitro. Chromatin immunoprecipitation and immunocytological analysis showed that SUVR4 in vivo specifically converts H3K9me1 to H3K9me3 at transposons and pseudogenes and has a locus-specific repressive effect on the expression of such elements. Bisulfite sequencing indicates that this repression involves both DNA methylation-dependent and -independent mechanisms. Transcribed genes with high endogenous levels of H3K4me3, H3K9me3, and H2Bub1, but low H3K9me1, are generally unaffected by SUVR4 activity. Our results imply that SUVR4 is involved in the epigenetic defense mechanism by trimethylating H3K9 to suppress potentially harmful transposon activity.
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http://dx.doi.org/10.1371/journal.pgen.1001325DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3053343PMC
March 2011

Genome-wide transcript profiling of endosperm without paternal contribution identifies parent-of-origin-dependent regulation of AGAMOUS-LIKE36.

PLoS Genet 2011 Feb 17;7(2):e1001303. Epub 2011 Feb 17.

Department of Molecular Biosciences (IMBV), University of Oslo, Oslo, Norway.

Seed development in angiosperms is dependent on the interplay among different transcriptional programs operating in the embryo, the endosperm, and the maternally-derived seed coat. In angiosperms, the embryo and the endosperm are products of double fertilization during which the two pollen sperm cells fuse with the egg cell and the central cell of the female gametophyte. In Arabidopsis, analyses of mutants in the cell-cycle regulator CYCLIN DEPENDENT KINASE A;1 (CKDA;1) have revealed the importance of a paternal genome for the effective development of the endosperm and ultimately the seed. Here we have exploited cdka;1 fertilization as a novel tool for the identification of seed regulators and factors involved in parent-of-origin-specific regulation during seed development. We have generated genome-wide transcription profiles of cdka;1 fertilized seeds and identified approximately 600 genes that are downregulated in the absence of a paternal genome. Among those, AGAMOUS-LIKE (AGL) genes encoding Type-I MADS-box transcription factors were significantly overrepresented. Here, AGL36 was chosen for an in-depth study and shown to be imprinted. We demonstrate that AGL36 parent-of-origin-dependent expression is controlled by the activity of METHYLTRANSFERASE1 (MET1) maintenance DNA methyltransferase and DEMETER (DME) DNA glycosylase. Interestingly, our data also show that the active maternal allele of AGL36 is regulated throughout endosperm development by components of the FIS Polycomb Repressive Complex 2 (PRC2), revealing a new type of dual epigenetic regulation in seeds.
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http://dx.doi.org/10.1371/journal.pgen.1001303DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3040660PMC
February 2011

The ASH1 HOMOLOG 2 (ASHH2) histone H3 methyltransferase is required for ovule and anther development in Arabidopsis.

PLoS One 2009 Nov 12;4(11):e7817. Epub 2009 Nov 12.

Department of Molecular Biosciences, University of Oslo, Oslo, Norway.

Background: SET-domain proteins are histone lysine (K) methyltransferases (HMTase) implicated in defining transcriptionally permissive or repressive chromatin. The Arabidopsis ASH1 HOMOLOG 2 (ASHH2) protein (also called SDG8, EFS and CCR1) has been suggested to methylate H3K4 and/or H3K36 and is similar to Drosophila ASH1, a positive maintainer of gene expression, and yeast Set2, a H3K36 HMTase. Mutation of the ASHH2 gene has pleiotropic developmental effects. Here we focus on the role of ASHH2 in plant reproduction.

Methodology/principal Findings: A slightly reduced transmission of the ashh2 allele in reciprocal crosses implied involvement in gametogenesis or gamete function. However, the main requirement of ASHH2 is sporophytic. On the female side, close to 80% of mature ovules lack embryo sac. On the male side, anthers frequently develop without pollen sacs or with specific defects in the tapetum layer, resulting in reduction in the number of functional pollen per anther by up to approximately 90%. In consistence with the phenotypic findings, an ASHH2 promoter-reporter gene was expressed at the site of megaspore mother cell formation as well as tapetum layers and pollen. ashh2 mutations also result in homeotic changes in floral organ identity. Transcriptional profiling identified more than 300 up-regulated and 600 down-regulated genes in ashh2 mutant inflorescences, whereof the latter included genes involved in determination of floral organ identity, embryo sac and anther/pollen development. This was confirmed by real-time PCR. In the chromatin of such genes (AP1, AtDMC1 and MYB99) we observed a reduction of H3K36 trimethylation (me3), but not H3K4me3 or H3K36me2.

Conclusions/significance: The severe distortion of reproductive organ development in ashh2 mutants, argues that ASHH2 is required for the correct expression of genes essential to reproductive development. The reduction in the ashh2 mutant of H3K36me3 on down-regulated genes relevant to the observed defects, implicates ASHH2 in regulation of gene expression via H3K36 trimethylation in chromatin of Arabidopsis inflorescences.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0007817PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2772814PMC
November 2009

Identification of a putative receptor-ligand pair controlling cell separation in plants.

Plant Signal Behav 2008 Dec;3(12):1109-10

Department of Molecular Biosciences; University of Oslo; Oslo Norway.

Cell separation events are important throughout the lifespan of a plant. To assure that the plant's integrity is not compromised, such events, which depend on cell wall degradation, have to be tightly controlled both in time and space. The final step of floral organ abscission in Arabidopsis is controlled by INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), in that mutation of IDA causes a block in abscission. Overexpression results in early abscission of floral organs. In a recent article we show that this is also the case when overexpressing the related IDA-LIKE (IDL) proteins, indicating a degree of functional redundancy. Based on gene swap and deletion constructs introduced in the ida mutant and synthetic peptide assays we demonstrated that the conserved C-terminal motif (EPIP) of IDA and IDL1 was sufficient to replace IDA function. This function is dependent on the presence of the receptor-like kinases (RLK) HAESA (HAE) and HAESA-LIKE2 (HSL2), suggesting that an IDA peptide acts as a ligand interacting with these receptors. Our study further revealed that the five IDL genes are expressed at various sites where cell separation takes place. We suggest that the IDL proteins constitute a family of ligands that act through RLKs similar to HAESA and control cell separation at different sites and development stages during the life of the plant.
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http://dx.doi.org/10.4161/psb.3.12.7009DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2634470PMC
December 2008

AtMBD8 is involved in control of flowering time in the C24 ecotype of Arabidopsis thaliana.

Physiol Plant 2009 May;136(1):110-26

Department of Molecular Biosciences, University of Oslo, Blindern, N-0316 Oslo, Norway.

The Arabidopsis thaliana accession C24 is a vernalization-responsive, moderately late flowering ecotype. We report that a mutation in AtMBD8, which encodes a protein with a putative Methyl-CpG-Binding Domain (MBD), in C24 background, results in a delay in flowering time during both long and short days. The atmbd8-1 mutant responded to vernalization as wild type (wt) plants. Consistent with a role in modulation of flowering time, an AtMBD8::GUS-reporter construct was expressed in the shoot meristem region and developing leaves. Full-genome transcriptional profiling revealed very few changes in gene expression between atmbd8-1 and wt plants. The expression level of FLC, the major repressor of transition to flowering, was unchanged in atmbd8-1, and in accordance with that, genes upstream of FLC were unaffected by the mutation. The expression level of CONSTANS, involved in photoperiodic control of flowering, was very similar in atmbd8-1 and wt plants. In contrast, the major promoters of flowering, FT and SOC1, were both downregulated. As FT is a regulator of SOC1, we conclude that AtMBD8 is a novel promoter of flowering that acts upstream of FT in the C24 accession. In contrast to atmbd8-1, the Colombia (Col) SALK T-DNA insertion line, atmbd8-2, did not display a delayed transition to flowering. Transcriptional profiling revealed that a substantial number of genes were differentially expressed between C24 and Col wt seedlings. Several of these genes are also differentially expressed in late flowering mutants. We suggest that these differences contribute to the contrasting effect of a mutation in AtMBD8 in the two ecotypes.
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http://dx.doi.org/10.1111/j.1399-3054.2009.01218.xDOI Listing
May 2009

Plant peptides in signalling: looking for new partners.

Trends Plant Sci 2009 May 9;14(5):255-63. Epub 2009 Apr 9.

Department of Molecular Biosciences, University of Oslo, N-0316 Oslo, Norway.

A novel candidate ligand-receptor system, INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) and the related receptor-like kinases (RLKs) HAESA (HAE) and HAESA-LIKE (HSL)2, has been shown to control floral abscission in Arabidopsis thaliana. Furthermore, several IDA-LIKE (IDL) proteins, which contain a conserved C-terminal domain resembling that of the CLAVATA (CLV)3-ENDOSPERM SURROUNDING REGION (ESR)-RELATED (CLE) protein family, have been shown to be partially redundant with IDA. Here, we use the genetic similarities between the IDA and CLV3 signalling systems to hypothesize that closely related peptide ligands are likely to interact with families of closely related RLKs. Guided by this hypothesis and with the aid of genetics and novel methods, ligand-receptor systems can be identified to improve our understanding of developmental processes in plants.
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http://dx.doi.org/10.1016/j.tplants.2009.02.002DOI Listing
May 2009

The EPIP peptide of INFLORESCENCE DEFICIENT IN ABSCISSION is sufficient to induce abscission in arabidopsis through the receptor-like kinases HAESA and HAESA-LIKE2.

Plant Cell 2008 Jul 25;20(7):1805-17. Epub 2008 Jul 25.

Department of Molecular Biosciences, University of Oslo, N-0316 Oslo, Norway.

In Arabidopsis thaliana, the final step of floral organ abscission is regulated by INFLORESCENCE DEFICIENT IN ABSCISSION (IDA): ida mutants fail to abscise floral organs, and plants overexpressing IDA display earlier abscission. We show that five IDA-LIKE (IDL) genes are expressed in different tissues, but plants overexpressing these genes have phenotypes similar to IDA-overexpressing plants, suggesting functional redundancy. IDA/IDL proteins have N-terminal signal peptides and a C-terminal conserved motif (extended PIP [EPIP]) at the C terminus (EPIP-C). IDA can, similar to CLAVATA3, be processed by an activity from cauliflower meristems. The EPIP-C of IDA and IDL1 replaced IDA function in vivo, when the signal peptide was present. In addition, synthetic IDA and IDL1 EPIP peptides rescued ida and induced early floral abscission in wild-type flowers. The EPIP-C of the other IDL proteins could partially substitute for IDA function. Similarly to ida, a double mutant between the receptor-like kinases (RLKs) HAESA (HAE) and HAESA-LIKE2 (HSL2) displays nonabscising flowers. Neither overexpression of IDA nor synthetic EPIP or EPIP-C peptides could rescue the hae hsl2 abscission deficiency. We propose that IDA and the IDL proteins constitute a family of putative ligands that act through RLKs to regulate different events during plant development.
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http://dx.doi.org/10.1105/tpc.108.059139DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2518227PMC
July 2008

The BLADE-ON-PETIOLE genes are essential for abscission zone formation in Arabidopsis.

Development 2008 Apr 13;135(8):1537-46. Epub 2008 Mar 13.

Department of Botany, University of British Columbia, Vancouver, British Columbia, V6T 1Z4, Canada.

The Arabidopsis BLADE-ON-PETIOLE 1 (BOP1) and BOP2 genes encode redundant transcription factors that promote morphological asymmetry during leaf and floral development. Loss-of-function bop1 bop2 mutants display a range of developmental defects, including a loss of floral organ abscission. Abscission occurs along specialised cell files, called abscission zones (AZs) that develop at the junction between the leaving organ and main plant body. We have characterized the bop1 bop2 abscission phenotype to determine how BOP1 and BOP2 contribute to the known abscission developmental framework. Histological analysis and petal breakstrength measurements of bop1 bop2 flowers show no differentiation of floral AZs. Furthermore, vestigial cauline leaf AZs are also undifferentiated in bop1 bop2 mutants, suggesting that BOP proteins are essential to establish AZ cells in different tissues. In support of this hypothesis, BOP1/BOP2 activity is required for both premature floral organ abscission and the ectopic abscission of cauline leaves promoted by the INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) gene under the control of the constitutive CaMV 35S promoter. Expression of several abscission-related marker genes, including IDA, is relatively unperturbed in bop1 bop2 mutants, indicating that these AZ genes respond to positional cues that are independent of BOP1/BOP2 activity. We also show that BOP1 and BOP2 promote growth of nectary glands, which normally develop at the receptacle adjacent to developing AZs. Taken together, these data suggest that BOP1/BOP2 activity is required for multiple cell differentiation events in the proximal regions of inflorescence lateral organs.
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http://dx.doi.org/10.1242/dev.012807DOI Listing
April 2008

The Arabidopsis SET-domain protein ASHR3 is involved in stamen development and interacts with the bHLH transcription factor ABORTED MICROSPORES (AMS).

Plant Mol Biol 2008 Jan 3;66(1-2):47-59. Epub 2007 Nov 3.

Department of Molecular Biosciences, University of Oslo, PO Box 1041, Blindern, 0316 Oslo, Norway.

The Arabidopsis thaliana genome contains more than 30 genes encoding SET-domain proteins that are thought to be epigenetic regulators of gene expression and chromatin structure. SET-domain proteins can be divided into subgroups, and members of the Polycomb group (PcG) and trithorax group (trxG) have been shown to be important regulators of development. Both in animals and plants some of these proteins are components of multimeric protein complexes. Here, we have analyzed the Arabidopsis trxG protein ASHR3 which has a SET domain and pre- and post-SET domains similar to that of Ash1 in Drosophila. In addition to the SET domain, a divergent PHD finger is found in the N-terminus of the ASHR3 protein. As expected from SET-domain proteins involved in transcriptional activation, ASHR3 (coupled to GFP) localizes to euchromatin. A yeast two-hybrid screening revealed that the ASHR3 protein interacts with the putative basic helix-loop-helix (bHLH) transcription factor ABORTED MICROSPORES (AMS), which is involved in anther and stamen development in Arabidopsis. Deletion mapping indicated that both the PHD finger and the SET domain mediate the interaction between the two proteins. Overexpression of ASHR3 led in general to growth arrest, and specifically to degenerated anthers and male sterility. Expression analyses demonstrated that ASHR3 like AMS is expressed in the anther and in stamen filaments. We therefore propose that AMS can target ASHR3 to chromatin and regulate genes involved in stamen development and function.
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http://dx.doi.org/10.1007/s11103-007-9251-yDOI Listing
January 2008

Drosophila dSet2 functions in H3-K36 methylation and is required for development.

Biochem Biophys Res Commun 2007 Aug 4;359(3):784-9. Epub 2007 Jun 4.

Institute of Molecular Biosciences, University of Oslo, PO Box 1041 Blindern, NO-0316 Oslo, Norway.

Lysine methylation has important functions in biological processes that range from heterochromatin formation to transcription regulation. Here, we demonstrate that Drosophila dSet2 encodes a developmentally essential histone H3 lysine 36 (K36) methyltransferase. Larvae subjected to RNA interference-mediated (RNAi) suppression of dSet2 lack dSet2 expression and H3-K36 methylation, indicating that dSet2 is the sole enzyme responsible for this modification in Drosophila melanogaster. dSet2 RNAi blocks puparium formation and adult development, and causes partial (blister) separation of the dorsal and ventral wing epithelia, defects suggesting a failure of the ecdysone-controlled genetic program. A transheterozygous EcR null mutation/dSet2 RNAi combination produces a complete (balloon) separation of the wing surfaces, revealing a genetic interaction between EcR and dSet2. Using immunoprecipitation, we demonstrate that dSet2 associates with the hyperphosphorylated form of RNA polymerase II (RNAPII).
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http://dx.doi.org/10.1016/j.bbrc.2007.05.189DOI Listing
August 2007

The Drosophila SET domain encoding gene dEset is essential for proper development.

Hereditas 2006 Dec;143(2006):177-88

Institute of Molecular Biosciences, University of Oslo, P.O.B. 1041 Blindern, NO-0316 Oslo, Norway.

We have identified dEset, the fly homolog of human SETDB1 and mouse ESET histone lysine methyltransferases (HKMTases) that methylates the lysine 9 residue of histone 3 (H3-K9) and negatively regulates transcription of target genes. By using spatio-temporal RNA interference we show that dEset is required at several stages of development coinciding with ecdysone pulses, possibly as a repressor of transcription of target genes. Several interacting partners, for example USP, spire, and cut up were identified in a yeast two-hybrid screen. The spatio-temporal expression profiles of dEset and its potential partners suggest that they may act together or even in a larger complex.
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http://dx.doi.org/10.1111/j.2006.0018-0661.01970.xDOI Listing
December 2006

The Arabidopsis SUVR4 protein is a nucleolar histone methyltransferase with preference for monomethylated H3K9.

Nucleic Acids Res 2006 4;34(19):5461-70. Epub 2006 Oct 4.

Department of Molecular Biosciences, University of Oslo, P.O. Box 1041 Blindern, N-0316 Oslo, Norway.

Proteins containing the evolutionarily conserved SET domain are involved in regulation of eukaryotic gene expression and chromatin structure through their histone lysine methyltransferase (HMTase) activity. The Drosophila SU(VAR)3-9 protein and related proteins of other organisms have been associated with gene repression and heterochromatinization. In Arabidopsis there are 10 SUVH and 5 SUVR genes encoding proteins similar to SU(VAR)3-9, and 4 SUVH proteins have been shown to control heterochromatic silencing by its HMTase activity and by directing DNA methylation. The SUVR proteins differ from the SUVH proteins in their domain structure, and we show that the closely related SUVR1, SUVR2 and SUVR4 proteins contain a novel domain at their N-terminus, and a SUVR specific region preceding the SET domain. Green fluorescent protein (GFP)-fusions of these SUVR proteins preferably localize to the nucleolus, suggesting involvement in regulation of rRNA expression, in contrast to other SET-domain proteins studied so far. A novel HMTase specificity was demonstrated for SUVR4, in that monomethylated histone H3K9 is its preferred substrate in vitro.
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http://dx.doi.org/10.1093/nar/gkl687DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1636477PMC
December 2006

Ethylene-dependent and -independent pathways controlling floral abscission are revealed to converge using promoter::reporter gene constructs in the ida abscission mutant.

J Exp Bot 2006 21;57(14):3627-37. Epub 2006 Sep 21.

Department of Molecular Biosciences, University of Oslo, PO Box 1041 Blindern, N-0316 Oslo, Norway.

The process of floral organ abscission in Arabidopsis thaliana can be modulated by ethylene and involves numerous genes contributing to cell separation. One gene that is absolutely required for abscission is INFLORESCENCE DEFICIENT IN ABSCISSION, IDA, as the ida mutant is completely blocked in abscission. To elucidate the genetic pathways regulating floral abscission, molecular markers expressed in the floral abscission zone have been studied in an ida mutant background. Using plants with promoter-reporter gene constructs including promoters of a novel FLORAL ABSCISSION ASSOCIATED gene (FAA) encoding a putative single-stranded binding protein (BASIL), chitinase (CHIT::GUS) and cellulase (BAC::GUS), it is shown that IDA acts in the last steps of the abscission process. These markers, as well as HAESA, encoding a receptor-like kinase, were unaffected in their temporal expression patterns in ida compared with wild-type plants; thus showing that different regulatory pathways are active in the abscission process. In contrast to BASIL, CHIT::GUS and BAC::GUS showed, however, much weaker induction of expression in an ida background, consistent with a reduction in pathogen-associated responses and a lack of total dissolution of cell walls in the mutant. IDA, encoding a putative secreted peptide ligand, and HAESA appeared to have identical patterns of expression in floral abscission zones. Lastly, to address the role of ethylene, IDA::GUS expression in the wild type and the ethylene-insensitive mutant etr1-1 was compared. Similar temporal patterns, yet restricted spatial expression patterns were observed in etr1-1, suggesting that the pathways regulated by IDA and by ethylene act in parallel, but are, to some degree, interdependent.
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http://dx.doi.org/10.1093/jxb/erl130DOI Listing
February 2007
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