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Programmed Cell Death in Xylem Development
Umeå University, Faculty of Science and Technology, Umeå Plant Science Centre.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Concerns about climate changes and scarcity of fossil fuels are rising. Hence wood is becoming an attractive source of renewable energy and raw material and these new dimensions have prompted increasing interest in wood formation in trees, in both the scientific community and wider public. In this thesis, the focus is on a key process in wood development: programmed cell death (PCD) in the development of xylem elements. Since secondary cell wall formation is dependent, inter alia, upon the life time of xylem elements, the qualitative features of wood will be affected by PCD in xylem, about which there is little information.

This thesis focuses on the anatomical, morphological and transcriptional features of PCD during xylem development in both the stem of hybrid aspen, Populus tremula (L.) x tremuloides (Michx.) and the hypocotyl of the herbaceous model system Arabidopsis thaliana (L. Heynh.). In Populus, the progressive removal of organelles from the cytoplasm before the time of death (vacuolar bursts) and the slowness of the cell death process, illustrated by DNA fragmentation assays (such as TUNEL and Comet assays), have been ascertained in the xylem fibres by microscopic analyses. Furthermore, candidate genes for the regulation of fibre cell death were identified either from a Populus EST library obtained from woody tissues undergoing fibre cell death or from microarray experiments in Populus stem, and further assessed in an in silico comparative transcriptomic analysis of Arabidopsis thaliana. These candidate genes were either putative novel regulators of fibre cell death or members of previously described families of cell death-related genes, such as autophagy-related genes. The induction of the latter and the previous microscopic observations suggest the importance of autophagy in the degradation of the cytoplasmic contents specifically in the xylem fibres. Vacuolar bursts in the vessels were the only previously described triggers of PCD in the xylem, which induce the very rapid degradation of the nuclei and surrounding cytoplasmic contents, therefore unravelling a unique previously unrecorded type of PCD in the xylem fibres, principally involving autophagy. Arabidopsis is an attractive alternative model plant for exploring some aspects of wood formation, such as the characterisation of negative regulators of PCD. Therefore, the anatomy of Arabidopsis hypocotyls was also investigated and the ACAULIS5 (ACL5) gene, encoding an enzyme involved in polyamine biosynthesis, was identified as a key regulator of xylem specification, specifically in the vessel elements, though its negative effect on the cell death process.

Taken together, PCD in xylem development seems to be a highly specific process, involving unique cell death morphology and molecular machinery. In addition, the technical challenges posed by the complexity of the woody tissues examined highlighted the need for specific methods for assessing PCD and related phenomena in wood.

Abstract [sv]

Oron för klimatförändringar och brist på fossila bränslen har ökat påtagligt under de senaste åren. De enorma möjligheter som skogsråvaran erbjuder som alternativ källa för förnyelsebar energi och råmaterial har väckt ett stort intresse också för den biologiska processen bakom vedbildning i träd. Denna avhandling fokuserar på en viktig process i vedbildning: programmerad celldöd (PCD) i xylemet. Xylemcellernas livstid påverkar bildningen av sekundära cellväggar, vilket i sin tur påverkar vedens kvalitativa egenskaperna, så som veddensitet. Trots dess betydelse för viktiga egenskaper hos vedråvaran existerar fortfarande väldigt lite information om xylem PCD på cellulär eller molekylär nivå.

I den här avhandlingen belyses de anatomiska, morfologiska och genetiska aspekterna av PCD i xylemutveckling i både stam av hybridasp, Populus tremula (L.) x tremuloides (Michx.) och hypokotyl av det örtartade modellsystemet Arabidopsis thaliana (L. Heynh.). Xylemet i både Populus och Arabidopsis består av två olika celltyper; de vattentransporterade kärlen och de stödjande fibrerna. Det är känt att celldöd i kärlen pågår mycket snabbt efter att den centrala vakuolen brister och de hydrolytiska enzymer släpps in i cytoplasman. I den här avhandlingen ligger fokus på fibrerna i Populus xylemet. Med hjälp av mikroskopianalyser av cellmorfologin (elektronmikroskopi) och DNA-fragmentering i cellkärnan (TUNEL- och Comet-analyser) kunde vi konstatera att till skillnad från kärlen så uppvisar fibrerna en långsam och progressiv nedbrytning av organellerna och cellkärnans DNA före vakuolbristning. Dessutom har kandidatgener för reglering av fibercelldöd identifierats antingen från ett Populus EST bibliotek från vedartade vävnader som genomgår fibercelldöd eller från mikroarray experiment i Populus stam. Dessa kandidatgener är antingen potentiella nya regulatorer av fibercelldöd eller medlemmar av tidigare beskrivna familjer av celldödsrelaterade gener. Bland de sistnämnda finns autofagi-relaterade gener, vilket stöder funktionen av autofagi i samband med autolys av cellinnehållet i xylemfibrerna. Dessa studier pekar därför på en typ av PCD som har inte tidigare beskrivits för xylemet. Arabidopsis är ett alternativt växtmodellsystem för studier av vissa aspekter av vedbildningen, såsom karakteriseringen av negativa regulatorer av PCD. Därför har också hypokotylanatomin analyserats, och ACAULIS5 (ACL5) genen, som kodar för ett enzym i biosyntesen av polyaminer, har visats vara en viktig regulator av xylemspecifikation genom dess negativa effekt på kärlens celldöd.

Sammantaget visar denna avhandling att PCD i xylemutvecklingen verkar involvera unika morfologiska och molekylära mekanismer. Vi visar dessutom att komplexiteten hos de vedartade vävnaderna leder till ett behov av bättre anpassade verktyg för att djupare kunna bedöma PCD och liknande fenomen i veden.

Place, publisher, year, edition, pages
Plant Physiology Department - Umeå Plant Science Centre , 2008. , 58 p.
Keyword [en]
PCD (Programmed Cell Death), Xylem, Apoptosis, Autophagy, Secondary Cell Walls, Microscopy, Microarrays, Comet Assay, TUNEL Assay
National Category
Biochemistry and Molecular Biology
Identifiers
URN: urn:nbn:se:umu:diva-1831ISBN: 978-91-7264-640-7 (print)OAI: oai:DiVA.org:umu-1831DiVA: diva2:142146
Public defence
2008-10-10, KB3B1, KBC-Huset, floor 3, Umeå University, Umeå - Sweden, 13:00 (English)
Opponent
Supervisors
Note
Även med namnet Moreau-Courtois, Charleen L. samt Moreau, Charleen.Available from: 2008-09-19 Created: 2008-09-18 Last updated: 2009-06-22Bibliographically approved
List of papers
1. A genomic approach to investigate developmental cell death in woody tissues of Populus trees
Open this publication in new window or tab >>A genomic approach to investigate developmental cell death in woody tissues of Populus trees
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2005 (English)In: Genome Biology, ISSN 1465-6906, Vol. 6, no 4, R34:1-14 p.Article in journal (Refereed) Published
Abstract [en]

Background

Poplar (Populus sp.) has emerged as the main model system for molecular and genetic studies of forest trees. A Populus expressed sequence tag (EST) database (POPULUSDB) was previously created from 19 cDNA libraries each originating from different Populus tree tissues, and opened to the public in September 2004. We used this dataset for in silico transcript profiling of a particular process in the woody tissues of the Populus stem: the programmed death of xylem fibers.

Results

One EST library in POPULUSDB originates from woody tissues of the Populus stem where xylem fibers undergo cell death. Analysis of EST abundances and library distribution within the POPULUSDB revealed a large number of previously uncharacterized transcripts that were unique in this library and possibly related to the death of xylem fibers. The in silico analysis was complemented by a microarray analysis utilizing a novel Populus cDNA array with a unigene set of 25,000 sequences.

Conclusions

In silico analysis, combined with the microarray analysis, revealed the usefulness of non-normalized EST libraries in elucidating transcriptional regulation of previously uncharacterized physiological processes. The data suggested the involvement of two novel extracellular serine proteases, nodulin-like proteins and an Arabidopsis thaliana OPEN STOMATA 1 (AtOST1) homolog in signaling fiber-cell death, as well as mechanisms responsible for hormonal control, nutrient remobilization, regulation of vacuolar integrity and autolysis of the dying fibers.

Identifiers
urn:nbn:se:umu:diva-13063 (URN)doi:10.1186/gb-2005-6-4-r34 (DOI)
Note
Även med namnet Courtois-Moreau, Charleen, Laetitia samt Moreau-Courtois, Charleen L.Available from: 2007-05-23 Created: 2007-05-23 Last updated: 2015-04-29Bibliographically approved
2. ACAULIS5 controls Arabidopsis xylem specification through the prevention of premature cell death.
Open this publication in new window or tab >>ACAULIS5 controls Arabidopsis xylem specification through the prevention of premature cell death.
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2008 (English)In: Development, ISSN 0950-1991, E-ISSN 1477-9129, Vol. 135, no 15, 2573-82 p.Article in journal (Refereed) Published
Abstract [en]

Cell size and secondary cell wall patterning are crucial for the proper functioning of xylem vessel elements in the vascular tissues of plants. Through detailed anatomical characterization of Arabidopsis thaliana hypocotyls, we observed that mutations in the putative spermine biosynthetic gene ACL5 severely affected xylem specification: the xylem vessel elements of the acl5 mutant were small and mainly of the spiral type, and the normally predominant pitted vessels as well as the xylem fibers were completely missing. The cell-specific expression of ACL5 in the early developing vessel elements, as detected by in situ hybridization and reporter gene analyses, suggested that the observed xylem vessel defects were caused directly by the acl5 mutation. Exogenous spermine prolonged xylem element differentiation and stimulated cell expansion and cell wall elaboration in xylogenic cell cultures of Zinnia elegans, suggesting that ACL5 prevents premature death of the developing vessel elements to allow complete expansion and secondary cell wall patterning. This was further supported by our observations that the vessel elements of acl5 seemed to initiate the cell death program too early and that the xylem defects associated with acl5 could be largely phenocopied by induction of premature, diphtheria toxin-mediated cell death in the ACL5-expressing vessel elements. We therefore provide, for the first time, mechanistic evidence for the function of ACL5 in xylem specification through its action on the duration of xylem element differentiation.

Keyword
Arabidopsis/*cytology/genetics/growth & development/*metabolism, Arabidopsis Proteins/genetics/*metabolism, Asteraceae/genetics/metabolism, Biological Markers, Carrier Proteins/genetics/metabolism, Cell Death, Cell Differentiation, Cell Line, Gene Expression Regulation; Plant, Mutation/genetics, Vacuoles/metabolism, Xylem/*metabolism
Identifiers
urn:nbn:se:umu:diva-10576 (URN)doi:10.1242/dev.019349 (DOI)18599510 (PubMedID)
Note
Även med namnet Courtois-Moreau, Charleen L samt Moreau, Charleen.Available from: 2008-10-02 Created: 2008-10-02 Last updated: 2015-04-29Bibliographically approved
3. A unique program for cell death in xylem fibers of Populus stem
Open this publication in new window or tab >>A unique program for cell death in xylem fibers of Populus stem
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2009 (English)In: The Plant Journal, ISSN 0960-7412, E-ISSN 1365-313X, Vol. 58, no 2, 260-274 p.Article in journal (Refereed) Published
Abstract [en]

Maturation of the xylem elements involves extensive deposition of secondary cell-wall material and autolytic processes resulting in cell death. We describe here a unique type of cell-death program in xylem fibers of hybrid aspen (Populus tremula x P. tremuloides) stems, including gradual degradative processes in both the nucleus and cytoplasm concurrently with the phase of active cell-wall deposition. Nuclear DNA integrity, as determined by TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) and Comet (single-cell gel electrophoresis) assays, was compromised early during fiber maturation. In addition, degradation of the cytoplasmic contents, as detected by electron microscopy of samples fixed by high-pressure freezing/freeze substitution (HPF-FS), was gradual and resulted in complete loss of the cytoplasmic contents well before the loss of vacuolar integrity, which is considered to be the moment of death. This type of cell death differs significantly from that seen in xylem vessels. The loss of vacuolar integrity, which is thought to initiate cell degradative processes in the xylem vessels, is one of the last processes to occur before the final autolysis of the remaining cell contents in xylem fibers. High-resolution microarray analysis in the vascular tissues of Populus stem, combined with in silico analysis of publicly available data repositories, suggests the involvement of several previously uncharacterized transcription factors, ethylene, sphingolipids and light signaling as well as autophagy in the control of fiber cell death.

Keyword
autophagy, DNA degradation, in silico transcriptional analysis, Populus, programmed cell death, xylem fiber development
National Category
Biological Sciences
Identifiers
urn:nbn:se:umu:diva-22104 (URN)10.1111/j.1365-313X.2008.03777.x (DOI)000265040700007 ()19175765 (PubMedID)
Note

Även med namnet Moreau-Courtois, Charleen L. samt Moreau, Charleen.

Available from: 2009-04-23 Created: 2009-04-23 Last updated: 2015-04-29Bibliographically approved

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