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Shoot Development A. Function and organization of the apical meristem B. Origin of primary meristems C. Gene regulation at the meristem D. Leaf Initiation.

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Presentation on theme: "Shoot Development A. Function and organization of the apical meristem B. Origin of primary meristems C. Gene regulation at the meristem D. Leaf Initiation."— Presentation transcript:

1 Shoot Development A. Function and organization of the apical meristem B. Origin of primary meristems C. Gene regulation at the meristem D. Leaf Initiation E. Secondary Growth in stems?

2 Shoot organization - Phytomeres Phytomer = modular unit of the Shoot

3 Shoot organization Protoderm Apical Meristem Primary Meristems Protoderm Procambium Ground Meristem

4 Shoot apical meristem - Importance Center of postembryonic growth & development Source of all primary meristems –Protoderm, ground meristem & procambium Source of –Leaves –Branches –Tendrils –Thorns Self-renewing mass of cells  stem cells Balance cell division and cell differentiation

5 Apical Meristem Examples Equisetum ConifersAngiosperms

6 Shoot apical meristem organization L1 = tunica L2 = tunica Peripheral Zone Pith or Rib Meristem Central ZoneStem CellsOrganizing Center L3 = corpus

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8 Chimeras & Organization of the Shoot Apical Meristem

9 Shoot Development Organization of the Meristem 1.Periclinal chimeras at the SAM tell us that SAM layers have distinct lineages 1.Nuclear chimeras 2.Genetic markers 1.L1 marker: Arabidopsis thaliana meristem layer 1 (ATML1) 2.Mericlinal chimeras at the SAM tell us 1.Central zone stem cells are not permanent 2.A few cells (vs many) populate the central zone 3.Domains/tiers of cells may populate the SAM

10 Copyright ©2004 American Society of Plant Biologists Laux, T., et al. Plant Cell 2004;16:S190-S202 Gene Expression in the Apical Embryo Domain WUSCHEL (WUS), CLAVATA (CLV) AND SHOOT MERISTEMLESS (STM)

11 Shoot Development II: Genetics WUSCHEL gene phenotype Wild Type wus mutant Wild type SAM wus SAM

12 Shoot Development II: Genetics WUSCHEL (WUS) gene Maintains Stem Cell Population Localization of WUS gene product in organizing center (OC) of shoot L1 L2 L3 Fig 6 wus mutants result in 1. early termination of SAM  wus defective in maintaining SAM 2. aberrant SAM organization  wus defective in maintaining SAM integrity WUS function: 1. WUS Protein product is a homeodomain transcription factor 2. Gene regulation 3. Positional influence of once cell type by another

13 Normal SAM clavata SAM Shoot Development II: Genetics CLAVATA gene phenotype 1. CLV localization in Central Zone of SAM 2. Mutant phenotype: Huge apical meristems  CLV wild type restricts stem cell accumulation CLV localization in Central Zone of SAM

14 CLAVATA GENE Characteristics CLV1 – Extracellular polypeptide: 96 amino acids Restricted to L1, L2 of SAM Central Zone CLV2 – Membrane-bound protein receptor with a protein-binding motif CLV3 – Membrane-bound protein receptor with a protein-binding motif and Restricted to L1, L2 of SAM Central Zone Kinase activity… signaling… Kinase cascade Inhibitory to WUS expression Shoot Development II: Genetics CLAVATA gene mechanism Protein-binding motif Signal transduction pathway

15 Gene Interaction: WUSCHEL and CLAVATA Initiation of an Organizing Center in the shoot apical meristem 1. OC precursor lineage established in 4 subepidermal cells of 16 cell proembryo as indicated by expression of WUS (red) 2. Stem cells of Central Zone induced by heart stage as indicated by expression of CLV3 gene (blue) CLV3 expression WUS expression

16 mRNA Expression Domains and gene interaction for CLV1 CLV3 and WUS clv mutant Wild type Wild-type mRNA expression domains illustrate location of gene expression. 1. WUS – under stem cells of Central Zone 2. CLV3 – stem cells of Central Zone above OC (produces extracellular protein) 3. CLV1 – Organizing Center (OC) & vicinity (produces membrane-bound protein) wus mutant Overexpressed WUS mutant

17 1.WUS gene 1.Where? Organizing Center of Central Zone (just a few cells) 2.Function? Molecular: Encodes homeodomain protein 3.Function? Molecular Genetic: Induces Expression of CLV3 4.Function? Developmental: WUS specifies stem cells of the SAM, i.e. maintains stem cells and maintains stem cell identity. 2.CLV3 gene: 1.Where? Stem cells above Organizing Center 2.Function? Molecular: Encodes peptide secreted in extracellular space 3.Function? Molecular Genetic: Inhibits WUS expression. 4.Function? Developmental: CLV3 restricts size of Central Zone, i.e. CLV3 restricts size of the stem cell population. Gene interaction CLAVATA and WUSCHEL in the shoot apical meristem

18 Gene Regulation: stm mutation STM wild-type prevents cell differentiation in Peripheral Zone STM gene Where? Stem cells of Central Zone and peripheral Zone. Function? Molecular: Molecular:Encodes homeodomain protein – KNOTTED Class Function? Molecular Genetic: Transcription factor Function? Developmental: Prevents premature differentiation of cells from Peripheral Zone. stm mutant Wild type shoot apex Wild type SAM stm mutant SAM Shoot Meristemless (stm) phenotype 1. SAM terminates prematurely 2. rapid depletion of stem cells, faster than they are replenished

19 WUS, CLV and STM expression in the shoot apex 1.WUS gene 1.Where? Organizing Center of Central Cells (just a few cells) 2.Function? Molecular: Encodes homeodomain protein 3.Function? Molecular Genetic: Induces Expression of CLV3 4.Function? Developmental: WUS specifies stem cells of the SAM, i.e. maintains stem cells and maintains their identity. CLV3 gene 1.Where? Stem cells of Central Zone 2.Function? Molecular: Encodes peptide secreted in extracellular space 3.Function? Molecular Genetic: Inhibits WUS expression. 4.Function? Developmental: CLV3 restricts size of Central Cells, i.e. CLV3 restricts size of the stem cell population. 1.STM gene 1.Where? Through SAM apical “dome” of cells: central zone and peripheral zone. 2.Function? Molecular:Encodes homeodomain protein 3.Function? Molecular Genetic: Blocks organ formation genes (AS1, AS2) 4.Function? Developmental: Prevents premature differentiation of cells from Peripheral Zone… thus prevents premature organ initiation.

20 Summary: Shoot Apical Meristem Genetic Influences on Development 1. Organizing Center (OC): Maintains Stem Cells a. cells expressing WUS gene confers stem cell fate to overlying stem cells b. cells above OC target stem cells by preferred connections through plasmodesmata 2. Stem Cells control boundaries of stem cells a. Surgical expts. Demonstrate “release” from inhibition of differentiating “daughter” cells b. CLV3 gene inhibits WUS gene expression 3. Daughter cells of stem kept in undifferentiated state a. STM gene keeps daughter cells in an undifferentiated state. b. Daughter cells increase to sufficient numbers before organ formation

21 End Shoot Development


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