shoot development a. function and organization of the apical meristem b. origin of primary meristems...
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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
E. Secondary Growth in stems?
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
Shoot apical meristem organization
L1 = tunica
L2 = tunica
Peripheral Zone
Pith or Rib Meristem
Central Zone
Stem Cells
Organizing Center
L3 = corpus
Shoot DevelopmentOrganization of the Meristem
1. Periclinal chimeras at the SAM tell us that SAM layers have distinct lineages
1. Nuclear chimeras
2. Genetic markers1. L1 marker: Arabidopsis thaliana meristem layer 1 (ATML1)
2. Mericlinal chimeras at the SAM tell us1. 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
Copyright ©2004 American Society of Plant Biologists
Laux, T., et al. Plant Cell 2004;16:S190-S202
Gene Expression in the Apical Embryo DomainWUSCHEL (WUS), CLAVATA (CLV) AND SHOOT MERISTEMLESS (STM)
Shoot Development II: GeneticsWUSCHEL (WUS) gene Maintains Stem Cell Population
Localization of WUS gene product in
organizing center (OC) of shoot
L1L2L3
Fig 6
wus mutants result in1. early termination of SAM
wus defective in maintaining SAM2. 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
Normal SAM
clavata SAM
Shoot Development II: GeneticsCLAVATA 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
CLAVATA GENE CharacteristicsCLV1 – Extracellular polypeptide: 96 amino acids
Restricted to L1, L2 of SAM Central ZoneCLV2 – Membrane-bound protein receptor with a protein-binding motifCLV3 – Membrane-bound protein receptor with a protein-binding motif and
Restricted to L1, L2 of SAM Central ZoneKinase activity… signaling… Kinase cascadeInhibitory to WUS expression
Shoot Development II: GeneticsCLAVATA gene mechanism
Protein-binding motif
Signal transduction pathway
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
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
OverexpressedWUS mutant
1. WUS gene1. Where? Organizing Center of Central Zone (just a few cells)2. Function? Molecular: Encodes homeodomain protein3. Function? Molecular Genetic: Induces Expression of CLV34. 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 space3. 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
Gene Regulation: stm mutationSTM wild-type prevents cell
differentiation in Peripheral Zone
STM geneWhere? Stem cells of Central Zone and peripheral Zone.Function? Molecular: Molecular:Encodes homeodomain protein – KNOTTED ClassFunction? Molecular Genetic: Transcription factorFunction? Developmental: Prevents premature differentiation of cells from Peripheral Zone.
stm mutant
Wild type shoot apex
Wild type SAM
stm mutant SAM
Shoot Meristemless (stm) phenotype1. SAM terminates prematurely2. rapid depletion of stem cells, faster than they are replenished
WUS, CLV and STM expression in the shoot apex
1. WUS gene1. Where? Organizing Center of Central Cells (just a few cells)2. Function? Molecular: Encodes homeodomain protein3. Function? Molecular Genetic: Induces Expression of CLV34. 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 Zone2. Function? Molecular: Encodes peptide secreted in extracellular space3. 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 protein3. 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.
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 cellsb. cells above OC target stem cells by preferred connections through plasmodesmata
2. Stem Cells control boundaries of stem cellsa. Surgical expts. Demonstrate “release” from inhibition of differentiating “daughter” cellsb. CLV3 gene inhibits WUS gene expression
3. Daughter cells of stem kept in undifferentiated statea. STM gene keeps daughter cells in an undifferentiated state.b. Daughter cells increase to sufficient numbers before organ formation
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