Download Shoot apical meristem

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 organization - Phytomeres
Phytomer =
modular unit
of the Shoot
Shoot
organization
Protoderm
Apical
Meristem
Primary
Meristems
Protoderm
Procambium
Ground Meristem
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
Apical Meristem
Examples
Equisetum
Conifers
Angiosperms
Shoot apical meristem organization
Stem Cells
Central Zone
Organizing Center
L1 = tunica
L2 = tunica
L3 = corpus
Peripheral Zone
Pith or Rib
Meristem
Chimeras & Organization of the Shoot Apical Meristem
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
Gene Expression in the Apical Embryo Domain
WUSCHEL (WUS), CLAVATA (CLV) AND SHOOT MERISTEMLESS (STM)
Laux, T., et al. Plant Cell 2004;16:S190-S202
Copyright ©2004 American Society of Plant Biologists
Shoot Development II: Genetics
WUSCHEL gene phenotype
Wild Type
wus mutant
Wild type SAM
wus SAM
Shoot Development II: Genetics
WUSCHEL (WUS) gene Maintains Stem Cell Population
wus mutants result in
1. early termination of SAM
 wus defective in maintaining SAM
2. aberrant SAM organization
 wus defective in maintaining SAM integrity
Fig 6
L1
L2
L3
WUS function:
1. WUS Protein
product is a
homeodomain
transcription factor
2. Gene regulation
Localization of WUS
gene product in
organizing center
(OC) of shoot
3. Positional influence
of once cell type by
another
Shoot Development II: Genetics
CLAVATA gene phenotype
Normal SAM
CLV localization in
Central Zone of SAM
1. CLV localization in Central Zone of SAM
clavata SAM
2. Mutant phenotype: Huge apical meristems
 CLV wild type restricts stem cell accumulation
Shoot Development II: Genetics
CLAVATA gene mechanism
Protein-binding
motif
Signal transduction pathway
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
Gene Interaction: WUSCHEL and CLAVATA
Initiation of an Organizing Center in the shoot apical meristem
CLV3 expression
WUS expression
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)
mRNA Expression Domains and gene interaction
for CLV1 CLV3 and WUS
Wild type
clv
mutant
wus
mutant
Overexpressed
WUS mutant
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)
Gene interaction CLAVATA and WUSCHEL in the shoot apical meristem
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 Regulation: stm mutation
Wild type shoot apex
STM wild-type prevents cell
differentiation in Peripheral Zone
Wild type SAM
Shoot Meristemless (stm) phenotype
1. SAM terminates prematurely
2. rapid depletion of stem cells,
faster than they are replenished
STM gene
stm mutant
stm mutant SAM
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.
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.
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
End Shoot
Development