TILAK. I SPG14AGR6074

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Introduction

Meristem

SAM

RAM

Conclusions

contentsMolecular Analysis

Of SAM & RAM

INTRODUCTION

• Plant Tissues fall into two large categories:

Meristematic.

Permanent (Non-Meristematic)

Term”meristem” was first used in

1858 by Carl Wilh. V. Nagell

Greek word ”merizein ” to divide.

Contains undifferentiated cells (meristematic cells).

Found in zones of the plant where growth can take place.

Epidermal (L1) Subepidermal (L2) Tunica.

(L3) Inner most layer corpus

meristem

Tunica determine the physical characteristics of the leaf edge and margin.

Positional information (auxin

accumulation) precedes leaf initiation

Auxin accumulation precedes leaf initiation

Indole-3-acetic acid (IAA) a naturally occurring auxin

A boundary forms by the action of

mutually antagonistic genes

ARP

KNOX1

KNOX1 genes, expressed in the

meristem,

ARP genes, expressed in the leaf

primordia, are mutually repressive,

and help establish a separate identity

for the emerging leaf primordium

Apical meristems(Growing tip)

• Found in the buds and growing tips of roots in plants.

1) Growth of new cells in young seedlings at the tips of roots and shoots .

2) An active apical meristem lays down a growing root or shoot behind itself.

Apical Meristems

• In plants belonging to the MONOCOT , apical meristems are located ONLY in the root tips.

• REVOLUTA gene is necessary for Apical meristemdevelopment

STM and CUC1 Expression (Apical Meristem)

• CUC1 redundantly required for embryonic apical meristemformation, cotyledon separation and expression of STM..

Cup shaped cotyledon

Intercalary Meristems

• Plants in the MONOCOT class have special meristems called intercalary meristems.

• Lateral meristems

• Cause SECONDARY GROWTH.

• meristems located in the shoot and root tips, plants in the DICOT class have lateral meristems.

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(Jean et al.,2007) 12

Shoot apical meristems

• 14 days later, leaves have developed (right).

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

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The shoot apical meristem consists of 4 distinct cell groups:

Stem cells.

The immediate daughter cells of the stem cells.

A subjacent organising centre.

Founder cells for organ initiation in surrounding regions.

are maintained by a complex signalling pathway

Shoot apical meristem organization

L1 = tunica

L2 = tunica

Peripheral Zone

Pith or Rib Meristem

Central Zone

Stem Cells

Organizing Center

L3 = corpus

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Development involves positional

informationAuxin gradients are regulated by auxintransporters)

Movement of a signal away from a source

… selective destruction of a signal by miRNAcleavage of mRNAs)

REVOLUTA gene is necessary for Apical meristem development

Development involves boundaries

Shoot apical meristem

Boundaries form between the shoot apical meristem and leaf primordia, and between the upper and lower surfaces of the leaf

Patterning of cells in the epidermis also involves production of inhibitory signals

Genes necessary for (SAM)

SHOOT- MERISTEMLESS (STM) Late globular embryo.

WUSCHEL (WUS) Early Globular stage @ cells below the stem cells

CLAVATA1 (CLV1)

CLAVATA3 (CLV3)

• SAM becomes distinquishable at torpedo stage.

Early Heart – shaped embryo Size ,Cell divisionSignalling

Clavata’s Molecular Mechanism

• CLAVATA 1, 2 and 3 mutants have identical phenotypes of enlarged meristems

• CLAVATA3 protein acts as a signal molecule.

• The CLAVATA receptor is a leucine rich repeat (lrr) serine/threonine kinasereceptor.

24(Thomas et al.,2008)

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 : GeneticsCLAVATA gene mechanism

Protein-binding motif

Signal transduction pathway

WUSCHEL(WUS) molecular mechanism

• wus mutants result in1. 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 homeodomaintranscription factor

2. Gene regulation

3. Positional influence of once cell type by another

Localization of WUS geneproduct in organizingcenter (OC) of shoot

L1L2L3

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Shoot Development : GeneticsWUSCHEL gene phenotype

Wild Type

wus mutant

Wild type SAM

wus SAM

mutants but it fails to develop

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 ( Gene Regulation )

( Cell division )

mRNA Expression Domains and gene interactionfor 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

A feedback look between WUS and CLV exists because:

1. Expansion of the WUS expression domain in a clvmutant suggests a feedback loop between the twogenes.

2. wus mutants have downregulated (smaller) CLV3expression domain.

3. WUS overexpressed mutants have a broader CLV3expression domain.

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

STM molecular mechanism

– Stem cells of Central Zone andperipheral Zone.

– Encodes homeodomain proteinKNOTTED Class

– Transcription factor

– Prevents prematuredifferentiation of cells fromPeripheral Zone.

Wild type Stm mutant

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Some Genes Involved in Root-Shoot Formation

Both shoot and root meristems are apical meristems, but are independently controlled

-Shootmeristemless (STM) is necessary for shoot formation, but not root development

STM wild type

stm mutant

-STM encodes a transcription factorwith homeobox region Cotyledons not mature leaves

are shown

Laux, T., et al. Plant Cell 2004;16:S190-S202

Gene Expression in the Apical Embryo DomainWUSCHEL (WUS), CLAVATA (CLV) AND SHOOT MERISTEMLESS (STM)

Root apical meristem

1 - Quiescent center

2 - Calyptrogen (live rootcap cells)

3 – Rootcap

4 - Rootcap cells

5 - Procambium

• The root apical meristem produces cells in two dimensions.

• It harbors two pools of stem cells around an organizing center called the quiescent center(QC) cells and together produce most of the cells in an adult root.

• QC : low mitotic activity.

Act as a reservoir of stem cells

• Root cap: protects and guide its growth

RAM

Root Cells

• Root structure

Root cap

• composed of inner columella cells and lateral root cap cells

• New root cap produced when existing cap is removed

• Zone of maturation

– cells differentiate into specific cell types

• root surface cells mature into epidermal hairs, each with root hair

• cortex produced by parenchyma cells– inner boundary differentiates into endodermis

» surrounded by Casparian strips composed of suberin

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The HOBBIT gene is required for root meristem, but not shoot meristem formation

Hobbit is a protein that inhibits another protein that stops the gene expression of the genes that Auxin causes to be made!!!!

Some Genes Involved in Root-Shoot Formation

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Two Internal Proteins Responsible for the Development of a Structure Cause Similar

Phenotypes if their corresponding genes are mutated

Has a basal peg not a rootAbnormal celldivision create stubrather than a root

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Auxin and Monopteros Promote Root Development

One way that auxin induces

gene expression is

by activating the

MONOPTEROS

(MP) protein

-Auxin releases the

repressor from MP

-MP then activates the

transcription of a

root development gene

• ”Auxin maximum” is required for RAM development.

• Root apical meristem protected by root cap.

Strigolactones (SLs) : Regulates shoot & root development.

• RAM size is controlled by DA1-Related Protein2 in Arabidopsis.

• ERECTA genes regulate auxin transport in SAM.

• SCARECROW function reveals a radial patterning in root & shoot.

• MADS box genes : Helps in development of Roots, Leaf, Flower, Ovule & fruit.

Conclusion

• SAM

SHOOT-MERISTEMLESS (STM),

WUSCHEL (WUS),

CLAVATA1 (CLV1)

CLAVATA3 (CLV3).

ERECTA genes

• RAM

DA1-Related Protein2

MONOPTEROS (MP)protein

HOBBIT gene

MADS box genes

SCR Mutants

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References

Plant roots – Amaram Eshel.

Plant physiology – A.S.Gontia.

www.wikipedia.com

www.DocStock.com

www.ncbi.nlm.nih.gov

www.researchgate.net

www.sciencedirect.com

Reviews & Research articles.

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