The mode of development in animals and plantsis different

Outcome of animal embryogenesis is a mini edition ofthe adult

Outcome of plant embryogenesis is a simple structurewith-root apical meristem-embryonic root-hypocotyl-1 or 2 cotyledons-shoot apical meristemAll the other organs of the plant are formed postembryogenicallyin the meristems

In contrast with animal development, plant development isan ongoing process

“General Sherman” sequoia-2,200 years old and stillgrowing

WHY?

Plants cannot move out of their environment so theyreact to environmental changes (e.g. changes in light,temperature, soil conditions, water availability, daylength) by changing their growth and development

Plants grown underdifferent lightconditions

This weekIntroduction to meristems

1. What are meristems?

2. Where are the meristems in a plant?

3. When do meristems develop?

(4. How do we know which cells give rise to each tissue?)

What are Meristems?-’growing points’ in the plant from which developnew tissues and organs develop e.g the shootapical meristem gives rise to the stems, leaves,and reproductive structures

Meristems are tiny(rarely more than 250 micrometres)

1000 shoot apical meristems in an eppendorf test tube

Meristem sizes vary but are all very small

Meristems are tiny(rarely more than 250 micrometres)

1000 shoot apical meristems in an eppendorf test tube

The shoot apical meristem stays the same sizeas cells continually leave it and begin todifferentiate.

Meristems can be indeterminate (show nopredetermined limit to growth)

-in some species the meristems can grow forthousands of years

Methuselah (aBristlecone Pine,Pinus longaeva) isabout 4,841 yearsold (it was morethan 200years oldwhen the pyramidswere built).

California

As long as conditions are favorable, thevegetative apical meristems will form

phytomers

phytomermodular unitsconsisting of aleaf, bud andinternode (whichcontains ameristem).

How can meristems keep making newtissues and at the same time regenerate

themselves (self-perpetuate)?

Some meristematic cells remainundifferentiated and retain the capacityfor cell division indefinitely. These arecalled “stem cells” (or “initial cells”)

What are stem-cells?

Relatively undifferentiated cells that can divideto give rise to one daughter cell that continuesto be a stem cell and another that willdifferentiate into plant organs and tissues

Stem cell division and differentiation

How is the fate of the daughter cells decidedafter stem cell division?

The fate of each individual daughter cell isdetermined by its relative position. Those thatstay at the most apical position of the shootmeristem, that is in a “stem cell niche”, renewthe stem cell population, whereas those thatare displaced from this position differentiate.

Characteristics of plant stem-cells

-are found in meristems

-divide slowly

-are the ultimate source of the tissues thatmake up the plant body

-identity of stem cells is not an inherentproperty of the cell-lineage, but conferredby positional cues

Animal and plant stem-cells have differenttasks

Animal stem-cells - replenish highly specialized bodycells with limited life spans e.g. blood and skin cells

Plant stem-cells - provide material for formation ofentire new organs e.g. leaves, flowers and roots

-are found in “stem-cell niches”

-are maintained in an undifferentiated stateby signals from surrounding cells

Stem cells in plants have different tasksfrom those in animals but may be

regulated in a similar way.

2. Where are the meristems in a plant?

There are two categories of meristemsfound in plants

Apical

Lateral

"the meristem makes the plant."

Responsible forincrease ingirth

Responsible forincrease inlength

Apical and lateral meristems

Shoot apical meristems

-produces stems, leaves, and reproductive structures

Shoot apical meristems

-located at the extreme tip of the shoot

-flat or mounded region with small thin-walled cellscontaining dense cytoplasm but no large centralvacuoles

Cross section ofa shoot apicalmeristem

Cross section of a cabbage showing the shootapical meristem

Shoot apical meristems have different zones

CZ central zone contains the stem cellsRib zone generates the central tissues of the stemPZ peripheral zone generates organ primordia

CZ

Rib PZ

Primordia

-localised regions of the apical meristemthat have high levels of cell division andgive rise to cells with an identifiablefunction e.g. leaves

Leaves

Organ primordia form at the edge of themeristem

Cross section ofa shoot apicalmeristem

Primordia forming at the edge of themeristem

Scanningelectronmicrograph of ashoot apicalmeristem fromabove

Cross section of a shoot apical meristem

Shoot apical meristems are composed of up tothree distinct layers of cells: L1, L2, and

L3/corpus

L1 (outermost layer, 1 cell thick)L2 (lies beneath L1, 1 cell thick)L3 (inner most layer)

Cell division in the apical meristem

L1 and L2 cells divide anticlinaly to generate the epidermisand ground tissueL3 cells divide in any plane and generate the internaltissues of the shoot

L1L2L3

Each of the meristem layers (L1-L3) probablycontains 1-3 stem cells (shown darkly shaded)

Where are the stem cells in the shoot apicalmeristem?

Root apical meristems-produces the primary root only (no lateral organs)

primary root

Secondaryroot

In the root the meristem lies behind the root cap(in Arabidopsis it is about 0.25mm long)

Root cap (protects meristem)

Root Apical meristem

Epidermis

cortex

Vascular tissueCelldifferentiation

Cell elongation

Cell division

In the root of a young seedling stem cells in the rootdivide rapidly to produce long files (rows) of cells

Cross section ofa root apicalmeristem

There are two categories of meristemsfound in plants

Apical

LateralResponsible forincrease ingirth

Responsible forincrease inlength

Lateral Meristems

- cylindrical meristems found in shoots androots that result in secondary growth e.g.the vascular cambium located between thexylem and phloem in woody plants

- found in all woody and some herbaceousplants

Lateral Meristemslocated between thephloem and xylem.On its inside surface,vascular cambiumproduces new xylemcells, on its outsidenew phloem cells.

Cross section of a portion of treetrunk

Tree rings are generated by thevascular cambium

Intercalary meristem- found in monocot stems (particularly grass stems)

- meristem at the base of the internode

Rows of cells being produced by theintercalary meristem

Cross section ofa monocot stem

Intercalarymeristem

-only the apicalmeristem (node) isactive.  

-If the tip of the stemis removed, theuppermost intactintercalary meristembecomes the apicalmeristem Cross section of monocot stem

When do the meristems develop?

Primary meristemsShoot and root apical meristems arise duringembryogenesis.

Secondary meristemsOther meristems e.g.intercalary meristemsarise during post-embryonic development.This shows that in plants differentiated cellscan reverse their state and go back to beingundifferentiated meristem cells.

Not all meristems are indeterminate

As plants change from a vegetative to areproductive state so does the meristem.

vegetative reproductive

The transition to flowering involvesmajor changes in the pattern ofmorphogenesis and cell differentiationat the shoot apical meristem

During vegetative growth the Arabidopsis shootapical meristem produces phytomeres with veryshort internodes.

As reproductive development is initiated, thevegetative meristem is transformed into andeterminate inflorescence meristem.

Floral meristems are produced on the sides of theinflorescence meristem.

Vegetative meristem to Inflorescence meristemto Floral meristem

Arabidopsis inflorescence meristem withfloral meristems produced at the sides

Scanning electron micrograph of an Arabidopsisinfloresense meristem

Stem cellsOrgan primordiaforming (in thiscase floral budswith floralmeristem )

floral bud

sepals

petalanther style

ovary

sepal

stigma

1 mm

The anatomy of an Arabidopsis flower

filament

stamencarpels

4. How do we know which cellsgive rise to each tissue?

Label a single cell and follow its fate

To label a single cell

-make transgenicplants with the35S::Ac::GUSconstruct

-when the mobile Acelement is excised the35S promoter will“switch on” the GUSgene

Label a single cell and follow its fateA single cell in an inflorescence meristem is labeled,this results in one floral meristem being labeled.

L1 L2 L3

GUS staining

-of crosssections offloralmeristem

-wholeflowers

Contributions of the meristem layers toArabidopsis flowers

ovaryL1L2L3

Diagram of the contributions of themeristem layers to Arabidopsis flowers

anther style

sepal

stigma

1 mm

filament

petal

Next lecture: How does the plantmaintain a stem cell population?