the mode of development in animals and plants -...
TRANSCRIPT
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
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.
There are two categories of meristemsfound in plants
Apical
Lateral
"the meristem makes the plant."
Responsible forincrease ingirth
Responsible forincrease inlength
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
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
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?
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.
Intercalary meristem- found in monocot stems (particularly grass stems)
- meristem at the base of the internode
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.
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
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