Cell and Developmental BiologyModule BS1003

Plant Cell and Developmental Biology

Pat Heslop-Harrison phh4@le.ac.uk#BS1003 on Google+

Plant Development and Meristems

From Jim Haseloff / Gerd Jürgens, Tübingen

FernsGymnospermsAmborellaWaterliliesBasal Magnoliids

Monocots

Eudicots

Seed plant Tree of LifeSearch APGIII

http://www.mobot.org/mobot/research/apweb/

Where does a multicellular organism come from?

• Single-celled embryo

• Plant: usually a seed

• What to study?– Model species: principles are

universal• Comparative analysis

Embryogenesis in Arabidopsis

A. Asymmetric first division B-D. Cells have different FATES Suspensor > transverse divisionsEmbryo > precise cell divisions D. OCTANT: suspensor & embryo lineagesE-F. GLOBULAR: protoderm forms> future epidermis;G. HEART: cotyledons initiated& root meristemH. TORPEDO: axis extension > future hypocotylI. COTYLEDONARY: cotyledon greening, vascular tissues

A D E F

GH I

B C

SKILL: Drawing, emphasizing theimportant parts, not artistic quality!TERMINOLOGY:

Zygote

• From Chun-Ming Liu via biology.kenyon.edu http://tinyurl.com/embryogenesis Arabidopsis Embryogenesis

From Jim Haseloff / Gerd Jürgens, Tübingenhttp://www.plantsci.cam.ac.uk/Haseloff/teaching/PlantSci2_index/notes/notes.html

http://tinyurl.com/Haseloff Lectures 1 and 3 and http://tinyurl.com/embryogenesis

Disturbed apical basal polarity & No bilateral symmetryEarliest defect at first division of zygote > symmetrical

divisionDivision symmetry linked to a change in cell fateSupports hypothesis of asymmetric distribution of cell

fate determinants

wt

gnomgnom

wt

Early embryo development in gnom

gnomForms ball-like embryos

without apical/basal organs

Lacks ability to establish polarity & morphogenesis

GNOM GENE INVOLVED IN EMBRYO DEVELOPMENT (POLARITY)

Mutants are useful tools to understand development

Gerd Jürgens, Tübingen

Summary: Two components of embryogenesis1. PATTERN FORMATION Embryo must establish Polarity (apical-basal &

radial) > Control of cell divisionEmbryo must achieve correct shape

(morphogenesis)> Control of cell division & expansion

2. DIFFERENTIATION(Cells in different regions become specialized

Eg. Chloroplast differentiation in cotyledons/vascular tissues in hypocotyl, radicle & cotyledons)

> Control of Cell Fate???

Building the plant bodyplan…….Complex processes repeated with great precision in

every developing embryoControl mechanisms must also be very carefully co-

ordinated.

Is plant cell fate controlled by:

(1) Segregation (or inheritance) of determinants at each division?

(2) Positional information?

By the end of this lecture you will:

1. Use four ways to understand and study developmental processes

2. Understand pattern formation and plant embryogenesis

3. Know about the structure and development of plant meristems

4. Know about totipotency and cell development

5. Have insight into cell function and communication

Meristems & Organogenesis

Does form reflect function?

What is growth? How is growth

controlled? Where do the major

organ systems of the plant

originate?

How are they generated?

Brooker Chapters 35 & 36

Campbell & Reece Section 6

Raven section VI Chapters 36 & 42

Questions…

Evidence for the role of positional information in specifying cell fate

PLANT TISSUE CULTURES

Cells within differentiated tissues, such as leaf tissue can be induced to REDIFFERENTIATE into a completely new embryo or plant, containing the FULL RANGE OF CELL TYPES

ie Change the relative position (local signals) of cells in the leaf

totipotency!Haberlandt (Austria): The results of attempts to culture isolated vegetative cells from higher plants in simple nutrient should give insight to the properties and potentialities which the cell as an elementary organism possesses … “I am not making too bold a prediction if I point to the possibility that, in this way, one should successfully cultivate artificial embryos from vegetative cells” (1902)

PLANT TISSUE CULTURE

Single cell regeneration demonstrates totipotency PLANT CLONING

Herbert Street (Leicester), Ted Cocking/Mike Davey (Nottingham), Nitsch, Steward, Maheshwari, Skoog: defining nutritional and developmental aspects of cultured plant cells: early 1970s

What does this mean for plants?

1. Differentiated plant cells are usually NOT irreversibly committed

2. They contain all the genetic information necessary for all aspects of plant development.

3. There is no loss of genetic information during development

4. Their relative ‘position’ is important in signalling to maintain their fate.

A cell is totipotent if it is has the ability to divide and re-differentiate to form a whole

organism

In contrast we easily cannot change the fate of differentiated animal cells without drastic measures

Nuclear transplant > genetic re-programming of udder cell nucleus in enucleated egg cytoplasmI am a clone

Plant Cells > change of position (external signals) sufficient to re-programme the nucleusCloning in plants is comparatively easy

What you need to know:• Pattern formation (embryogenesis)

– Materials and methods• Model species (Arabidopsis, tobacco)• Experimental biology• Mutants• Evolution• DNA sequence analysis

– Growth and development• Essential processes

– From the single-celled zygote to the embryo

• Asymmetrical first division• Pattern formation and polar/radial symmerty

– From the embryo to plant with reiteration of patterns

• Positional information in cell fate– Totipotency and regeneration

MERISTEM A spatially restricted region within an

organ in which cell division for growth

occurs

Leaf primordiumShoot apical meristem

Meristems are vital!All POSTEMBRYONIC development in plants

occurs from meristemsGive rise to all major organ systems > roots, stems, leaves, flowers

Primary meristems

• Shoot Apical Meristem• Root Apical Meristem

• SAM and RAM produce additional meristematic tissue that increases plant length and produces new organs

• Primary meristems produce primary tissues and organs of diverse types

Tissues in plants

D: (Epi-)DermisV: VascularG: Ground

• SAM and RAM both produce– Protoderm – generates dermal tissue– Procambium – produces vascular tissues– Ground meristem – produces ground

tissues defined by location• Plant cell specialization and tissue

development do not depend much on the lineage of a cell or tissue

• Chemical influences are much more important

Stem development and structure• New primary stem tissues arise by the cell

division activities of primary meristems located near the bases of SAMs

• Epidermis develops at the stem surface– Produces a waxy cuticle (reduces water loss,

protects plant) • Cortex – composed of parenchyma tissue

– Composed of only one cell type, parenchyma cells

– Stores starch in plastids • Stem parenchyma also has the ability to

undergo cell division (meristematic capacity) to heal damage

Vegetative growth

• Production of tissues by SAM and RAM and growth of mature plant

• Plant shoots produce vegetative buds – miniature shoots having a dormant SAM

• Under favorable conditions, buds produce new stems and leaves

• Indeterminate growth – SAMs continuously produce new stem tissue and leaves as long as conditions are favorable

Plant growth & morphogenesis

requires co-ordination of 3 key cellular processes which occur within

and around the meristem

RATE of CELL DIVISIONPLANE of CELL DIVISION

DIRECTION of CELL EXPANSION

Fasciation - loss of control of meristem size

How might cell division and expansion be co-ordinated to maintain meristem size and activity?

Positional information might be exchanged between cells

Question…

Are cells in the meristem interconnected?

Communication between meristem cell layers

PlasmodesmataMembranes from adjacent cellsconnect through a pore inthe cell wall

Transmission electron micrograph

Summary

• SAM is the site of organ initiation

• Major activity of the SAM is cell division

• Meristem size/shape must be maintained• Otherwise there would be CHAOS!

• Involves coordinated control of rates and

planes of cell division in different regions

• Communication between different cell

layers via plasmodesmata which traverse

the cell wall

http://www2.mcdaniel.edu/Biology/botf99/tissimages/meristematic.html

Root meristems

ROOT APICAL MERISTEM (RAM) ORGANISATION

SIMPLER IN ORGANIZATION THAN SHOOT APEX

CELLS ARRANGED IN FILES

NO LATERAL ORGANS FORMED

NEAR APEX

ROOT CAP PRESENT

HOW DO CELL FILES ARISE?

Most division in apical regionLess division below the apex A group of cells that divide

infrequently:

QUIESCENT CENTRE

Divisions take place at the PERIPHERY of the QUIESCENT CENTRE

INITIAL OR STEM CELLS

Cells DIFFERENTIATE as they expand.

ROOT BRANCHINGLATERAL ROOTS emerge

further back behind the apical meristem from the PERICYCLE CELL LAYER

Establishment of a NEW MERISTEM

DEVELOPMENTAL RESPONSE TO AN ENVIRONMENTAL SIGNAL water/nutrient suppliesAuxin signalling

EpidermisCortexEndodermisPERICYCLEStele

Other meristems [Allow propagation

via cloning]

KalanchoeMeristems formed at the leaf

marginsGenetically identical progeny

(mitotic divisions)Vegetative reproduction

• Roots, stems and leaves can function in asexual reproduction– Kalanchoe leaves

form plantlets, sucker shoots, potato “eyes”, banana suckers or spears

• Apomixis – fruits and seeds are produced in the absence of fertilization– Meiosis produces

diploid megaspores (no meiosis II)

Meristems & Organogenesis

Where do the major organ

systems of the plant

originate?Meristem: A spatially restricted region within an organ in which cell division for growth occursPOSTEMBRYONIC development in plants occurs from meristemsRoot and Shoot Apical MeristemsRate & Plane of cell division; direction of expansionCell communication