Lecture 9 on plant development and meristems. Heslop-Harrison for BS1003 Cell and Developmental Biology, University of Leicester

1. Cell and Developmental Biology
Module BS1003
Plant Cell and Developmental Biology
Pat Heslop-Harrison
phh4@le.ac.uk
#BS1003 on Google+
Plant Development and Meristems

3. From Jim Haseloff / Gerd Jürgens, Tübingen

6. Ferns
Gymnosperms
Amborella
Waterlilies
Basal Magnoliids
Monocots
Eudicots
Seed plant Tree of Life
Search APGIII
http://www.mobot.org/mobot
/research/apweb/

8. Where does a multicellular
organism come from?
• Single-celled embryo
• Plant: usually a seed
• What to study?
– Model species: principles are universal
• Comparative analysis

9. Embryogenesis E
Zygote A B C D F
in Arabidopsis
A. Asymmetric first division
B-D. Cells have different FATES
Suspensor > transverse divisions
Embryo > precise cell divisions
D. OCTANT:
suspensor & embryo lineages
E-F. GLOBULAR: protoderm forms
> future epidermis;
G. HEART: cotyledons initiated
I
& root meristem H
H. TORPEDO: G
axis extension > future hypocotyl
I. COTYLEDONARY:
cotyledon greening, vascular tissues
SKILL: Drawing, emphasizing the
important parts, not artistic quality!
TERMINOLOGY:

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

11. From Jim Haseloff / Gerd Jürgens, Tübingen
http://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

12. wt
wt
gnom gnom
Disturbed apical basal polarity & No bilateral symmetry
Earliest defect at first division of zygote > symmetrical division
Division symmetry linked to a change in cell fate
Supports hypothesis of asymmetric distribution of cell fate
determinants

13. Early embryo development in gnom

14. Mutants are useful
tools to understand
development
gnom
Forms ball-like embryos
without apical/basal
organs
Lacks ability to establish
polarity & morphogenesis
GNOM GENE INVOLVED IN
EMBRYO DEVELOPMENT
(POLARITY)
Gerd Jürgens, Tübingen

15. Summary: Two components of embryogenesis
1. PATTERN FORMATION
Embryo must establish Polarity (apical-basal & radial)
> Control of cell division
Embryo 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???

16. Building the plant bodyplan…….
Complex processes repeated with great precision in
every developing embryo
Control 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?

17. 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

18. Meristems & Organogenesis
Questions…
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

19. 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)

20. 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

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

22. 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 cytoplasm
I am a clone
Plant Cells > change of
position (external
signals) sufficient to re-
programme the nucleus
Cloning in plants is
comparatively easy

23. 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

24. MERISTEM
A spatially restricted region within an
organ in which cell division for growth
occurs
Shoot apical meristem Leaf primordium

25. Meristems are vital!
All POSTEMBRYONIC development in plants occurs
from meristems
Give rise to all major organ systems
> roots, stems, leaves, flowers

28. 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

29. Tissues in plants
D: (Epi-)Dermis
V: Vascular
G: Ground

30. • 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

31. 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

32. 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

33. Plant growth &
morphogenesis requires
co-ordination of 3 key
cellular processes which
occur within and around
the meristem
RATE of CELL DIVISION
PLANE of CELL DIVISION
DIRECTION of CELL EXPANSION

36. Fasciation - loss of control of
meristem size

37. 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?

38. Communication between
meristem cell layers
Plasmodesmata
Membranes from adjacent cells
connect through a pore in
the cell wall

39. Transmission electron micrograph

40. 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

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

42. Root meristems

43. ROOT APICAL MERISTEM (RAM)
ORGANISATION
SIMPLER IN ORGANIZATION
THAN SHOOT APEX
CELLS ARRANGED IN FILES
NO LATERAL ORGANS FORMED
NEAR APEX
ROOT CAP PRESENT

44. HOW DO CELL
FILES ARISE?
Most division in apical region
Less 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.

45. ROOT BRANCHING
LATERAL ROOTS emerge
further back behind the
apical meristem from the
PERICYCLE CELL LAYER
Establishment of a NEW
MERISTEM
DEVELOPMENTAL RESPONSE
TO AN ENVIRONMENTAL
Epidermis
SIGNAL water/nutrient
Cortex
supplies Endodermis
Auxin signalling PERICYCLE
Stele

46. Other meristems
[Allow propagation
via cloning]
Kalanchoe
Meristems formed at the leaf
margins
Genetically identical progeny
(mitotic divisions)
Vegetative reproduction

47. • 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)

48. 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 occurs
POSTEMBRYONIC development in plants
occurs from meristems
Root and Shoot Apical Meristems
Rate & Plane of cell division; direction of
expansion
Cell communication