Oldenburgia grandis, Botany RU
a quick lesson in embryogenesis
sporophyte
seed
2N
MEIOSIS
FERTILIZATION
N
megaspores
sperm
egg
megagametophyte
microgametophyte
microspores
Where do we start?
gametogenesis
seed
embryo
totipotency
[differentiation, maturation]
initiation of SAM, RAM (polarity?)
[differentiation]
cell division, formation of axes
expansion growth
axis & coleoptiles form
-----------------
Seed to plant
Seed
monocotyledon
dicotyledon
activation of SAM & RAM
Embryo proper of
Capsella
cell division
(genes, hormones)
procambium
expansion, cell elongation
graviperception
phototropism
Post-Germination: Rapid growth
2h
2h 20min
Growth in this bean plant is due in part
to cell division, but, to a larger extent, to
longitudinal elongation of cells – all
controlled by genes.
2h 40
min
Apical organization
Organization in plants is dependent upon
programmed, controlled cell division, followed
by growth, further cell division and ultimately,
differentiation.
Programmed and controlled cell
division occurs within the domain of the
vegetative apex.
the apex
All the tissues within the apex differentiate rapidly. By
about 150 µm, cells within the apical region are
starting to differentiate. In the pine apex (above), you
can see developing leaflets.
The Coleus apex to the right, shows rapidly
developing leaflets beneath the apical dome.
cell division
Cell division is responsible for
the formation of all cells and
tissues in the primary plant
body as well as in the
secondary plant body.
Cell source
apical and sub apical primary division
undifferentiated
generative
source
protoderm
primary
lineage
epidermis
Secondary cell
lineage
apical meristem
provascular
tissue
primary
xylem
primary
phloem
fascicular
cambium
ground
meristem
pith
cortex
the ground tissue
cortex
parenchyma
filling tissue
collenchyma
sclerenchyma
mechanical, supportive
the secondary lineage
COMPLETE RING OF CAMBIUM
fascicular
cambium
ASSOCIATED WITH THE
VASCULAR BUNDLE
ONLY
vascular
cambium
secondary
xylem
secondary
phloem
cork
cambium
the secondary protective lineage
subepidermal
layers
the
cork cambium
(bark layer)
phellogen
phellem
the periderm a protective barrier
phelloderm
Development of the periderm
subepidermal
layers
The first periderm is
formed just beneath
the epidermis
phellogen
phellem
a waterproof, fireproof
insulator
phelloder
m
phellem
phellogen
phelloderm
primary organization
ground
meristem
PITH
CORTEX
fascicular
cambium
interfascicular
cambium
vascular
cambium
secondary
xylem
secondary
phloem
cork
cambium
Click for Filling spaces notes
phellem/cork
cambium
primary mechanical tissues
ground
meristem
ground
meristem
PITH
CORTEX
CORTEX
collenchyma
interfascicular
cambium
PITH
phellem/cork
cambium
sclerenchyma
collenchyma
(rare)
sclerenchyma
development of the vascular cambium
fascicular
cambium
fascicular
cambium
fusiform
initials
vascular
cambium
secondary
xylem
secondary
phloem
axial
xylem
ray
initials
axial
phloem
cork
cambium
xylem
rays
to cambial derivatives notes pages
phloem
rays
initial
axial
cambial division
Cell division within the ray
and fusiform initials
results in the formation of
derivative cells that are
placed either on the
outside of the mother
cell, in which case they
add to the secondary
phloem, or on the inside
endarch to) the mother
cell, thus adding to the
secondary xylem
Cell source

The apical meristem is the principle source of new cells in the
primary as well as within the secondary plant body. All cell
division linked to vegetative growth, involves mitosis, and, as a
result, the cells that are produced are exact copies of each
other. Lineage depends on the position of the initial within the
meristem.
the periderm a protective barrier




During secondary growth, the diameter of stems and roots
increases rapidly, which results in tension and splitting of the
existing dermal tissues, which subsequently, will stretch and
become disrupted.
The generative layer of the first periderm (phellogen) is
initiated within parenchymatous elements in the outer cortex
of stems and roots. It offers protection from invasion by insects,
pathogens and fungi.
As the stem or root continues to increase in diameter, so
successive periderms are formed. These are formed within
the secondary phloem.
The periderm is a natural waterproof, fireproof insulator.
Filling spaces

Within all plants the primary packaging tissues are composed of cells
that either fill in spaces, or support other areas of the stem, root or
leaf. Thus, the parenchymatic elements that are produced (and have
lineage back to the apical meristems) are produced from what is
termed the ground meristem. In simple terms, the ground meristem
is that region of a shoot or root apical meristem that is NOT involved
in the production of vascular tissue.
cambial derivatives


The vascular cambium is the source of all needed (secondary)
differentiation in plants. It contains two systems, the secondary
xylem, and the secondary phloem tissue. Each of these tissues is
complex, and is developed and has evolved for specific functions –
the xylem for the transport of water and water soluble
molecules, the phloem for the transport of assimilated, and the,
which consist of sugars and related carbohydrates translocated in
water.
Physiologically, the transport xylem is dead at maturity, has
secondarily-lignified cell walls, and functions under extreme
negative pressure potentials. Transport phloem on the other
hand, contains a majority of living cells, with specialized sieve
elements, which are geared for rapid, long-distance
translocation of the assimilated carbohydrate pool. These
transport elements, have thickened walls, are living at maturity
and function under a high positive pressure potential.
click here for the next page
transport functionality

The xylem and phloem conduits form axial tubes.
These tubes facilitate rapid, long-distance movement of
water and dissolved materials. It follows therefore that
the fascicular cambial derivatives that form these
transport cells are longer than they are wide, and that
the cells will, depending on position form either xylem
or phloem.
click here for cambial derivatives
click the need for lateral communication
back
the need for lateral communication


As the secondary plant body enlarges, so the carbohydrate
conducting, and water transporting systems become laterally
spatially and physiologically further removed from each other.
The core of a stem or root, for example, may well contain a
number of living cells, that not only require water and a
supply of assimilate and other carbohydrates, in order to
maintain their functional state. If this does not happen or if the
supply is cut off for some reason, then the core will die.
Lateral communication, and the production of these cells in
the lateral communication pathway, is due to the activity of
specialised cambial cells, called the ray cells. These cells are
sort, often cubic in shape and the produce rows (files) of
parenchymatous living cells, that interconnect the phloem with
the inner xylem core, thereby facilitating exchange of
carbohydrate inwards, and water outwards in the living
plant.
Regulation of SAM & RAM
some examples recap
Cytokinins (CKs), naturally occurring plant hormones that promote cell division, are
essential for normal plant growth and development [1,2].
TOPLESS gene appears to be involved in establishing apical/basal polarity in the
embryo3
PINHEAD regulates SHOOTMERISTEMLESS. PINHEAD gene possibly plays a
role in promoting the translation of genes such as the SHOOTMERISTEMLESS
gene that are required for meristem maintenance3.
ARGONAUTE genes - involved in RNA interference (RNAi) which is an
evolutionarily conserved process through which double-stranded RNA (dsRNA)
induces the silencing of cognate genes
1 D.W. Mok and M.C. Mok, Cytokinin metabolism and action, Annu Rev Plant Physiol
Plant Mol Biol 52 (2001), pp. 900–908.
2 S.H. Howell, S. Lall and P. Che, Cytokinins and shoot development, Trends Plant
Sci 8 (2003), pp. 453–459.
3 http://carnegiedpb.stanford.edu/research/research_barton.php
more
zipping organ formation

Arabidopsis class III
homeodomain-leucine
zipper (HD-Zip III) proteins
play overlapping, distinct,
and antagonistic roles in
key aspects of
development that have
evolved during land plant
evolution.
Plant signals
Figure 1 Model of how CLASS III HD-ZIP1 and KANADI activities pattern lateral organs and vasculature. A centrally
derived signal (red) activates CLASS III HD-ZIP genes, whose activity is antagonistic with that of KANADI activity.
Both KANADI and MIR165/166 negatively regulate CLASS III HD-ZIP genes, (relationship between the two is not
presently known). In lateral organs, CLASS III HD-ZIP activity promotes adaxial fates and KANADI activity
promotes abaxial fates. In the vascular bundles, interactions between the two gene classes pattern the arrangement
of xylem and phloem tissues. The vascular bundle shown is already differentiated, but the initial patterning events
likely occur just below the apical meristem where provascular cells are being specified.
–––––––––––––––-–
1Class
III homeodomain-leucine zipper proteins See http://www.nature.com/nrm/journal/v5/n5/full/nrm1364.html
Homework:

Spend a bit of time researching other gene
systems (in Arabidopsis, or higher plants) that are
involved in SAM or RAM development,
expression of morphology, size and shape.

Insert into a word doc, CITE the references as well
please and send them to me – I will collate and
redistribute useful information back to you via
RUConnected
Gene control - a list of some important role players 1.
Details are in the attached notes (below)
Formation of the meristem & separation of the cotyledons:
CUP-SHAPED COTYLEDON 1 (CUC1) and CUP-SHAPED COTYLEDON 2. (CUC2) as well as SHOOT
MERISTEMLESS (STM)
Regulated cell division:
TEBICHI (TEB) gene, regulates and involved in differentiation in meristems. Conversely, teb mutants show
morphological defects
Control of cell division and endodermis formation:
SHORT-ROOT (SHR) and SCARECROW (SCR) are involved in control this asymmetric cell division and
the differentiation of endodermal cells
Organization of division of initials (called stem cells (ugh!))
Fate of initials [stem] and associated CLAVATA3 (CLV3) gene expression maintained by the underlying
organizing center (initials) expressing the WUSCHEL (WUS) gene. Conversely, CLV3 restricts the size of
the organizing center by repressing the expression of WUS, and clv3 mutants show expansion of the
SAM and the WUS-expressing region
Regulation of SAM/RAM size:
Loss-of-function mutations of ULTRAPETALA1, (encodes a transcriptional regulatory protein) results in
enlargement of the SAM and expansion of the WUS expression domain; ULTRAPETALA1 protein is a
negative regulator of division in the initials
Gene control - a list of some important role players 2.
SAM initials – population size regulation:
CORONA, PHABULOSA, PHAVOLUTA, and REVOLUTA, (Class III homeodomain–leucine zipper
transcription factors) regulate the population size of the initials and size of the SAM. The
mutants, mgoun1 (mgo1) and mgo2 result in perturbed organ primordia formation and
enlargement and disorganization of the SAM
fasciata1 (fas1) and fas2 mutants show stem fasciation, abnormal phyllotaxy, and short roots.
fas mutants do not express WUS in the SAM and SCR in the RAM. FAS complex may control
the state of gene expression by regulating the structure of chromatin
Loss of function control:
TONSOKU (TSK)/MGO3 involved in the maintenance of meristem structure, and a
loss-of-function mutation in this gene disrupts the control of cell division and
cellular arrangement in the SAM and the RAM.
tsk/mgo3 mutant shows altered expression of WUS in the SAM and SCR in the
RAM as well as expression of developmental phenotypes similar to those of the fas
mutants (including short roots, abnormal phyllotaxy, and stem fasciation).
From Wikipedia, the free encyclopaedia
Fasciation is a condition of plant growth in which the apical meristem, normally concentrated around a single point,
producing approximately cylindrical tissue, becomes elongated perpendicularly to the direction of growth, producing
flattened, ribbon-like, crested, or elaborately contorted tissue. The phenomenon may occur in the stem, root, fruit, or flower
head.
Fasciation (also: cresting) can be caused by a mutation in the meristematic cells, bacterial infection, mite or insect attack,
or chemical or mechanical damage. Some plants may inherit the trait.
end
Observational Skills
If you cannot find this link: try these links :
1.
http://ruconnected.ru.ac.za/mod/resource/index.php?id=574
2. http://virtualplant.ru.ac.za/Botany2/workbook/Add-material-intro.htm