Oldenburgia grandis, Botany RUOldenburgia grandis, Botany RU

a quick lesson in embryogenesis

FERTILIZATION MEIOSIS

seed

sporophyte

megaspores microspores

megagametophyte

microgametophyte

sperm egg

2N

N

Where do we start?

embryo

initiation of SAM, RAM (polarity?)

cell division, formation of axes

expansion growth

axis & coleoptiles form

gametogenesisgametogenesis seed

[differentiation, maturation]

[differentiation]

- - - - - - - - - - - - - - - - -

totipotency

Seed to plant

graviperception

phototropism

Seed

monocotyledon

dicotyledon

expansion, cell elongation

activation of SAM & RAM

(genes, hormones)cell division

procambium

Embryo proper ofCapsella

Post-Germination: Rapid growth

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.

2h2h

2h20min 2h20min

2h 40 min2h 40 min

Apical organizationApical 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 apexthe 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 divisioncell 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 divisionapical and sub apical primary division

apical meristemapical meristem

provascular tissue

provascular tissue

epidermisepidermis pithpith cortexcortexprimary phloem

primary phloem

primary xylem

primary xylem

ground meristem

ground meristemprotodermprotoderm

undifferentiated

generative source

Secondary celllineage

fascicular cambium

fascicular cambium

primary lineage

the ground tissuethe ground tissue

cortexcortex

parenchyma collenchyma sclerenchyma

filling tissue mechanical, supportive

the secondary lineagethe secondary lineage

fascicular cambium

fascicular cambium

secondary xylem

secondary xylem

vascular cambium

vascular cambium

secondary phloem

secondary phloem

cork cambium

cork cambium

ASSOCIATED WITH THE VASCULAR BUNDLE ONLY

COMPLETE RING OF CAMBIUM

the secondary protective lineagethe secondary protective lineage

sub-epidermal

layers

sub-epidermal

layers

phellemphellem

phellogenphellogen

phellodermphelloderm

thecork cambium(bark layer)

the periderm a protective barrier

Development of the peridermDevelopment of the periderm

sub-epidermal

layers

sub-epidermal

layers

phellemphellem

phellogenphellogen

phelloderm

phelloderm

The first periderm is formed just beneath the epidermis

phellem

phellogenphelloderm

a waterproof, fireproofinsulator

primary organizationprimary organization

groundmeristem

groundmeristem

PITHPITH CORTEXCORTEX

interfascicular

cambium

interfascicular

cambium

phellem/cork cambium

phellem/cork cambium

Click for Filling spaces notes

fascicular cambium

fascicular cambium

secondary xylem

secondary xylem

vascular cambium

vascular cambium

secondary phloem

secondary phloem

cork cambium

cork cambium

primary mechanical tissuesprimary mechanical tissues

groundmeristem

groundmeristem

PITHPITHCORTEXCORTEX

collenchymacollenchyma

sclerenchymasclerenchyma sclerenchymasclerenchyma

collenchyma(rare)

collenchyma(rare)

groundmeristem

groundmeristem

PITHPITH CORTEX

CORTEX

interfascicularcambium

interfascicularcambium

phellem/cork cambium

phellem/cork cambium

development of the vascular cambiumdevelopment of the vascular cambium

fascicular cambium

fascicular cambium

fusiform initials

fusiform initials

ray initials

ray initials

axialxylem

axialxylem

axial phloem

axial phloem

fascicular cambium

fascicular cambium

secondary xylem

secondary xylem

vascular cambium

vascular cambium

secondary phloem

secondary phloem

cork cambium

cork cambium

xylemrays

xylemrays

phloemrays

phloemrays

to cambial derivatives notes pages

cambial divisioncambial division

radial

axia

linitial

xylem 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 sourceCell 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 barrierthe 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 spacesFilling 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 derivativescambial 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.

click here for the next page

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.

transport functionalitytransport 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

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the need for lateral communicationthe 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

Cytokinins (CKs), naturally occurring plant hormones that promote cell division, are essential for normal plant growth and development [1,2].

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. 3http://carnegiedpb.stanford.edu/research/research_barton.php

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

more

some examples recap

zipping organ formation

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

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.

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.

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)

Details are in the attached notes (below)

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 encyclopaediaFasciation 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

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1. http://ruconnected.ru.ac.za/mod/resource/index.php?id=574

2. http://virtualplant.ru.ac.za/Botany2/workbook/Add-material-intro.htm