Plant Development

How is the cell fate determined?
(Regulative or mosaic)
Patterns of division or interactions?
Fate can be altered by its position within the meristem (must be some regulation) but little is known about plant cell-cell communication.
Plant growth hormones are very small molecules that can pass through cell walls.
Adult plant cells retain totipotency which means that the determined state can be escaped.

Embryonic development:
First division often divides the zygote into apical and basal regions (A/B axis) in an unequal asymmetric cell division.

Electrical currents are involved in polarizing the Fucus zygote
Fucus (multicellular brown alga) has a body consisting of flattened thallus divided into fronds and anchored to a rock by the rhizoid or holdfast.
Eggs of Fucus are fertilized externally and zygotes float until they find a suitable surface.
First division gives a small cell that gives rise to the rhizoid (~root) and thallus (leafy part).
Apical-basal polarity is set up before the cleavage begins by environmental signals (external signal) such as light, pH gradients and water flow.
The signals direct calcium into the cell.
In response, calcium pumps form on the opposite side of the cell.
The flow of calcium localizes cellular components to the site of calcium entry.
Ion currents establish the apical/basal axis but other processes stabilize it.

Hypotheses:  The polarization of the Fucus egg may depend upon calcium currents to localize the cytoskeleton (actin) at the future site of the rhizoid and to encourage insertion of polysaccharides into the cell wall.

Fate in early Fucus is determined by the cell wall
If the cell wall of the basal cell of the two cell Fucus embryo is destroyed, the basal cell first extrudes as a spherical protoplast and can develop into a normal embryo by itself.
Therefore the basal cell is not yet determined but the cell wall must direct the development of a basal cell.
Apical cell without basal cell wall develop (for a while) as thallus cells
but if the basal cell wall is still attached, cells that would normally become thallus, adopt the rhizoid fate.
Position-dependent differentiation of the two cell Fucus embryo must be linked to cell wall factors.

Difference in cell size resulting from unequal divisions in the Volvox embryo
Volvox (green alga) lives as a simple multicellular organism, asexual colony.
2 cell types:
2000 biflagellate somatic cells surface of gelatinous sphere and
16 asexual reproductive cells (gonidia).
Each gonidium gives rise to a colony!
5 cleavages are symmetric but in the
sixth cleavage 16 anterior cells divide to give 16 large (become gonida) and
16 small (become some of the somatic cells).
The difference is an unknown "determining factor".

Both asymmetric cell divisions and cell position pattern the early embryos of flowering plants
The first division often divides into apical and basal cells.
Often the basal cells only gives rise to the suspensor (a few cells) while the apical cells under goes a complex pattern of division to form the
embryo.

The patterning of the Arabidopsis embryo can be altered by mutation
Although microsurgery is difficult, mutations in Arabidopsis that alter the heart-shaped embryo have been found & suggest that genes control apical/basal axis.
apical mutants (gurke):  missing cotyledons and shoot meristem.
central mutants (fackel):  no hypocotyl and cotyledons are attached to root.
basal mutants (monopterous):  no hypocotyl nor root.

Rough fate map of Arabidopsis
Octant stage (8 apical cells) can be mapped to the heart-stage at which a clear map can be made.
In the heart-stage,
the apical region gives rise to the shoot meristem and cotyledons,
the central region gives rise to hypocotyl (the embryonic stem) and
the basal region gives the root.

radial pattern (concentric rings of tissue layers):
epidermis, ground tissue (cortex & endodermis) and vascular tissue.
Cell lineage does not appear to be critical.
Note that adult tissue can be cultured and callus can undergo redifferentiation.

In monocotyledons (i.e. Zea mays) cell divisions are irregular (difficult to map a fate mate).
Cell position seems to play a big role in establishing and interpreting positional information.

Radial pattern formation...
begins at the 8-cell stage involves oriented cell division.
periclinal (radial) divisions give new tissue layers.
anticlinal divisions increase the number cells in a layer.
Some mutants demonstrate that tissue fate is determined early.

In keule mutants, epidermal cells enlarged.
In short root mutants, the endodermis is absent.
Plant somatic cells can give rise to embryos and seedlings plants have amazing abilities to regenerate.
Unlike animals, many somatic plant cells remain totipotent.

Meristems

The fate of a cell in the shoot meristem is dependent on its position
The shoot meristem of dicotyledons, which gives rise to the stalk and leaves, is comprised of three layers:
L1 (outermost layer, 1 cell thick)
L2 (lies beneath L1, 1 cell thick)
L3 (inner most layer)
L1 & L2 comprise the tunica and divide by anticlinal divisions (perpendicular to layer).
L3 cells divide in any plane and make up the corpus.
Cell fate has been determined by generating chimeric tissues.
Chimeras are composed of cells of different genetoypes and are made by treatment with radiation or chemicals (colchicine).
Periclinal chimeras have one of the three layers marked differently.
In angiosperms, L1 becomes the epidermis while L2 & L3 produce cortex and vascular tissue.
Occassionly L1 or L2 cells divide periclinally, invade a new layer and adopt the fate of the new layer (regulative).
Mericlinal chimeras, the result of irradiation or mobilization of a transposon, are plants that have an entire sector marked by a clone.
These have been used to produce a probabilistic fate map in maize and Arabidopsis.

Meristem development is dependent on signals from the plant
In maize, the apical meristem gives rise to number of nodes (16 -22) and the tassel.
Isolated meristems do not retain memory of how many nodes produced and will generate a complete set of nodes.
The number of nodes is determined by interaction of the meristem with the plant.
Pea seedlingís meristems when bisected will regulate into 2 complete meristems.
Removal of part of a meristem will result in regeneration of a complete meristem.
Removal of a complete meristem results in an incipient meristem (at the base of the leaf) to develop.
Thus growing meristems inhibit the growth of nearby ones.
Leaf positioning (phyllotaxy) involves lateral inhibition and often produces a helical pattern of leaves on a stalk.
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Root tissues are produced from root apical meristems by a highly sterotypical pattern of cell divisions
Root meristems resemble shoot meristems but have two important differences:
1) the Root Cap covers the root meristem (protection) &
2) no segmental arrangement as seen with the node-internode-node module.
The root is set up early in the late heart-stage embryo by a set of initial cells.
Each column of root cells originate with a specific cell in the meristem via a specific patten of cell division.
Nevertheless, this process is under regulatory control as laser ablation of developing root cells result in normal tissue.
At the centre of the root meristem is a quiescent centre of cells that do not divide.
There is no obvious segmental arrangement of the root as is seen with node-internode-leaf module of the shoot.

Flower Development

Homeotic genes control organ identity
Shoot meristem converts to inflorescence meristem which can form one or more floral meristems.
The floral organ primordia arise from floral meristem by cell differentiation and enlargement.
4 concentric whorls reflect the order within the floral meristem.
Sepal (whorl 1) from the outer ring.
Petals (whorl 2) from the next ring.
Stamens (whorl 3; male reproduction) from the inner ring.
Carpels (whorl 4; female reproduction) from the centre.
In Arabidopsis there are 15 separate primordia (4 sepals, 4 petals, 6 stamens and 1 pistil [with 2 carpels]).

Homeotic mutants have abnormal flowers that have parts of the flower replaced with other parts.
These identify floral organ identity genes and help determine heir mode of action.
There are three classes of homeotic floral mutations, each which affect the organs of two adjacent whorls.
Class One:  such as apetala2, effects whorls 1 & 2 such that sepals are replaced by carpels (whorl 1) and petals by stamens (whorl 2).
Class Two: such as pistillata and apetala3, effects whorls 2 & 3 and have petals replaced by sepals (whorl 2) and stamens by carpels (whorl 3).
Class Three: such as agamous affects whorls 3 & 4 and have the reproductive organs replaced by sepals and petals.

Model of gene activity in floral organ development.
The floral meristem is divided into three overlappng regions, A, B and C.
Region A contains whorls 1 & 2, B contains whorls 2 & 3, and C covers whorls 3 & 4,
This combination of  a, b and c regulatory functions give each whorl an unique identity.
In addition, the aand c regulatory functions must be mutually exclusive such that a prevents c activity and vise versa.
A floral meristem region with ...
a function only produces sepals,
a & b functions produce petals,
b & c functions produce stamens and
c function only produces carpels.
Homeotic mutants eliminate either a, b or c function.

A model of gene action controlling flower patterning is consistent with combinatorial organ identity specification.
Class One:  a is disrupted and c is expressed in all whorls -> carpel, stamin, stamin, carpel patern (ie. apetala2)
Class Two: only a in whorls 1 & 2 and c in whorls 3 & 4 -> sepal, sepal, carpel, carpel (ie. pistillata and apetala3)
Class Three: c is disrupted and a is expressed in all whorls -> sepal, petal, petal, sepal (ie. agamous).
The combination of functions modify the ground state to cause flower development.
The AGAMOUS protein is expressed in the central whorls while APETALA3 is expressed in the outer whorls.

The floral identity genes encode homeotic proteins.
APETALA1 and AGAMOUS share a conserved DNA binding domain, the MADS box.
MADS box proteins act as transcription factors.
MADS box genes are present in plants, fungi and animals.
These act as homeotic selector genes in a manner similar to HOX genes is animals.
In addition the CURLY LEAF gene of Arabidopsis is similar to the Polycomb family of genes in Drosophila
CURLY LEAF proteins act to cause the stable repression of hometoic genes in plants.
SUPERMAN is a cadastral gene acts to maintain the boundary between the 3 &  4th whorls.

The transition to a floral meristem is under environmental and genetic control
Flowering in Arabidopsis is controlled by internal and external factors.
The apical meristem generates leaves during the vegetative phase but undergoes a transition to a floral meristem in one of two ways.
Determinate transition: the inflorescence meristem becomes the terminal flower.
Indeterminate transition: the inflorescence meristem becomes a number of floral meristems (as in Arabidopsis).
Leafy and apetala1 are floral meristem identity genes.
Mutants in these partially transform flowers into shoots.