Transcript tunica (CZT) - The Virtual Plant

Domains in apical development
The apical meristem is one of
the simplest-looking structures
in the higher plant, yet, the
processes controlling its
differentiation sequencing is
not yet fully understood.
We recognize that changes
have to be effected in the way in
which neighbouring cells
communicate (or stop
communicating) prior to, during
and after a cell division event in
this structure. This topic
explores the concept of domain
control in higher plants,
specifically in the shoot apex.
AM
Shoot apical meristem type – increasing complexity
monoplex
Here, all subsequent
cells are related to
one single AM cell.
Common in lower
order plants
simplex
Here a number (possibly
three) AM cells are
involved in the formation
of new initials and
derivatives
duplex
Here several AM cells
are involved in
production of new
initials and derivatives
– However zonation
becomes apparent and
easier to explain.
monoplex
Monoplex shoot apical meristems have a
single top cell, often tetrahedral and
produces daughter cells by lateral cell
division. A relatively simple structure,
where all cells have direct lineage to the
apical mother cell.
Separation into cortex and stele requires
isolation of derivatives to allow for
periclinal and anticlinal cell division
monoplex
Remember: ANTICLINAL means perpendicular to a surface;
PERICLINAL is parallel to a surface.
= plane of division
simplex
The simplex apical meristem has a zone of
initials in an unstable sub-superficial layer.
Cells may divide in the horizontal and the
vertical plane. Not all cell have the same
lineage. A slightly more complex structure
evolves.
simplex
simplex
Alternative division plane
duplex
The duplex apical meristem has two
layers of sub-superficial cells.
These give rise to two lineage
compartments – the tunica and
corpus. This results in an apical
meristem with two distinct cellular
features (recognizable quite early
on in development) and will give
rise, through the to the two major
cell lineages, to the cortex and the
stele, and its associated tissues.
The black box – two domains
This system (common in higher plants)
allows for independent cell division in
the two compartments. It is initiated
through closed-gating of plasmodesmata.
= plasmodesma closed
construction…and the need for continuity .. sometimes!
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symplasmic continuity
tunica (CZT)
CZT = cell zone: tunica
CZC = cell zone: corpus
CZTP=lateral cell zone: peripheral tunica
peripheral
tunica (CZPT)
corpus
CZC
Three zones can be recognized within the apex:
(1). the tunica, (2) the peripheral tunica zone and (3) the
central corpus zone. All are in symplasmic contact. This
is thus a single domain.
(2) tunica and corpus symplasmically connected
tunica (CZT)
peripheral
tunica (CZPT)
symplasmic continuum here, means that all
corpus
the cells are in contact and that small
CZC
molecules and signals may traverse the
whole developing apex, via plasmodesma.
Conceptually, a signal gradient can be
established
(3) tunica in symplasmic continuity, corpus isolated
tunica (CZT)
= plasmodesma closed
CZT = cell zone: tunica
CZC = cell zone: corpus
CZTP=cell zone: peripheral tunica
Here, tunica as well as peripheral tunica
are symplasmically connected, but isolated
from the corpus. Corpus could engage in
non-synchronous cell division, to produce
cells without the influence of the tunica.
peripheral
tunica (CZTP)
corpus
(CZC)
(4) tunica in symplasmic continuity, corpus isolated,
signaling divisionary processes
tunica (CZT)
= plasmodesma closed
CZT = cell zone: tunica
CZC = cell zone: corpus
CZTP=cell zone: peripheral tunica
peripheral
tunica (CZPT)
corpus
(CZC)
(5) CZPT zone becomes isolated
New event can occur
tunica (CZT)
= plasmodesma closed
CZT = cell zone: tunica
CZC = cell zone: corpus
CZTP=cell zone: peripheral tunica
peripheral
tunica (CZPT)
corpus
(CZC)
The apex, simple cells, complex arrangement, new form and
function
epidermal and subepidermal development – step one
Cortex and stele emerges – step 2
Vascular differentiation – step 3
Foliar buttress
Conclusion:
It is possible to apply this model to the development of a leaf as well. Clearly,
Cell division can be synchronous (cell compartments in harmony) or
asynchronous (dividing cell compartments isolated). Synchrony or
asynchrony can thus determine the (a) type of derivative cell formed (b) the
type of tissue formed and its position.
So what happens in the apex is that the puzzle pieces are simply(!) put
together and orchestrated during the early developmental stages….
Plasmodesma are the key
An extension of and to, the regulatory pathway?
Whether we deal with the apex or a
developing leaf, it makes good sense to
recognise that domains exist in mature
tissues and that these domains are functional
and operate to regulate not only the flow of
information, but also, as in this example, the
flow of assimilates into the phloem in a
source leaf.
From: Mette La Cour Petersen, Gary A. Thompson and Alexander Schulz
. Long-distance and Cell-to-cell Transport of phloem proteins.
The Royal Veterinary and Agricultural University (KVL), Denmark &
University of Arkansas at Little Rock, USA.
Presented at Plasmodesma 2001: 19-24 August Waterfront
Cape Town.
Spheres of influence – movement of signals?
This diagram shows that there
is a degree of influence
possible if there are
overlapping domains in our
system. The points of ‘overlap’
– (really domain boundaries)
will possibly influence
neighbouring cells under
specific conditions, and at set
point during the development
of new cells within the duplex
apical meristem. The red and
blue arrows simply show two
possibilities for a
multidirectional signalling
potential.
Plasmodesmal channels in intercellular communication
NR = neck region; MP = movement protein; Dt = desmotubule; CC = cytoplasmic sleeve;
ER = endoplasmic reticulum; PM = plasmamembrane; CW = cell wall.
End Domains
What is the regulatory pathway?