The role of the apex in differentiation growth:

The development of leaves

The foliar buttress and the formation of new leaves

The foliar buttress develops in close proximity to the apex

The leaf expands rapidly, in width and in length, through division of meristematic cells called initials

Newly-formed cells are initially very similar to each other, but signs of differentiation can soon be seen. This image is a section of part of a leaf and has been sectioned parallel to the surface, so that we can see part of the developing vascular network.

Veins are formed by a special subgroup of meristematic cells, called the submarginal initials.

Becoming different -- differentiation

The two groups of initialsThe two groups of initials

MARGINAL INITIALS: Responsible for the formation of the epidermis & hypodermis

SUBMARGINAL INITIALS: RSUBMARGINAL INITIALS: Responsible for the formation of the mesophyll and by a separate route, the VASCULAR TISSUE

The influence of photosynthetic type on leaf differentiation

Whether the plant is a C3, or a C4, or CAM, photosynthetic type will affect the shape, size and internal structure (development; differentiation) of the leaf.

In C3 plants, chloroplasts structure is the same in all photosynthetic tissue.

In C3 dicots chloroplasts are di --> polymorphic, if mesophyll is differentiated then this forms into palisade and spongy mesophyll, else undifferentiated mesophyll.

In CAM plants, water conservation is critical and spongy mesophyll is centrally-located and stores water

In C4 monocots, mesophyll is differentiated into Kranz and non-Kranz mesophyll.

In C4 dicots, the mesophyll is also differentiated, this time into non-Kranz and Kranz mesophyll

SISIMI

Adaxial epidermisAdaxial epidermis

Abaxial epidermisAbaxial epidermis

Bundle sheathBundle sheath

Kranz mesophyllKranz mesophyll

procambium Vascular tissueVascular tissue

xylemxylem

phloemphloem

mesophyll between bundles is undifferentiated

Leaf differentiation in a C4 monocot

SISIMI

Adaxial palisade mesophyll

Abaxial palisade mesophyll

Adaxial epidermis

Abaxial epidermis

Bundle sheath

procambium Vascular tissueVascular tissue

xylem

phloem

Bundle sheath

Leaf differentiation in a C3 dicot

Controlling development and differentiationdicotyledonous foliage leaves

adaxial epidermis

adaxial palisade mesophyll

procambium vascular bundles

bundle sheath (parenchymatous)

abaxial (spongy) mesophyll

abaxial epidermis

MI SI

Controlling development and differentiationdicotyledonous foliage leaves C4

adaxial epidermis

vascular bundles

abaxial epidermis

procambium

MI SI

adaxial palisade mesophyll (non-Kranz)

abaxial palisade mesophyll (non-Kranz)

bundle sheath

bundle sheath

Kranz mesophyll

Kranz mesophyll

So, how does differentiation work?Where do cells originate?Where do tissues form?

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 simplex duplex

Here, all subsequent cells are related to one single AM cell. Common in lower order plants.

Here a number (possibly three) AM cells are involved in the formation of new initials and derivatives.

Here several AM cells are involved in production of new initials and derivatives – however, zonation becomes apparent and easier to explain.

monoplex

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

d3d2

d1

d1l

d2l

d1r

d2r

= plane of division

NB: ANTICLINAL means perpendicular to a surface;PERICLINAL is parallel to a surface.

simplex

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 can evolve than in monoplex systems.

How does it function?

The working simplex

simplex

zone

1

Alternative division plane

The 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 duplex – a “black box” – two domains

outer z

one

domai

n 2

inne

r zone

dom

ain

1

= plasmodesma closed

This system (common in higher plants) allows for independent cell division in the two compartments. It is initiated through closed-gating of plasmodesmata.

construction…and the need for continuity .. sometimes!

12

2 1

1

2

21

1

1

1

symplasmic continuity

tunica (CZT)

corpusCZC

peripheraltunica (CZPT)

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.

CZT = cell zone: tunicaCZC = cell zone: corpusCZTP=lateral cell zone: peripheral tunica

(2) tunica and corpus symplasmically connected

tunica (CZT)

corpusCZC

peripheraltunica (CZPT)

symplasmic continuum here, means that all the cells are in contact and that small molecules and signals may be transported through all cells in the developing apex, via plasmodesmata. The concept of a signal gradient can be argued.

(3) tunica in symplasmic continuity, corpus isolated

tunica (CZT)

corpus(CZC)

peripheraltunica (CZTP)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.

= plasmodesma closed

CZT = cell zone: tunicaCZC = cell zone: corpusCZTP=cell zone: peripheral tunica

(4) Tunica has role in mediating in symplasmic continuity if corpus isolated, division processes signaled

tunica (CZT)

corpus(CZC)

peripheraltunica (CZPT)

= plasmodesma closed

CZT = cell zone: tunicaCZC = cell zone: corpusCZTP=cell zone: peripheral tunica

(5) Zonation: When a CZPT region becomes isolated

tunica (CZT)

corpus(CZC)

peripheraltunica (CZPT)

= plasmodesma closed

Signal gradient

Sign

al g

radi

ent i

sola

tion

New event can occur

CZT = cell zone: tunicaCZC = cell zone: corpusCZTP=cell zone: peripheral tunica

Here….1/23/2008

Summary:The apex, simple cells, complex arrangement, new form and function

Apical meristem

epidermal and subepidermal development – step one establishing gradients

Cortex and stele emerges – step 2 refining and defining

Vascular differentiation – step 3 differentiation begins

Step 4 - The foliar buttress

Step 5 - emerging leaf

Conclusion:

tun

ica (CZ

T)

corp

us

CZ

C perip

hera

ltu

nica

(P

TZ

)

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, the flow of assimilates into the phloem in a source leaf.

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.

End Domains