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 sub marginal initials. Becoming different -- differentiation

The two groups of initials MARGINAL INITIALS: Responsible for the formation of the epidermis & hypodermis SUBMARGINAL INITIALS: R SUBMARGINAL 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 C 3, or a C 4, or CAM, photosynthetic type will affect the shape, size and internal structure (development; differentiation) of the leaf. In C 3 plants, chloroplasts structure is the same in all photosynthetic tissue. In C 3 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 C 4 monocots, mesophyll is differentiated into Kranz and non-Kranz mesophyll. In C 4 dicots, the mesophyll is also differentiated, this time into non- Kranz and Kranz mesophyll

SI MI Adaxial epidermis Abaxial epidermis Bundle sheath Kranz mesophyll procambium Vascular tissue xylem phloem mesophyll between bundles is undifferentiated Leaf differentiation in a C 4 monocot

SI MI Adaxial palisade mesophyll Abaxial palisade mesophyll Adaxial epidermis Abaxial epidermis Bundle sheath procambium Vascular tissue xylem phloem Bundle sheath Leaf differentiation in a C 3 dicot

Controlling development and differentiation dicotyledonous foliage leaves adaxial epidermis adaxial palisade mesophyll procambium vascular bundles bundle sheath (parenchymatous) abaxial (spongy) mesophyll abaxial epidermis MI SI

Controlling development and differentiation dicotyledonous foliage leaves C 4 adaxial epidermis vascular bundles abaxial epidermis procambium MI SI adaxial palisade mesophyll (non-Kranz) abaxial palisade mesophyll (non-Kranz) bundle sheath 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 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 d3 d2 d1 d1 l d2 l d1 r d2 r = plane of division NB: ANTICLINAL means perpendicular to a surface; PERICLINAL is parallel to a surface.

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 zone domain 2 inner zone domain 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!

symplasmic continuity tunica (CZT) corpus CZC peripheral tunica (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: tunica CZC = cell zone: corpus CZTP=lateral cell zone: peripheral tunica

(2) tunica and corpus symplasmically connected tunica (CZT) corpus CZC peripheral tunica (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) peripheral tunica (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: tunica CZC = cell zone: corpus CZTP=cell zone: peripheral tunica

(4) Tunica has role in mediating in symplasmic continuity if corpus isolated, division processes signaled tunica (CZT) corpus (CZC) peripheral tunica (CZPT) = plasmodesma closed CZT = cell zone: tunica CZC = cell zone: corpus CZTP=cell zone: peripheral tunica

(5) Zonation: When a CZPT region becomes isolated tunica (CZT) corpus (CZC) peripheral tunica (CZPT) = plasmodesma closed Signal gradient Signal gradient isolation New event can occur CZT = cell zone: tunica CZC = cell zone: corpus CZTP=cell zone: peripheral tunica Here….1/23/2008

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

epidermal and sub epidermal 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: tunica (CZT) corpus CZC peripheral tunica (PTZ) 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

What is the regulatory pathway?

Plasmodesmal channels in intercellular communication NR = neck region; MP = movement protein; Dt = desmotubule; CC = cytoplasmic sleeve; ER = endoplasmic reticulum; PM = plasmamembrane; CW = cell wall.