1. 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

2. 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.

3. 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 Remember: ANTICLINAL means perpendicular to a surface; PERICLINAL is parallel to a surface. d3 d2 d1 d1 l d2 l d1 r d2 r = plane of division

4. 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.

5. simplex zone 1 Alternative division plane

6. 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.

7. The 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.

8. construction…and the need for continuity.. sometimes! 1 2 2 1 1 2 2 1 1 1 1

9. 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

10. (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 traverse the whole developing apex, via plasmodesma. Conceptually, a signal gradient can be established

11. (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

12. (4) tunica in symplasmic continuity, corpus isolated, signaling divisionary processes tunica (CZT) corpus (CZC) peripheral tunica (CZPT) = plasmodesma closed CZT = cell zone: tunica CZC = cell zone: corpus CZTP=cell zone: peripheral tunica

13. (5) CZPT zone 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

14. The apex, simple cells, complex arrangement, new form and function

15. epidermal and subepidermal development – step one

16. Cortex and stele emerges – step 2

17. Vascular differentiation – step 3

18. Foliar buttress

20. 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

21. An extension of and to, the regulatory pathway? Whether we deal with the apex, or a 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.

23. 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.