The Black Box controlled plasmodesmally mediated cell division.


Inspiration for the black box concept, comes from the article by Rinne and van der Schoot (1998).  

Diagrams are redrawn and modified where appropriate to the present discussion


The shoot apex may act as a multi-domain system. This system is protected from direct contact with the environment, by the overlying developing leaf tissues as well as the protoderm. Irrespective of the apex being arranged as a simplex or a duplex system, the process involved in the regulation of the domains within the apex, are apparently controlled by gating the plasmodesma that interconnect these cell systems.



In most plants, the shoot apex can be divided into three zones: An outside protective layer called the protoderm, a region where active cell division takes place, called the apical meristem and a zone in which elongation and enlargement of cells takes place.


Within the tunica, most cell division is going to be associated with the formation of new protoderm cells, and very little interaction can be expected between the tunica and corpus layers.

The corpus, on the other hand is going to be involved in the formation of the body of the developing plant. Here more programmed cell division is necessary

The are several types of apices recognised in vascular plants.

The first is called the simplex apex (see Rinne and van der Schoot 1998) may require more complex plasmodesmal regulation, to maintain periclinal and anticlinal cell division. In contrast, the duplex shown on the right allows for more control of the morphogenetic process. 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 to the two major cell lineages the cortex and the stele, and its associated tissues.

Many of the illustrations in this Factfile are based upon information that was transformed from the listed





In this case, periclinal cell division will be responsible for initiating a number of rows of cells that are parallel to the surface, essentially, these could either be in the same or in different domains. Anticlinal cell division will be necessary to establish subsequent (different) domains in the apex, as an example, that which must be formed to separate cortex from stele. Here things could be a little more simple, as the two layered approach essentially means that the cells could by pre-existence, already be located in separate domains, and for example, the central lower one could be the precursor of all the stelar tissues the upper pair of the cortex including the protoderm. There is a need for continuity sometimes!



Three zones can be may be recognized within the apex:

(1). the tunica,

(2) the peripheral tunica zone and

(3) the central corpus zone. The single colour is used to indicate that all are in symplasmic contact

The above diagram illustrates a a single domain.







Partial or total isolation can be achieved by gating plasmodesma ( various states of openness or closedness) at a particular interface. The diagram below shows several domains -- three in the outer region, the tunica (CZT) the peripheral tunica (CZPT)  and internally, is the corpus. Potential lines of isolation exist, if the plasmodesmal connections (shown as thin black lines) are gated closed. If gated closed, then the tunica may become symplasmically isolated from the corpus.  The corpus would then be free to undergo cell division, in isolation from the tunica, the corpus domain would thus be capable of developing asynchronously with respect to the tunica


Isolation leads to division 


Closing (gating) the plasmodesma between Tunica and Corpus is shown in the Figure below. The red dots indicate where plasmodesma are gated closed. The Tunica zone remains in symplasmic continuity.



It is possible to explain additional isolations. For example, the peripheral tunica zone on one side of the apex, may become isolated prior to the cell division necessary for the formation of a foliar buttress.  


These diagrams can help us understand the complexities that make up the shoot apex. Cell divisions are sometimes synchronous, at other times regions must be separated symplasmically, to allow for asynchronous cell division to take place.






Periclinal cell division. Parallel to the surface Go Back to place marker in text.


Anticlinal cell division. Perpendicular to the surface. Go Back to place marker in text.


The following references are highly recommended reading.


J Pfluger and PC Zambryski 2001.The Power of Symplastic Isolation. Current Biology R 436-R 439.


KJ Oparka and AG Roberts 2001. Plasmodesmata. A not so open and shut case. Plant Physiol 125: 123-126.

PLH Rinne and C van der Schoot 1998. Symplasmic fields in the tunica of the shoot apical meristem coordinate morphogenetic events. Development 144: 1477-1485.

CEJ Botha and RHM Cross 2001. Regulation within the supracellular highway plasmodesma are the key. SAJ Bot. 67: 1-9.