Cell fate determination

The question of what the derivative cell will become after it leaves the meristem is governed by its position within the organ. The question of cell fate has been of interest to plant scientists for centuries, but there was little way to test the competing hypotheses of cell lineage and cell position. In an elegant series of experiments, researchers have shown that plant cells can change their fate by being forced into a new position, taking their cues for which kind of cell to form based on their neighbors rather than the identity of their mother cell. Since that set of experiments, researchers have started to identify the signals that transmit this identity information between adjacent cells, largely through microscopic analysis of mutants.

At the broad end of the spectrum, all plant cells fall into one of three tissues with respect to identity: vascular, dermal, and ground tissues. Vascular tissue is composed of cells involved in transport and cells that support transport. Dermal tissue is composed of cells on the surface of the plant. Ground tissue is composed of cells that make up the rest of the plant – in the interior of leaves, stems, and roots but not having a specific role in transport. Thus, the fate of any particular derivative to become a cell in either of these three tissue systems is determined by the location the derivative finds itself within the meristem.

Because of the centrality of position in determining cell fate, one of the most important facets of plant development is the specification of the plane of cell division. For any given mother cell undergoing division, the orientation of the plane of division could mean the difference between a derivative becoming an epidermal cell (dermal tissue) or a cortex cell (ground tissue). This is the case not only because plant cells don’t migrate and adhere, and not only because of the primary role of position in determining cell fate, but also because of the presence of the thick, rigid, cellulosic cell wall. Once wall deposition begins during cytokinesis, there is no chance for a cell to change its position, and hence its fate. For an example of this relationship, see this visualization of cell fate and division plane.

Within the meristem, cells pass through the cell cycle with relatively high frequency. While the process of mitosis has some notable differences from other eukaryotic cells, cytokinesis in plant cells is almost wholly unlike that seen in other eukaryotes. The “textbook” cell undergoes cytokinesis through the familiar ‘pinching in’ of the plasma membrane, but the rigid wall of plant cells is not conducive to such a process. Rather, cytokinesis progresses by the deposition of polysaccharides and membrane lipids at the site of the new cell wall through vesicle trafficking. These vesicles travel along a specialized arrangement of microtubules known as the phragmoplast. Imagine dividing a room in two by suspending a piece of plaster in mid-air, then adding to it until you had built a complete wall, and you would have some idea of how cytokinesis in plants occurs.