What are Stomata and How are they Produced
Stomata have a very important role to play in the survival of a plant as they are responsible for the control of both carbon dioxide assimilation and of the way that water is allowed to harnessed. Stomata can be thought of as epidermal valves that heighten a plants ability to perform.
Stomata are produced by a specialised cell lineage found within developing cells in places such as young leaves and other shoot epidermis. Once an epidermal area has matured then the development of stomata comes to a stop. Most of the epidermal cells that are produced in developing leaves are a consequence of the stomatal cell lineage.
The first part of stoma production involves the conversion of postprotodermal cells into meristemoid mother cells; an asymmetric division of the meristemoid mother cells produces a stomatal precusser cell known as a meristemoid. The meristemoid undergoes several rounds of asymmetric division to create a guard mother cell. This cell then divides once more symmetrically to produce two guard cells of the stoma. The asymmetric division that creates a meristemoid also produces a sister cell. Morphogenesis and formation of the pore will lead to the creation of the stoma.
The divisions that occur in the meristemoid mother are classified into three different classes.
1. Entry divisions: these occur in meristemoid mother cells. This indicated the start of the linage.
2. Amplification divisions: these occur in meristemoid cells and increase the number of epidermal cells
3. Spacing divisions: These occur in cells adjacent to stoma or precursor cells. This helps the stomata develop in the shoot epidermis in a way that they will not touch each other.
The number of stomata that are produced are dependent upon the spacing divisions. If no spacing divisions occur then no matter how many times a daughter cell divides in other ways only one stomata will develop. However if the daughter cells divides by a spacing division then many stomata can be produced. It is these spacing divisions of the daughter cells that allow a plant to have different numbers of stomatal on different parts of its epidermis. For example it enabled different amounts of stomata above and below the leaf, or even a different way of developing for shade tolerant plants.
Genes involved in stomatal patterning
Stomata can be thought of as valves that allow a plant to control the intake of carbon dioxide and to control water retention. They develop by a cell linage that produces meristemoids and guard cells. The stomata cell linage is responsible for most of the epidermal content of a plant, with cells from the lineage being able to divide to produce stomata or non-stomata containing tissue.
Many genes are involved in the production of epidermal tissue and stomata. The earlier acting genes act to control the asymmetric divisions of the meristemoid and are responsible for the final number and the patterning of the stomata that are produced. Later acting genes are more involved in halting the production of the stomata and of its morphogenesis.
Stomata are generally separated by at least one other cell. Theses cells act as reservoirs for solutes and enables stomata to act on an individual basis. These cells are produced by asymmetrical divisions of cells next to storna. This pattern of stomata spacing enables optimisation of gas and water diffusion. The division that takes place to produce the daughter cell requires that division is attuned so that the mew cell wall is not next to the stoma.
The patterning of stomata is dependent upon spatial cues; this enables them to be adjusted correctly within the epidermal. It is thought that patterning is influenced by intercellular signalling through the apoplasm. Many cells in the epidermal tissue are controlled by lateral inhibition (eg trichomes, root hairs); this is not the case with stomata as neighboring cells are able to acquire their own cell fate. A type of secondary spacing may also also occur in some plant species such as Arabidopsis, this produces an inward spiral that is not invariant.
Many genes have been shown to control stomatal development in Arabidopsis; they are thought to act by controlling the patterning and quantity of asymmetric stomatal cell divisions.
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Geisler et al (2000). Oriented asymmetric divisions that generate the stomatal spacing pattern in Arabidopsis are disrupted by the too many mouths mutation. Plant Cell 12:2075–86
Schroeder et al. (2001). Guard cell signal transduction. Annu. Rev. Plant Physiol. Plant Mol. Biol. 52:627–58