Genes that are involved in the Development of Stomata
Early acting genes in stomatal development
In stomata early acting development genes that act early are able to control the asymmetric division of meristemoid cells. They are therefore able to directly control both the placement and the quantity of stomata that are produced. Much work has been done in Arabidopsis thaliana in the stomata field this has helped to identify many of the earlier acting genes; these include those which code proteins that act as as receptors, kinases and proteases.
Receptor mediated stomata genes
One of the genes that is involved in receptor mediated control of stomata development is TMM (Too Many Mouths). This gene was identified through a mutation that led to changes in asymmetric cell divisions and resulted in too many leaf stoma being produced. it was found that the mutant form of too many mouths resulted in a failure to control spacing divisions and to stop the division of cells adjacent to two stomata or precursor cells. Additionally of TMM led to reduction in amplification division, in turn producing more guard mother cells. TMM is thought to encode a cell surface receptor that is involved in asymmetric divisions and is expressed in guard mother cells and meristemoids, as well as daughter cells that produce meristemoids. It is thought that the gene acts with erecta family members to control the tissue specific regulation that underlies the formation of stomata.
MAP kinase signaling pathway genes that influence stomatal cell development
It is known that a MAP Kinase pathway is involved in stomatal development. A gene known as YODA is able to alter both the spacing and density of stomata. The YODA mutation leads to an increased production of guard cells. Mutations of this MAP kinase kinase kinase gene lead to an increase in entry divisions and a lack of spacing division alignment. Additionally YODA mutations often results in stomata having no adjacent reservoir cells between them.
In addition to MAP kinase pathways and receptor mediated mechanisms many other early acting genes are known to be involved in stomatal development. These include those that are involved in ligand interactions such as SDD1 and those that have a role in stem cell compartmentation.
Late Acting Genes in Stomatal Development
The stomata is involved in both the control of gas exchange and water retention in plants. It acts like a valve an is located in the leaf epidermal layers of plants. The development of stoma acts as a cell linage and is under the influence of both early and late acting genes. Following the assymetric divisions of the meristemoid that help to establish the patterning of spacing and quantity of stomata produced it is time for the late acting genes to take over. These genes help to end the growth of cells and to regulate the timing of guard cell differentiation and of their specification.
Late acting genes that end cell proliferation
In cell linages precursor cells can only divide a finite amount of times after this they must differentiate into a specialised cell. The final process in the development of the stomata is the symmetrical differentiation of the guard mother cell; this results in the two guard cells that guard the stomatal opening. In Arabidopsis three genes are known to be involved in the ending of stomata lineage cell cycling. These are the genes Myb88, FAMA and Four Lips (FLP). These encode proteins that act as transcription regulators. Mutations of Four lips results in clustering with normal and arrested stomata, this can be enhanced by mutation of the four lips paralogue Myb88. Fama encodes a protein that s a bHLH results to similar clusters. It is therefore thought that all three of these late acting genes limit the symmetrical division of genes at the end of the stomata cell lineage. Additionally FAMA is able to play a role in guard cell identity.
Late genes that act as cell cycle regulators
It is thought that genes such as Four lips and FAMA interact with the machinery that regulates the cell cycle. Unfortunately little is known about genes involved in the cell cycle machinery, this is a consequence of the lethality that is caused by many of the genes that are involved in it.
Three classes of cell cycle regulators are known to be involved in the stomatal developmental lineage.
1. Those that positively regulate the production of stomata, and also promote guard mother cell mitosis and cytokineses (eg. CDKB1)
2. Those that regulate the origins of replication (CDT1 and CDC6)
3. Those that repress the transcription factor complex E2F-DP (RBR)
It can be seen that many genes both early and late acting have an influence on the control of stomatal development; the later acting ones have a role to play in the ending of cell proliferation, cell cycling regulation and the division and differentiation of stomatal development. For more detailed information it is recommended that you read some of the papers listed.
How the Environment Affects Stomatal Development
It is well known that the stomata helps the plant to interact with the environment by regulation the flow of gases and water through valve like mechanisms. Although many genes have been shown to play a role in the stomatal cell linage, many of these genes themselves are under the influence of environmental stimuli. Additionally many of the plant hormones are also thought to be impacted by environmental factors.
The environment can lead to a long range control of genes (eg control of flowering time by light period). It is thought that many of the genes responsible for stomatal control are themselves under go similar control. A mutation of the SDD1 gene results in a plant that has an increase in the density of stomata at ambient temperatures; this suggests that the gene may not be able to read environmental cues correctly.
A Conclusion of Stomatal Development
The development of stomata involve a developmental cell linage. This involves the production of meristemoid by asymmetrical division. These can divide to produce daughter cells that act as reservoir cells or go on to produce guard cells. The production of stomata involves many genes that may act to create, pattern and locate stomata at early stages of development or later on to stop development and play a role in morphogenesis and the cell cycle. In addition to genes it is thought that the environment also has a key role to play in stomata development. Many of the genes that are involved in stomatal development are yet to be identified and much is still unknown about he relationship between assymetric division and cell fate in plants it is helped that future research will lead to a better understanding of stomatal development in plants.
Bergmann and Sack (2007). Stomatal Development. Annu, Rev. Plant. Biology 58: 163 to 181Bergmann et al ( 2004).
Stomatal development and pattern controlled by a MAPKK kinase. Science 304:1494–97
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