COLUMBUS, Ohio – Researchers at Ohio State University have identified the gene that controls the distribution of stomatal cells on leaves, key components for the healthy growth of all plants.
The discovery may have implications for enhancing the growth of important crop plants and might even provide an alternative of sorts for research into human stem cells.
The finding, reported in the current issue of the journal Science, linked the TMM gene in Arabidopsis plants to the formation and distribution of stomatal cells on the surfaces of leaves.
Arabidopsis thaliana is for botanists what fruit flies are for animal geneticists. It is one of the most important – and best understood – models for scientific study. While this common mustard plant has little economic value, it is widely used to better understand the processes governing plant growth.
Fred Sack, a professor of plant biology at Ohio State University, and Jeanette Nadeau, a postdoctoral fellow in the same department, spent several two years trying to understand how the TMM gene altered the number and arrangement of stomatal cells on leaves. Now they have discovered that TMM is only the second known gene to be involved in stomatal development in plants.
"Genes like TMM exist in crop plants such as rice," Sack said. "If TMM has the same function in crop plants that it does in Arabidopsis, then it could be a key regulator."
Stomatal cells function as pores on the underside of leaves and control the entry of gases into the leaf for photosynthesis.
Normally, stem cells – cells that are self-renewing and that form specific types of cells – in the Arabidopsis epidermis will divide asymmetrically into a smaller, daughter cell, which eventually form stomata, and a larger daughter cell. Signalling between cells tells the smaller cell to become located away from existing stomata, insuring a fairly even distribution of stoma across the leaf surface.
But when Sack and Nadeau found mutations in the TMM gene, they realized that the programming failed. Stomatal cells began to increase in number and clump together rather than maintain their characteristic spacing. (The name of the gene – TMM – stands for "Too Many Mouths. "Stoma" is the Greek word for mouth.)
"This tells us that the TMM protein probably receives signals that orient the position of asymmetric cell division," Sack said. "The TMM protein is found in a set of stem cells and their progeny. It apparently controls which cells divide during leaf development."
Sack said that the developmental pathway that leads to stomatal cells is responsible for the formation of the majority of plant cells on the leaves. This gives the plant a way to adjust the number of stomata it may need to thrive in any given environment.
Aside from its potential importance in enhancing crop plants, the work may be valuable in other ways.
"In animal science, the investigation of asymmetric cell division is a major area of study but in plant science, almost nothing is known about it," he said. "Here we have a cell type that is patterned by asymmetric division and we have a gene that affects the orientation of those divisions."
Sack also said that while stem cell research has been widely discussed recently, "most people aren’t used to thinking that plants have stem cells. But these fit all the hallmarks of stem cells.
"It raises interesting questions about what we can learn from stem cells in plants versus what we can learn from animal stem cells," he said.
The research was supported in part by the National Science Foundation.
Ohio State University. June 2002.