The choice of a “star” model organism for plants is simple: Arabidopsis thaliana easily beats out all others. This tiny angiosperm, a mustard relative, has become a staple of plant biology laboratories because of its small size, easy cultivation, and prolific reproduction. In addition, biologists can use a bacterium called Agrobacterium tumefaciens to carry new genes into Arabidopsis cells.
The amount of attention devoted to this plant may seem extravagant, considering its insignificance as a commercial plant. Because angiosperms are closely related to one another, however, discoveries in Arabidopsis will likely apply directly to economically important crop plants. For example, genetic and molecular studies of Arabidopsis can help researchers identify genes that enable plants to grow on poor soil. Understanding these genes may help plant scientists develop improved varieties of food crops such as rice, barley, wheat, and corn. Moreover, many genes in Arabidopsis have counterparts in humans and other organisms, so research on this plant can have farreaching applications. For example, scientists have discovered that Arabidopsis has at least 139 genes that correspond to human disease genes. Studies of Arabidopsis can therefore help us understand illnesses from colitis to Alzheimer disease to arthritis.
The following list includes a few of the important discoveries resulting from work on Arabidopsis:
Control of gene expression
Each cell type in a multicellular organism turns on a different combination of genes. One way that cells regulate gene expression is to attach methyl groups to unneeded DNA; another is to produce small pieces of RNA that bind to genes that have already been transcribed. Researchers have studied both processes in Arabidopsis, in part because problems in gene expression cause some types of cancer in humans.
The Arabidopsis genome sequencing project was completed in 2000. Analysis of the DNA suggests that the entire genome has duplicated two or three times, fueling speculation that all plants are polyploids. This finding may help shed light on the evolutionary history of plants.
Some plants construct a sort of “fire break” around the spot where a bacterium or fungus has entered. Smal areas of surrounding plant tissue die, and this zone of dead cells prevents the invader from spreading throughout the plant. Study of this response in Arabidopsis has led to new insights into how genes regulate apoptosis (programmed cell death); this research may one day help plant breeders improve disease resistance in other crops as well.
Response to the environment
A plant cannot avoid extremes of temperature, light availability, and salinity by moving to a better location. Instead, it must adjust its physiology. Arabidopsis has genes that control its response when the weather turns cold, a finding that could help crop plants survive freezing.
Ethylene is a gas that helps control fruit ripening and plant senescence (aging). Mutant Arabidopsis plants that do not respond to ethylene have helped researchers find ethylene receptor proteins. Researchers have also discovered that the ethylene response requires copper, which the plant transports using a protein similar to one that transports copper in humans. (When faulty in humans, this protein causes Menkes disease.)
Circadian processes occur in 24-hour cycles. In Arabidopsis, for example, proteins encoded by clock genes ensure that the expression of genes needed for photosynthesis peaks at around noon. The same genes are repressed at night. Pigments called phytochromes “reset” the clock each day.
Angiosperms delay flowering until they reach reproductive maturity. How do they “know” when the time comes, and how do they build flower parts in the right places? Arabidopsis research has revealed genes that control the timing of flowering, the differentiation of cells that give rise to flowers, the development of individual flower parts, and the development of ovules inside the flower. For example, some genetic mutations induce flowers to develop into shoots; others promote early flowering.