Plant Responses to the Environment — Six Graduate Student Projects

Blue Roots

"Agricultural plants have a hard time acquiring phosphorus because it's bound up in the soil," says graduate student Jennifer Tomscha. "Farmers get around this by applying concentrated phosphate fertilizers, but those fertilizers can pollute waterways. Also, the mineral deposits used to make fertilizers are nonrenewable resources and will inevitably run out, possibly in the next 100 to 200 years. We need alternate ways for plants to acquire phosphorus."

Microorganisms can sense when phosphorus availability in their environment is low, Tomscha explains. At that point, they begin secreting two enzymes, a phosphatase and a ribonuclease, which liberate phosphorus from larger organic molecules in the environment. The organisms also begin pumping up the system that transports phosphorus across their cell membranes into their cells. Fungi living in the soil can do this. So can bacteria that invade the human bloodstream. "When pathogenic bacteria invade our bodies, we lower the level of phosphorus in our blood to try to starve them out," she says. "To survive, the pathogen must be able to adjust."

The genes for the secreted phosphatase and ribonuclease, as well as the system that transports phosphate across the cell membrane, have been identified in a yeast, a fungus, and a bacterium. Interestingly, all three genes are turned on by the same switch. "All we know about the switch is that it's made up of a group of proteins," Tomscha says. "One protein acts as the phosphorus sensor. It communicates to the other proteins in the group that the phosphorus concentration is dropping. Then, two of these proteins release a 'transcription factor,' which travels to the cell's DNA and prepares the phosphatase, ribonuclease, and transporter genes to be read."

Plant roots also secrete a phosphatase, but researchers don't know yet how the system is controlled. By working with Arabidopsis thaliana, Tomscha hopes to find the analogous switch in plants. Arabidopsis thaliana is a common roadside mustard called mouse ear cress the "lab rat" of plant geneticists. Tomscha will cross normal plants with mutant plants that always secrete acid phosphatase. By observing how the mutations travel through the offspring, she hopes to learn how many genes are involved in the response and where in the DNA those genes are located.

To screen for mutants, she grows thousands of seedlings on a medium that contains a chemical indicator for phosphatase activity. Because the medium contains adequate phosphorus, normal plants wouldn't need to secrete the enzyme. "But if the plants are defective for sensing phosphorus, they will secrete phosphatase and the indicator will turn their roots blue," says Tomscha. Among the 70 thousand seeds screened, Tomscha and Alison Dewald, an undergraduate in biochemistry and molecular biology whose work is supported by Penn State's Women In the Sciences and Engineering (WISE) Institute, have found 11 mutants. They will continue to use this medium to study the genetics of the mutant plants.