Plant Responses to the Environment — Six Graduate Student Projects

Root Architects

When plants face something undesirable, they can't walk away but they can grow away, says graduate student Jeremiah Fasano. "Since Darwin's day, we've known that plants will grow toward or away from things that are important," Fasano says. "Some of these 'movements,' which contribute to the final shape of the plant, are called tropisms. Shoots that track light and roots that grow toward water are exhibiting tropisms. There also are movements in response to nutrients, such as nitrogen and phosphorous. It's this combination of inherent form and substances in the soil that ultimately determines a root's architecture."

What particularly interests Fasano is "gravitropism," or growth in response to gravity. If it weren't for gravitropism, roots might grow sideways and shoots might grow down. "At about the same time Darwin published The Power of Movement in Plants," he says, "people noticed that certain cells contain little rocks that settle in the direction of gravity. And they thought, 'Aha! It's these rocks that tell the plant where down is!' But a century later, we still don't understand how this movement becomes a biochemical signal that plants can perceive. We have to get inside the cell and begin piecing together various biochemical events, and the techniques to do this are only now becoming available. But in 10 years, I think we'll have it."

Only certain tissues in a plant can sense gravity. In shoots, these tissues are found along the xylem and the phloem, the plant's bloodstream. In roots, these tissues are found in the root cap, the gravity-sensing organ at the tip of the root. "In corn, you can yank off the root cap without damage and show that the root can no longer sense gravity," Fasano says. "You can turn the root on its side and it continues growing horizontally it no longer knows to curve down."

The root cap houses the cluster of cells that contain the rocks called amyloplasts which are composed of starch crystals. By using infrared lasers to systematically blow up individual cells, Fasano and postdoctoral researcher Elison Blancaflor were able to measure how much each cell contributed to the gravitropism. Then, by correlating a map of the gravitropic response with a map of sedimentation velocity, they found that the faster the amyloplasts settle, the stronger the signal to curve.

Now Fasano wants to learn how the movement of the amyloplasts generates the signal. "The amyloplasts aren't completely free in the cell," he explains. "If you look at them under a microscope, you can actually see them dancing. They're surrounded by a very fine, dynamic mesh called the cytoskeleton that constantly pumps energy into them. This mesh seems to be connected to membranes around the periphery of the cell. And because the sensing event appears to begin sometime before the amyloplasts reach the lower wall, we no longer think it's the simple weight of them that creates the signal. We believe a physical force is translated to a chemical signal through the cytoskeleton."

"We don't know yet what happens when the force reaches the membrane," he says. "That's what I'm going to try to figure out."

Researchers know of certain molecules that affect signaling pathways inside cells in various and subtle ways. By keeping these molecules caged and inactive, then releasing them with a UV laser at the membrane to do their work, Fasano can observe how the cell responds. He will pay special attention to the movements of calcium and hydrogen ions, because they are known to be involved in the gravitropic response at the tissue level. After watching the cell's response to key combinations of chemicals, Fasano hopes to figure out what is happening biochemically.

"Studying gravitropism is interesting on two levels," Fasano says. "For one thing, anything we learn about plant perception is likely to apply to other sensory systems, particularly touch. Second, once we understand how root architecture is generated, we potentially can control how plants respond to various stimuli. We might even modify a plant's natural responses to make it grow properly in bizarre settings, such as space."