FORT COLLINS--A.S.N. Reddy's experiment on the space shuttle Columbia should help explain how plants tell up from down.
Colorado State University's Reddy wants to look at what happens at the molecular level that tells a plant's root to grow down and a shoot to grow upward. It's an important question: Thirty years after humans first walked on the moon, the emphasis has shifted to the space station and longer manned space flights, possibly to other planets.
The problem is that plants -- an important source of food and oxygen and a means of recycling carbon dioxide -- aren't ready for the weightless condition known as microgravity.
"A root always grows down, a process known as gravitropism, which means root cells have some mechanism to know where gravity is," said Reddy, an associate professor of biology. "The relevant cells are in a one-millimeter area at the root tip called a root cap. But at this point we don't know which genes are involved in sensing gravity's signal."
After three days of growth aboard the Columbia, 240,000 tiny seedlings of Arabidopsis, a common weed closely related to mustard, will be frozen in liquid nitrogen, stopping all biological action and preserving molecular information. Reddy says about 13,000 genes--some 65 percent of the plant's total--have been sequenced (that is, the order of each gene's nucleotides is known). The investigators will use that and other information to determine which genes are activated or deactivated by gravity.
Reddy, in collaboration with Donald Mykles, professor of biology, and the late Willy Sadeh, former director of Colorado State's Center for Engineering in Spaces Sciences, has received $280,000 for his work from NASA. Some of the experiments will be performed with Monsanto Chemical Co., which has equipment with the capability to examine thousands of genes at a time. Then the real work begins--an analysis of genetic changes that Reddy estimates could take two years.
Joining Reddy at NASA's Kennedy Space Center were Mykles and Farida Safadi-Chamberlain, a postdoctoral research associate. Mykles shares with Reddy an interest in the role of calcium in cell function.
"With this experiment we'll be cataloging every gene that responds to gravity," said Mykles. "We currently have just a few pieces of this immense jigsaw puzzle, but with this project we hope to get most of them."
According to Reddy, "Our main goal is to find out how plants can sense the gravity signal, to determine what genes are turned on or off by gravity. If we find that a certain gene is regulated by gravity, we expect to find its function in gravitropism."
Arabidopsis won't be the first plant grown in space; Russian cosmonauts, for example, grew wheat aboard MIR. And Reddy expects roots to sprout without specific direction. What excites him is the opportunity to observe at the genetic level why this takes place.
He suspects two likely discoveries: the project will eventually reveal what proteins are involved in sensing gravity signals, and the investigators may discover how calcium ions mediate gravity-signal sensing. In fact, while the experiment is rooted in up-and-down, Reddy says the effort actually involves three major projects:
- To understand how plants respond to gravity's signal and their later development;
- To chart what changes take place in response to the gravity signal at the genetic level, and to take a particular look at the role calcium plays in this so-called signal transduction (or transfer); and
- To identify the role of proteins regulated by gravity in gravity-signal transduction.
About 20 hours before launch, Reddy and Safadi-Chamberlain spread the seeds, 10,000 to a dish, on 24 Petri dishes containing a growth medium. They're carried in a container on the shuttle's middeck (the cargo bay will carry the Chandra x-ray satellite) and won't need light to germinate, Mykles said.
"Like clockwork, at a certain temperature the germination of the seeds occurs under microgravity conditions," he said. "We're treating a similar batch of seeds that has been put in an orbital environmental simulation chamber on the ground with a real-time linkage to the seed container aboard Columbia.
"What we're trying to do is to treat these two batches of seedlings the same, except that one will be exposed to one G of gravity and the other to microgravity."
Reddy shows a certain yen to fly, at least for scientific purposes, because metabolic changes will start on the ground, he said. However, they shouldn't affect the experiment.
He adds, somewhat wistfully, "the ideal thing would be to do all this in space, but it is not possible."
The team has had a tough schedule after the mission was delayed twice from its scheduled July 20 launch. For all three planned launches, they began seed preparation about 1:30 a.m. the day before.
"NASA advised us to be prepared for up to five "scrubs," or cancellations, of the mission, so each day until launch we'd have to prepare a batch of seedlings ready to place on the shuttle," Safadi-Chamberlain said.
The results of the experiment should be novel and worth the effort, however. A plant physiologist, Reddy has long been interested in how plants transmit signals at the molecular level.
"Nobody really knows how roots sense gravity, but there are papers in the literature suggesting that calcium is involved," he said. "Recently, mutants with altered or no gravitropic response have been isolated, suggesting that there are specific genes involved in gravity signal transduction."
"That makes me think that in microgravity we might be able to learn about this whole mechanism by identifying the genes regulated by gravity."
"We're ready to go, and also very excited about the whole project," Reddy said. "It's one of the first attempts to look at gene expression in microgravity."