An experiment on the space shuttle Columbia by an associate professor of biology at Colorado State University should help explain how plants tell up from down.
A.S.N. 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. 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. "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."
To find out, Reddy and colleagues sent 240,000 tiny seedlings of Arabidopsis, a common weed closely related to mustard, aloft on Columbia. After three days the seedlings were 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 were 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 Infrastructure and Science in Space, has received $280,000 from NASA for his work. Some of the experiments will be performed with Monsanto Chemical Co., which has equipment with the capabiliby 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 asssociate. 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 the rest 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 how this takes place at the genetic level.
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 how that affects 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.
About 20 hours before the July 23 launch, Reddy and Safadi-Chamberlain spread the seeds, 10,000 to a dish, on 24 Petri dishes containing a growth medium. They were carried in a container on the shuttle’s middeck and didn’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 treated a similar batch of seeds that were put in an orbital environmental simulation chamber on the ground with a real-time linkage to the seed container aboard Columbia.
"What we were trying to do is to treat these two batches of seedlings the same, except that one was exposed to one G of gravity and the other to microgravity."
The team had a tough schedule. They began preparations about 1:30 a.m. the day prior to launch, originally scheduled for July 20, and repeated the procedure twice until Columbia finally took off after a false warning signal and later weather cancelled two launches, acccording to Safadi-Chamberlain.
"NASA advised us to be prepared for up to five ‘scrubs,’ or cancellations, of the mission, so each day until launch we had 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."
In other words, said Reddy, some plants appear not to respond to gravity, but these mutations are naturally occurring and not at all understood. It does, however, suggest that plants have a genetic basis for gravitropism.
"That makes me think that in microgravity we might be able to learn about this whole mechanism by identifying the genes regulated by gravity, " Reddy said. "We’re very excited about the whole project. It’s one of the first attempts to look at gene expression in microgravity."