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Just the fibers please.

Just the Fibers, Please

If the useful part of a cotton plant is the cotton fiber, why not grow just the fiber - in a laboratory dish?

Growing them in the lab instead of in the field offers researchers an opportunity to understand the mysterious cellular and biochemical factory that makes cotton fibers.

That's what ARS scientists Robert W. Seagull and Barbara A. Triplett at the Southern Regional Research Center in New Orleans began working on 5 years ago.

"Fibers grown in the lab provide us with a good model for studying the biological mechanisms that regulate important economic traits such as fiber length and strength without the complications of having to examine the rest of the plant," says cell biologist Seagull.

Their aim and expectation is not to produce commercial quantities of fiber. Seagull says, "But once we understand what biochemical signals from what structures in the cells direct specific traits, we may be able to achieve higher quality fibers by either classical breeding or molecular genetic manipulation."

In cellular terms, cotton fiber cells are unusual giants, easily reaching several centimeters in length; most plant cells grow no larger than a few hundred micrometers.

"Yet if fiber cells can grow that long, why can't they grow longer? What shuts off the lengthening process? These are basic questions we are just beginning to answer," says Seagull.

To unravel the mystery of length, he is studying how the cells become fibers by looking at how and where cell wall is laid down.

It has been believed that elongating cotton cells grow primarily from the end of the cell protruding out from the seed thus creating a growth tip.

"On the contrary, while a somewhat higher percentage of growth may take place at the fiber tip, I've found significant amounts of growth taking place along the whole length of the fiber," says Seagull.

In order to directly examine the fiber and pin down exactly where growth takes place, Seagull had to devise a new way to prepare cells to be studied under the electron microscope.

In his method, fibers are plunged into liquid propane at minus 196 [degrees] C, which freezes cells in microseconds. This fast-freeze causes cells to solidify before the water in them can form ice crystals. Crystallizing water disrupts delicate cell structures.

The frozen cells are gradually warmed to minus 80 [degrees] C, and the cell's water is replaced with an acetone-osmium mixture, which preserves the cell's ultrastructure.

"This technique has given us images of a cotton fiber that are more lifelike than any we've seen before. Using relatively high magnifications of 50,000 times, I can directly find the sites of primary cell growth," Seagull explains.

Cell structures called secretory vesicles have been found all along the length of the fiber in the process of fusing to the cell membrane. Secretory vesicles supply new membrane to accommodate cell lengthening and bring the building blocks for cell wall formation from inside the cell to the membrane. Growth takes place where vesicles fuse in the cell.

Length is not the only trait of interest. Seagull's improved preservation technique was also used to confirm when cuticle, the waxy outer coating of a fiber, appears. The cuticle is responsible for much of the way cotton reacts to dyes and other chemical finishings during textile processing.

Previous thought was that cuticle is synthesized late in the cell's life around 21 days after the flower blooms and after secondary wall is laid down in the fiber.

But molecular biologist Triplett found evidence that cuticle appears very early in fiber formation, as early as 10 days after flowering.

"Bob was able to confirm what I had found biochemically by finding a cuticle structure," says Triplett. "Cuticle in its early stages is there even before the fibers are long enough for me to pluck them from the seed with tweezers to examine the composition biochemically."

This early cuticle production could change some thinking on how to go about altering cuticle's response to chemical finishings like permanent press and dyeing. Triplett explains.

"It means researchers are probably going to have to look much earlier in the fiber growth process to make any real changes in how cotton responds to chemical finishing processes," she says.

Seagull and Triplett are also looking at many other pieces of the cellular manufacturing process such as the layering of microfibrils in the cell wall. Microfibrils are filaments of cellulose responsible for much of a cotton fiber's strength.

"Throughout much of the fiber elongation phase, microfibrils are deposited like bands around a barrel circling through the primary wall." says Seagull. "As the elongation slows toward the end of growth. microfibrils of the secondary wall are deposited in helixes and ultimately in parallel, longitudinal lines. This multilayered wall improves the strength of the mature fiber.

"We are examining a set of structural elements in the cell cytoplasm involved in controlling the deposition and organization of the microfibrils. These elements, called microtubules. control the patterns of the micro-fibrils."

Now the researchers are looking for biochemical signals that direct this multilayered wall through an analysis of the protein components of the microtubules.

"We haven't isolated any proteins with identified functions yet, but we expect to," Seagull says.

The combination of ultrastructure and biochemical analysis is bringing new information into focus all the time.

Recently, they've begun to pinpoint where messenger RNA localizes for production of proteins specified by particular genes.

"This information will bridge the gap from biochemistry to cell biology," Triplett says. "Knowing when and where pivotal developmental points are will help us identify genes that control specific important processes or the signals that turn the gene off or on at a particular time. Some day we may be able to tell a breeder you need to select for cotton that turns on a specific set of genes at a particular time or at a different level to improve the quality of cotton fiber. - By J. Kim Kaplan, ARS.

PHOTO : Cotton fibers growing in ovule culture. Magnified about 170 times.

Robert W. Seagull and Barbara A. Triplett are at the ARS Southern Regional Research Center, 1100 Robert E. Lee Blvd., New Orleans. LA 70176. Phone (504) 286-4275.
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Title Annotation:US Department of Agriculture researchers try to understand the cellular and biochemical aspects of cotton
Author:Kaplan, J. Kim
Publication:Agricultural Research
Date:Jun 1, 1991
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