Scientists find evidence for crucial root in the history of plant evolution

March 25, 2003

NEW ORLEANS, March 25 -- If ancient plants had not migrated from the shallow seas of early Earth to the barren land of the continents, life as we know it might never have emerged. And now it appears this massive floral colonization may have been spurred by a single genetic mutation that allowed primitive plants to make lignin, a chemical process that leads to the formation of a cell wall. The new findings were presented today at the 225th national meeting of the American Chemical Society, the world's largest scientific society, in New Orleans.

Using an advanced analytical technique, applied for the first time to fossils, a team of researchers studied an extinct plant species called Asteroxylon -- thought to be one of the first plants to inhabit the land of the early continents. The new method, which allows scientists to analyze fossils without altering their spatial context, could provide paleontologists a way to answer some of the looming questions in Earth's history. It also offers wide-ranging applications in fields such as petroleum exploration and meteorite chemistry.

"A critical question is whether Asteroxylon in fact had the capacity to biosynthesize lignin," says George Cody, Ph.D., a chemist at the Carnegie Institute of Washington who presented the research. "If it did, it starts to beg an interesting question: If one of the earliest plants had this capacity, then is it that capacity that allowed plants to colonize the continents? And that, of course, could have enormous significance, because that was probably one of the many truly defining events in Earth history."

Asteroxylon was found fossilized in the beds of the Rhynie Chert -- a rock formation in northeast Scotland. Fossils from this site have revealed much about Earth as it might have been 400 million years ago, in the early Devonian period.

"What we came up with is evidence that really can't be explained any other way than the fact that this plant, when it lived, had two structural biopolymers in its cell wall," Cody says. "The differences that you see in the spectra are consistent with a greater amount of lignin being in one region of the cell wall than the other, which is consistent with what we see in modern plants."

Plants originally evolved from some kind of algae, which, even in modern forms, do not have the ability to make lignin. "At what point did plants become plantlike, and less like algae?" Cody asks. A clue may be found in determining if there is a chemical difference in the cell walls of the earliest plants, hinting at the presence of lignin.

In the early Devonian period, Earth is thought to have been an aggregate mass of land in the Southern Hemisphere with smaller continents in the equatorial region. Europe was near the equator, so the Rhynie Chert beds were in a tropical climate consisting mostly of flatlands and shallow pools of fresh water.

Rhynie fossils were preserved when trapped in silica precipitated from hot spring fluids. Over time, the amorphous silica crystallized to form a fine-grained rock known as chert, which protected the fossils.

"Different thermal events over 400 million years have transformed some of this material, but its spatial context has never changed," Cody says. Normal analytical methods require dissolving the rock matrix to study the organic material in the chert. "The problem is, when you're done, all the organic matter basically disintegrates into this incredibly fine black powder," Cody continues. "So you've obtained your organic matter, but all the spatial context related to these beautiful fossils is gone."

To develop a method of studying the organic matter in the fossils while still preserving the spatial context, Cody turned to his collaborator Kevin Boyce, Ph.D., a paleobotanist at Harvard University. "What Kevin does is polish a surface of rock where the interesting fossils are, etch it in hydrofluoric acid, then put a gel on top," Cody says. "You allow the thing to dry and then pull it off like you're pulling a bandage off your leg." With it comes all the organic matter that was embedded in the chert, while completely preserving the anatomical detail.

Paleobotanists have been using this method for years. "The entire phylogeny of plants is based on this technique," Cody continues. "I said, well, that's organic matter; we can actually analyze that."

To do so, the researchers used scanning transmission X-ray microscopy (STXM, or "stixum"). This involved mounting the fossil impression on a copper grid and bombarding it with a brilliant focused beam of X-rays in the "soft" range -- between 200-900 electron volts. Biological elements, such as carbon, nitrogen and oxygen, exhibit characteristic absorption in this unusual energy range, Cody says.

This is the first time STXM has been used to study archaeological fossils, and it could be applied to other lingering questions of paleontology, Cody says. For example, it could be used to determine whether what some scientists have called extremely ancient micro-fossils -- on the order of 3.5 billion years old -- are actually biological fossils at all, or just organic matter.

The technique also has potential uses in the area of petroleum and natural gas exploration, and it could even be used to study the complex organic chemistry found in meteorites. "It turns out that biologically derived organic matter is chemically and structurally complex," Cody says. "This is a very nice tool to try to interrogate that complexity while obtaining chemical information."
The paper on this research, GEOC 106, will be presented at 9 a.m., Tuesday, March 25, at the Morial Convention Center, Room 387, during the symposium, "Ancient Biomolecules: New Perspectives in Archaeology and Paleobiology."

-- Jason Gorss

American Chemical Society

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