Moss Helps Chart The Conquest Of Land By Plants
Recent work at Washington University in St. Louis sheds light on one of the most important events in earth-history, the conquest of land by plants 480 million years ago.
No would-be colonizer could have survived on dry land without the ability to deal with dehydration, a major threat for organisms accustomed to soaking in water.
Clues to how the first land plants managed to avoid drying out might be provided by bryophytes, a group that includes the mosses, many of which retain remarkable drought tolerance. Some mosses can become so dry they crumble in the hand, but, if remoistened, will begin making proteins within minutes.
The work, published in the Jan. 29 issue of the journal Science, reveals several components of the signaling pathway that underlies the moss's Lazarus-like behavior.
Intriguingly, the flowering plants haven't entirely lost the ability to tolerate dessication: this tolerance has just been limited to their seeds, which dry out over winter and yet come to life in the spring.
The seed strategy
"We began by asking whether the moss Physcomitrella patens, which we have been using as a model system, employs different molecules or different regulatory systems than seeds. Or does the same mechanism underlie its desiccation tolerance and that of seeds?" says Ralph Quatrano, Ph.D., the Spencer T. Olin Professor of biology at WUSTL and senior author on the paper.
Earlier, Quatrano and others had showed that seeds depended on both the plant hormone ABA and the regulatory molecule ABI3 to survive drying.
Plant hormones, much like human hormones, are chemicals produced in small amounts that have a profound effect on growth and development. The release of one plant hormone, for example, causes bolting, the sudden growth of a floral stalk that signals the end of a leaf lettuce's tasty days.
ABI3 is a transcription factor, a molecule that binds to a specific DNA sequence next to the gene it regulates and controls the copying of that gene into messenger RNA, the first step in making a protein from the gene.
"The assumption," says Quatrano, "was that in the presence of ABI3, ABA triggers some genes to make proteins and then when you dry out the seed, those gene products protect the cells."
"The earliest known land plant is Cooksonia pertoni, a tiny plant with a simple structure (no roots or leaves) that lived in the Silurian period 425 million years ago. (Credit: Hans Steur, Ellecom, The Netherlands)"
Source: Washington University in St. Louis
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