Study: How Plants Evolved to Weather the Cold

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Dec 22, 2013, 01:00 ET

Researchers Create Largest Evolutionary 'Timetree' of Land Plants; Results Published in Nature

WASHINGTON, Dec. 22, 2013 /PRNewswire-USNewswire/ -- A team of researchers studying plants has assembled the largest dated evolutionary tree, using it to show the order in which flowering plants evolved specific strategies, such as the seasonal shedding of leaves, to move into areas with cold winters. The results will be published Dec. 22 in the journal Nature.

Early flowering plants are thought to have been restricted to warm, wet tropical environments. But they have since put down roots in chillier climates. How they managed this expansion has long vexed researchers searching for plants' equivalent to the winter parka.

"Freezing is a challenge for plants. Their living tissues can be damaged. It's like a plant's equivalent to frostbite," said Amy Zanne, the study's lead author and an assistant professor of biology in the George Washington University. "So over time, if plants moved into colder climates, they've had to figure out how to get around these problems."

Researchers identified three repeated evolutionary shifts they believe flowering plants made to fight the cold. Plants either:

dropped their leaves seasonally, shutting down the pathways that would normally carry water between roots and leaves;
made skinnier water-conducting pathways, allowing them to keep their leaves while reducing the risk of air bubbles developing during freezing and thawing, which would shut down those pathways; or
avoided the cold seasons altogether as herbs, losing aboveground stems and leaves and retreating as seeds or storage organs underground.

Identifying these evolutionary adaptations and likely paths to them required the team to build two robust sets of data. First, Dr. Zanne and colleagues created a database of 49,064 species, detailing whether each species maintains a stem above ground over time, whether it loses or keeps its leaves and the width of its water-carrying pathways. To these they added whether it is ever exposed to freezing. Then, researchers took that information and combined it with an unprecedented dated evolutionary tree with 32,223 species of plants, allowing them to model the evolution of species' traits and climate surroundings. This "timetree," which can be viewed at OneZoom here, is the most comprehensive view into the evolutionary history of flowering plants.

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