How Plants Evolved to Weather the Cold

Researchers Create Largest Evolutionary ‘Timetree’ of Land Plants to Investigate Traits That Permit Survival in Cold Climates; Results Published in Nature

Released: 12/17/2013 9:45 AM EST
Embargo expired: 12/22/2013 1:00 PM EST
Source Newsroom: George Washington University
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Citations Nature Materials; National Science Foundation Grant EF-0905606

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**For photos and captions that illustrate the findings of the study, email Kurtis Hiatt at kkhiatt@gwu.edu
**To view the evolutionary tree, visit: http://www.onezoom.org/vascularplants_tank2013nature.htm
**To download photos of Amy Zanne, visit: http://www.flickr.com/photos/georgewashingtonuniversity/
sets/72157638647492816/

**To request an embargoed copy of the full study, email Kurtis Hiatt at kkhiatt@gwu.edu

Newswise — WASHINGTON—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 woody—which maintain a prominent stem above ground across years and changing weather conditions, such as maple trees—and restricted to warm, wet tropical environments. But they have since put down roots in chillier climates, dominating large swaths of the globe where freezing occurs. 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. Their water-conducting pipes can also be blocked by air bubbles as water freezes and thaws,” said Amy Zanne, the study’s lead author and an assistant professor of biology in the George Washington University’s Columbian College of Arts and Sciences. “So over time, if plants moved into colder climates, they’ve had to figure out how to get around these problems.”

Dr. Zanne and a team of 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 (the fatter the pathways, the higher the risk); or
- avoided the cold seasons altogether as herbs, losing aboveground stems and leaves and retreating as seeds or storage organs underground, such as tulips or tomatoes.

The researchers also identified the order of evolutionary events. Most often woody plants became herbs or developed skinnier pathways before moving into freezing climates. In contrast, plants usually began dropping their leaves after moving into freezing climates.

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, using resources from the Global Biodiversity Information Facility and a global climate database. 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 yet into the evolutionary history of flowering plants.

“Until now, we haven’t had a compelling narrative about how leaf and stem traits have evolved to tolerate cold temperatures,” Dr. Zanne said. “Our research gives us this insight, showing us the whens, hows and whys behind plant species’ trait evolution and movements around the globe.”

To build on these findings, Dr. Zanne and others will use the massive tree to explore other aspects of the evolutionary history of plants, especially to examine how plants respond to additional environmental pressures besides just freezing.

The team will make available at Dryad the data and tools developed for this study for other researchers’ use. The National Evolutionary Synthesis Center, National Science Foundation (grant number EF-0905606) and Australia-based Macquarie University’s Genes to Geoscience Research Centre funded this study.

The Columbian College of Arts and Sciences
Established in 1821 in the heart of the nation’s capital, the George Washington University Columbian College of Arts and Sciences is the largest of GW’s academic units. It encompasses the School of Media and Public Affairs, the Trachtenberg School of Public Policy and Public Administration and more than 40 departments and programs for undergraduate, graduate and professional studies. The Columbian College provides the foundation for GW’s commitment to the liberal arts and a broad education for all students. An internationally recognized faculty and active partnerships with prestigious research institutions place Columbian College at the forefront in advancing policy, enhancing culture and transforming lives through research and discovery.

The George Washington University
In the heart of the nation's capital with additional programs in Virginia, the George Washington University was created by an Act of Congress in 1821. Today, GW is the largest institution of higher education in the District of Columbia. The university offers comprehensive programs of undergraduate and graduate liberal arts study, as well as degree programs in medicine, public health, law, engineering, education, business and international affairs. Each year, GW enrolls a diverse population of undergraduate, graduate and professional students from all 50 states, the District of Columbia and more than 130 countries.


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