Impact of evolutionary ‘handshake’ demonstrated

Published On Tue 10 Jun 2014 by Grant Hill

An international team of scientists, including a University of Dundee researcher, has shown how a single evolutionary ‘handshake’ between plants and bacteria helped create the world as we know it today.

Bees pollinate plants in return for nectar, ants protect trees in return for housing, and our own bodies house bacteria that help us digest our food. These types of beneficial relationships, called mutualisms, are at the heart of the world’s biodiversity.

Now the team, led by researchers from VU University in the Netherlands, has discovered how to reconstruct the ancient history of these partnerships and pinpoint the major events leading to the emergence of such ancient collaborations.

They have revealed the key evolutionary steps towards the emergence of the mutualism that transformed global nutrient cycles. Plants need nitrogen to survive but cannot extract it from the air unaided. They do, however, produce sugar, which is required by various kinds of bacteria living in structures  called nodules. These bacteria harness atmospheric nitrogen and hand it over to the plant to make proteins, DNA and other vital compounds.

Not all plants can form partnerships with certain bacteria, however, and by creating the first complete evolutionary reconstruction of this symbiosis, the researchers hope to find out why. They developed the world’s largest database of global nitrogen-fixing plant species and used a newly developed mathematical model to reconstruct the ‘deep’ history of the nitrogen-fixing symbiosis.

Janet Sprent, Emeritus Professor of Plant Biology at the University of Dundee and one of the scientists involved in the study, said, “I have been collating data on nodulation for decades, which is now being entered in a database called ILDON (http://www.ildon.org) and am delighted that it has been used in this study. It was like looking for a needle in a haystack but we were eventually able to pinpoint a single innovation evolving in plants over 100 million years ago that made symbiotic nitrogen-fixation possible, confirming a previous hypothesis made verbally almost two decades ago.”

The researchers were able to reveal more details of the complicated evolutionary history of symbiotic nitrogen fixation. For instance, they found that in some plant groups the interaction is extraordinarily stable, while in others it is easily lost.

They also found that plants without the ancient innovation never form the symbiosis, explaining why it is not more commonly found. These discoveries are important because this symbiosis is crucial for the nitrogen supply of many ecosystems and farming systems, making it one of the planet’s most important symbioses. The evolution of symbiotic nitrogen-fixing in plants transformed global nitrogen cycles, meaning the world would have looked very different had it not occurred.

PhD student Gijsbert Werner, the study’s lead author, said, “There are enormous potential benefits of housing a nitrogen-producing factory in your roots, so it has always been a puzzle why this symbiosis is not more common. Now we finally have the tools to ask why.

“After this innovation, symbiotic nitrogen-fixation became possible, evolving repeatedly in descendant plant species. With reconstructions like these, we can literally look back hundreds of millions of years, and identify when and where crucial steps in the evolution of complex traits happened.”

A long-standing goal in agriculture is the introduction of the nitrogen fixing partnership into non-fixing crops, like maize and wheat. The newly published research is a key step in determining if and how such symbiotic partnerships can be transferred. The researchers found that none of the major cereal grains are among the species “primed” for the symbiosis, so modifying them for symbiosis will prove difficult until the specific molecular machinery allowing for this priming is discovered.

The paper has been published in the latest edition of the journal Nature Communications. The researchers will now use the evolutionary reconstruction to understand the underlying genetics of the symbiosis.

“The history is only the first step,” said VU Professor of Mutualistic Interactions Toby Kiers. “The million-dollar question is identifying what this innovation actually is. If we solve that puzzle, we are getting closer to the goal of transferring the symbiosis. The beauty of the symbiosis is its complexity, it is never easy to force partners to cooperate.”

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