What is the limiting factor in life

10.06.2014 13:32

Nitrogen fixation in plants - a symbiosis that is millions of years old

Dr. Eberhard Fritz Press and public relations
Max Planck Institute for Biogeochemistry

When plants with nitrogen-fixing bacteria form a community of convenience, both partners gain competitive advantages from this symbiosis: The plants receive nitrogen from the bacteria, which they need for growth but cannot fix themselves. The microorganisms in turn find food and a protected habitat in the plant roots. It was not previously known how these symbioses between bacteria and plants developed during evolution. An international team of experts has now found that the basis for all symbioses between plant species and nitrogen-fixing bacteria was laid at a very early stage of evolution.

Nitrogen is an essential component of biological molecules, especially macromolecules such as proteins and nucleic acids. Although it occurs in huge quantities in the atmosphere as molecular nitrogen (N2), it cannot be processed or used by plants in this form. Only certain bacteria, so-called nitrogen fixers, are able to bind elemental nitrogen dissolved in the soil water. In the course of evolution, various symbiotic relationships have developed between the bacterial nitrogen fixers and certain plant families in the soil. In this community, which is beneficial for both partners, the bacteria supply the essential nitrogen as ammonium to the plants, which in turn release carbohydrates and other nutrients to the bacteria. This symbiosis is usually visible as root nodules - collections of bacterial colonies that adhere to the roots of their host plants.

But when, how and with which representatives have such profound developmental steps, which are highly beneficial for plants, taken place in the course of evolution? A team of experts from Amsterdam, Dundee and Jena investigated these questions. Their research approach was based on a suitable database containing all known nitrogen-fixing plant species. "In our global database on plant traits, we have included the trait of nitrogen symbiosis in addition to many other plant traits" says Dr. Jens Kattge from the Max Planck Institute for Biogeochemistry in Jena, co-author of the study. This database, which is also funded by the German Center for Integrative Biodiversity Research (iDiv), has around 70,000 plant species recorded worldwide and has been supplemented with further data on root nodule symbioses. The result was the first comprehensive database of all known plant species that live in a symbiotic relationship with bacterial nitrogen fixers.

Combined with genetic relationship analyzes, a comprehensive family tree of all plant species with regard to their ability to fix nitrogen was created on the basis of the database. Mathematical calculations provided the astonishing history of the genesis of the symbiosis: all nitrogen-fixing plants are most likely based on a single common, fundamental change that enables them to live with bacterial symbionts. Building on this property, acquired over 100 million years ago, various symbiotic relationships have subsequently developed between some plant families and various bacterial nitrogen fixers. "With the evolution of this symbiosis, the global nitrogen cycle was fundamentally changed 100 million years ago," says Jens Kattge. “The world would look different today without them.” In some groups of plants, these symbioses were stably continued in the course of evolution, but were also lost again in other plants.

With the help of this reconstruction of the evolutionary development, the detailed genetic basis and thus the mechanism can be researched, which enables plants to enter into a symbiosis with the nitrogen fixers. With the new findings, hope increases that the mechanisms of action of the symbiosis can possibly be transferred to agricultural crops. Because in food plants such as maize and wheat, the ability for symbiosis is not genetically anchored. These plants usually have to be offered mineral nitrogen as an artificial fertilizer to increase their yield.

Since nitrogen is usually only present in small quantities in soils, it is a limiting factor for plant growth. Plants that live symbiotically with nitrogen fixers, on the other hand, can also thrive on poor, nutrient-poor soils and thus have selection advantages. Such plants are grown in agriculture to enrich the soil with nitrogen. These include, for example, the legumes (pulses) that live with bacterial rhizobia.

These symbioses play an important role in biogeochemical material cycles: They are essential for the availability of nitrogen in many natural ecosystems and make a decisive contribution to the repopulation of ecosystems and their biodiversity. As a limiting factor in plant growth, nitrogen is also important for plant biomass production and thus for the global carbon cycle. The symbiosis is therefore considered to be one of the most important on our planet.

Werner, G.D.A. et al. A single evolutionary innovation drives the deep evolution of symbiotic N2 fixation in angiosperms. Nat. Commun. 5: 4087, doi: 10.1038 / ncomms5087 (2014).

Dr. Jens Kattge
Max Planck Institute for Biogeochemistry
Research group “Functional Biogeography”
Hans-Knöll-Strasse 10, 07745 Jena
Tel: +49 3641 576226
email: jkattge (at) bgc-jena.mpg.de

Additional Information:

http://www.bgc-jena.mpg.de/index.php/Main/HomePage Max Planck Institute for Biogeochemistry
http://dx.doi.org/10.1038/ncomms5087 Publication at Nature Communications

Features of this press release:
Biology, environment / ecology
Research results, research projects