The soybean is one of the world's most significant crop plants. It has been cultivated for thousands of years; today it serves mainly as a food source and protein-rich animal feed, and it is an important model organism for research into root symbioses. More than 400 million tonnes of soybeans are harvested worldwide each year; the largest growing countries are Brazil, the USA and Argentina. Around 6,000 to 9,000 years ago it was domesticated in East Asia from its wild form (Glycine soja).

A partnership underground. Every plant needs nitrogen in order to grow. About 78 percent of our atmosphere consists of nitrogen, but in elemental form, which plants cannot use. If soybean plants form a symbiosis with bradyrhizobia, they can receive nitrogen-containing compounds from them. The plant can then use these for growth and for building DNA and proteins. In return, the soybean plant uses sunlight to produce energy-rich photosynthetic products, part of which it passes on to the bacteria.

Bradyrhizobium. In the symbiosis the bradyrhizobia do not live freely in the soil but inside what are known as root nodules. These special thickenings of the root form in order to house the bacteria and to create conditions in which they can convert nitrogen from the atmosphere. This reaction, however, is highly sensitive to oxygen. Inside the root nodule the protein leghemoglobin binds the oxygen and thereby enables the bradyrhizobia to fix nitrogen. At the same time it gives active root nodules their characteristic pink colour.

Colonisation. Signal exchange: the soybean root releases signalling substances into the soil, the bradyrhizobia recognise these signals and respond with signalling substances of their own. The root hair grows and curls: it grows towards the bradyrhizobia until it curls around them and encloses them. An infection thread forms: inside the root hair the plant forms a tube that grows from the root hair tip into the root; the bradyrhizobia multiply within it and thus reach the interior of the root. A root nodule forms: there the plant forms a new organ in which the bradyrhizobia are taken up into the plant cells, multiply and begin to supply the plant with nitrogen-containing compounds.

Nitrogen fixation. Bradyrhizobia can make elemental nitrogen (N₂) from the air available to plants. With the help of an enzyme called nitrogenase it is converted into plant-available ammonium (NH₄⁺). This process is known as nitrogen fixation.

The partnership between plants and rhizobia can strengthen sustainable agriculture and support the achievement of several UN Sustainable Development Goals.

Goal 2 (Zero Hunger): the symbiosis supplies legumes with biologically fixed nitrogen and thus contributes to food security.

Goal 6 (Clean Water and Sanitation): because the nitrogen is provided close to the plant and as needed, nitrate leaching into ground and surface water is reduced compared with mineral fertilisation.

Goal 12 (Responsible Consumption and Production): the symbiosis can partly replace synthetic nitrogen fertilisers, whose production is energy-intensive and fossil-based.

Goal 13 (Climate Action): nitrogen fixation in nodules avoids part of the greenhouse gas emissions from the production and use of mineral fertiliser, in particular nitrous oxide (N₂O).

Goal 15 (Life on Land): the symbiosis raises soil fertility and lowers the need for soil-damaging over-fertilisation.

Legumes can form a symbiosis with soil bacteria of the rhizobia group. The exhibition shows it using the soybean (Glycine max) and the genus Bradyrhizobium as an example. Alongside maize, wheat and rice, soybean is one of the world's most important crop plants; more than 400 million tonnes are harvested worldwide each year, and the largest growing countries are Brazil, the USA and Argentina. Around 6,000 to 9,000 years ago it was domesticated from its wild form (Glycine soja); archaeological and genetic studies suggest that this began in northern China.

Like all crop plants, soybean needs large amounts of nitrogen for healthy growth and high yields. About 78 percent of the Earth's atmosphere consists of nitrogen, but in elemental form, which plants cannot use directly. Soybean plants, however, can form a symbiosis with bradyrhizobia and receive nitrogen-containing compounds from them, which they use for growth and for building DNA and proteins. In the opposite direction the plant passes on part of its energy-rich photosynthetic products to the bacteria.

In the symbiosis the bradyrhizobia do not live freely in the soil but inside the root nodules. These thickenings of the root house the bacteria and create the conditions under which they can convert nitrogen from the atmosphere. With the help of the enzyme nitrogenase, elemental nitrogen (N₂) is converted into plant-available ammonium (NH₄⁺) — a process known as nitrogen fixation. The reaction is highly sensitive to oxygen. The iron-containing protein leghemoglobin binds the oxygen inside the nodule and at the same time gives active root nodules their characteristic pink to reddish colour.

Colonisation proceeds in four steps. The soybean root releases signalling substances into the soil; the bradyrhizobia recognise these signals and respond with signalling substances of their own. A root hair grows towards the bacteria, curls around them and encloses them. Inside it an infection thread forms, growing from the root hair tip into the root; the bradyrhizobia multiply within it and thus reach the interior of the root. There the plant forms the root nodule as a new organ, takes the bacteria up into its cells, and the supply of nitrogen-containing compounds begins.

Soybean is among the best-studied crop plants and serves as a model organism; many of the findings obtained in it can also be transferred to other plants. The two microscopic images at this station, however, do not show soybean but Lotus japonicus with Mesorhizobium loti and a bean — both likewise legumes.