"The secrets of symbiosis: how plants decide to collaborate with fungi under stress conditions"

How are alliances formed between plants and fungi? How do plants decide whether or not to cooperate with fungi? Researchers from the Max Planck Institute of Molecular Plant Physiology in Potsdam, led by Professor Caroline Gutjahr, conducted a study shedding light on this underground plant alliance. The research findings were published in the journal Nature Communications.

The team of scientists discovered how the plant-fungus alliance changes when the plant is under stress and produces stress hormones. This research could lead to new approaches in agriculture, where plants and fungi are seen as equal partners.

Right beneath our feet, hidden in the soil, lies a complex system of interactions: almost all plants live in a close symbiosis with fungi. These fungi do not form the typical mushroom fruiting bodies that we know from forests and certain places where we consume them. Instead, they create a dense network of thin threads that permeate the soil, known as mycelium.

In just one cubic centimeter of soil, fungal mycelium can reach lengths of up to 100 meters. While many fungi decompose dead organic matter in the soil, there are specialized species that live in symbiosis with plants and rely on the photosynthesis products of living plants. In return, they provide plants with water and mineral nutrients. This interaction system has existed for hundreds of millions of years and is crucial for many terrestrial plants.

One particularly intimate form of symbiosis is called arbuscular mycorrhiza. In this type of alliance, fungal mycelium penetrates the roots of the plant, and sometimes even its cells. This cooperation has become one of the most important interactions that has evolved over time.

The close partnership between plants and fungi, involving the exchange of water and nutrients, is already embedded in their genetic code. However, under certain conditions, plants may refuse to cooperate with fungi.

Professor Caroline Gutjahr and her team at the Max Planck Institute of Molecular Plant Physiology in Potsdam study the processes that occur in plant roots when entering into or refusing symbiosis with fungi. Their discovery reveals that hormones play a crucial role in this interaction.

"For over 40 years, we have known that plants produce the gaseous hormone ethylene when they are under stress, for example, due to flooding, and this hormone suppresses the alliance between plants and fungi," explains Gutjahr. "Now we have identified the processes that take place in plants and how different plant hormones interact. Finally, we understand how plants decide to enter into or reject this partnership."

Experiments conducted by Debatosha Das and Kartikye Varshney showed that, contrary to common belief, ethylene does not trigger a plant's immune system defense reaction against the fungus. Instead, the plant hormone stimulates the accumulation of a central regulatory protein known as SMAX1.

This may suppress the expression of several genes responsible for the formation of symbiosis in the plant. Thus, when environmental conditions are unfavorable, the plant starts producing a hormone that inhibits the symbiosis-related genes. The partnership formation decreases or ceases.

If conditions change, other plant hormones take control over the SMAX1 protein, initiate its degradation, and thus re-activate the symbiosis-related genes. The plant is ready to cooperate again.

Collaborating with David K. Nelson's team from the University of California, Riverside, researchers also demonstrated that ethylene contributes to the accumulation of the SMAX1 protein in plants that have lost the ability to form symbiosis with fungi. Therefore, the significance of this discovery goes beyond the plant-fungus alliance. It will be interesting to study the role of this phenomenon in other contexts in the future.

Understanding how plants regulate their alliances with fungi under stress conditions can provide insights on developing crops that will form beneficial partnerships with fungi even under changing stress or climatic conditions.

In the future, this knowledge may help ensure an adequate supply of water and nutrients to plants in new climatic conditions and guarantee a harvest.