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Signals that trigger symbiotic relationships

Some fungi kill plants by wearing them down. Other fungi, however, are on “friendly” terms with plants. Arbuscular mycorrhiza is a symbiotic partnership between plants and fungi in which the two partners exchange nutrients. If such partnerships were to be cultivated in a controlled way – for example as part of a sustainable agriculture programme – plant growers would no longer need to use fertilisers and pesticides. The research group led by Prof. Dr. Natalia Requena from the Karlsruhe Institute of Technology (KIT) investigates the molecular mechanisms that enable fungi and plants to initiate contact with each other. How do plants differentiate “good” from “bad” fungi? How do they manage to set up a functional and effective symbiotic relationship?

Plants and fungi have different strengths and weaknesses. So, combining forces could be a good idea. The majority of plants live in symbiotic association with so-called mycorrhizal fungi which help them capture nutrients and water from the soil and even protect them against pests. In return, the plants provide the fungi with sugar and other carbohydrates produced by photosynthesis. This is a win-win situation; the roots of the vast majority of plants are colonised by arbuscular mycorrhizae. “The mutual relationship is hugely advantageous for both plants and fungi. We envisage that controlled, symbiotic mycorrhizal cultures have the potential to reduce the use of chemical fertilisers and pesticides, which are still required in large quantities to protect crops against pests and to boost their growth,” said Prof. Dr. Natalia Requena, head of the Plant-Microbe Interactions research group at the Institute of Applied Biosciences at the Karlsruhe Institute of Technology (KIT). “But before we are able to use mycorrhizal fungi for sustained agriculture, we need to understand how the complex relationships are controlled.”

Couplers between plants and fungi

Bird’s foot trefoil (Lotus corniculatus) has a symbiotic relationship with mycorrhizal fungi and is one of the areas currently being investigated by KIT researchers. © Johannes Kühnel

Requena and her team are mainly interested in the early molecular processes occurring at the interface between plant and fungus. Which signals are exchanged by the symbiotes to enable mutual recognition? How do plants differentiate beneficial mycorrhizal fungi from pathogenic relatives? Which genetic programmes are triggered in order to enable a symbiotic relationship to be established? To find answers to these questions, the researchers from Karlsruhe take model plants, such as bird's foot trefoil, tomatoes or potatoes, and label the different genes with fluorescent proteins. The plants are then put into contact with the fungus Glomus intradices and the changes in the plant genes' fluorescent activity are monitored as the mycorrhizal fungus colonises the plants. This method led to the discovery of genes that are switched on when mycorrhizal fungi start to colonise plant roots and initiate a symbiotic relationship. 

One of the genes that was discovered by the researchers from Karlsruhe encodes a protein located in the endoplasmic reticulum of plant cells. The researchers assume that the protein is a receptor for steroids that are able to act as second messengers. Switching off the protein by modifying the encoding gene leads to the death of the fungi inside the plant cells due to the breakdown of the symbiotic relationship. "We assume that this protein binds fungal signals and sends a message to the nucleus to initiate the genetic symbiosis programme," said Requena. Future investigations will focus on testing this hypothesis. Once the researchers have understood how recognition and signalling mechanisms of this kind function, they might be able to manipulate them in order to control the course of fungal-plant symbioses.

Who retains control?

Plant roots covered with mycorrhizal fungi. © Wikipedia

But what kinds of signals are used by mycorrhizal fungi to attract the attention of plants? In another experiment, Requena's research group found that plants activate their symbiotic programmes before the fungus actually comes into direct contact with the plants. "We cultivated the plants together with mycorrhizal fungi, but placed a membrane between the two species which prevented the fungal protrusions from contacting the plants. We found that the genes that were required for establishing a symbiotic relationship were switched on," said Requena. The researchers conclude that the fungus sends out signals that are able to freely move through nutrient solutions or soil. Physical contact is established once the plant has received the fungal signals. Requena and her team now have plans to use biochemical experiments to find out what kind of substances these signals are.

There is no guarantee that a symbiotic relationship, once established, will be permanent. A crucial question relates to how the mutual exchange of substances is controlled. Who decides how much sugar, minerals and water are exchanged? "We believe that it is the plant that controls the degree of the symbiotic relationship," said Requena referring to soon-to-be-published experiments that have provided initial evidence for this assumption. Phosphate is one of the substances plants receive from the fungi. It has long been known that plants have their own mechanisms for taking up phosphate. If sufficient amounts of phosphate are present in the soil, plants do not tend to engage in very intensive relationships with mycorrhizal fungi. They only intensify their symbiotic relationship with fungi when the amount of phosphate starts to decline. Requena's research group recently showed that the amount of sugar produced by a plant is more or less equal to the amount of phosphate received. It appears that the plant accurately calculates how much it loses when it gives away sugar and how much it benefits by receiving phosphate in return.

Targets and sustainable agriculture

The researchers from Karlsruhe are pursuing different approaches in the development of symbiosis models. “Our most interesting project at the moment is the investigation of a fungal protein that migrates into the nuclei of plant cells during the early colonisation phase,” said Requena. “We assume that this molecule switches off the plant’s defence mechanisms, which are normally triggered when foreign organisms come in contact with plant roots.” Such defence mechanisms normally protect plants against pathogenic fungi and bacteria. Mycorrhizal fungi need to find ways to evade the plant’s defence processes. Molecules that enable mycorrhizal fungi to do this are known as effectors, which have been discovered and described in many pathogenic microorganisms. It was not previously known that they were present in mycorrhizal fungi. “We are therefore the first group to discover an effector in a mycorrhizal fungus,” said Requena whose team is now investigating the molecule’s function in the nucleus of plant cells.

Requena and her team see themselves mainly as basic researchers. They investigate molecular mechanisms and signalling pathways that enable symbiotic organisms to interact. “However, I have always been interested in the applied aspects of basic research, even before I came to Karlsruhe,” said the scientist. It would be a huge step towards a sustainable and ecological agriculture if it were possible to grow crops without chemical fertilisers and pesticides. However, before the biological interactions can be exploited for human purposes, it is necessary to understand the biological interactions of ecosystems as a whole.

Further information:
Prof. Dr. Natalia Requena
Plant-Microbe Interactions Group
Karlsruhe Institute of Technology (KIT)
Hertzstrasse 16
D-76187 Karlsruhe
Tel. +49 (0)721 6084626
E-mail: natalia.requena(at)kit.edu
Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/signals-that-trigger-symbiotic-relationships