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A ray of light for fungi

Breaking through the protective darkness of the soil can be very uncomfortable for fungi because it requires them to adapt quickly to UV radiation or moisture fluctuations. But how do they know that they are on the soil surface? An important parameter is light. Researchers led by Prof. Dr. Reinhard Fischer at the Karlsruhe Institute of Technology (KIT) are investigating how the mould Aspergillus nidulans perceives light and how this governs its behaviour. Over the last few years, growing evidence has shown that many molecular processes in fungi are influenced by light. Since Aspergillus moulds are used for many applications in the food industry and in biotechnology, the researchers’ results are also of great importance for these areas.

The photo shows the environmental influences on fungi below and above the soil © Prof. Dr. Reinhard Fischer
Plants are able to “see” and turn towards the direction of the light that they require for photosynthesis. Animals also know when it is light or dark, adapting physiologically to the day’s rhythm. Light is one of the most important sources of information in nature. Relatively simple organisms such as fungi already have systems that enable them to detect the presence of light and its spectral composition. This is of great importance for fungi for different reasons. For example, the fungi need to know whether they are growing below or above ground since the harmful UV rays, the generation of oxygen radicals as a result of exposure to light or temperature fluctuations require completely different physiological adaptations than those required for growth below the soil’s surface. In common with other plants, fungi also have an “internal clock” which they need to adapt to the rhythm of day and night. Light is also crucial for reproduction. The mould Aspergillus nidulans produces asexual spores when exposed to light. While the asexual spores are adapted for survival, the production of sexual spores during darkness are adapted for surviving times of unfavourable conditions and to mix the genetic material of two individuals.

A light perception centre

“Researchers have recently found out that the activity of around five per cent of all genes in the Aspergillus moulds under investigation are influenced by light,” said Prof. Dr. Reinhard Fischer from the Institute of Applied Biosciences at the Karlsruhe Institute of Technology (KIT). “This is not only of great interest for basic researchers, but might also be of great interest for the food industry.” Fungi produce mycotoxins that can spoil food. Mycotoxin formation is influenced by light. Is light therefore a more important factor during the production processes than has previously been assumed? And how can the light-regulated physiological processes be regulated more efficiently? Fischer and his team hope to shed light on these questions by finding out what happens in Aspergillus on the molecular level when it is exposed to light. The Karlsruhe researchers have recently discovered a protein complex that functions as a light perception centre and which translates this information into physiological changes.

It has been known for quite some time that fungi can perceive light with the same molecules that plants and animals use. These are proteins that can change their structure when capturing the light quanta of a certain wavelength. In addition, fungi have different pigments for the different light colours that react to light irradiation. For example, blue light is perceived by flavin-containing proteins, green light by opsin, which also detects light in the human eye, and red light is perceived by phytochromes, which were initially discovered in plants where their role is to regulate light-dependent growth.

Surprising difference to animals and plants

Spore carrier of Aspergillus nidulans with hundreds of asexual spores. © Prof. Dr. Reinhard Fischer

"Both phytochromes and flavin-containing proteins are found in the A. nidulans mould light regulation complex," said Fischer. This complex is therefore able to react to different wavelengths. When the complex has captured light, it interacts with the nuclear DNA of the mould and controls the transcription rate of different genes. This has an effect on the physiology and development of the mould. Fischer's team are now working to solve the following questions: Which signalling cascades are triggered? Which nuclear genes are regulated, which other molecules are involved in this process and what are the physiological reactions? In the long term, the KIT researchers also hope to clarify fundamental questions such as how light governs the developmental processes in A. nidulans and makes the fungus produce asexual and sexual spores.

The fact that the ultimate perception of light and the subsequent reaction is different between A. nidulans and plants and animals is a very interesting aspect. In addition, these processes occur in one and the same cell compartment in fungi. In plants, the phytochromes are found in the cytoplasm and are transported to the nucleus upon excitation. In contrast, the fungal light regulation complex is located in the nucleus itself. The researchers assume that this change of location is due to the fact that the fungus is small and transparent, enabling the light to reach the nucleus directly. The detector (i.e. the component that perceives light) and the effector (the component that triggers a reaction on the genetic level) are located in close vicinity to each other.

In a cooperative project with biochemists, Fischer and his team hope to find out what happens inside the light detector when it captures light quanta. Which parts of the molecule are dislocated against each other? How dynamic is this process? Which forces are involved? “This project is still a dream of the future”, said Fischer, which suggests that the researchers still have a lot of work ahead of them. But one thing is clear: without dreams of the future, science would never be able to glimpse the light.

Prof. Dr. Reinhard Fischer
University of Karlsruhe
Institute of Applied Biosciences
Hertzstrasse 16
D-76187 Karlsruhe
Tel.: 49-721-608-4630
Mobile: 49-152-0160-1108
Fax: 49-721-608-4509
E-mail: reinhard.fischer(at)KIT.edu
Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/a-ray-of-light-for-fungi