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Many microorganisms, such as filamentous fungi, use light as a source of information. They are equipped with several photosensory systems and can respond to different light intensities and wavelengths (colours). Fungi have a crucial role in the remineralization of organic matter, are important plant and animal pathogens and are frequently used in industry, for instance, in the production of secondary metabolites or catalytic enzymes. Therefore, advances in the study of fungal biology should contribute to further understanding of their pathogenicity and improve their application in biotechnology. Structural basis of fungal photosensitivity.
Most cellular pathways are driven by proteins, and thus physical signals need to be transmitted to the protein level. In the case of light sensing, signal amplification and decoding of small conformational changes in the chromophore (which sometimes occurs in the range of picoseconds) into a code that cells can understand are required. Proteins without chromophores absorb ultra violet light through aromatic amino acids. Likewise, the photoreceptor UVR8 in Arabidopsis thaliana contains specific tryptophan residues, and ultraviolet-light absorption by those residues leads to the conversion of a UVR8 dimer into monomers that shuttle into nuclei. In comparison, visible-light sensing depends on a chromophore with aromatic residues or other systems with conjugated π-electrons. Such chromophores include the isoalloxazin ring systems in flavins, the retinal molecule or linear tetrapyrroles such as biliverdin.
Fig1. Structure of fungal photoreceptors and their chromophores. (Yu Z, et al., 2019)
Different photoreceptors vary considerably in structure and locate either to the cytoplasmic membrane (opsins) or to the cytoplasm and to nuclei (phytochrome and WC proteins). In all photoreceptors, light causes primary conformational changes of the protein, followed by different output mechanisms.such as control of protein-protein interactions or control of phosphorylation events leading to different signalling mechanisms. Most light responses ultimately depend on gene regulation. For instance, the production of reproductive, spore-producing structures from simple hyphae requires the control of hundreds, or even thousands, of genes at specific points in time. This is achieved by transcription factors that control certain steps in development. Linking their expression to light control thus enables coupling of the entire morphogenetic pathway to light. Likewise, metabolic processes may be controlled in a similar manner. Depending on the location of the photoreceptor, different signalling cascades are required.
Blue-light signalling mechanisms. Blue light controls all photoresponses in N. crassa and also has a role in other fungi with a pronounced red-light response.
Green-light signalling mechanisms. Green-light sensing and signalling are much less understood than blue-light sensing. There is good evidence that the green-light photoreceptor NOP-1 in N. crassa is involved in the regulation of developmental processes, possibly through the control of the oxidative state of the cell. These findings suggest a signalling cascade from the membrane-bound protein to the nucleus.
Red-light signalling mechanisms. Red-light and phytochrome signalling have been mainly studied in A. nidulans, A. fumigatus and B. cinerea. Some major phenomena controlled by red light in A. nidulans are the balance between asexual and sexual development and the germination process. Phytochromes were characterized by reverse geneticsafter their discovery in bacteria and the identification of partial sequences in fungal genome projects. Similar to blue-light signalling via the WC proteins, a phytochrome-dependent signalling cascade could be very short because red light also penetrates the cytoplasm and reaches the nucleus.
Fungi may use light – perhaps in combination with a circadian clock-to adapt to stressful conditions and for orientation in the environment to produce reproductive structures at the right place and the right time. It seems that fungi can also perceive light and adapt to stressful conditions without the described photoreceptors. Light also controls developmental decisions of the mycelium.
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