Abstract
The fungal cell wall is the external and first layer that fungi use to interact with the environment. Every stress signal, before being translated into an appropriate stress response, needs to overtake this layer. Many signaling pathways are involved in translating stress signals, but the cell wall integrity (CWI) signaling pathway is the one responsible for the maintenance and biosynthesis of the fungal cell wall. In fungi, the CWI signal is composed of a mitogen-activated protein kinase (MAPK) module. After the start of the phosphorylation cascade, the CWI signal induces the expression of cell-wall-related genes. However, the function of the CWI signal is not merely the activation of cell wall biosynthesis, but also the regulation of expression and production of specific molecules that are used by fungi to better compete in the environment. These molecules are normally defined as secondary metabolites or natural products. This review is focused on secondary metabolites affected by the CWI signal pathway with a special focus on relevant natural products such as melanins, mycotoxins, and antibacterial compounds.
Highlights
Leibniz Research Group Biobricks of Microbial Natural Product Syntheses, Leibniz Institute for Natural Product Research and Infection Biology-Hans Knöll Institute (HKI), Department of General Microbiology and Microbial Genetics, Institute of Microbiology, Faculty of Biology and Pharmacy, Friedrich Schiller University Jena, Neugasse 24, 07743 Jena, Germany
Fungi can produce a large arsenal of compounds that are synthetized to increase their fitness. Many of these active molecules are produced in secondary metabolism and include a wide range of low-molecular-weight chemicals [1]. These chemicals are normally classified as natural products (NPs) and they belong to the class of polyketides, small peptides, terpenoids, alkaloids, and phenols [1]
The most studied signaling pathways in fungi are: the cAMP pathway, which exploits the formation of cyclic AMP to amplify the activity of responsive kinases; the calcineurin pathway, which responds to intracellular calcium homeostasis; the TOR pathway, which mainly regulates cellular nutrient and energy levels; and the mitogen-activated protein kinase (MAPK) signaling pathways [7,8,9]
Summary
Fungi are organisms able to colonize every habitat across the world. Like plants, fungi do not actively move, and they spread in the environment by producing small spores that can be transported by atmospheric carriers (e.g., wind and water flows) and animals. Fungi can produce a large arsenal of compounds that are synthetized to increase their fitness Many of these active molecules are produced in secondary metabolism and include a wide range of low-molecular-weight chemicals [1]. A lot of effort has been spent on understanding the regulation of NP biosynthesis These efforts were focused on promoting the increase of NP production, relevant for industrial applications, and on inducing the expression of silent biosynthesis gene clusters which can potentially disclose new molecules with valuable functions [4]. The large and increasing availability of genome data revealed that the knowledge about NPs is very limited if compared with that concerning the computationally identified biosynthetic pathways This is true even for those organisms that have been extensively studied, which do not produce all their potential chemicals during lab fermenting conditions
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