The role of light-protective pigments in stress tolerance of lichens

Lichens synthesize a large number of secondary metabolites, including “lichen substances”. The presence of lichen substances allows lichens to grow in diverse, often stressful, ecological niches. Among the secondary metabolites of lichens, light-protective pigments are of particular interest. The dark brown pigment melanin plays a significant role in protecting lichens from UV-B stress, but the key drivers of melanization remain unexplored. Melanins are hydrophobic heterogeneous polymers formed by sequential reactions of oxidation of phenolic/indole precursors and subsequent polymerization of intermediate phenols and quinones. The formation of a melanin layer on the surface of the thallus in response to UV exposure is one of the key mechanisms of high tolerance of lichens not only to light stress, but also to desiccation. We showed that the binding of water molecules to melanin depends on the activity of specific functional groups in the structure of this polymer, elemental composition, and the presence of associated compounds, including polysaccharides, and the ultrastructure of melanin particles. The anthraquinone parietin is the dominant cortical pigment of the lichens Caloplaca and Xanthoria of the family Teloschistaceae. Along with providing protection to the thallus from high photosynthetically active radiation and UV radiation, parietin has antioxidant properties and helps to protect lichens from cadmium toxicity and desiccation. Parietin protects lichen thalli from abiotic stress by maintaining membrane stability, providing antioxidant defense, forming a structural barrier, and sustaining water in the cortex of lichen thalli. The unique properties of lichen pigments make these natural polymers promising objects for fundamental and applied research, in particular in medicine, biotechnology and ‘‘green electronics’’.