Abstract
The main role of lichen anthraquinones is in protection against biotic and abiotic stresses, such as UV radiation. These compounds are frequently deposited as crystals outside the fungal hyphae and most of them emit visible fluorescence when excited by UV. We wondered whether the conversion of UV into visible fluorescence might be photosynthetically used by the photobiont, thereby converting UV into useful energy. To address this question, thalli of Xanthoria parietina were used as a model system. In this species the anthraquinone parietin accumulates in the outer upper cortex, conferring the species its characteristic yellow-orange colouration. In ethanol, parietin absorbed strongly in the blue and UV-B and emitted fluorescence in the range 480–540 nm, which partially matches with the absorption spectra of photosynthetic pigments. In intact thalli, it was determined by confocal microscopy that fluorescence emission spectra shifted 90 nm towards longer wavelengths. Then, to study energy transfer from parietin, we compared the response to UV of untreated and parietin-free thalli (removed with acetone). A chlorophyll fluorescence kinetic assessment provided evidence of UV-induced electron transport, though independently of the presence of parietin. Thus, a role for anthraquinones in energy harvesting is not supported for X. parietina under presented experimental conditions.
Highlights
Anthraquinones are a wide family of lichen secondary metabolites, conspicuous in the family Teloschistaceae, where they are frequently deposited as crystals outside the fungal hyphae, conferring species a characteristic yellow or orange colouration
Absorption spectra of acetonic extracts from X. parietina resuspended in ethanol showed two maxima, one in the UV-B region and a second one in the blue region
Confocal laser microscopy confirmed these optical properties in vivo (Figure 2), for parietin there was a shift towards the orange region, with a maximum emission at 610 nm
Summary
Anthraquinones are a wide family of lichen secondary metabolites, conspicuous in the family Teloschistaceae, where they are frequently deposited as crystals outside the fungal hyphae, conferring species a characteristic yellow or orange colouration. Parietin is one of them, found in Aspergillus and Penicillium fungi and in Ventilago, Rheum, and Rumex among vascular plants [1] The function of these compounds has been frequently studied using the deposition of parietin in Xanthoria parietina as a model system. In the present work we wondered whether lichen secondary metabolites may transfer radiative energy to photosynthetic pigments and if fluorescence itself could represent an adaptive trait that conferred any biological advantage. To address this question, field-collected thalli of X. parietina were used as a model system
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