Usnea taylorii
Creators: name: Gadea, Alice
name: Charrier, Maryvonne
name: Fanuel, Mathieu
name: Clerc, Philippe
name: Daugan, Corentin
name: Sauvager, Aurélie
name: Rogniaux, Hélène
name: Boustie, Joël
name: Lamer, Anne-Cécile Le
name: Devehat, Françoise Lohézic - Le
Publisher: name: Zenodo
Year: 2019
Subjects: Biodiversity
Taxonomy
Fungi
Ascomycota
Lecanoromycetes
Lecanorales
Parmeliaceae
Usnea
Usnea taylorii

description: 2.1. Morphological and chemical characterisation of Usnea taylorii
2.1.1. Description of Usnea taylorii morphology
Microscopic analysis of the fruticose lichen Usnea taylorii reveals a specific morphology that deserves a detailed description to better understand the location of metabolites (Fig. 1a; a complete morphological description of the lichen is available in Text S1). Apothecia (the reproductive parts of the lichen) are located at the apex of the Usnea branches and are characterised by their large diameter (2–17 mm) and their jet-black pigmented disc. In cross-section, underneath the black epithecium, asci containing spores are present in the hymenium, covering the algal layer (Fig. 1b). The thallus in cross-section shows a very thin cortex, protecting a discontinuous algal layer. The central axis of U.
taylorii is divided into several smaller axial strands by the protruding medullae, which are surrounded by some algae. Between these axial strands, lax medulla is observed (Fig. 1c).
2.1.2. Chemical characterisation of Usnea taylorii
The extraction yields with acetone ranged between 0.5 and 1.0% of specialised metabolites for the six replicates of dried lichen materials with a mean value of 0.8 ± 0.2%. Preliminary experiments showed that more than 95% of the specialised compounds were extracted from the lichen U. taylorii by rinsing intact air-dried thalli in acetone. Thalli had only one or two main specialised metabolites. The concentration of usnic acid ranged from 2.5 to 5.4 mg g −1 dry mass (DM) of lichen, with a mean value ± s.d. of 4.1 ± 1.1 mg g −1 DM (n = 6). Sugar and polyol profiling was also performed. A low diversity was observed, with only four metabolites quantified. Arabitol was the most important polyol, reaching 138.4 ± 25.8 mg g −1 DM (mean value ± s.d., Table S1, Fig. 2).
Preliminary LDI-MS analysis of an acetone extract of Usnea taylorii confirmed that the (+)-usnic acid was the only specialised metabolite detected through its deprotonated molecule (m/z 343) along with a fragment ion at m/z 329 ([M-Me] -) (Fig. S1). Usnic acid was consequently imaged through the m/z 343 ion. D-arabitol, the major polyol quantified by GC, was also imaged by mass spectrometry. However, because D-arabitol does not absorb at the wavelength of the laser used for LDI-MSI, a MALDI matrix solution was sprayed on U. taylorii slices and images were obtained using MALDI-MSI (through its sodium adduct observed at m/z 175).
In situ LDI-MSI experiments applied to a slice of Usnea taylorii 's branch revealed that usnic acid was located in the peripheral layer of the thallus, i.e., in the cortex and in the uppermost parts of the medulla including the photobiont layer (Fig. 3a). Likewise, usnic acid was allocated to the external layers of the apothecium (epithecium, hymenium and the underside of the apothecium) (Fig. 3c). D-arabitol was present mainly in the lax medulla, the cortex and the algal layer (Fig. 4a), but occurred in lower intensities in the axial strands of the lichen branches. In apothecia, the signal corresponding to D-arabitol was stronger in the layer containing the lax medulla and algae than in the external layers of apothecium (epithecium and hymenium) (Fig. 4b). To perform a comparison between lichen parts eaten by snails or not, some branches and apothecia were sliced and analysed by LDI-MSI. Snails consumed parts of the cortex, the algal layer and the lax medulla of the branches. In apothecia, epithecium, hymenium as well as the underside of the apothecium were grazed. All these tissues contain usnic acid (Fig. 3b and d) and D-arabitol (Fig. 4).
description: Published as part of Gadea, Alice, Charrier, Maryvonne, Fanuel, Mathieu, Clerc, Philippe, Daugan, Corentin, Sauvager, Aurélie, Rogniaux, Hélène, Boustie, Joël, Lamer, Anne-Cécile Le & Devehat, Françoise Lohézic - Le, 2019, Overcoming deterrent metabolites by gaining essential nutrients: A lichen / snail case study, pp. 86-93 in Phytochemistry 164 on pages 87-88, DOI: 10.1016/j.phytochem.2019.04.019, http://zenodo.org/record/10483014