Mass Spectrometry Imaging of Specialized Metabolites for Predicting Lichen Fitness and Snail Foraging

Abstract

Lichens are slow-growing organisms supposed to synthetize specialized metabolites to protect themselves against diverse grazers. As predicted by the optimal defense theory (ODT), lichens are expected to invest specialized metabolites in higher levels in reproductive tissues compared to thallus. We investigated whether Laser Desorption Ionization coupled to Mass Spectrometry Imaging (LDI-MSI) could be a relevant tool for chemical ecology issues such as ODT. In the present study, this method was applied to cross-sections of thalli and reproductive tissues of the lichen Pseudocyphellaria crocata. Spatial mapping revealed phenolic families of metabolites. A quantification of these metabolites was carried out in addition to spatial imaging. By this method, accumulation of specialized metabolites was observed in both reproductive parts (apothecia and soralia) of P. crocata, but their nature depended on the lichen organs: apothecia concentrated norstictic acid, tenuiorin, and pulvinic acid derivatives, whereas soralia mainly contained tenuiorin and pulvinic acid. Stictic acid, tenuiorin and calycin, tested in no-choices feeding experiments, were deterrent for N. hookeri while entire thalli were consumed by the snail. To improve better knowledge in relationships between grazed and grazing organisms, LDI-MSI appears to be a complementary tool in ecological studies

Mass spectrometry analysis of saponins

“…This procedure permits to detect and localize ions of interest directly on tissue sections and nearly without any preparation. MALDI imaging experiments performed on specialized metabolites start to be reported in the literature (Gadea et al, 2020), since the great added value of using MALDI‐MS direct tissue analysis is to detect specific molecules in a spatially‐resolved analysis. By this way, MALDI‐MSI or MALDI profiling is recognized to open the door for important breakthroughs in the field of chemical ecology (Le Pogam et al, 2016).…”

Discovery and excavation of lichen bioactive natural products

“…However, although MS is the most sensitive and powerful method which detects and elucidates extremely low-abundance metabolites occurring in natural product research, it does not provide any information concerning the spatial and temporal distribution of metabolites. Mass Spectrometry Imaging (MSI) visualizes the production, location, and distribution of metabolites, which is newly used in lichens to visualize the accumulation of various polyketides such as parietin, physodic acid, atranorin, and pinastric acid in different tissues of the lichen and localize the phenolic compounds (Gadea et al, 2020 ). Desorption electrospray ionization-imaging mass spectrometry (DESI-IMS) realized the spatial distribution of usnic acid in cross-sections of the lichen thallus (Xu et al, 2022 ).…”

“…Ultimately, NMR can provide information on how many atoms are present in each of these environments (Altemimi et al, 2017 ). Other imaging techniques such as Raman microscopic analysis can provide time-resolved information about the distribution of major compounds in lichens (Edwards et al, 1997 ; Liao et al, 2010 ; Gadea et al, 2020 ; Xu et al, 2022 ). FTIR imaging and Raman microscopy were used to localize the presence of usnic acid in Cladonia arbuscular, C. Uncialis , and C. sulphurina (Liao et al, 2010 ).…”