Titre de la dissertation: Plant specialized metabolites as a resource for bee conservation: chemical profiling of medicinal and melliferous plant species.

Promoteurs: Monsieur Pascal Gerbaux et Pierre Duez

Résumé de la dissertation: Today more than ever, bee populations are in danger. The use of noxious pesticides can partially explain their high mortality rates, but bees can also be fatally infected by parasites like bacteria, viruses or protozoa. One way to help them fight against those infections is to investigate what bees feed on throughout their life (i.e. floral resources), and specially specialized metabolites from pollen as they could show medicinal properties. Moreover, this approach is already supported by encouraging literature data.

This work is part of the “Metaflore” project aiming to identify indigenous and melliferous plant species that harbor medicinal compounds effective against Crithidia bombi, a gut parasite of bumblebees. Nevertheless, before conducting biological tests, metabolites need to be fully characterized. The present thesis covers the specialized metabolite extraction and characterization using mass spectrometry hyphenated to liquid chromatography (LC-MS), an efficient technique for the analysis of complex natural extracts. Two metabolite families have been targeted, namely phenolamides and flavonoids, in four plant parts of five selected species, i.e. leaves, corollas, nectar and pollen. Commercial bee pollen, when available, has also been studied.

Due to the structural diversity of specialized metabolites, we first developed analytical methods for both families of compounds. The research has been initiated by the synthesis and characterization of standard phenolamides, which allowed acquiring data on their fragmentation in LC-MS/MS experiments in negative and positive ionization modes in relation with their structural characterization. This knowledge has been combined with the analysis of commercial flavonoid reference compounds and previous studies reported in literature to characterize and quantify the targeted metabolites in plant extracts. For the selected plant species, a comparison between the metabolite profiles of each plant part has thus been achieved, and the phenolamide and flavonoid contents have been determined.

All these analytical developments and chemical profiling support the research of biologists who currently investigate the specific impact of pollen specialized metabolites