Lichens are compound entities of a fungal partner (''mycobiont'') and one or more photosynthetically active algae or cyanobacteria (''photobionts''). The organisms live in an intimate, symbiotic association which has been classified as a mutualistic or controlled parasitic relationship. Several metabolites from lichens display unique structures with unknown functions, and only a few model species have been analysed comprehensively. The complex metabolic interplay between the organisms in lichens is also incompletely understood. Earlier experiments with 14 C-labelled precursors indicated that the photobionts produce from CO 2 glucose or sugar alcohols (e.g. ribitol and arabitol) which are then transferred to the mycobionts. In the fungi, these compounds are believed to be converted into mannitol serving as the carbon and energy source in the downstream metabolic processes. Recent methodological developments in spectroscopy and ''systems biology'' now enable a concise analysis of the metabolite profiles, networks and fluxes by non-targeted quantitative approaches. In this review, we summarize the current knowledge about lichen metabolism and report on the potential of the advanced methods to reinvestigate lichen chemistry and metabolism on a quantitative basis.

Abstract:The natural formation of the bioactive C 17 -polyacetylenes (−)-(R)-panaxynol and panaxydol was analyzed by 13 C-labeling experiments. For this purpose, plants of Panax ginseng were supplied with 13 CO 2 under field conditions or, alternatively, sterile root cultures of P. ginseng were supplemented with [U- 13C 6 ]glucose. The polyynes were isolated from the labeled roots or hairy root cultures, respectively, and analyzed by quantitative NMR spectroscopy. The same mixtures of eight doubly 13 C-labeled isotopologues and one single labeled isotopologue were observed in the C 17 -polyacetylenes obtained from the two experiments. The polyketide-type labeling pattern is in line with the biosynthetic origin of the compounds via decarboxylation of fatty acids, probably of crepenynic acid. The 13 C-study now provides experimental evidence for the biosynthesis of panaxynol and related polyacetylenes in P. ginseng under in planta conditions as well as in root cultures. The data also show that 13 CO 2 experiments under field conditions are useful to elucidate the biosynthetic pathways of metabolites, including those from roots.

The biosynthesis of lupeol-3-(3'R-hydroxy)-stearate (procrim b, 1) was investigated in the Mexican medicinal plant Pentalinon andrieuxii by (13)CO2 pulse-chase experiments. NMR analyses revealed positional enrichments of (13)C2-isotopologues in both the triterpenoid and the hydroxystearate moieties of 1. Five of the six isoprene units reflected a pattern with [1,2-(13)C2]- and [3,5-(13)C2]-isotopologues from the respective C5-precursors, IPP and DMAPP, whereas one isoprene unit in the ring E of 1 showed only the [3,5-(13)C2]-connectivity of the original C5-precursor, due to rearrangement of the dammarenyl cation intermediate during the cyclization process. The presence of (13)C2-isotopologues was indicative of [(13)C2]acetyl-CoA being the precursor units in the formation of the fatty acid moiety and of the triterpene via the mevalonate route. The observed labeling pattern was in agreement with a chair-chair-chair-boat conformation of the (S)-2,3-oxidosqualene precursor during the cyclization process, suggesting that the lupeol synthase from P. andrieuxii is of the same type as that from Olea europea and Taraxacum officinale, but different from that of Arabidopsis thaliana. The study shows that (13)CO2 pulse-chase experiments are powerful in elucidating, under in vivo conditions and in a single experiment, the biosynthesis of complex plant products including higher terpenes.

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