Switchgrass metabolomics reveals striking genotypic and developmental differences in saponins

Switchgrass ( L.) is a bioenergy crop that grows productively on low-fertility lands not suitable for food production. We hypothesize that traits such as low soil nitrogen demand, tolerance to water limitation and resistance to insect pests and microbial pathogens are influenced by low molecular weight compounds known as specialized metabolites. We leveraged untargeted liquid chromatography-mass spectrometry (LC-MS) and quantitative gas chromatography-mass spectrometry (GC-MS) to identify differences in above- and below-ground metabolomes of three northern upland and three southern lowland switchgrass cultivars. This analysis documented abundant steroidal saponins and terpenoid glycosides as well as varied phenolic compounds in switchgrass extracts. We identified many metabolite ‘features’ (annotated as retention time/mass-to-charge ratio pairs), which differentially accumulated between upland and lowland ecotypes. These include saponins built on at least five different steroidal sapogenin cores. The total saponin concentrations were statistically different between roots of the two switchgrass ecotypes. In contrast, flavonoids such as quercetin mainly exhibited a tissue-specific accumulation pattern and predominantly accumulated in shoots. These results set the stage for testing the impacts of differentially accumulating metabolites on biotic and abiotic stress tolerance and inform development of low-input bioenergy crops.