Switchgrass metabolomics reveals striking genotypic and developmental differences in specialized metabolic phenotypes

Switchgrass (Panicum virgatum L.) is a bioenergy crop that grows
productively on lands not suitable for food production and is an excellent
target for low-pesticide input biomass production. We hypothesize that
resistance to insect pests and microbial pathogens is influenced by
low-molecular-weight compounds known as specialized metabolites. We
employed untargeted liquid chromatography–mass spectrometry, quantitative
gas chromatography–mass spectrometry (GC–MS), and nuclear magnetic
resonance spectroscopy to identify differences in switchgrass ecotype
metabolomes. This analysis revealed striking differences between upland
and lowland switchgrass metabolomes as well as distinct developmental
profiles. Terpenoid- and polyphenol-derived specialized metabolites were
identified, including steroidal saponins, di- and sesqui-terpenoids, and
flavonoids. The saponins are particularly abundant in switchgrass extracts
and have diverse aglycone cores and sugar moieties. We report seven
structurally distinct steroidal saponin classes with unique steroidal
cores and glycosylated at one or two positions. Quantitative GC–MS
revealed differences in total saponin concentrations in the leaf blade,
leaf sheath, stem, rhizome, and root (2.3 ± 0.10, 0.5 ± 0.01, 2.5 ± 0.5,
3.0 ± 0.7, and 0.3 ± 0.01 μg/mg of dw, respectively). The quantitative
data also demonstrated that saponin concentrations are higher in roots of
lowland (ranging from 3.0 to 6.6 μg/mg of dw) than in upland (from 0.9 to
1.9 μg/mg of dw) ecotype plants, suggesting ecotypic-specific biosynthesis
and/or biological functions. These results enable future testing of these
specialized metabolites on biotic and abiotic stress tolerance and can
provide information on the development of low-input bioenergy crops.