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

Abstract

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 (LC-MS), quantitative gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) 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 especially 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 leaf blade, leaf sheath, stem, rhizome and root. The quantitative data also demonstrated that saponin concentrations is higher in roots of lowland than upland 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 inform development of low-input bioenergy crops. One sentence summaryIntegrated mass-spectrometry and nuclear magnetic resonance spectroscopy based metabolomics reveal that switchgrass accumulates structurally diverse terpenoids and phenolics, which vary in abundance and structure in a tissue- and ecotype-specific manner.