Lee Laboratory
for Biological Mass Spectrometry
Department of Chemistry
Iowa State University
Mass Spectrometry Imaging of Plant Metabolites
The multicellular nature of higher plants presents many challenges to deciphering how biological processes are distributed among different organs and tissues. We are developing and applying mass spectrometry imaging technique to understand plant metabolic biology down to cellular and eventually subcellular level high-spatial resolution.
High-spatial resolution epicuticular surface lipid images on the flower of Arabidopsis thaliana obtained with 12 um laser beam and raster size and using colloidal graphite as a matrix. Jun et al, Anal. Chem. 2010.
Left: [C29 alkane + 107Ag]+, Center: [C29 ketone + 107Ag]+, Right: [C26 fatty acid + 107Ag]+
The lack of chromatographic separation is a critical limitation in mass spectrometry imaging to confidently identify metabolites. To overcome this limitation, we have developed multiplex mass spectrometry imaging technique that acquires high-resolution mass spectra and tandem mass spectra at the same time directly on the tissue in a single data acquisition (Perdian, Anal. Chem. 2010). We have further expanded this technique to include polarity switching so that both positive and negative ion mass spectrometric images can be acquired at the same time (Korte et al., J. Am. Soc. Mass Spectrom. 2013).
In multiplex MS imaging, each raster scan is split into multiple spiral steps and each independent MS scans are acquired for each spiral step. In this example, for each raster scan, one high-mass resolution orbitrap scan (FT), four low-mass resolution ion trap scan (IT), and four independent MS/MS scans are acquired. Accurate mass information can be used to determine the chemical compositions and tandem mass spectra can be used to differentiate structural isomers.
We have demonstrated this technique for various plant systems, including high-spatial resolution expression of functional genomics (Korte and Lee, Anal. Methods, 2012), single cell level lipid distributions in cotton embryos (Horn et al., Plant Cells, 2012), and flavonoid and epiecuticular lipid distributions on Arabidopsis flower (Jun et al., Anal. Chem. 2010). We expanded the application to understand chemical interfaces between plant and pathogens (Klein et al., Anal. Chem. 2015).
We have further improved our spatial resolution down to 5 um and subcellular level mass spectrometry imaging and demonstrated for maize leaves and roots (Korte et al., Anal. Bioanal. Chem. 2015; Feenstra et al., J. Am. Soc. Mass Spectrom. 2017).
Lipid imaging of a cross-sectional maize leaf obtained with 5 um spatial resolution. PG(34:x) appears only on single layer of bundle sheath cells or both bundle sheath and mesophyll cells depending on unsaturation, while SQDG(34:3) is shown on both photosynthetic cell types. Furthermore, this cellular heterogeneity has genetic dependence.(Dueñas et al., Plant J. 2017).
More recently, we demonstrated the localization of Arabidopside A in chloroplast from its co-localization with chlorophyll a. (Hansen et al., Plant J. 2019)
