MALDI-TOF/TOF Tandem Mass Spectrometry Imaging Reveals Non-uniform Distribution of Disaccharide Isomers in Plant Tissues

Mass spectrometry imaging (MSI) has been increasingly utilized in investigating the locations of biomolecules within tissues. However, the isomeric compounds are rarely distinguished in the MS images, due to inability of MSI methods to differentiate isomers in the probing area. Coupling tandem mass spectrometry with MSI can facilitate differentiating isomeric compounds in ion images. Here we apply MALDI-TOF/TOF tandem mass spectrometry imaging approach to revealing the spatial distributions of isomeric disaccharides in plant tissues. First, the MS/MS imaging analysis of disaccharide-matrix droplet spots demonstrated the feasibility of distinguishing isomeric species in tissues, by measuring the relative intensity of specific fragments. Then, we conducted tandem MS imaging of disaccharides in onion bulb tissues, which indicated that sucrose and other unknown non-sucrose disaccharides exhibit heterogeneous locations throughout the tissues. This method enables us to image disaccharide isomers differentially in biological tissues, and to discover new saccharide species in plant. This work also emphasizes the necessity of considering isobaric compounds when interpreting MSI results.