ABSTRACT
Tracing in vivo isotope-labeled metabolites has been used to study metabolic pathways or flux analysis. However, metabolic differences between the cells have been often ignored in these studies due to the limitation of solvent-based extraction. Here we demonstrate that the mass spectrometry imaging of in vivo isotope-labeled metabolites, referred to as MSIi, can provide important insights into metabolic dynamics with cellular resolution that may supplement the traditional metabolomics and flux analysis. Developing maize root tips are adopted as a model system for MSIi by supplementing 200 mM [U-13C]glucose in 0.1x Hoagland medium. MSIi data sets were acquired for longitudinal sections of newly grown maize root tips after growing 5 days in the medium. A total of 56 metabolite features were determined to have been 13C-labeled based on accurate mass and the number of carbon matching with the metabolite databases. Simple sugars and their derivatives were fully labeled, but some small metabolites were partially labeled with a significant amount of fully unlabeled metabolites still present, suggesting the recycling of "old" metabolites in the newly grown tissues. Some distinct localizations were found, including the low abundance of hexose and its derivatives in the meristem, the high abundance of amino acids in the meristem, and the localization to epidermal and endodermal cells for lipids and their intermediates. Fatty acids and lipids were slow in metabolic turnover and showed various isotopologue distributions with intermediate building blocks, which may provide flux information for their biosynthesis.
