Lipidomics in situ: insights into plant lipid metabolism from high resolution spatial maps of metabolites.

The emergence of 'omics' technologies (i.e. genomics, proteomics, metabolomics, etc.) have revealed new avenues for exploring plant metabolism through data-rich experimentation and integration of complementary methodologies. Over the past decade, the lipidomics field has benefited from advances in instrumentation, especially mass spectrometry (MS)-based approaches that are well-suited for detailed lipid analysis. The broad classification of what constitutes a lipid lends itself to a structurally diverse range of molecules that contribute to a variety of biological processes in plants including membrane structure and transport, primary and secondary metabolism, abiotic and biotic stress tolerances, extracellular and intracellular signaling, and energy-rich storage of carbon. Progress in these research areas has been advanced in part through approaches analyzing chemical compositions of lipids in extracts from cells, tissues and/or whole organisms (e.g. shotgun lipidomics), and through visualization approaches primarily through microscopy-based methodologies (e.g. fluorescence, bright field, electron microscopy, etc.). While these techniques on their own provide rich biochemical and biological information, coordinated analyses of the complexity of lipid composition with the localization of these lipids at a high spatial resolution will help to develop a new level of understanding of lipid metabolism within the context of tissue/cellular compartmentation. This review will elaborate on recent advances of one such approach--mass spectrometry imaging (MSI)--that integrates in situ visualization with chemical-based lipidomics. We will illustrate, with an emphasis on oilseed lipid metabolism, how MS imaging can provide new insights and questions related to the spatial compartmentation of lipid metabolism in plants. Further it will be apparent that this MS imaging approach has broad application in plant metabolic research well beyond that of triacylglycerol biosynthesis in oilseeds.