There is a significant need to resolve the biomolecular processes within single cells and tissue functional units of plants, as these measurements can provide a more accurate portrayal of the true metabolic complexity of these systems. For example, employment of such assays could be used to improve our ability to create better bioenergy crops. Our group focuses on developing and deploying spatially resolved mass spectrometry (MS) methods, where MS imaging (MSI) is a subclass of these techniques. These approaches can reveal the molecular distributions within samples of interest <i>in situ</i>. A striking advantage of MSI over other imaging techniques is that the creation of highly detailed chemical maps can help correlate the function and genesis of detected compounds within individual plant cells and cell clusters. One such technique we have advanced is laser-ablation electrospray ionization (LAESI)-MS. This method can be performed under ambient conditions and permits measuring biomolecules of all types in samples that are inherently water-rich. Accordingly, LAESI-MS allows us to molecularly profile native plant tissue, and has thus shown promise in high-throughput spatial metabolomics of living plants down to the single-cell level. We recently created the LAESI-based ‘molecular microscope’ that utilizes a long working distance reflective objective. This configuration allows us to simultaneously visualize the sample and target single cells of interest with optical microscopy, and then obtain metabolomics data directly for selected regions by LAESI-MS analysis. We can also perform high lateral resolution MSI of native plant tissue under ambient condition using this technique. Our team has also created new matrix-assisted laser desorption/ionization (MALDI)-MSI methods, which allow us to image a broad array of biomolecular classes in an untargeted fashion (even three-dimensionally). Using enzyme-assisted MALDI-MSI, we are now able to map the N-gylcome of plant tissues for the first time. Our findings show that N-glycosylation might play a significant role in the recognition of plant cells with symbiotic rhizobia that are able to perform biological nitrogen fixation. Lastly, we are in the process of developing new sample preparation methods that allow us to visualize the abundance and distribution of phytohormones, among other small molecules, within plant tissues.

Spatially resolved mass spectrometry approaches for biomolecular cartography of plant cells and tissues

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