LAUSR

This Research Topic portrays some current developments in plant single cell metabolic profiling/ imaging and offers current technological solutions for a better understanding of plant biology/ biochemistry at the single cell level. This Research Topic has progressed from being an emerging technology to one of the more innovative tools for probing metabolism in plants. From previous studies in single cell transcriptomics, it was clear that each cell type may show unique transcriptomic profiles that will ultimately result in differences in cell-specific metabolic signatures. Such metabolic signatures can be monitored at the single cell level and correlated with transcriptomics data for each single cell type. Metabolomics profiles might be altered upon stress (e.g., via an environmental change and mechanical damage), and an understanding of such metabolic changes can potentially, for example, lead to discovery of cures for plant diseases. There are seven research articles in this Research Topic focusing on several aspects of single cell metabolic profiling and mass spectrometric imaging (MSI) of plant cells. These articles cover methods development, comparative study of different MSI technologies, and successful application of MSI to address cell specific metabolic heterogeneity. Studies herein were with either individual algal cells or whole plant tissues. The sample type analyzed mostly dictated choice of method. If individual cells were available, we can directly extract the metabolites using a special mechanical probe (Sun et al.) or a focused UV-laser beam (Baumeister et al.). Both approaches detected a similar set of metabolites that encompassed primary and secondary metabolites including diverse lipid classes. Of these two approaches, the laser-based method had higher throughput, as scanning with a laser is much faster than probing cells mechanically. However, sampling the exact cell of interest is not trivial when working with whole tissue. The Kertesz laboratory has suggested use of an automatic optics-guided laser capture microdissection (LCM) of individual cells with subsequent sample liquid vortex extraction followed by metabolite sampling and analysis using the mass spectrometer (Cahill and Kertesz). While this system is capable of handling several hundred cells from Allium cepa or cultured cells automatically, it works only on a monolayer of cells or a tissue cross-section, whereas multilayer and multicellular samples cannot be analyzed. To examine the metabolic makeup of individual cells, the analysis should be done in a reasonable timeframe and under an undisturbed condition; it should be as close to the in vivo condition as possible and not postmortem. Matrix-assisted laser desorption ionization (MALDI) in atmospheric pressure (AP) condition has advantages in this regard compared to vacuum MALDI with much less fragmentation and the analysis of volatile compounds. However, as Li and colleagues show