Single-cell metabolomics is of paramount interest, given that the sum of the functions and interactions of individual cells translates into the function of tissues, organs, and whole organism. At present, research on single-cell MS methods focuses on the development of ionization techniques and the corresponding sample pre-treatment methods. There are several types of single-cell MS methods with varying ionization techniques: MALDI imaging (Kaspar et al.
2011), nano-electrospray ionization mass spectrometry (nanoESI-MS), laser desorption ionization mass spectrometry (LDI-MS) (Hölscher et al.
2009), and secondary ion mass spectrometry (SIMS) (Moore et al.
2012). MALDI imaging is the most extensively used MS Imaging (MSI) technique (Bjarnholt et al.
2014; Hansen and Lee
2018), and has been used for the molecular imaging of plant tissues (Kaspar et al.
2011). Besides MALDI, SIMS and ambient ionization techniques (Venter et al.
2008), such as desorption ESI (DESI) (Takáts et al.
2004) and laser ablation ESI (LAESI) (Nemes et al.
2009), are the other popular sampling probes that have been employed in MS-based imaging measurements. Furthermore, integrating novel microsampling techniques with MALDI-MS and ESI-MS analyses can facilitate the development of single-cell metabolomics in plants. For example, anthocyanins in single petal cells of wishbone flower (
Torenia hybrida) were analyzed using laser microsampling (Kajiyama et al.
2006). Additionally, in onion (
Allium cepa) and daffodil (
Narcissus pseudonarcissus), LAESI-MS analysis of individual cells in the bulb epidermis led to the identification of 35 metabolites (Shrestha and Vertes
2009). Recently, the combined application of a cell pressure probe and ultraviolet (UV)-MALDI-TOF MS allowed in situ picoliter-scale sampling and metabolite profiling in the single leaf and bulb cells of tulip (
Tulipa suaveolens) (Gholipour et al.
2012). Furthermore, direct single-cell analysis via nano-ESI tip aspiration under a video-microscope has been developed to measure metabolites in their native environment. Aspiration of Wilde Malva (
Pelargonium zonale) leaf stalk single-mesophyll-cell contents using a nano-ESI tip, followed by MS, revealed over 1,000 features from a sample of 1-5 pL in volume (Tejedor et al.
2009). Similarly, a nano-ESI tip was used to extract the cellular contents from live single cells in the leaf, stem, and petal tissues of Wilde Malva. Subsequently, nanoESI-MS enabled the identification of terpenoid hydrocarbons and isoprenoids, among several other compounds, reported for the first time in Wilde Malva (Lorenzo Tejedor et al.
2012). In addition, a combination of laser microdissection (LMD) sampling and LDI-ToF-MS allowed the localization of specialized metabolites at a resolution of 10 μm in St. John’s wort (
Hypericum perforatum),
Hypericum reflexum, and
Arabidopsis (Hölscher et al.
2009). Recently, with the advances in sample separation and MS techniques, single-cell capillary electrophoresis mass spectrometry (CE-MS) has become a promising platform for analyzing cellular contents and probe cell heterogeneity (DeLaney et al.
2018; Kristoff et al.
2020; Yan et al.
2022). Hundreds of metabolites in a single red-onion cell have been successfully separated and putatively identified using an online single-cell CE-MS platform (Huang et al.
2021). Although progress has been made in the development of single-cell MS approaches, none of these methods uses chromatographic separation before MS analysis, greatly limiting the accurate structural assignment and quantification of metabolites. To address these limitations, ultra-high liquid chromatography-mass spectrometry (UPLC-MS) was used to perform the single-cell metabolomics analysis of individual leaf cells of
Catharanthus roseus, which led to the identification of several metabolites, including anhydrovinblastine (AHVB), vinblastine, catharanthine, serpentine, vindoline, and secoiridoid secologanin (Li et al. 2023b).