Single-cell mass spectrometry reveals heterogeneous triterpenic acid accumulation in apple callus-derived cells

Abstract

The use of plant cell cultures for large scale production of natural compounds, although promising, has been hindered by their genetic instability and heterogeneity. Here, we show how single cell mass spectrometry can be used to characterize the natural product profile of a callus culture at a highly resolved level. We identify and quantify triterpenic acids in a population of callus cells derived from Annurca apple (Malus pumila Miller cv Annurca) leaf. The analysis demonstrated that a high degree of metabolic heterogeneity exists in the cell population, with the levels of detected metabolites varying significantly across the callus cells. This metabolic heterogeneity was underpinned by variable expression levels of key biosynthetic genes in the single cells. The application of an abiotic stress, near ultraviolet radiation (NUV), to the callus culture resulted in increased levels of triterpenic acids. Single cell mass spectrometry analysis revealed that after treatment, a larger percentage of callus cells produced detectable amounts of these metabolites, ultimately resulting in a more homogeneous production of the metabolites. Furthermore, it showed that intracellular concentrations of ursolic acid derivatives can reach more than 100 mM. Single cell mass spectrometry analyses provide a starting foundation for understanding the molecular mechanisms responsible for metabolic heterogeneity in plant cell cultures, which could in turn facilitate efforts to improve these cell cultures for commercial purposes.