Study of the dynamic changes in the chemical constituents of Soapberry (Sapindus mukorossi Gaertn.) pericarp during fruit development by a non-targeted metabolomics approach

Soapberry ( Sapindus mukorossi Gaertn.) is a multi-functional tree, which is widely used in daily chemicals, biomedicine, biomass energy and landscaping. The pericarp of soapberry can be used as medicine or detergent. However, there is no systematic study on chemical constituents of soapberry pericarp in fruit development, and the dynamic changes of these constituents are far from clear. In this study, we applied a non-targeted metabolomics approach using an ultra-high performance liquid chromatography-Q Exactive HF hybrid quadrupole-Orbitrap mass spectrometer (UHPLC-QE-HF-MS) to comprehensively profile the variations of metabolites in soapberry pericarp at eight fruit development stages. The metabolome coverage of UHPLC-QE-HF-MS on a HILIC column was higher than that of a C18 column. A total of 111 metabolites were putatively identified, and these metabolites showed three accumulation patterns (pre-accumulation, mid-accumulation and post-accumulation) with fruit development. Twenty-five of these 111 metabolites (including amino acids and their derivatives, flavonoids, organic acids, fatty acids, nucleotides and their derivatives, alkaloids, carbohydrates, terpenoids, vitamins, phosphorylated intermediates) were present at significantly different levels between the two adjacent stages, which were involved in 13 KEGG pathways, among them 5 pathways (flavonoid biosynthesis; histidine metabolism; aminoacyl-tRNA biosynthesis; flavone and flavonol biosynthesis; and phenylalanine, tyrosine and tryptophan biosynthesis) were most relevant. S8 stage (fruit ripening stage) is the most suitable stage for fruit harvesting to utilize the pericarp, during which the accumulation of many bioactive and valuable metabolites (e.g., furamizole, alpha-tocopherol quinone, sucrose) in the pericarp was highest. To the best of our knowledge, this was the first time that the metabolomics in soapberry pericarp during the whole fruit development was profiled. This study will be beneficial to guide the harvesting, processing and application, and pave the way for further studies on the biosynthesis mechanism of the main metabolites of the soapberry pericarp.