Metabolomics and transcriptomics analyses provides insights into S-alk(en)yl cysteine sulfoxides (CSOs) accumulation in onion (Allium cepa)

Primary and secondary metabolites are responsible for the nutritional value of onions. S-Alk(en)ylcysteine sulfoxides (CSOs) are the precursors of onion flavour and medicinal components. The purpose of this study was to investigate the dynamics in metabolites and CSOs biosynthesis during onion bulb expansion by conducting metabolomic and transcriptomic analyses. A total of 633 metabolites were detected in SA1. Lipids, flavonoids and amino acids mainly accumulated in the rapid-expansion stage of onion bulb, whereas alkaloids and phenolic acids accumulated in the late stage of bulb expansion. The concentration of S-methylcysteine sulfoxide (MCSO) was increased in SA1 during onion bulb development, however, there was no significant changes in S-propylcysteine sulfoxide (PeCSO) and S-allylcysteine sulfoxide (ACSO). Candidate genes involving in CSOs biosynthesis were screened by RNA-Seq. Two AcSULTRs and AcCSs, a AcATPS, AcSAT, AcGL, AcGS, AcGGT and AcFMO might play core roles in CSOs biosynthesis. The results offer a theoretical basis for determining the regulation of organosulfur compound synthesis in onion.