Transcriptome analysis reveals the genes involved in S-alk(en)ylcysteine sulfoxide biosynthesis and its biosynthetic location in postharvest chive (Allium schoenoprasum L.)

Abstract

The biosynthetic mechanism of S-alk(en)ylcysteine sulfoxides (CSOs), a flavor precursor and nonvolatile medicinal compound in chive is still poorly understood. In the present study, transcriptomic analysis was used to investigate the biosynthetic mechanism of S-alk(en)ylcysteine sulfoxides (CSOs) in green leaves of postharvest chive stored under normal temperature (20 °C) for 5 d and low-temperature (3 °C) for 12 d. The de novo assembly of the transcriptome enabled the identification of unigenes involved in the sulfur assimilation and CSOs biosynthesis. The RNA-seq data showed that the unigenes related to sulfur assimilation were down-regulated during storage under 20 °C and 3 °C. The low temperature did not affect cysteine biosynthesis and the expression of γ-glutamyl transpeptidase (GGT) and flavin-containing monooxygenase (FMO) involved in CSOs biosynthesis; nonetheless, it prolonged CSOs synthesis by sustaining the chive quality during the storage period. The qPCR data revealed that the expressions of genes related to sulfur assimilation were mainly in the white stalk. In contrast, CSOs biosynthetic genes had higher expression levels in green leaf. The results indicate the CSOs were mainly synthesized in green leaf while cysteine, the primary substrate for CSOs synthesis, was from de novo synthesis and proteolysis. The study presents discrete evidence that CSOs biosynthesis in postharvest chives occurs in green leaves and is translocated to the white stalk for storage.