Research progress on metabolic pathway of S-alk(en)ylcysteine sulfoxides in Allium

Allium L. is one of the largest genera of angiosperms, including garlic, onion, green Chinese onion, Chinese chive and other important vegetable crops with unique spicy flavor. S-alk(en)ylcysteine sulfoxides, the unique secondary metabolites of Allium, which giving Allium spicy flavor and medicinal value, are the precursors of various volatile sulfur compounds. Therefore, it is of great significance to study the metabolic pathway of S-alk(en)ylcysteine sulfoxides in Allium. Seven S-alk(en)ylcysteine sulfoxides have been found in Allium plants. These S-alk(en)ylcysteine sulfoxides are mainly synthesized in leaves through glutathione pathway, and then transported to the cytoplasm of storage organs such as bulbs for accumulation. At present, there are many studies on the catabolism of S-alk(en)ylcysteine sulfoxides in Allium, while few studies on the biosynthesis of S-alk(en)ylcysteine sulfoxides. Only two biosynthetic enzymes, γ-glutamyl transpeptidase (GGT) and flavin-containing monooxygenase (FMO), have been confirmed at the molecular level. In addition, S-alk(en)ylcysteine sulfoxides are the downstream products of plant sulfur metabolism. The upstream of S-alk(en)ylcysteine sulfoxide metabolism involves the absorption and transport of sulfur-containing compounds, the metabolism of cysteine and glutathione, and the changes of these metabolic processes may also affect the biosynthesis of S-alk(en)ylcysteine sulfoxides. With the rapid development of omics technology, genomics, transcriptomics and metabolomics have been applied in the study of Allium plants, especially the completion of garlic genome sequence assembly, which provides great convenience for the study of S-alk(en)ylcysteine sulfoxides metabolism pathway in Allium plants. Two aspects of research should be strengthened in the future: one is to continue to clone and identify the key enzyme genes in the biosynthesis pathway of S-alk(en)ylcysteine sulfoxides and study their functions; another is to strengthen the study of sulfur metabolism in Allium plants, so as to lay a foundation for the study of the regulation of S-alk(en)ylcysteine sulfoxides biosynthesis. These studies will provide a reference for further analyzing the metabolic pathway of S-alk(en)ylcysteine sulfoxides in Allium and regulating the flavor of Allium by molecular breeding technology.