Cellular and subcellular localization of flavin-monooxygenases involved in glucosinolate biosynthesis

Glucosinolates are amino acid-derived secondary metabolites with diverse biological activities dependent on chemical modifications of the side chain. Five flavin-monooxygenases FMOGS-OX1-5 have recently been identified as aliphatic glucosinolate side chain modification enzymes in Arabidopsis thaliana that catalyse the generation of methylsulphinylalkyl glucosinolates, which can be hydrolysed to products with distinctive benefits for human health and plant defence. Though the localization of most aliphatic glucosinolate biosynthetic enzymes has been determined, little is known about where the side chain modifications take place despite their importance. Hence, the spatial expression pattern of FMOGS-OX1-5 genes in Arabidopsis was investigated by expressing green fluorescent protein (GFP) and beta-glucuronidase (GUS) fusion genes controlled by FMOGS-OX1-5 promoters. The cellular compartmentation of FMOGS-OX1 was also detected by transiently expressing a FMOGS-OX1-yellow fluorescent protein (YFP) fusion protein in tobacco leaves. The results showed that FMOGS-OX1-5 were expressed basically in vascular tissues, especially in phloem cells, like other glucosinolate biosynthetic genes. They were also found in endodermis-like cells in flower stalk and epidermal cells in leaf, which is a location that has not been reported for other glucosinolate biosynthetic genes. It is suggested that the spatial expression pattern of FMOGS-OX1-5 determines the access of enzymes to their substrate and therefore affects the glucosinolate profile. FMOGS-OX1-YFP fusion protein analysis identified FMOGS-OX1 as a cytosolic protein. Together with the subcellular locations of the other biosynthetic enzymes, an integrated map of the multicompartmentalized aliphatic glucosinolate biosynthetic pathway is discussed.