Abstract: Glucosinolates(GSLs)are amino acid-derived secondary metabolites,which play an important role in biotic defense reactions in plants.The bioactivity of GSLs is closely related with its chain structure.There are five Flavin-containing Monooxygenase(FMO)enzymes,FMO_GS-OX1-5,involved in GSL biosynthesis catalyzing the S-oxygenation from Methylthioalkyl GSLs to Methylsulfinylalkyl GSLs.Our previous work showed that all fmo_gs-ox mutants,except fmo_gs-ox4,presented altered GSLs phenotypes.To reveal the expression characteristics and functions in GSLs biosynthesis and chain structure modification,GFP and GUS were used as reporter genes,expression pattern of FMO_GS-OX4 in different tissue was analyzed and results showed that FMO_GS-OX4 was expressed mainly in the vascular tissue of the leaf,stem and silique.Under normal conditions,spatial distribution of FMO_GS-OX4 did not overlap with the distribution of its substrates,which was considered as the reason why fmo_gs-ox4 lacked altered GSLs phenotype.