Two Novel Flavin-Containing Monooxygenases Involved in Biosynthesis of Aliphatic Glucosinolates

Glucosinolates, a class of secondary metabolites from cruciferous plants, are derived from amino acids and have diverse biological activities, such as in biotic defense, depending on their side chain modification. The first structural modification step in the synthesis of aliphatic (methionine-derived) glucosinolates—S-oxygenation of methylthioalkyl glucosinolates to methylsulfinylalkyl glucosinolates—was found to be catalyzed by five flavin-containing monooxygenases (FMOs), FMO(GS-OX1-5). Here, we report two additional FMO(GS-OX) enzymes, FMO(GS-OX6), and FMO(GS-OX7), encoded by At1g12130 and At1g12160, respectively. The overexpression of both FMO(GS-OX6) and FMO(GS-OX7) decreased the ratio of methylthioalkyl glucosinolates to the sum of methylthioalkyl and methylsulfinylalkyl glucosinolates, suggesting that the introduction of the two genes converted methylthioalkyl glucosinolates into methylsulfinylalkyl glucosinolates. Analysis of expression pattern revealed that the spatial expression of the two genes is quite similar and partially overlapped with the other FMO(GS-OX) genes, which are primarily expressed in vascular tissue. We further analyzed the responsive expression pattern of all the seven FMO(GS-OX) genes to exogenous treatment with abscisic acid, 1-aminocyclopropane-1-carboxylic acid (ACC), jasmonic acid (JA), salicylic acid, indole-3-acetic acid (IAA), and low and high temperatures. Although these genes showed same tendency toward the changing stimulus, the sensitivity of each gene was quite different. The variety in spatial expression among the FMO(GS-OX) genes while responding to environmental stimulus indicated a complex and finely tuned regulation of glucosinolates modifications. Identification of these two novel FMO(GS-OX) enzymes will enhance the understanding of glucosinolates modifications and the importance of evolution of these duplicated genes.