Abscisic acid-mediated induction of FLAVIN-CONTAINING MONOOXYGENASE 2 leads to reduced accumulation of methylthioalkyl glucosinolates in Arabidopsis thaliana.

Side-chain modification contributes to the structural diversity of aliphatic glucosinolates (GSLs), a class of sulfur-containing secondary metabolites found in Brassicales. The first step in side-chain modification of aliphatic GSLs is the S-oxygenation of the methylthioalkyl (MT) moiety to the methylsulfinylalkyl (MS) moiety. This reaction is catalyzed by flavin-containing monooxygenase (FMO), which is encoded by seven genes in Arabidopsis thaliana. Therefore, the regulation of FMO gene expression is key to controlling side-chain structural diversity. In this study, we demonstrated that the expression of FMO and FMO was induced by glucose treatment, independent of MYB28/29 and MYC2/3/4, the transcription factors that positively regulate aliphatic GSL biosynthesis. Glucose treatment of the abscisic acid (ABA)-related mutants indicated that glucose-triggered upregulation of FMO and FMO was partially regulated by ABA through the key negative regulators ABI1 and ABI2, and the positive regulator SnRK2, but not via the transcription factor ABI5. In wild-type plants, glucose treatment drastically reduced the accumulation of 4-methylthiobutyl (4MT) GSL, whereas a decrease in 4MT GSL was not observed in the fmo, abi1-1, abi2-1, aba2-1, or aba3-1 mutants. This result indicated that the decreased accumulation of 4MT GSL by glucose treatment was attributed to upregulation of FMO via the ABA signaling pathway.