Flavin-dependent oxidoreductase involved in the biosynthetic pathway to 4-hydroxyindole-3-carbonyl nitrile (4-OH-ICN), a cyanogenic metabolite required for inducible pathogen defense. Converts indole cyanohydrin into indole-3-carbonyl nitrile (ICN). Binds 1 FAD per subunit in a bicovalent manner. Up-regulated by MKK4. The FAD cofactor is bound via a bicovalent 6-S-cysteinyl, 8alpha-N1-histidyl FAD linkage. Three- to fivefold reduction in levels of ICN metabolites and accumulation of indole cyanogenic glycosides. Belongs to the oxygen-dependent FAD-linked oxidoreductase family.