The Glucosinolate–myrosinase complex or mustard oil bomb is a chemical herbivory defense system found in members of the Brassicaceae (or cabbage family). The complex requires the activation of a common plant secondary metabolite, glucosinolate, by an enzyme, myrosinase. The defense complex is unusual among plant defenses to herbivory in that the two molecules are stored in different compartments in the leaves of plants until the leaf is torn by an herbivore. The glucosinolate has a β-glucose and a sulfonated oxime. The myrosinase removes the β-glucose to form an unstable compound that further breaks down into substances that are toxic to herbivores.
Countermeasures
The diamondback moth, Plutella xylostella, is a worldwide crop pest that feeds exclusively on members of the Brassicaceae and has developed a defense against the glucosinolate-myrosinase complex. The moth has an enzyme, a sulfatase, that it uses to desulfate the glucosinolate, meaning the myrosinase cannot locate and remove the β-glucose to form the necessary chemical precursors to the toxic defense products. Plant hosts contain a variety of glucosinolates; while all of them have the β-glucose and sulfonated oxime, they contain different amino acid side chains. The diamondback moth, however, can desulfate any Brassicaceae glucosinolate.