Insect herbivore counteradaptations to the plant glucosinolate-myrosinase system.

The glucosinolate-myrosinase system found in plants of the Brassicales order is one of the best studied plant chemical defenses. Glucosinolates and their hydrolytic enzymes, myrosinases, are stored in separate compartments in the intact plant tissue. Upon tissue disruption, bioactivation of glucosinolates is initiated, i.e. myrosinases get access to their glucosinolate substrates, and glucosinolate hydrolysis results in the formation of toxic isothiocyanates and other biologically active products. The defensive function of the glucosinolate-myrosinase system has been demonstrated in a variety of studies with different insect herbivores. However, a number of generalist as well as specialist herbivores uses glucosinolate-containing plants as hosts causing large agronomical losses in oil seed rape and other crops of the Brassicaceae. While our knowledge of counteradaptations in generalist insect herbivores is still very limited, considerable progress has been made in understanding how specialist insect herbivores overcome the glucosinolate-myrosinase system and even exploit it for their own defense. All mechanisms of counteradaptation identified to date in insect herbivores specialized on glucosinolate-containing plants ensure that glucosinolate breakdown to toxic isothiocyanates is avoided. This is accomplished in many different ways including avoidance of cell disruption, rapid absorption of intact glucosinolates, rapid metabolic conversion of glucosinolates to harmless compounds that are not substrates for myrosinases, and diversion of plant myrosinase-catalyzed glucosinolate hydrolysis. One of these counteradaptations, the nitrile-specifier protein identified in Pierid species, has been used to demonstrate mechanisms of coevolution of plants and their insect herbivores.