Formation of Simple Nitriles upon Glucosinolate Hydrolysis Affects Direct and Indirect Defense Against the Specialist Herbivore, Pieris rapae

The glucosinolate-myrosinase system, found in plants of the order Brassicales, has long been considered an effective defense system against herbivores. The defensive potential of glucosinolates is mainly due to the products formed after myrosinase-catalyzed hydrolysis upon tissue damage. The most prominent hydrolysis products, the isothiocyanates, are toxic to a wide range of organisms, including herbivorous lepidopterans. In contrast, little is known about the biological activities of alternative hydrolysis products such as simple nitriles and epithionitriles that are formed at the expense of isothiocyanates in the presence of epithiospecifier proteins (ESPs). Here, we used transgenic Arabidopsis thaliana (Brassicaceae) plants overexpressing ESP (35S:ESP plants) to investigate the effects of simple nitriles on direct and indirect defense against the specialist cabbage white butterfly Pieris rapae L. (Lepidoptera, Pieridae). In the 35S:ESP plants, glucosinolates are hydrolyzed mainly to simple nitriles upon tissue disruption, while isothiocyanates are the predominant hydrolysis products in Columbia-0 (Col-0) wild-type plants. The parasitoid Cotesia rubecula (Hymenoptera, Braconidae), a specialist on P. rapae larvae, was significantly more attracted to P. rapae-infested 35S:ESP plants than to P. rapae-infested Col-0 wild-type plants in a wind tunnel setup. Furthermore, female P. rapae butterflies laid more eggs on Col-0 wild-type plants than on 35S:ESP plants when the plants had been damaged previously. However, when given a choice to feed on 35S:ESP or Col-0 plants, caterpillars did not discriminate between the two genotypes. Growth rate and developmental time were not significantly different between caterpillars that were reared on 35S:ESP or Col-0 plants. Thus, the production of simple nitriles instead of isothiocyanates, as catalyzed by ESP, can promote both direct and indirect defense against the specialist herbivore P. rapae.