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

Abstract

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. rapaeinfested 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.

Effects of glucosinolates on a generalist and specialist leaf-chewing herbivore and an associated parasitoid

“…Volatile breakdown products of GLS have been shown to attract several specialist 334 parasitoids of herbivores that feed on GLS-containing plants (Blande et al, 2007;Bradburne 335 and Mithen, 2000; Mumm et al, 2008). Whether this is also true for H. ebeninus is presently 336 not known.…”

The Genetic Basis of Constitutive and Herbivore-Induced ESP-Independent Nitrile Formation in Arabidopsis

“…Feeding tests show that simple nitrile production does not function as a direct defense against generalist lepidopteran herbivores, because these herbivores prefer plants with simple nitriles over those with isothiocyanates and complete development faster on nitrile-producing than on isothiocyanateproducing plants (Lambrix et al, 2001;Burow et al, 2006b;Zhang et al, 2006). Recent studies with Pieris rapae (Lepidoptera) suggest that simple nitriles are involved in direct and indirect defense responses against this specialist herbivore (De Vos et al, 2008;Mumm et al, 2008). The biological function of epithionitriles is unknown (Wittstock et al, 2003).…”

The potential for modifying plant volatile composition to enhance resistance to arthropod pests.

“…Thus nitriles may be a cue for its parasitoid Cotesia rubecula, and indeed this natural enemy was more attracted to the nitrile Arabidopsis type than to the isothiocyanate type. Further, P. rapae females avoided these plants for oviposition, possibly because nitrile emission indicated that they were already occupied by conspecific larvae [75]. Down-regulation of volatile-related genes is possible via mutations in those genes or transformation of plants with antisense constructs or via cosuppression [33].…”