Glucosinolates are a diverse group of defensive secondary metabolites that is characteristic of the Brassicales. Arabidopsis thaliana (L.) Heynh. (Brassicaceae) lines with mutations that greatly reduce abundance of indole glucosinolates (cyp79B2 cyp79B3), aliphatic glucosinolates (myb28 myb29), or both (cyp79B2 cyp79B3 myb28 myb29) make it possible to test the in vivo defensive function of these two major glucosinolate classes. In experiments with Lepidoptera that are not crucifer-feeding specialists, aliphatic and indole glucosinolates had an additive effect on Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) larval growth, whereas Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae) and Manduca sexta (L.) (Lepidoptera: Sphingidae) were affected only by the absence of aliphatic glucosinolates. In the case of two crucifer-feeding specialists, Pieris rapae (L.) (Lepidoptera: Pieridae) and Plutella xylostella (L.) (Lepidoptera: Plutellidae), there were no major changes in larval performance due to decreased aliphatic and/or indole glucosinolate content. Nevertheless, choice tests show that aliphatic and indole glucosinolates act in an additive manner to promote larval feeding of both species and P. rapae oviposition. Together, these results support the hypothesis that a diversity of glucosinolates is required to limit the growth of multiple insect herbivores.

The antiherbivore potential of the glucosinolate-myrosinase defense system found in plants of the order Capparales is heavily influenced by the types of hydrolysis products (e.g. isothiocyanates, nitriles) formed from the parent glucosinolates upon plant damage. However, comparison of the effects of glucosinolate hydrolysis products on insect herbivores has been hampered by the lack of suitable experimental tools for rigorous bioassays, such as intact plants differing only in the types of hydrolysis products they produce, or artificial diets that can accurately simulate glucosinolate hydrolysis. The wide array of molecular resources for Arabidopsis thaliana has facilitated the identification of several genes that play a role in glucosinolate hydrolysis. One of these encodes the epithio-specifier protein (ESP) that promotes the formation of nitriles at the expense of isothiocyanates in certain ecotypes of A. thaliana. We overexpressed the ESP cDNA from the nitrile-producing ecotype Landsberg erecta in the isothiocyanate-producing ecotype Columbia-0 to generate transgenic lines of A. thaliana that differed from wild-type plants in the type of glucosinolate hydrolysis products formed upon tissue damage, whereas parent glucosinolate profile and myrosinase activity levels, as well as plant morphology and growth habit, remained unchanged. Bioassays with the model generalist herbivore Spodoptera littoralis (Lepidoptera: Noctuidae) demonstrated that larvae reared on the nitrile-producing lines on average gained weight faster in the first larval stages than larvae that fed on isothiocyanate-producing control plants. Furthermore, larvae with medial growth rates showed a tendency to pupate earlier on the ESP-overexpressing plant lines. Together with the results of previous studies, these findings suggest that isothiocyanates are more effective defenses against insect herbivores than nitriles, and raise questions about what conditions select for nitrile formation in plants.

Glucosinolates are a group of thioglucosides that are components of an activated chemical defense found in the Brassicales. Plant tissue damage results in hydrolysis of glucosinolates by endogenous thioglucosidases known as myrosinases. Spontaneous rearrangement of the aglucone yields reactive isothiocyanates that are toxic to many organisms. In the presence of specifier proteins, alternative products, namely epithionitriles, simple nitriles, and thiocyanates with different biological activities, are formed at the expense of isothiocyanates. Recently, simple nitriles were recognized to serve distinct functions in plant-insect interactions. Here, we show that simple nitrile formation in Arabidopsis (Arabidopsis thaliana) ecotype Columbia-0 rosette leaves increases in response to herbivory and that this increase is independent of the known epithiospecifier protein (ESP). We combined phylogenetic analysis, a screen of Arabidopsis mutants, recombinant protein characterization, and expression quantitative trait locus mapping to identify a gene encoding a nitrile-specifier protein (NSP) responsible for constitutive and herbivore-induced simple nitrile formation in Columbia-0 rosette leaves. AtNSP1 is one of five Arabidopsis ESP homologues that promote simple nitrile, but not epithionitrile or thiocyanate, formation. Four of these homologues possess one or two lectin-like jacalin domains, which share a common ancestry with the jacalin domains of the putative Arabidopsis myrosinase-binding proteins MBP1 and MBP2. A sixth ESP homologue lacked specifier activity and likely represents the ancestor of the gene family with a different biochemical function. By illuminating the genetic and biochemical bases of simple nitrile formation, our study provides new insights into the evolution of metabolic diversity in a complex plant defense system.

Halorhodopsin of Natronobacterium pharaonis which acts as a light-driven chloride pump is studied by time-resolved resonance Raman spectroscopy. In single-beam experiments, resonance Raman spectra were obtained of the parent state HR578 and the first thermal intermediate HR520. The parent state is structural heterogeneous including ca. 80% all-trans and 20% 13-cis isomers. The resonance Raman spectra indicate that the all-trans conformer exhibits essentially the same chromophoric structure as in the parent states of bacteriorhodopsin or halorhodopsin from Halobacterium salinarium. Special emphasis of the resonance Raman spectroscopic analysis was laid on the C=C and C=N stretching region in order to probe the interactions between the protonated Schiff base and various bound anions (chloride, bromide, iodide). These investigations were paralleled by spectroscopic studies of retinal Schiff base model complexes in different solvents in an attempt to determine the various parameters which control the C=C and C=N stretching frequencies. From these data, it was concluded that in the parent state the anion is not involved in hydrogen bonding interactions with the Schiff base proton but is presumably bound to a nearby (positively charged) amino acid residue. On the other hand, the anion still exerts an appreciable effect on the chromophore structure which is, for instance, reflected by the variation of the isomer composition in the presence of different anions and in the anion-depleted form. In contrast to the parent state, the intermediate HR520 reveals frequency shifts of the C=N stretching in the presence of different anions. These findings indicate a closer proximity of the bound anion to the Schiff base proton which is sufficient for hydrogen bonding interactions. These changes of the anion-chromophore interaction upon transition from HR578 to HR520 may be related to the coupling of the chromophore movement with the anion translocation.