Glucosinolates

Abstract

Plants resist herbivorous insects and pathogens by employing specialised metabolites as chemical defences. In the case of constitutively present activated defences, an inactive precursor is stored and activated upon damage or attack by biotic stressors releasing high amounts of toxic and bioactive products. Glucosinolates are a group of defence compounds found in cruciferous vegetables. They consist of a nitrogen‐ and sulfur‐containing core structure and an amino acid‐derived, variable side chain. Enzymatic activation of glucosinolates further amplifies their chemical diversity, as one glucosinolate can generate different bioactive metabolites. These serve diverse functions in plant–insect and plant–pathogen interactions. Insects, however, have evolved many creative adaptations to avoid toxic glucosinolate‐derived metabolites. For humans, glucosinolates and their activation products have many beneficial health effects and promising applications in sustainable agriculture.

Key Concepts

  • Plants of the Brassicales order store nontoxic glucosinolates as precursors of bioactive compounds to defend themselves against herbivores and pathogens.
  • Glucosinolate composition varies across and within plant species as well as between tissues, developmental stages and environmental conditions.
  • Upon attack, enzymatic activation of glucosinolates initiated by myrosinase enzymes further amplifies their structural diversity, which is pivotal for the plants' resistance against a wide range of biotic enemies.
  • Against harmful bacteria and fungi, activation of particularly indolic glucosinolates in intact plant tissue can also occur through atypical myrosinases.
  • Glucosinolate‐derived bioactive compounds function in both direct and indirect defence against insect herbivores.
  • Many herbivores have adapted towards feeding on glucosinolate‐containing plants and have developed creative counteradaptations against the glucosinolate–myrosinase system, while nonadapted herbivores rely on detoxification of glucosinolate‐derived active compounds.
  • Apart from their defensive functions, glucosinolates serve as molecular signal to feedback regulate plant metabolism, growth and defence.
  • The chemical diversity of glucosinolates and their hydrolysis products provides benefits for human health and promising applications for sustainable agriculture.

Keywords: plant‐specialised metabolism; plant defence; plant–insect interaction; coevolution; sulfur metabolism; abiotic stress; biotic stress; insect herbivore; activated defence

Figure 1. The structural diversity of glucosinolates. (1) p‐Hydroxybenzylglucosinolate (sinalbin); (2) Indol‐3‐ylmethylglucosinolate (glucobrassicin); (3) 4‐Methoxy‐indol‐3‐ylmethylglucosinolate; (4) Methylthioalkylglucosinolate (n=1–6), for n=2: 4‐methylthiobutylglucosinolate (glucoerucin); (5) Methylsulfinylalkylglucosinolate (n=1–6), for n=2: 4‐methylsulfinylbutylglucosinolate (glucoraphanin); (6) 2R‐hydroxy‐3‐butenylglucosinolate (progoitrin); (7) Hydroxyalkylglucosinolate (n=1–2), for n=1: 3‐hydroxypropyl glucosinolate; (8) Alkenylglucosinolate (n=1–3), for n=1: allylglucosinolate. Glc, glucose.
Figure 2. Activation of glucosinolates. In intact plant tissue, glucosinolates are spatially separated from their activating enzymes (myrosinases). Loss of cellular integrity (i.e. by chewing) causes hydrolysis of glucosinolates and generates unstable aglucones. Depending on the presence of specifier proteins, the side chain chemistry (R) and the chemical environment, different hydrolysis products are formed. ESP, epithiospecifier protein; NSP, nitrile‐specifier protein; TFP, thiocyanate‐forming protein.
Figure 3. Adaptations of insect herbivores against the glucosinolate–myrosinase system. (1) Modification of the glucosinolate core structure generates a stable, nontoxic product that cannot be hydrolysed by myrosinases. (2) Excretion of a myrosinase‐interacting protein redirects the rearrangement of the aglucone towards a less‐toxic metabolite. (3) When toxic metabolites have been generated, the conjugation with glutathione (GSH) allows excretion of the toxin. Glc, glucose; GSH, glutathione.
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Further Reading

Agerbirk N and Olsen CE (2012) Glucosinolate structures in evolution. Phytochemistry 77: 16–45.

Burow M and Halkier BA (2017) How does a plant orchestrate defense in time and space? Using glucosinolates in Arabidopsis as case study. Current Opinion in Plant Biology 38: 142–147.

Fahey JW, Zalcmann AT and Talalay P (2001) The chemical diversity and distribution of glucosinolates and isothiocyanates among plants. Phytochemistry 56 (1): 5–51.

Halkier BA and Gershenzon J (2006) Biology and biochemistry of glucosinolates. Annual Review of Plant Biology 57: 303–333.

Harborne JB (2014) Introduction to Ecological Biochemistry. Academic press.

Iason GR, Dicke M and Hartley SE (eds) (2012) The Ecology of Plant Secondary Metabolites: From Genes to Global Processes. Ecological Reviews. Cambridge, UK: Cambridge University Press.

Jensen LM, Halkier BA and Burow M (2014) How to discover a metabolic pathway? An update on gene identification in aliphatic glucosinolate biosynthesis, regulation and transport. Biological Chemistry 395 (5): 529–543.

Kopriva S (ed) (2016) Advances in Botanical Research: Glucosinolates, vol. 80. Elsevier.

Schoonhoven LM, Van Loon JJ and Dicke M (2005) Insect‐Plant Biology. Oxford, UK:: Oxford University Press.

Voelckel C and Jander G (eds) (2014) Annual Plant Reviews volume 47: Insect‐Plant Interactions. Chichester: John Wiley & Sons, Ltd..

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Jeschke, Verena, and Burow, Meike(May 2018) Glucosinolates. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0027968]