Biosynthesis of glucosinolates – gene discovery and beyond

doi:10.1016/j.tplants.2010.02.005

Trends in Plant Science

Volume 15, Issue 5, May 2010, Pages 283-290

https://doi.org/10.1016/j.tplants.2010.02.005 Get rights and content

Glucosinolates are sulfur-rich secondary metabolites characteristic of the Brassicales order with important biological and economic roles in plant defense and human nutrition. Application of systems biology tools continues to identify genes involved in the biosynthesis of glucosinolates. Recent progress includes genes in all three phases of the pathway, i.e. side-chain elongation of precursor amino acids, formation of the core glucosinolate structure and side-chain decoration. Major breakthroughs include the ability to produce glucosinolates in Nicotiana benthamiana, the finding that specific glucosinolates play a key role in Arabidopsis innate immune response, and a better understanding of the link between primary sulfur metabolism and glucosinolate biosynthesis.

Section snippets

Expanding the glucosinolate gene inventory

Glucosinolates are secondary metabolites well-known for their role in plant resistance to insects and pathogens and for their cancer-preventive properties. They are largely found in the order Brassicales which includes the economically and nutritionally important Brassica crops, such as oilseed rape (Brassica napus), cabbage (Brassica oleracea) and the model plant Arabidopsis (Arabidopsis thaliana) [1].

Glucosinolates have obtained status as ‘model’ secondary metabolites, and scientists in the

Biosynthesis overview

Glucosinolates are derived from amino acids and can thus be divided into three groups according to their amino acid precursor: aliphatic glucosinolates, derived from Ala, Leu, Ile, Val, and Met; benzenic glucosinolates, derived from Phe or Tyr; and indolic glucosinolates, derived from Trp. Biosynthesis proceeds through three independent stages: (i) chain elongation of selected precursor amino acids (only Met and Phe), (ii) formation of the core glucosinolate structure, and (iii) secondary

Side-chain elongation of Met – the resemblance to Leu biosynthesis

Before entering the core structure pathway, Met undergoes chain elongation (Figure 1a) in a process similar to the conversion of the branched-chain amino acid Val to its chain-elongated homolog Leu. The process starts with a deamination by a branched-chain amino acid aminotransferase (BCAT), which gives rise to a 2-oxo acid (Figure 1, step 1 to 2). The 2-oxo acid then enters a cycle of three successive transformations: condensation with acetyl-CoA by a methylthioalkylmalate synthase (MAM) (

Compartmentalization – the need for transport of intermediates

BCAT4 catalyzes the first reaction in the chain elongation process, and is localized in the cytosol [19], whereas the remaining enzymes involved in chain elongation are localized in the chloroplast 6, 10, 20, 30, 33, 34. This indicates the need for import of 2-oxo acids into the chloroplast and for export of chain-elongated amino acids into the cytosol, where biosynthesis of the core glucosinolate structure is believed to take place [1]. The chloroplast-localized bile acid transporter BAT5 was

Constructing the glucosinolate core – the importance of sulfur incorporation

The formation of the glucosinolate core structure seemed almost completely elucidated by 2006, where 13 enzymes representing five different biochemical steps had already been characterized 1, 35. Briefly, precursor amino acids are converted to aldoximes by cytochromes P450 of the CYP79 family (Figure 1, step 6 to 7). CYP79B2 and CYP79B3 both metabolize Trp 36, 37, CYP79A2 uses Phe as a substrate [38], CYP79F1 converts all chain-elongated Met derivatives, and CYP79F2 only converts the

Secondary modifications – the decoration that creates structural biodiversity

The biological activity of glucosinolates is to a large extent determined by the structure of the side chain [21], which makes secondary modifications particularly interesting from an application perspective. For aliphatic glucosinolates, secondary modifications include oxygenations, hydroxylations, alkenylations and benzoylations (Figure 1c). Indolic glucosinolates, in turn, can undergo hydroxylations and methoxylations (Figure 1c).

What is beyond gene discovery?

With the near-complete inventory of glucosinolate biosynthetic genes, one of the next goals in glucosinolate research is to understand the channeling of intermediates to the final product, which may be enabled by the presence of a biosynthetic multi-enzyme complex, also termed ‘metabolon’ [64]. Furthermore, virtually nothing is known about the transport of glucosinolates from their production site to the proper storage site. Regulation of flux through the pathway might be affected by

Acknowledgements

This work was supported by the Villum Kann Rasmussen (VKR) Foundation grant to VKR Research Centre for Pro-Active Plants.

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Trends in Plant Science

ReviewBiosynthesis of glucosinolates – gene discovery and beyond

Section snippets

Expanding the glucosinolate gene inventory

Biosynthesis overview

Side-chain elongation of Met – the resemblance to Leu biosynthesis

Compartmentalization – the need for transport of intermediates

Constructing the glucosinolate core – the importance of sulfur incorporation

Secondary modifications – the decoration that creates structural biodiversity

What is beyond gene discovery?

Acknowledgements

J. Biol. Chem.

Trends Plant Sci.

Curr. Opin. Plant Biol.

Phytochemistry

Phytochemistry

Trends Plant Sci.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Phytochemistry

Phytochemistry

Biochem. Biophys. Acta

Phytochemistry

Biology and biochemistry of glucosinolates

Annu. Rev. Plant Biol.

Transcriptional co-regulation of secondary metabolism enzymes in Arabidopsis: functional and evolutionary implications

Plant Mol. Biol.

Omics-based identification of Arabidopsis Myb transcription factors regulating aliphatic glucosinolate biosynthesis

Proc. Natl. Acad. Sci. U. S. A.

A systems biology approach identifies a R2R3 MYB gene subfamily with distinct and overlapping functions in regulation of aliphatic glucosinolates

PLoS ONE.

Omics-based approaches to methionine side-chain elongation in Arabidopsis: characterization of the genes encoding methylthioalkylmalate isomerase and methylthioalkylmalate dehydrogenase

Plant Cell Physiol.

Arabidopsis bile acid: sodium symporter family protein 5 is involved in methionine-derived glucosinolate biosynthesis

Plant Cell Physiol.

Glucosinolate engineering identifies a gamma-glutamyl peptidase

Nat. Chem. Biol.

Arabidopsis branched-chain aminotransferase 3 functions in both amino acid and glucosinolate biosynthesis

Plant Physiol.

A redox-active isopropylmalate dehydrogenase functions in the biosynthesis of glucosinolates and leucine in Arabidopsis

Plant J.

Glucosinolate metabolites required for an Arabidopsis innate immune response

Science

Identification of a flavin monoxygenase as the S-oxygenating enzyme in aliphatic glucosinolate biosynthesis in Arabidopsis

Plant J.

A gene controlling variation in Arabidopsis glucosinolate composition is part of the methionine chain elongation pathway

Plant Physiol.

The gene controlling the indole glucosinolate modifier1 quantitative trait locus alters indole glucosinolate structures and aphid resistance in Arabidopsis

Plant Cell

Gene duplication in the diversification of secondary metabolism: tandem 2-oxoglutarate-dependent dioxygenases control glucosinolate biosynthesis in Arabidopsis

Plant Cell

A novel 2-oxoacid-dependent dioxygenase involved in the formation of the goiterogenic 2-hydroxybut-3-enyl glucosinolate and generalist insect resistance in Arabidopsis

Plant Physiol.

BRANCHED-CHAIN AMINOTRANSFERASE4 is part of the chain elongation pathway in the biosynthesis of methionine-derived glucosinolates in Arabidopsis

Plant Cell

Arabidopsis thaliana encodes a bacterial-type heterodimeric isopropylmalate isomerase involved in both Leu biosynthesis and the Met chain elongation pathway of glucosinolate formation

Plant Mol. Biol.

Role of glucosinolates in insect-plant relationships and multitrophic interactions

Annu. Rev. Entomol.

Molecular basis for chemoprevention by sulforaphane: a comprehensive review

Cell Mol. Life Sci.

Glucosinolates, isothiocyanates and human health

Phytochem. Rev.

Review
Biosynthesis of glucosinolates – gene discovery and beyond