Arabidopsis thaliana accumulates a number of aromatic glucosinolates in addition to those derived from methionine (Haughn et al, 1991) (see Glucosinolates under Sulfur uptake and assimilation). These include glucosinolates with heterocyclic side-chains derived from tryptophan: 3-indolylmethyl-, 4-methoxy-3-indolylmethyl-, and 1-methoxy-3-indolylmethyl-.

The pathway of synthesis of the indole glucosinolates in Arabidopsis thaliana is thought to proceed via the catalytic action of O₂ and NADPH-dependent cytochrome P540's acting on tryptophan and indole-3-acetaldoxime (Celenza, 2001; Bak et al, 2001). CYP79B2 and CYP79B3 N-hydroxylate tryptophan, converting it to indole-3-acetaldoxime [a reaction analogous to the first step in the synthesis of cyanogenic glucosides] (Celenza, 2001). CYP83B1 subsequently N-hydroxylates indole-3-acetaldoxime to indole-3-acetaldoxime N-oxide (1-aci-nitro-2-indolyl-ethane) (Hansen et al, 2001; Bak et al, 2001).

Mutants (superroot2 (sur2); runt1) defective in CYP83B1 have elevated levels of IAA, and decreased levels of indole glucosinolates (Bak et al, 2001; Celenza, 2001). In contrast, overexpression of CYP83B1 causes a bushy, short phenotype consistent with reduced levels of IAA, suggesting that the glucosinolate pathway competes with the auxin biosynthesis pathway for the indole-3-acetaldoxime intermediate (Bak et al, 2001). Overexpression of CYP79B2 causes increased levels of indole glucosinolates without perturbing IAA levels (Celenza, 2001; Mikkelsen et al, 2000).

The benzoic acid glucosinolate esters with non-heterocyclic side-chains (Haughn et al, 1991):

• R = C₆H₅COO-(CH₂)₃- (3-benzoyloxypropyl)
• R = C₆H₅COO-(CH₂)₄- (4-benzoyloxybutyl)
• R = C₆H₅COO-(CH₂)₅- (5-benzoyloxypentyl)
• R = C₆H₅COO-(CH₂)₆- (6-benzoyloxyhexyl)

Note that these benzoic acid glucosinolate esters have a distinct origin from "benzylglucosinolate" (R = C₆H₅(CH₂)₁) which is derived from phenylalanine via phenylacetaldoxime in Arabidopsis thaliana (Wittstock and Halkier, 2000). The 2-phenylethyl glucosinolate (R = C₆H₅(CH₂)₂) is derived from the chain-elongated phenylalanine homolog, homophenylalanine (Wittsock and Halkier, 2000). Synthesis of phenylacetaldoxime from phenylalanine is catalyzed by cytochrome P-450 CYP79A2 in Arabidopsis thaliana. This enzyme does not act on homophenylalanine, tyrosine, methionine or tryptophan (Wittsock and Halkier, 2000).

Considerable effort has gone into breeding out glucosinolates from the seeds of commercial varieties of canola (Brassica campestris) and oilseed rape (Brassica napus) because these compounds reduce the nutritional value of rapeseed meal. However, this is detrimental because low glucosinolate content of the cotyledons of emerging seedlings encourages feeding by slugs and other nonspecialist herbivores (Bennett et al, 1997). Breakdown products of glucosinolates may also be toxic to fungi (Osbourn, 1996).

Bak S, Tax FE, Feldmann KA, Galbraith DW, Feyereisen R 2001 CYP83B1, a cytochrome P450 at the metabolic branch point in auxin and indole glucosinolate biosynthesis in Arabidopsis. Plant Cell 13: 101-111.

Bennett RN, Kiddle G, Wallsgrove RM 1997 Involvement of cytochrome P450 in glucosinolate biosynthesis in white mustard: a biochemical anomaly. Plant Physiol. 114: 1283-1291.

Celenza JL 2001 Metabolism of tyrosine and tryptophan - new genes for old pathways. Curr. Opin. Plant Biol. 4: 234-240.

Hansen CH, Du L, Naur P, Olsen CE, Axelsen KB, Hick AJ, Pickett JA, Halkier BA 2001 CYP83B1 is the oxime-metabolizing enzyme in the glucosinolate pathway in Arabidopsis. J. Biol. Chem. 276: 24790-24796.

Haughn GW, Davin L, Giblin M, Underhill EW 1991 Biochemical genetics of plant secondary metabolites in Arabidopsis thaliana. The glucosinolates. Plant Physiol. 97: 217-226.

Mikkelsen MD, Hansen CH, Wittstock U, Halkier BA 2000 Cytochrome P450 CYP79B2 from Arabidopsis catalyzes the conversion of tryptophan to indole-3-acetaldoxime, a precursor of indole glucosinolates and indole-3-acetic acid. J. Biol. Chem. 275: 33712-33717.

Osbourn AE 1996 Preformed antimicrobial compounds and plant defense against fungal attack. Plant Cell 8: 1821-1831.

Wittstock U, Halkier BA 2000 Cytochrome P450 CYP79A2 from Arabidopsis thaliana L. Catalyzes the conversion of L-phenylalanine to phenylacetaldoxime in the biosynthesis of benzylglucosinolate. J. Biol. Chem. 275: 14659-14666.