Abstract
BackgroundBrassica vegetables contain a class of secondary metabolites, the glucosinolates (GS), whose specific degradation products determine the characteristic flavor and smell. While some of the respective degradation products of particular GS are recognized as health promoting substances for humans, recent studies also show evidence that namely the 1-methoxy-indol-3-ylmethyl GS might be deleterious by forming characteristic DNA adducts. Therefore, a deeper knowledge of aspects involved in the biosynthesis of indole GS is crucial to design vegetables with an improved secondary metabolite profile.ResultsInitially the leafy Brassica vegetable pak choi (Brassica rapa ssp. chinensis) was established as suitable tool to elicit very high concentrations of 1-methoxy-indol-3-ylmethyl GS by application of methyl jasmonate. Differentially expressed candidate genes were discovered in a comparative microarray analysis using the 2 × 104 K format Brassica Array and compared to available gene expression data from the Arabidopsis AtGenExpress effort. Arabidopsis knock out mutants of the respective candidate gene homologs were subjected to a comprehensive examination of their GS profiles and confirmed the exclusive involvement of polypeptide 4 of the cytochrome P450 monooxygenase subfamily CYP81F in 1-methoxy-indol-3-ylmethyl GS biosynthesis. Functional characterization of the two identified isoforms coding for CYP81F4 in the Brassica rapa genome was performed using expression analysis and heterologous complementation of the respective Arabidopsis mutant.ConclusionsSpecific differences discovered in a comparative microarray and glucosinolate profiling analysis enables the functional attribution of Brassica rapa ssp. chinensis genes coding for polypeptide 4 of the cytochrome P450 monooxygenase subfamily CYP81F to their metabolic role in indole glucosinolate biosynthesis. These new identified Brassica genes will enable the development of genetic tools for breeding vegetables with improved GS composition in the near future.
Highlights
Brassica vegetables contain a class of secondary metabolites, the glucosinolates (GS), whose specific degradation products determine the characteristic flavor and smell
Indole GS can undergo hydroxylations and methoxylations, with CYP81F2 identified as the gene responsible for 4-hydroxylation of indol-3-ylmethyl GS (I3M) in Arabidopsis [9,10,11] (Figure 1), together with further
Increased indole GS biosynthesis in pak choi treated with methyl jasmonate In a previous study it was shown that different cultivars of the leafy vegetable pak choi (Brassica rapa ssp. chinensis) contain a certain amount of indole GS in their green leaf tissue [30]
Summary
Brassica vegetables contain a class of secondary metabolites, the glucosinolates (GS), whose specific degradation products determine the characteristic flavor and smell. Glucosinolates (GS) are amino acid-derived nitrogen- and sulphur-containing plant secondary metabolites characteristic for most families of the order Brassicales [1,2]. The biosynthesis of GS proceeds through three separate phases, the chain elongation of selected precursor amino acids, the formation of the core GS structure, and modifications of the side chain. The formation of the GS core structure is widely elucidated and genes responsible for secondary modifications of aliphatic GS via oxygenations, hydroxylations, alkenylations and benzoylations have been identified [8]. Indole GS can undergo hydroxylations and methoxylations, with CYP81F2 identified as the gene responsible for 4-hydroxylation of indol-3-ylmethyl GS (I3M) in Arabidopsis [9,10,11] (Figure 1), together with further
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