Metabolic engineering of p-hydroxybenzylglucosinolate in Arabidopsis by expression of the cyanogenic CYP79A1 from Sorghum bicolor

Long-distance phloem transport of glucosinolates in Arabidopsis

Glucosinolates are a large group of plant secondary metabolites found mainly in the order Capparales, which includes a large number of economically important Brassica crops and the model plant Arabidopsis. In the present study, several lines of evidence are provided for phloem transport of glucosinolates in Arabidopsis. When radiolabeled p-hydroxybenzylglucosinolate (p-OHBG) and sucrose were co-applied to the tip of detached leaves, both tracers were collected in the phloem exudates at the petioles. Long-distance transport of [(14)C]p-OHBG was investigated in wild-type and transgenic 35S::CYP79A1 plants, synthesizing high amounts of p-OHBG, which is not a natural constituent of wild-type Arabidopsis. In both wild-type and 35S::CYP79A1 plants, radiolabeled p-OHBG was rapidly transported from the application site into the whole plant and intact p-OHBG was recovered from different tissues. The pattern of distribution of the radioactivity corresponded to that expected for transport of photoassimilates such as sucrose, and was consistent with translocation in phloem following the source-sink relationship. Radiolabeled p-OHBG was shown to accumulate in the seeds of wild-type and 35S::CYP79A1 plants, where p-OHBG had been either exogenously applied or endogenously synthesized from Tyr in the leaves. p-OHBG was found in phloem exudates collected from cut petioles of leaves from both wild-type and 35S::CYP79A1 plants. Phloem exudates were shown to contain intact glucosinolates, and not desulphoglucosinolates, as the transport form. It is concluded that intact glucosinolates are readily loaded into and transported by the phloem.

The involvement of two p450 enzymes, CYP83B1 and CYP83A1, in auxin homeostasis and glucosinolate biosynthesis

The first committed step in the biosynthesis of indole glucosinolates is the conversion of indole-3-acetaldoxime into an indole-3-S-alkyl-thiohydroximate. The initial step in this conversion is catalyzed by CYP83B1 in Arabidopsis (S. Bak, F.E. Tax, K.A. Feldmann, D.A. Galbraith, R. Feyereisen [2001] Plant Cell 13: 101-111). The knockout mutant of the CYP83B1 gene (rnt1-1) shows a strong auxin excess phenotype and are allelic to sur-2. CYP83A1 is the closest relative to CYP83B1 and shares 63% amino acid sequence identity. Although expression of CYP83A1 under control of its endogenous promoter in the rnt1-1 background does not prevent the auxin excess and indole glucosinolate deficit phenotype caused by the lack of the CYP83B1 gene, ectopic overexpression of CYP83A1 using a 35S promoter rescues the rnt1-1 phenotype. CYP83A1 and CYP83B1 heterologously expressed in yeast (Saccharomyces cerevisiae) cells show marked differences in their substrate specificity. Both enzymes convert indole-3-acetaldoxime to a thiohydroximate adduct in the presence of NADPH and a nucleophilic thiol donor. However, indole-3-acetaldoxime has a 50-fold higher affinity toward CYP83B1 than toward CYP83A1. Both enzymes also metabolize the phenylalanine- and tyrosine-derived aldoximes. Enzyme kinetic comparisons of CYP83A1 and CYP83B1 show that indole-3-acetaldoxime is the physiological substrate for CYP83B1 but not for CYP83A1. Instead, CYP83A1 catalyzes the initial conversion of aldoximes to thiohydroximates in the synthesis of glucosinolates not derived from tryptophan. The two closely related CYP83 subfamily members therefore are not redundant. The presence of putative auxin responsive cis-acting elements in the CYP83B1 promoter but not in the CYP83A1 promoter supports the suggestion that CYP83B1 has evolved to selectively metabolize a tryptophan-derived aldoxime intermediate shared with the pathway of auxin biosynthesis in Arabidopsis.

CYP83b1 is the oxime-metabolizing enzyme in the glucosinolate pathway in Arabidopsis

CYP83B1 from Arabidopsis thaliana has been identified as the oxime-metabolizing enzyme in the biosynthetic pathway of glucosinolates. Biosynthetically active microsomes isolated from Sinapis alba converted p-hydroxyphenylacetaldoxime and cysteine into S-alkylated p-hydroxyphenylacetothiohydroximate, S-(p-hydroxyphenylacetohydroximoyl)-l-cysteine, the next proposed intermediate in the glucosinolate pathway. The production was shown to be dependent on a cytochrome P450 monooxygenase. We searched the genome of A. thaliana for homologues of CYP71E1 (P450ox), the only known oxime-metabolizing enzyme in the biosynthetic pathway of the evolutionarily related cyanogenic glucosides. By a combined use of bioinformatics, published expression data, and knock-out phenotypes, we identified the cytochrome P450 CYP83B1 as the oxime-metabolizing enzyme in the glucosinolate pathway as evidenced by characterization of the recombinant protein expressed in Escherichia coli. The data are consistent with the hypothesis that the oxime-metabolizing enzyme in the cyanogenic pathway (P450ox) was mutated into a "P450mox" that converted oximes into toxic compounds that the plant detoxified into glucosinolates.

Metabolism of tyrosine and tryptophan--new genes for old pathways

Tyrosine and tryptophan are precursors for the plant defense compounds dhurrin and indole glucosinolates, respectively. In addition, tryptophan is a precursor for the essential phytohormone indole-3-acetic acid. Recent advances in understanding the biosynthesis of these compounds have come from the characterization of enzymes that catalyze the N-hydroxylation of the precursor amino acid to the oxime intermediate. Furthermore, enzymes catalyzing subsequent biosynthetic steps have also been identified.

Cytochrome p450 CYP79F1 from arabidopsis catalyzes the conversion of dihomomethionine and trihomomethionine to the corresponding aldoximes in the biosynthesis of aliphatic glucosinolates

Glucosinolates are natural plant products that have received rising attention due to their role in interactions between pests and crop plants and as chemical protectors against cancer. Glucosinolates are derived from amino acids and have aldoximes as intermediates. We report that cytochrome P450 CYP79F1 catalyzes aldoxime formation in the biosynthesis of aliphatic glucosinolates in Arabidopsis thaliana. Using recombinant CYP79F1 functionally expressed in Escherichia coli, we show that both dihomomethionine and trihomomethionine are metabolized by CYP79F1 resulting in the formation of 5-methylthiopentanaldoxime and 6-methylthiohexanaldoxime, respectively. 5-methylthiopentanaldoxime is the precursor of the major glucosinolates in leaves of A. thaliana, i.e. 4-methylthiobutylglucosinolate and 4-methylsulfinylbutylglucosinolate, and a variety of other glucosinolates in Brassica sp. Transgenic A. thaliana with cosuppression of CYP79F1 have a reduced content of aliphatic glucosinolates and a highly increased level of dihomomethionine and trihomomethionine. The transgenic plants have a morphological phenotype showing loss of apical dominance and formation of multiple axillary shoots. Our data provide the first evidence that a cytochrome P450 catalyzes the N-hydroxylation of chain-elongated methionine homologues to the corresponding aldoximes in the biosynthesis of aliphatic glucosinolates.

Cytochrome P450s as genes for crop improvement

In the past year, several cytochrome P450 genes have been identified that will be important for generating crop protectants and natural medicinal products. Among these are the 2-hydroxyisoflavone synthase (CYP93C) and the indole-3-acetaldoxime N-hydroxylase (CYP83B1) genes, which catalyze the formation of isoflavones and glucosinolates, respectively.

Cytochrome P450 CYP79B2 from Arabidopsis catalyzes the conversion of tryptophan to indole-3-acetaldoxime, a precursor of indole glucosinolates and indole-3-acetic acid

Glucosinolates are natural plant products known as flavor compounds, cancer-preventing agents, and biopesticides. We report cloning and characterization of the cytochrome P450 CYP79B2 from Arabidopsis. Heterologous expression of CYP79B2 in Escherichia coli shows that CYP79B2 catalyzes the conversion of tryptophan to indole-3-acetaldoxime. Recombinant CYP79B2 has a K(m) of 21 microm and a V(max) of 7.78 nmol/h/ml culture. Inhibitor studies show that CYP79B2 is different from a previously described enzyme activity that converts tryptophan to indole-3-acetaldoxime (Ludwig-Müller, J. , and Hilgenberg, W. (1990) Phytochemistry, 29, 1397-1400). CYP79B2 is wound-inducible and expressed in leaves, stem, flowers, and roots, with the highest expression in roots. Arabidopsis overexpressing CYP79B2 has increased levels of indole glucosinolates, which strongly indicates that CYP79B2 is involved in indole glucosinolate biosynthesis. Our data show that oxime production by CYP79s is not restricted to those amino acids that are precursors for cyanogenic glucosides. Our data are consistent with the hypothesis that indole glucosinolates have evolved from cyanogenesis. Indole-3-acetaldoxime is a precursor of the plant hormone indole-3-acetic acid, which suggests that CYP79B2 might function in biosynthesis of indole-3-acetic acid. Identification of CYP79B2 provides an important tool for modification of the indole glucosinolate content to improve nutritional value and pest resistance.

Transgenic tobacco and Arabidopsis plants expressing the two multifunctional sorghum cytochrome P450 enzymes, CYP79A1 and CYP71E1, are cyanogenic and accumulate metabolites derived from intermediates in Dhurrin biosynthesis

Novel cyanogenic plants have been generated by the simultaneous expression of the two multifunctional sorghum (Sorghum bicolor [L.] Moench) cytochrome P450 enzymes CYP79A1 and CYP71E1 in tobacco (Nicotiana tabacum cv Xanthi) and Arabidopsis under the regulation of the constitutive 35S promoter. CYP79A1 and CYP71E1 catalyze the conversion of the parent amino acid tyrosine to p-hydroxymandelonitrile, the aglycone of the cyanogenic glucoside dhurrin. CYP79A1 catalyzes the conversion of tyrosine to p-hydroxyphenylacetaldoxime and CYP71E1, the subsequent conversion to p-hydroxymandelonitrile. p-Hydroxymandelonitrile is labile and dissociates into p-hydroxybenzaldehyde and hydrogen cyanide, the same products released from dhurrin upon cell disruption as a result of pest or herbivore attack. In transgenic plants expressing CYP79A1 as well as CYP71E1, the activity of CYP79A1 is higher than that of CYP71E1, resulting in the accumulation of several p-hydroxyphenylacetaldoxime-derived products in the addition to those derived from p-hydroxymandelonitrile. Transgenic tobacco and Arabidopsis plants expressing only CYP79A1 accumulate the same p-hydroxyphenylacetaldoxime-derived products as transgenic plants expressing both sorghum cytochrome P450 enzymes. In addition, the transgenic CYP79A1 Arabidopsis plants accumulate large amounts of p-hydroxybenzylglucosinolate. In transgenic Arabidopsis expressing CYP71E1, this enzyme and the enzymes of the pre-existing glucosinolate pathway compete for the p-hydroxyphenylacetaldoxime as substrate, resulting in the formation of small amounts of p-hydroxybenzylglucosinolate. Cyanogenic glucosides are phytoanticipins, and the present study demonstrates the feasibility of expressing cyanogenic compounds in new plant species by gene transfer technology to improve pest and disease resistance.