The bifurcation of the cyanogenic glucoside and glucosinolate biosynthetic pathways

Plant J. 2015 Nov;84(3):558-73. doi: 10.1111/tpj.13023.

Abstract

The biosynthetic pathway for the cyanogenic glucoside dhurrin in sorghum has previously been shown to involve the sequential production of (E)- and (Z)-p-hydroxyphenylacetaldoxime. In this study we used microsomes prepared from wild-type and mutant sorghum or transiently transformed Nicotiana benthamiana to demonstrate that CYP79A1 catalyzes conversion of tyrosine to (E)-p-hydroxyphenylacetaldoxime whereas CYP71E1 catalyzes conversion of (E)-p-hydroxyphenylacetaldoxime into the corresponding geometrical Z-isomer as required for its dehydration into a nitrile, the next intermediate in cyanogenic glucoside synthesis. Glucosinolate biosynthesis is also initiated by the action of a CYP79 family enzyme, but the next enzyme involved belongs to the CYP83 family. We demonstrate that CYP83B1 from Arabidopsis thaliana cannot convert the (E)-p-hydroxyphenylacetaldoxime to the (Z)-isomer, which blocks the route towards cyanogenic glucoside synthesis. Instead CYP83B1 catalyzes the conversion of the (E)-p-hydroxyphenylacetaldoxime into an S-alkyl-thiohydroximate with retention of the configuration of the E-oxime intermediate in the final glucosinolate core structure. Numerous microbial plant pathogens are able to detoxify Z-oximes but not E-oximes. The CYP79-derived E-oximes may play an important role in plant defense.

Keywords: CYP71E1; CYP79A1; CYP83B1; E- and Z-oxime metabolism; Sinapis alba; Sorghum bicolor; cytochrome P450; microbial Z-oxime-nitrile pathway; nitriles; oxime dehydration.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Arabidopsis Proteins / genetics
  • Arabidopsis Proteins / metabolism
  • Biosynthetic Pathways
  • Cytochrome P-450 Enzyme System / genetics
  • Cytochrome P-450 Enzyme System / metabolism*
  • Glucosinolates / metabolism*
  • Isomerism
  • Mutation
  • Nicotiana / genetics
  • Nicotiana / metabolism
  • Oximes / metabolism*
  • Plant Proteins / genetics
  • Plant Proteins / metabolism
  • Plants, Genetically Modified
  • Sorghum / genetics
  • Sorghum / metabolism*
  • Tyrosine / metabolism

Substances

  • Arabidopsis Proteins
  • Glucosinolates
  • Oximes
  • Plant Proteins
  • 4-hydroxyphenylacetaldoxime
  • Tyrosine
  • Cytochrome P-450 Enzyme System
  • CYP83B1 protein, Arabidopsis
  • cytochrome P450TYR
  • cytochrome P-450 CYP71E1 (sorghum)