Diversification of an ancient theme: Hydroxynitrile glucosides

© 2008 Published by Elsevier

Many plants produce cyanogenic glucosides as part of their chemical defense. They are α-hydroxynitrile glucosides, which release toxic hydrogen cyanide (HCN) upon cleavage by endogenous plant β-glucosidases. In addition to cyanogenic glucosides, several plant species produce β- and γ-hydroxynitrile glucosides. These do not release HCN upon hydrolysis by β-glucosidases and little is known about their biosynthesis and biological significance. We have isolated three β-hydroxynitrile glucosides, namely (2Z)-2-(β-d-glucopyranosyloxy)but-2-enenitrile and (2R,3R)- and (2R,3S)-2-methyl-3-(β-d-glucopyranosyloxy)butanenitrile, from leaves of Ribes uva-crispa. These compounds have not been identified previously. We show that in several species of the genera Ribes, Rhodiola and Lotus, these β-hydroxynitrile glucosides co-occur with the l-isoleucine-derived hydroxynitrile glucosides, lotaustralin (α-hydroxynitrile glucoside), rhodiocyanosides A (γ-hydroxynitrile glucoside) and D (β-hydroxynitrile glucoside) and in some cases with sarmentosin (a hydroxylated rhodiocyanoside A). Radiolabelling experiments demonstrated that the hydroxynitrile glucosides in R. uva-crispa and Hordeum vulgare are derived from l-isoleucine and l-leucine, respectively. Metabolite profiling of the natural variation in the content of cyanogenic glucosides and β- and γ-hydroxynitrile glucosides in wild accessions of Lotus japonicus in combination with genetic crosses and analyses of the metabolite profile of the F2 population provided evidence that a single recessive genetic trait is most likely responsible for the presence or absence of β- and γ-hydroxynitrile glucosides in L. japonicus. Our findings strongly support the notion that the β- and γ-hydroxynitrile glucosides are produced by diversification of the cyanogenic glucoside biosynthetic pathway at the level of the nitrile intermediate.

Nanna Bjarnholt, Fred Rook, Mohammed Saddik Motawia, Claus Cornett, Charlotte Jørgensen, Carl Erik Olsen, Jerzy W. Jaroszewski, Søren Bak and Birger Lindberg Møller