Diversification of an ancient theme: hydroxynitrile glucosides.
Many plants produce cyanogenic glucosides as part of their chemical defense. They are alpha-hydroxynitrile glucosides, which release toxic hydrogen cyanide (HCN) upon cleavage by endogenous plant beta-glucosidases. In addition to cyanogenic glucosides, several plant species produce beta- and gamma-hydroxynitrile glucosides. These do not release HCN upon hydrolysis by beta-glucosidases and little is known about their biosynthesis and biological significance. We have isolated three beta-hydroxynitrile glucosides, namely (2Z)-2-(beta-D-glucopyranosyloxy)but-2-enenitrile and (2R,3R)- and (2R,3S)-2-methyl-3-(beta-D-glucopyranosyloxy)butanenitrile, from leaves of Ribesuva-crispa. These compounds have not been identified previously. We show that in several species of the genera Ribes, Rhodiola and Lotus, these beta-hydroxynitrile glucosides co-occur with the L-isoleucine-derived hydroxynitrile glucosides, lotaustralin (alpha-hydroxynitrile glucoside), rhodiocyanosides A (gamma-hydroxynitrile glucoside) and D (beta-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 beta- and gamma-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 beta- and gamma-hydroxynitrile glucosides in L. japonicus. Our findings strongly support the notion that the beta- and gamma-hydroxynitrile glucosides are produced by diversification of the cyanogenic glucoside biosynthetic pathway at the level of the nitrile intermediate.
Bjarnholt
Nanna
N
Plant Biochemistry Laboratory and Center for Molecular Plant Physiology (PlaCe), Department of Plant Biology, Faculty of Life Sciences, University of Copenhagen, Frederiksberg C, Copenhagen, Denmark.
Rook
Fred
F
Motawia
Mohammed Saddik
MS
Cornett
Claus
C
Jørgensen
Charlotte
C
Olsen
Carl Erik
CE
Jaroszewski
Jerzy W
JW
Bak
Søren
S
Møller
Birger Lindberg
BL
D003486:Cyanides / Q000737:chemistry / Q000378:metabolism; D006027:Glycosides / Q000737:chemistry* / Q000378:metabolism; D001467:Hordeum / Q000378:metabolism; D006856:Hydrogen Cyanide / Q000737:chemistry / Q000378:metabolism; D007532:Isoleucine / Q000378:metabolism; D000070116:Lotus / Q000378:metabolism; D015394:Molecular Structure; D009570:Nitriles / Q000737:chemistry* / Q000378:metabolism; D029771:Rhodiola / Q000378:metabolism; D031965:Ribes / Q000378:metabolism
D003486:Cyanides; D006027:Glycosides; D009570:Nitriles; D007532:Isoleucine; D006856:Hydrogen Cyanide