Transport of cyanogenic glucosides: linustatin uptake by Hevea cotyledons

Abstract

The ¹⁴C-labelled cyanogenic glucosides linustatin (diglucoside of acetone cyanohydrin) and linamarin (monoglucoside of acetone cyanohydrin), prepared by feeding [¹⁴C]valine to plants of Linum usitatissimum L., were applied to cotyledons of Hevea brasiliensis Muell.-Arg. in order to study their transport. Both [¹⁴C]-linustatin and [¹⁴C]linamarin were efficiently taken up by the cotyledons. Whereas ¹⁴C was recovered completely when [¹⁴C]linustatin was applied to the seedling, only about one-half of the radioactivity fed as [¹⁴C]linamarin could be accounted for after incubation. This observation is in agreement with the finding that apoplasmic linamarase hydrolyzes linamarin but not the related diglucoside linustatin. These data prove that, in vivo, linamarin does not occur apoplasmically and that linustatin, which is exuded from the endosperm, is taken up by the cotyledons very efficiently. Thus, these findings confirm the linustatin pathway (Selmar et al. 1988, Plant Physiol. 86, 711–716), which describes mobilization and transport of the cyanogenic glucoside linamarin, initiated by the glucosylation of linamarin to yield linustatin. When linustatin is metabolized to non-cyanogenic compounds, in Hevea this cyanogenic diglucoside is hydrolyzed by a diglucosidase which splits off both glucose molecules simultaneously as one gentiobiose moiety (Selmar et al. 1988). In contrast, [¹⁴C]linustatin, which is taken up by the cotyledon, is not metabolized but is reconverted in high amounts to the monoglucosidic [¹⁴C]linamarin, which then is temporarily stored in the cotyledons. These data demonstrate that in Hevea, besides the simultaneous diglucosidase, there must be present a further diglucosidase which is able to hydrolyze cyanogenic diglucosides sequentially by splitting off only the terminal glucose moiety from linustatin to yield linamarin. From this, it is deduced that the metabolic fate of linustatin, which is transported into the source tissues, depends on the activities of the different diglucosidases. Whereas sequential cleavage — producing linamarin — is purely a part of the process of linamarin translocation (using linustatin as the transport vehicle), simultaneous cleavage, producing acetone cyanohydrin, is part of the process of linamarin metabolization in which the nitrogen from cyanogenic glucosides is used to synthesize non-cyanogenic compounds.