Abstract
Cassava (Manihot esculenta Crantz), a staple crop for millions of sub-Saharan Africans, contains high levels of cyanogenic glycosides which protect it against herbivory. However, cyanogens have also been proposed to play a role in nitrogen transport from leaves to roots. Consistent with this hypothesis, analyses of the distribution and activities of enzymes involved in cyanide metabolism provides evidence for cyanide assimilation, derived from linamarin, into amino acids in cassava roots. Both β-cyanoalanine synthase (CAS) and nitrilase (NIT), two enzymes involved in cyanide assimilation to produce asparagine, were observed to have higher activities in roots compared to leaves, consistent with their proposed role in reduced nitrogen assimilation. In addition, rhodanese activity was not detected in cassava roots, indicating that this competing means for cyanide metabolism was not a factor in cyanide detoxification. In contrast, leaves had sufficient rhodanese activity to compete with cyanide assimilation into amino acids. Using transgenic low cyanogen plants, it was shown that reducing root cyanogen levels is associated with elevated root nitrate reductase activity, presumably to compensate for the loss of reduced nitrogen from cyanogens. Finally, we overexpressed Arabidopsis CAS and NIT4 genes in cassava roots to study the feasibility of enhancing root cyanide assimilation into protein. Optimal overexpression of CAS and NIT4 resulted in up to a 50% increase in root total amino acids and a 9% increase in root protein accumulation. However, plant growth and morphology was altered in plants overexpressing these enzymes, demonstrating a complex interaction between cyanide metabolism and hormonal regulation of plant growth.
Highlights
Cyanide is ubiquitous in nature and as such most eukaryotic organisms have developed mechanisms for its detoxification
Cyanide is detoxified in plants either through condensation with cysteine, catalyzed by cyanoalanine synthase (CAS), or via condensation with thiosulfate, derived from sulfur metabolism, and catalyzed by the enzyme rhodanese (Figure 1; Hatzfeld and Saito, 2000)
Cyanogenic plants produce sufficient levels of cyanogens to provide protection against a variety of herbivores and pathogens (Nahrstedt, 1985; Jones, 1998). It is this function of cyanogens that has received the greatest attention since it can potentially impact human health
Summary
Cyanide is ubiquitous in nature and as such most eukaryotic organisms have developed mechanisms for its detoxification. Cyanogen Metabolism in Cassava Roots from the enzymes that degrade them (Kojima et al, 1979; Poulton, 1990; McMahon et al, 1995; Maruyama et al, 2001). The cyanogenic glycoside linamarin, is stored in vacuoles while its corresponding β-glucosidase, linamarase, is localized to the cell wall and laticifers (Mkpong et al, 1990; McMahon et al, 1995; Elias et al, 1997a). E.g., during mechanical damage, initiates hydrolysis of linamarin by the generalized β-glucosidase, linamarase, to produce acetone cyanohydrin. Acetone cyanohydrin can spontaneously decompose to yield cyanide and acetone at pH > 5.0 or temperatures > 35◦C, or is broken down by the enzyme hydroxynitrile lyase (HNL), which is expressed only in cassava leaves and stems and not in roots (White et al, 1998)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
