The cassava plant accumulates the cyanogenic glucosides linamarin and lotaustralin in its edible roots and leaves. Mechanical injury of the plant tissue causes the release of beta-glucosidases and beta-hydroxynitrile lyases which results in the hydrolysis of these Cyanogenic glucosides causing the liberation of cyanide. The presence 0E cyanide in cassava derived products constitutes a serious nutritional health hazard and results in cronic or acute cyanide poisoning in populations where the supply of other food elements is limited. To minimize these adverse effects, breeding Programmes have been carried out and some improved cassava lines with a lower content of cyanogenic glucosides obtained. The cyanide content is known to vary depending o the growth conditions and so far it has not been possible to obtain acyanogenic cassava varieties.
In the current STD 2 progranme we have (i) obtained a microsomal enzyme system from etiolated seedlings of cassava catalyzing the in vitro synthesi of cyanogenic glucosides; (ii) elucidated the pathway for biosynthesis of cyanogenic glucosides; (iii) demonstrated the involvement of cytochrome P45 in the pathway; (iv) purified the membrane proteins cytochrome P450, NADPH-cytochrome P450 oxidoreductase, NADH-cytochrome b5 oxidoreductase and cytochrome bs involved; (v) purified the soluble proteins UDPG-glucosyltransferase, beta-glucosidaseand.x-hydroxynitrilelyase; (vi) identified cdna clones for NADPH-cytochrome P450 oxidoreductase (also sequenced), UDPG-glucosyl transferase and beta-glucosidase (also sequenced); (vii) shown that beta-glucosidase and x-hydroxynitrilase genes are:expressed primarily in young latex vesicles. The overall aim has been to understand the biochemical, molecular biological and physiological processes behind the phenomenon ofcyanogenesis The present proposal constitutes an extension of these studies. It is important to isolate and characterize both structural- genes and control sequences for the components of cyanogenesis so that a rtional strategy for crop improvement can be produced. It is critical to understand the physiological factors important for regulation of cyanogenesis. The availability of the relevant gene probes and specific antibodies will permit efficient screening of new cassava cultivars obtained by traditional plant breeding.
Funding SchemeCSC - Cost-sharing contracts
NE2 4HH Newcastle Upon Tyne