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ISHS Acta Horticulturae 375: International Workshop on Cassava Safety

TRANSLOCATION OF CYANOGENIC GLUCOSIDES IN CASSAVA

Author:   D. Selmar
Keywords:   Cyanogenesis, cyanogenic glucosides, linamarin, linustatin, transport
DOI:   10.17660/ActaHortic.1994.375.3
Abstract:
In cassava, the cyanogenic glucoside, linamarin, is synthesized mainly in the leaves and petioles; little biosynthesis takes place in the roots. It was therefore deduced that cyanogenic glucosides must be translocated from the leaves to the roots. This hypothesis was supported by ringing experiments, the results of which suggested that transport of cyanogenic glucosides took place within the phloem.

Long distance transport generally takes place via the vascular system. It involves apoplasmic steps, either during xylem transport which is entirely apoplasmic, or in the course of apoplasmic phloem loading. Direct contact of glucosides with enzymes capable of hydrolyzing them leads to their cleavage. In the case of cyanogenic glucosides, HCN is liberated from the unstable aglycones. In intact cassava plants, such cyanogenesis does not occur, proving that in vivo, particularly during translocation of cyanogenic glucosides, linamarin does not come into contact with linamarase. Therefore, because of the high activities of apoplasmic linamarase, any apoplasmic transport of linamarin in cassava can be excluded. According to the hypothesis of the 'linustatin pathway', it is not the monoglucoside, linamarin, but its glucosylated derivative, the diglucoside linustatin, which is translocated via the apoplasm. The occurrence of linustatin in the sieve tubes is confirmed by analysis of phloem sap.

Based on these findings and on studies on the enzymes capable of hydrolyzing linustatin, the diglucosidases, it is proposed that in cassava, long distance transport of the cyanogenic glucoside, linamarin, is performed via the corresponding diglucoside, linustatin.

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