Cassava Plants with a Depleted Cyanogenic Glucoside Content in Leaves and Tubers. Distribution of Cyanogenic Glucosides, Their Site of Synthesis and Transport, and Blockage of the Biosynthesis by RNA Interference Technology

Abstract

Transgenic cassava (Manihot esculenta Crantz, cv MCol22) plants with a 92% reduction in cyanogenic glucoside content in tubers and acyanogenic (,1% of wild type) leaves were obtained by RNA interference to block expression of CYP79D1 and CYP79D2, the two paralogous genes encoding the first committed enzymes in linamarin and lotaustralin synthesis. About 180 independent lines with acyanogenic (,1% of wild type) leaves were obtained. Only a few of these were depleted with respect to cyanogenic glucoside content in tubers. In agreement with this observation, girdling experiments demonstrated that cyanogenic glucosides are synthesized in the shoot apex and transported to the root, resulting in a negative concentration gradient basipetal in the plant with the concentration of cyanogenic glucosides being highest in the shoot apex and the petiole of the first unfolded leaf. Supply of nitrogen increased the cyanogenic glucoside concentration in the shoot apex. In situ polymerase chain reaction studies demonstrated that CYP79D1 and CYP79D2 were preferentially expressed in leaf mesophyll cells positioned adjacent to the epidermis. In young petioles, preferential expression was observed in the epidermis, in the two first cortex cell layers, and in the endodermis together with pericycle cells and specific parenchymatic cells around the laticifers. These data demonstrate that it is possible to drastically reduce the linamarin and lotaustralin content in cassava tubers by blockage of cyanogenic glucoside synthesis in leaves and petioles. The reduced flux to the roots of reduced nitrogen in the form of cyanogenic glucosides did not prevent tuber formation.Cassava (Manihot esculenta Crantz) is the most important root crop in the world and ranks second among African staple crops (Nweke et al., 2002). Cassava is vegetatively propagated through stem cuttings and produces well on poor soils. The tubers may be kept in the soil for extended time periods. This secures rural farmers a carbohydrate source in years with adverse growth conditions where other crops fail and famine would otherwise prevail. These features and high crop yield contribute to the importance of cassava in Africa, South East Asia, and South America Industrial cassava starch production is important, especially in South East Asia (Bokanga, 1994). Cassava leaves are not widely used as a food source despite their high protein content (Ngudi et al., 2003).Major drawbacks of the cassava crop are the low tuber protein content, rapid tuber perishability following harvest, and high content of the cyanogenic glucosides linamarin and lotaustralin in all tissues. Upon tissue disruption, the cyanogenic glucosides are brought in contact with b-glucosidases and a-hydroxynitrile lyases and degraded into cyanohydrins, hydrogen cyanide, and ketones (Conn, 1980). When cassava products are used as a primary staple food, careful processing to remove these toxic constituents is required to avoid chronic cyanide intoxication (Onabolu et al., 2002). Incomplete processing may result in hi...

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“…Similar girdling experiments have previously been carried out in cassava and demonstrated the significance of transport of cyanogenic glucosides from the site of production in the actively growing shoot apexes and leaf laminae to the tuber. Thus in cassava, the cyanogenic glucoside content in the internode above the incision zone in the shoot apex increased by a factor of 75 (Jørgensen et al, 2005). This clearly demonstrates the feasibility of using the girdling method to assess transport of cyanogenic glucosides.…”

Quantitative Estimation of Hydrogen Cyanide in Fresh and Cooked Tuber Parenchyma (Pulp) of Three Cultivars of Sweet Cassava Cultivars Grown in Some Parts of Benue State, Nigeria

“…However, as earlier reported by Jogensen et al [5], the foremost shortcoming associated with cassava crop are: low protein content, rapid tuber perishability following harvest and high content of cyanogenic glycosides. The cyanogenic glycosides are a group of secondary metabolites in plants that yield cyanide upon enzyamatic metabolism which have been implicated in their role as herbivore deterrents and as transportable forms of reduced nitrogen [6].…”

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“…Another domestication trait is the decreased cyanide content in roots 4 . Every tissue of cassava contains cyanogenic glucosides 25 . Ketones, cyanohydrin, and hydrogen cyanide are the key toxic compounds formed during degradation of cyanogenic glucosides 25,26 .…”

“…Every tissue of cassava contains cyanogenic glucosides 25 . Ketones, cyanohydrin, and hydrogen cyanide are the key toxic compounds formed during degradation of cyanogenic glucosides 25,26 . These toxic compounds must be eliminated before human consumption.…”