Characterization of sucrose uptake system in cassava (Manihot esculenta Crantz)

A leaf disc system was developed to study sucrose uptake in cassava (Manihot esculenta Crantz). The uptake of (U-(14)C) sucrose by cassava leaf discs followed Michaelis-Menten kinetics with a K(m) value for sucrose of 1.3 mM. It was found to be strongly inhibited by sulfhydryl reagents, N-ethylmaleimide, p-chloromercuribenzene sulfonate and iodoacetate. Several metabolic inhibitors were also tested. Among these, dinitrophenol, chlorocarbonyl cyanide phenylhydrazone, phloridzin and vanadate inhibited the sucrose uptake by the leaf discs. Linamarin, the main cassava cyanogenic glucoside, strongly inhibited the sucrose uptake by the leaf discs, while other cyanogenic glycosides tested (prunasin and amygdalin) showed a much weaker inhibition. A linamarin analog, isopropylthioglucoside, was a weaker inhibitor than p-nitrophenyl glucosides (both alpha and beta forms). Cassava root discs were also capable of taking up sucrose, but linamarin activated its uptake. The observations suggested that linamarin may regulate the sucrose transport in cassava.