**4.5 Improved starch content and quality**

Starch quality is one of the key agronomic traits for selection of elite cassava genotypes. The ratio of amylose to amylopectin determines the property of starch granules, and influences the quality of various starch products utilized in the pharmaceutical, chemical, and paper-making industries. Starch synthesis is regulated by AGPase, starch synthase (SS), and starch-branching enzyme (SBE). These three main enzymes have been successfully cloned from cassava [85–87]. The inhibition of AGPase activity results in partial or complete termination of starch synthesis. Thus, improvement of the AGPase activity contributes to the conversion of sugar to starch, which subsequently increases the starch quantity. The reduction of allosteric feedback regulation by fructose-1,6-bisphosphate was detected by genetic modification of the *E. coli* glgC gene (encoding AGPase) by site directed mutagenesis (G336D) [57]. Transgenic cassava plants expressing the mutant glgC gene showed a 70% enhancement in AGPase activity, and up to a 2.6-fold increase in biomass. The quality of cassava starch depends on amylose and amylopectin content. Antisense RNA technology has been used to reduce GBSS expression in the potato resulting into decreased amylose content in potato tubers [88]. Using antisense GBSSI RNA under the control of the CaMV 35S promoter, waxy transgenic cassava plants were generated [89]. Waxy transgenic cassava clones have also been developed using the cassava vascular-specific p54/1.0 and CaMV 35S promoters to drive the expression of hairpin dsRNA homologous to cassava GBSSI. Starches from waxy transgenic plants revealed altered biological and physico-chemical properties [56]. Thus, the control of GBSS activity is an effective way to regulate amylose synthesis.
