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The efficacy of the ipt-type Multi-Auto-Transformation (MAT) vector system to transform the extensively grown cassava cultivar "KU50" was evaluated. This system utilizes the isopentenyltransferase (ipt) gene as morphological marker for visual selection of transgenic lines. The extreme shooty phenotype (ESP) of transgenic lines is lost due to the removal of ipt gene mediated by the yeast Rint/RS system. As a result, phenotypically normal shoots, considered marker-free transgenic plants, could be obtained. When transforming KU50 cassava cultivar with two different ipt-type MAT vectors, transformation frequency at 19-21% was observed. Among the total number of ESP explants, 32-38% regained normal extended shoot phenotype and 88-96% of which were confirmed to represent the marker-free transgenic plants. This is the first demonstration of the efficacy of Rint/RS system in promoting excision of ipt marker gene in cassava specie, with the consequent rapid production of marker-free transgenic plants. The high efficiency of this system should facilitate pyramiding a number of transgenes by repeated transformation without having to undergo through laborious, expensive and time-consuming processes of sexual crossing and seed production. The generation of marker-free, thus environmentally safe, genetically modified cassava clones should also ease the public concerns regarding the use of transgenic cassava in both food and nonfood industries.

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To improve the regeneration efficiency of cassava (Manihot esculenta Crantz) in vitro, the effect of silver nitrate (AgNO 3 ) on shoot organogenesis from somatic cotyledons was assessed. Adding AgNO 3 to the regeneration medium improved the regeneration frequency and reduced callus formation in all tested cultivars. Both the extent of the response to and the optimum concentration of AgNO 3 were cultivar dependent. In the model cultivar MCol22, the use of AgNO 3 at concentrations between 4 and 12 mg l −1 increased shoot organogenesis frequency and the number of shoot primordia per explant, the maximum effect being observed on a medium containing 12 mg l −1 AgNO 3 . At this concentration, the frequency of shoot organogenesis was enhanced from 60 to 90%, while callus formation decreased from 100 to 5%. The highest shoot organogenesis rates were obtained by supplementing the medium with 2 and 1 mg l −1 AgNO 3 in KU50 and Hanatee, respectively, while cultivar T5 showed an optimum response at 4 mg l −1 . The shoots regenerated from explants cultured on a medium containing AgNO 3 were more elongated than those cultured on a medium without AgNO 3 . The application of AgNO 3 did not change the dose response of shoot organogenesis for the selective agents hygromycin and mannose.