Generation of embryogenic callus is a key step in genetic engineering of many crop species, including cassava. Protocols for generation of friable embryogenic callus (FEC) have been lacking for Ugandan cassava genotypes, thereby delaying their genetic engineering for agronomic and other desirable traits. The objective of this study was to determine conditions suitable for production and regeneration of FEC in the Ugandan cassava genotypes; Aladu, Bukalasa and Ebwanateraka, and control cultivar 60444. Immature leaf lobe explants were established on Murashige and Skoog (MS) based media for initiation of organized embryogenic callus (OES). To produce FEC, resulting OES were established on Gresshoff and Doy based callus induction media with varying levels of sucrose, maltose, tyrosine, tryptophan, naphthalene acetic acid (NAA) under light and dark conditions. Subsequently, FEC was subcultured to MS-based embryo maturation and embryo regeneration media. All genotypes produced OES. All genotypes produced FEC except Bukalasa. The amino acid tyrosine favoured production of FEC in Aladu and Ebwanatereka, but not in 60444, while 20 g/L of sucrose trigged production of FEC in Aladu and 60444, but 40 g/L of sucrose was superior for Ebwanatereka. Media supplemented with 1 ml/L naphthalene acetic acid NAA facilitated embryo regeneration in Ebwanatereka and 60444, while Aladu responded better to 5 ml/L NAA. Light, tyrosine and sucrose were essential for FEC production in Uganda cultivars while NAA was required for regeneration of somatic embryos. Ability to produce FEC in these genotypes lays a foundation for their improvement through genetic transformation for the desired and agronomic traits.
Key words: Cassava (Manihot esculenta Crantz), somatic embryogenesis, amino acids, carbon sources.
OES, Organised embryogenic structures; FEC, friable embryogenic callus; MS, Murashige and Skoog Basal media; GD, Gresshoff and Doy basal media; NAA, naphthalene acetic acid; BAP, benzylaminopurine; CBSD, cassava brown streak disease; CBSV, cassava brown streak virus; UCBVS, Ugandan cassava brown streak virus; UFPCG, Ugandan farmer preferred cassava genotypes; GD250P, Gresshoff and Doy basal medium supplemented with the auxin picloram; MS2 50P, Murashige and Skoog basal medium supplemented with the auxin picloram.