Progress made in FEC transformation of cassava

In cassava friable embryogenic callus (FEC) has been used to obtain transgenic plants using particle bombardment, electroporation, and Agrobacterium tumefaciens. FEC cultures have been obtained in 6 of the10 tested genotypes. In all genotypes FEC could be regenerated into plants,however the efficiency differed between the genotypes. Almost all plants regenerated from 6 months old FEC cultures of TMS604444, Adira 4,Thai 5 and M7 were morphological similar to control plants. However, in R60 and R90 a large number of plants were not identical to control plants. Older FEC lines of TMS60444 have a reduced ability to regenerate plants and the plants show somaclonal variation. Somaclonal variation is observed in the same extend in transgenic and non-transgenic plants. The origin of this variation is both genetic and epigenetic. Luciferase based selection is less efficient in producing transgenic lines than chemical selection. Furthermore Agrobacterium tumefaciens mediated transformation is much more efficient than particle bombardment with respect to the production of transgenic lines. A tentative model is introduced which best describes the effect of different selection regimes on the time period required to produce transgenic plants. Kanamycin and stringent luciferase selection required a shorter period of time than selection based on hygromycin, phosphinothricin or non-stringent luciferase. However, a more significant reduction of time was obtained if young instead of old FEC lines of genotype TMS60444 were used for genetic modification. In accordance to the model these young FEC lines of TMS60444 produced transgenic plants within 4 months with both Agrobacterium tumefaciens combined with kanamycin selection and particle bombardment combined with stringent luciferase selection. This revised version was published online in July 2006 with corrections to the Cover Date.     

Isolation and characterisation of cDNAs encoding the large and small subunits of ADP-glucose pyrophosphorylase from cassava (Manihot esculenta Crantz)

Screening of a tuber specific cassava cDNA library resulted in the isolation of full length cDNA clones with homology to the genes encoding the small and large subunits of ADP glucose pyrophosphoryalse. Sequence analysis revealed that AGPase B the clone with homology to the small subunit shared 54% homology at amino acid level with the AGPase S clone that is more closely related to the large subunit. Segregation analysis of a cross between the cassava cultivars TMS 30572 and CM 2177-2 revealed that AGPase S is a single copy gene that is localised on the female derived linkage group E of the cassava genetic map. AGPase B is a low copy gene of which one member is localised on the female derived linkage group P. The two genes are expressed in all cassava tissues but AGPase B exhibits a higher steady state mRNA level than AGPase S and is highly expressed in leaf and tuber tissue. The AGPase enzyme activity was much higher in young cassava leaves as compared to older leaves and tubers. Cassava AGPase was activated by 3-PGA and inhibited by up to 90% in the presence of inorganic phosphate (Pi). The tuber enzyme was relatively unaffected by 3PGA but was highly inhibited by Pi. Transformation of potato (Solanum tuberosum) plants with an antisense AGPase B construct resulted in 10 out of 134 antisense AGPase B plants having on average 3.5 times more tubers than the control non transgenic plants. Analysis of these transgenic plants revealed they had greatly reduced levels of AGPase B mRNA, 1.5 to 3 times less starch, and five times higher levels of soluble sugars, sucrose, glucose and fructose, to those found in control plants. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cassava starch biosynthesis: new avenues for modifying starch quantity and quality

The genes coding for the main enzymes involved in cassava starch biosynthesis have been cloned and characterised. Molecular analysis of these genes revealed high amino acid sequence homology with other cloned genes from starch forming plant species. Use of these genes to modify the pathway of starch biosynthesis in cassava has become possible with the advent of a reproducible transformation and regeneration protocol for cassava. This would enable the development of new cassava cultivars able to produce starches with different physico-chemical properties and uses. This revised version was published online in July 2006 with corrections to the Cover Date.     

Resistance breeding and biocontrol of Striga asiatica (L.) Kuntze in maize: a review

Purpose: The aims of this article are to highlight pre-breeding procedures for identifying primary sources of Striga-resistance genes and to summarize complimentary breeding techniques that enhance partial resistance of maize varieties against Striga species.

Materials and methods: The paper presented a comprehensive account of Striga screening and controlling techniques and highlighted the potential of integrating partial resistance with FOS to boost maize production and productivity in SSA.

Results: Striga infestation is a major constraint to maize production and productivity in Sub-Saharan Africa (SSA). A lack of Striga-resistant maize varieties and the limited adoption of other control methods hinder effective and integrated control of the parasitic weed in maize and related cereal crops globally. Genetic resistance of maize should be complemented with the use of Fusarium oxysporum f.sp. strigea (FOS), a biocontrol agent known to suppress Striga.

Conclusions: A combined use of genetic resistance and FOS has remained largely unutilized in controlling Striga in Africa. A combination of conventional and molecular Striga-resistance breeding tools as well as the use of FOS are promising methods to effectively control Striga in SSA.