dc.description.abstract | Cassava (Manihot esculenta Crantz) is a major source of food for over 800 million
people worldwide. Although cassava is relatively easy to grow, even in poor soils and
under erratic rainfall conditions, its roots have a short shelf-life of only 24-72 hours
due to post-harvest physiological deterioration (PPD). PPD is a process initiated on
harvesting and mediated by reactive oxygen species (ROS) that ultimately renders
storage roots unpalatable and unmarketable. Control of PPD through conventional
breeding is difficult and do not appear to offer a solution due to lack of resistance
genes in existing germ plasm, high heterozygosity, poor flowering and low pollen
fertility. As an alternative, a transgenic approach focusing on preventing ROS
accumulation post-harvest has the potential to extend the shelf-life of cassava storage
roots. However, an essential pre-requisite for genetic transformation of cassava is the
availability of an efficient and reproducible transformation and regeneration system.
Therefore, the first objective of this study was to optimize a transformation protocol
for African farmer-preferred cassava cultivars using p-glucuronidase (GUS) reporter
gene. To establish an efficient transformation system, the conditions for production of
friable embryogenic calli (FEC) and Agrobacterium-mediated transformation were
optimized for a selection of African farmer-preferred cultivars (i.e. Albert,
Ebwanatereka, Kibaha, Kibandameno, Mkombozi, Serere and TME 14). FECs from
Ebwanatereka, Serere, Kibandameno and 60444 cultivars were transformed with
Agrobacterium strain LBA4404 harboring a binary vector pCAMBIA 130 1 and 17-28
transgenic plants per 100 mg of FEC were regenerated. Histochemical GUS assay
demonstrated transient and stable expression of uidA "gene in both calli and
regenerated plants. The presence, integratlbij'~and expression of the trsnsgenes were
confirmed by PCR, Southern blot and RT-PCR analysis, respectively. An efficient
transformation and regeneration protocol has been established in this study and will
provide a useful platform to transfer novel traits to farmer-preferred cassava cultivars
in Africa. The second objective was to modulate PPD through overexpression of
glutathione peroxidase (GPX) gene in cassava storage roots. Transgenic plants of
cultivar 60444 were generated expressing Ar~bid.opsis GPX (AtGPX) gene, driven by
a root-specific patatin promoter. Molecular analysis indicated that AtGPX had been
integrated in the genome of transgenic plants and functionally expressed. Transgenic
cassava storage roots showed a reduction in ROS accumulation and malondialdehyde
(MDA) content compared to non-transgenic plants, with an extension in shelf-life.
The third objective was to develop transgenic cassava expressing dehydroascorbate
reductase (DHAR) gene and evaluate the level of tolerance to oxidative stress. DHAR
is an important enzyme functioning in the regeneration of ascorbate (AsA), and since
AsA blocks ROS accumulation, the hypothesis that DHAR overexpression protects
cassava plants from abiotic stress was tested. Transgenic cassava plants of cultivar
60444 were generated expressing Arabidopsis DHARI (AtDHARl) gene driven by
CaMV35S promoter. Molecular analyses of transgenic lines revealed stable
integration and expression of AtDHARI gene. DHAR activity and AsA content were
significantly (p<0.05) increased in transgenic lines compajedto non-transgenic plants.
Overexpression of AtDHARl in cassava led to enhanced oxidative stress tolerance
measured by enhanced root elongation, reduced ROS actumulat-ion and reduced lipid
peroxidation products. This study has developed an effective transgenic approach for
delaying PPD and enhancing oxidative stress tolerance in cassava. | en |
dc.description.department | a
Department of Psychiatry, University of Nairobi, ; bDepartment of Mental Health, School of Medicine,
Moi University, Eldoret, Kenya | |