Abstract:

Cassava is staple food crop with potential to improve household food and income security for over
800 million people in tropical countries. However, cassava’s survival and ability to produce
reasonable yields under drought conditions is drastically affected. This calls for research to select
and develop cassava cultivars that are capable of yielding better in water constrained environments.
Genetic improvement of cassava requires clear understanding of physiological and morphological
responses of the crop to drought stress, and genetic diversity among genotypes. In addition,
identification of gene sources and genes involved in drought tolerance would enhance incorporation
of biotechnology in cassava improvement. This research was carried out to evaluate Ugandan
cassava germplasm for drought tolerance, determine genetic diversity that exists within the
germplasm, and identify candidate drought tolerance genes in cassava. A total of 66 cassava
genotypes were used in this study. Replicated field screening trials for cassava genotypes were
established in Buliisa, Nakasongola and Kabanyolo. Genetic diversity assessment among the
cassava genotypes was evaluated using 26 SSR markers, while gene expression studies often genes
were undertaken using Real Time PCR under stressed and well watered conditions in a tolerant and
a susceptible genotype.
Results from field trials indicated significant difference (p<0.05) among genotypes in response to
drought stress. Genotypes MM96/0686, Magana, Yellow, TME204, Nyamutukura, MH97/2961.
Akena and NASE12 were least affected by drought, while Bao, Nyalanda, Egabu. Icilicili and
Rwaburaru were more susceptible. Genetic diversity analysis revealed high genetic diversity
(ile-0.667) among the cassava genotypes grown in Uganda, suggesting sufficient genetic base that
can benefit cassava breeding. Genetic diversity assessment also indicated that landraces have
unique alleles and therefore, including them in hybridization could provide highly heterogeneous
base populations for selection. Furthermore, this study identified four (MeALDH, MeZFP, MeMSD
and MeRD28) drought tolerant associated genes that can be used as gene-based markers for drought
tolerance breeding. However, detailed studies to biologically validate these genes need to be
undertaken.
Contrary to the general belief that cassava is tolerant to water stress, this study demonstrated that
severe water stress is detrimental to cassava growth and productivity, and highlighted several
drought tolerant and susceptible cassava genotypes. The study also revealed high genetic diversity
among the cassava genotypes grown in Uganda, suggesting that genetic diversity and diversification of cassava has been maintained over years of clonal propagation and human selection. Furthermore,
as of this writing, this is the first report on gene expression in cassava using Real Time PCR and the
candidate drought tolerance genes identified in this study provides the most important step towards
improvement of cassava for drought tolerance through genetic engineering. Moreover, gene
expression data were collected under water stressed conditions that closely resemble the actual field
conditions, making it the first study of its kind and thus the methodologies developed will guide
scientists interested in similar studies either in cassava or other crop species. Overall, information
and materials developed from this research will significantly contribute towards improvement of
cassava for drought tolerance.