Towards improvement of Ugandan cassava germplasm for drought tolerance
Turyagenda, Laban Frank
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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 of ten 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 (He=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 x 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.