Evaluation of virus resistance and storage root yield in cassava germplasm
Date
2022
Authors
Nyoni, Emmanuel Edwards
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Abstract
Cassava mosaic disease (CMD) remains the biggest challenge to cassava cultivation and deployment of CMD resistant landraces has been one of the key strategies to minimize the spread of CMD in open fields and avert losses. The aim of the project was to screen drought tolerant South African cassava germplasm for responses to South Africa cassava mosaic virus (SACMV) challenge and evaluate previously transformed cassava lines, where model cultivar cv. 60444 was transformed with a hairpin DNA cassette transcribing a hairpin RNA (hp-RNA) targeting either plastidial adenylate kinase (ADK) or uridine monophosphate synthase (UMPS) under the control of a patatin root promoter in an attempt to increase starch production and storage root yield. Evaluation of infectivity in cassava germplasm indicated that Ukulinga 12 genotype was susceptible to SACMV, showing severe CMD symptoms and high viral load. Ukulinga 2 exhibited high levels of resistance to SACMV, and the CMD symptoms emerged very late post 32 days post inoculation (dpi) and remained mild, with low viral load. Ukulinga 8 germplasm did not exhibit any symptoms and viral load was minimal for the duration of the study. There was a weak Pearson’s correlation (r=0.37) between mean symptom severity index and viral load. Two trials were conducted in different growth environments to evaluate increased starch content and storage root yield. The majority of control and transformed plants from trial 1 did not produce storage roots, and a few plants produced small (below 2g) storage roots whilst trial 2 plants produced a high number of average to large (2g≤x≤5g) storage roots. There was statistically significantly (p<0.05) increase in percentage dry weight (29.4 ± 1.0%) of storage roots from UMP 26 line in comparison to untransformed cv. 60444 (22.8 ± 1.6%). Both ADK 5 and UMP 26 lines produced a significant increase (p<0.05) in total percentage starch content of storage roots of (32.3 ± 0.6g/100g and 40.2 ± 1.3g/100g, respectively), while cv. 60444 control had 27.9 ± 1.3g/100g. UMP 28 line had the least productivity, producing the least number of storage roots per plant, the lowest starch content and least percentage dry weight of storage roots. Principal component 1 (PC1) had standardized variation of 98.4% and 93.3% in trial 1 and 2 respectively and plant height was the biggest contributor to the separation of genotypes in both trials. ADK 5 line produced the highest total yield (53.9g) in all of trial 2 tested genotypes, in comparison to the high-yielding T200 landrace and model cv. 60444 (46.3g and 18.3g respectively). The correlation between total starch and percentage dry weight of the storage roots from trial 1 was (r=0.63, p=8.4 x 10-6 ) compared to (r=0.3, p=0.042) for trial 2. ADK 5 line produced the highest total yield (53.9g) in all of trial 2 tested genotypes, in comparison to the high-yielding T200 landrace and model cv. 60444 (46.3g and 18.3g respectively). The correlation between total starch and percentage dry weight of the storage roots from trial 1 was (r=0.63, p=8.4 x 10-6 ) compared to (r=0.3, p=0.042) for trial 2.
Description
A dissertation submitted in fulfilment of the requirements for the degree of Master of Science to the Faculty of Science, School of Molecular and Cell Biology, University of the Witwatersrand, Johannesburg, 2022