| Summary: | A Thesis Submitted to the Faculty of Science University
of the Witwatersrand, Johannesburg in Partial Fulfilment
of the Requirements for the Degree of Master of Science.
February 1995. === Cassava (Mamhot esculenta Crantz) is a perennial woody shrub grown mainly in
Latin America, Africa and Asia for its starchy tuberous roots. The cassava crop
provides a lot of calories for both animal and human consumption as well as starch for
industrial use. Because of its broad adaptability to a variety of soil and climatic
conditions (such as drought tolerance and ability to grow on depleted and marginal
soil), the crop is very important to the agroeconomy of several tropical countries.
The main threat to the crop's survival is the Cassava Mosaic Disease (CMO) caused
by the African Cassava Mosaic Virus (ACMV). This virus is a bipartite geminivirus.
Recently a new strain of ACMV (i.e, ACMV-SA) has been observed to infect
cassava crops in Kwazulu-Natal and Eastern Transvaal in South Africa. Initial studies of
a 793bp Polymerase chain reaction (PCR) generated fragment sequence of the
DNA-A component of the virus reveals that it shares a higher sequence homology
with a monopartite TYLCV than with its other bipartite ACMV counterparts. This
unique ACMV-strain is a cause for concel? which needs to be further investigated so
that an effective approach can be taken to combat its devastating influence
on cassava in South Africa.
In this project further characterization of DNA-A of ACMV-SA was pursued so
that: (1) antisense-Afll ORF (putative polymerase) expression could be developed
in genetically engineered virus-resistant cassava; (2) the new virus could be
compared with other similar geminiviruses. The characterization was done by: (1)
Partially sequencing a 607bp ACI PCR-generated fragment of ACMV-SA cloned
into Pstl siteof pBluescript SK(-) and comparing the sequence with ether
geminiviruses; (2) constructing a restriction map of the DNA-A of ACMV-SA cloned
into pBluescript KS so as to identify convenient restriction sites for future subcloning
experiments. For the Agrobacterium transformation studies-jj; vitro plantlets, callus
and suspension cell cultures for cassava an,.I tobacco were maintained. Protoplast
isolation from cell suspension cultures was carried out for purposes of inoculating
them with DNA-A of ACMV-SA. An attempt was made to clone the 607bp ACI
fragment of ACMV-SA into the plant transformation vectors pBINl9 and pBI121. === MT2017
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