Infection of sweet potato by multiple viruses is a common occurrence and
often results as a more severe disease than the sum effect of infection with
each virus alone. The sweet potato feathery mottle virus (SPFMV), sweet
potato chlorotic stunt virus (SPCSV), sweet potato virus G (SPVG) and Sweet
potato mild mottle virus (SPMMV) have caused a synergistic disease complex
that has devastated sweet potato in South Africa. In order to address the
problem, transgenic sweet potato (Ipomoea batatas Lam.) with broad virus
resistance is being developed through genetic engineering.

Transformation of apical tips of sweet potato cultivar Blesbok was achieved
through Agrobacterium tumefaciens strain LBA4404. Coat protein gene segments
of the four viruses were used to induce gene silencing in transgenic sweet
potato. Six transgenic plants were developed and these six were then
evaluated through graft inoculation with SPFMV, SPCSV, SPVG and
SPMMV-infected Ipomoea setosa. Virus presence was detected in the transgenic
sweet potatoes using nitrocellulose enzyme-linked immunosorbent assay.
However, all transgenic plants displayed delayed and milder symptoms of
chlorosis and mottling of lower leaves compared to the control plants.

These results showed promise and require further investigation on resistance
to virus infections under field conditions.

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