Browsing by Author "Halsey, Mark"

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    RNAi-derived field resistance to Cassava brown streak disease persists across the vegetative cropping cycle
    (GM Crops & Food, 2014) Odipio, John; Ogwok, Emmanuel; Taylor, Nigel J.; Halsey, Mark; Bua, Anton; Fauquet, Claude M.; Alicai, Titus
    A confined field trial was established to determine durability of RNAi-mediated resistance to Cassava brown streak disease (CBSD). Stem cuttings were obtained from field-grown cassava plants of cv 60444 transgenic for construct p718, consisting of an 894 bp inverted repeat sequence from the Ugandan Cassava brown streak virus (UCBSV) coat protein. Plants were established from three transgenic lines previously shown to provide complete resistance to UCBSV and differing levels of protection to the non-homologous virus species Cassava brown streak virus (CBSV), and grown for 11 months. CBSD symptoms were observed on shoots and storage roots of all non-transgenic cv 60444 control plants and transgenic lines p718–002 and p718–005, but not on p718–001. RT-PCR diagnostic showed tissues of plant lines p718–002 and p718–005 to be infected with CBSV, but free of UCBSV. All leaves and roots of p718–001 plants were confirmed to carry no detectable levels of either pathogen. Plants of cv 60444 in this field trial showed severe cassava mosaic disease symptoms, indicating that presence of replicating geminiviruses did not cause significant suppression of RNAi-mediated resistance to CBSD. Resistance to CBSD across a vegetative cropping cycle confirms earlier field data, and provides an important step in proof of concept for application of RNAi technology to control of CBSD under conditions encountered in farmers’ fields.
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    Transgenic RNA interference (RNAi)-derived field resistance to cassava brown streak disease
    (Molecular plant pathology, 2012) Ogwok, Emmanuel; Odipio, John; Halsey, Mark; Gaitán-Solís, Eliana; Bua, Anton; Taylor, Nigel J.; Fauquet, Claude M.; Alicai, Titus
    Cassava brown streak disease (CBSD), caused by the Ipomoviruses Cassava brown streak virus (CBSV) and Ugandan Cassava brown streak virus (UCBSV), is considered to be an imminent threat to food security in tropical Africa. Cassava plants were transgenically modified to generate small interfering RNAs (siRNAs) from truncated full-length (894-bp) and N-terminal (402-bp) portions of the UCBSV coat protein (DCP) sequence. Seven siRNA-producing lines from each gene construct were tested under confined field trials at Namulonge, Uganda. All nontransgenic control plants (n = 60) developed CBSD symptoms on aerial tissues by 6 months after planting, whereas plants transgenic for the full-length DCP sequence showed a 3-month delay in disease development, with 98% of clonal replicates within line 718-001 remaining symptom free over the 11-month trial. Reverse transcriptase-polymerase chain reaction (RT-PCR) diagnostics indicated the presence of UCBSV within the leaves of 57% of the nontransgenic controls, but in only two of 413 plants tested (0.5%) across the 14 transgenic lines. All transgenic plants showing CBSD were PCR positive for the presence of CBSV, except for line 781-001, in which 93% of plants were confirmed to be free of both pathogens. At harvest, 90% of storage roots from nontransgenic plants were severely affected by CBSD-induced necrosis. However, transgenic lines 718- 005 and 718-001 showed significant suppression of disease, with 95% of roots from the latter line remaining free from necrosis and RT-PCR negative for the presence of both viral pathogens. Crossprotection against CBSV by siRNAs generated from the full-length UCBSV DCP confirms a previous report in tobacco.The information presented provides proof of principle for the control of CBSD by RNA interference-mediated technology, and progress towards the potential control of this damaging disease.
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    The VIRCA Project: Virus resistant cassava for Africa
    (GM Crops & Food, 2012) Taylor, Nigel J.; Halsey, Mark; Gaitán-Solís, Eliana; Anderson, Paul; Gichuki, Simon; Miano, Douglas; Bua, Anton; Alicai, Titus; Fauquet, Claude M.
    The VIRCA (Virus Resistant Cassava for Africa) project is a collaborative program between the Donald Danforth Plant Science Center, USA the National Crops Resources Research Institute, Uganda and the Kenya Agricultural Research Institute, Kenya. VIRCA is structured to include all aspects of the intellectual property, technology, regulatory, biosafety, quality control, communication and distribution components required for a GM crop development and delivery process. VIRCA's goal is to improve cassava for resistance to the viral diseases cassava brown streak disease (CBSD) and cassava mosaic disease (CMD) using pathogen-derived RNAi technology, and to field test, obtain regulatory approval for and deliver these products to small landholder farmers. During Phase I of the project, proof of concept was achieved by production and testing of virus resistant plants under greenhouse and confined field trials in East Africa. In VIRCA Phase II, two farmer-preferred varieties will be modified for resistance to CBSD and CMD, and lead events identified after molecular and field screening. In addition to delivery of royalty-free improved planting materials for farmers, VIRCA capacity building activities are enhancing indigenous capability for crop biotechnology in East Africa.