Enhancement of drought tolerance in Sweetpotato by expressing xvaldl and xvsapl genes Isolated from xerophyta viscosa
Mbinda, Wilton Mwema
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Sweetpotato, Ipomoea batatas L. (Lam.), ranks among the top six most important food crops in the world. It is widely grown throughout the world especially in the developing countries with high and stable yield, strong adaptability and rich nutrient content with multiple uses. Despite its importance, drought stress is a critical delimit for increased productivity and cultivation expansion of sweetpotato. Unique plants such as Xerophyta viscosa, (a resurrection plant) which exploit a broad range of physiological and molecular responses to enable survival under extreme stress conditions, are valuable sources of useful genes for crop improvement. Accordingly, two genes; XvAldl and XvSapl that encode aldose reductase and a membrane signalling protein respectively, have previously been isolated from X viscosa under dehydration stress. The objective of this study was to establish somatic embryogenesis and regeneration protocol of Kenyan sweetpotato cultivars and develop transgenic drought tolerant sweetpotato plants expressing XvAld, XvSapl and XvSapl-XvAldl transgenes via Agrobacterium-mediated transformation and phosphomannose isomerase (pmi)/mannose selection system. Three• Kenyan sweetpotato cultivars; KSP36, Kemb36 and Mweu mutheke along with an exotic model cultivar Jewel were used. As a prerequisite for transformation, a reproducible somatic embyrogenesis and regeneration protocol was established for Kenyan sweetpotato cultivars using meristems as explants. In an attempt to produce highly drought tolerant sweetpotato plants, XvAldl XvSapl XvSapl-XvAldl transgenes under the control of a stressinducible XvPsapl promoter were introduced into the transformable sweetpotato cultivars via Agrobacterium-mediated transformation and transgenic events regenerated through somatic embryogenesis. Expression cassette of the trans genes were cloned into pNOV2819 binary vector modified to contain nptII gene to enable selection of transgenic plants using kanamycin. Polymerase chain reaction (PCk) and Southern blot hybridization confirmed integration of trans genes, whereas reverse transcription peR and quantitative real-time peR confirmed express transcription of the XvAldl XvSapl XvSapl-XvAldl transgenes. In order to evaluate drought tolerance, physiological and morphological responses of transgenic sweetpotato compared to I' wild type plants was examined under water deficit stress. From this study, an optimized and reproducible transformation protocol for Kenyan sweetpotato cultivars has been developed. The study established pmi/mannose selection system unsuitable for successful gene transfer in sweetpotato. In addition, transgenic plants expressing XvAldl XvSapl XvSapl-XvAldl transgenes survived p days of water deficiency and exhibited improved recovery after rehydration compared to wild type plants. This study demonstrates an effective and feasible approach to improve sweetpotato tolerance to drought stresses without causing any phenotypic defects. Production of sweetpotato transgenic lines with greatly improved drought tolerance will not only stabilizes yield production in unpredictable climates but also provides a novel germplasm for sweetpotato production on marginal lands.