Optimization of Agrobacterium-mediated genetic transformation of Fraxinus nigra and development of black ash for possible emerald ash borer resistance
Emerald ash borer (EAB; Agrilus planipennis Fairmaire) is the most devastating insect pest of North American ash species, including black ash (Fraxinus nigra Marsh.). As a first step in an effort to develop transgenic black ash plants resistant to EAB, we successfully established an efficient Agrobacterium-mediated transformation system for black ash hypocotyls. Kanamycin and timentin at 40 and 300 mg L−1, respectively, were most effective to select transformed explants and control excess Agrobacterium growth. Using a plant transformation vector harboring the enhanced green fluorescent protein (eGFP) gene, the effects of Agrobacterium strain, bacterial density, and the concentration of Silwet L-77 on transformation efficiency were evaluated. The best result was obtained when Agrobacterium strain EHA105 was used at a density of OD600 = 1.0. Silwet L-77 failed to promote transformation frequency and showed an adverse effect at higher concentrations (> 0.015%). Using this optimized transformation system, transgenic black ash shoots expressing a synthetic Bacillus thuringiensis toxin gene (cry8D2) were regenerated. Although no morphological abnormality was observed, transgenic shoots showed severe growth restriction. Three independent transgenic lines were selected for further assessment. All selected lines contained two copies of the cry8D2 gene, and the expression of the transgene was verified in transcript and protein levels. These transgenic shoots can be used for future bioassay to evaluate its efficacy against EAB.
KeywordsBlack ash Emerald ash borer Fraxinus Genetic transformation
This research was supported by partial funding from the USDA-APHIS-PPQ Center for Plant Health Science and Technology, the U.S. Endowment for Forestry and Communities, and members of the Indiana Hardwood Lumbermen’s Association. The authors gratefully acknowledge Dr. Dennis J. Gray, University of Florida, for the transformation vector pq35GR, and Dale Simpson, Natural Resources Canada, for the black ash seed. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that also may be suitable.
JHL and PMP conceived and designed the research. JHL conducted the experiments and analyzed the data. JHL and PMP wrote the manuscript. All authors read and approved the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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