Blueberry is an important fruit crop in the genus Vaccinium L. (for reviews, see Ratnaparkhe 2007; Song and Sink, 2005). Blueberry fruit are one of the richest sources of antioxidant phytonutrients among the fresh fruit (Conner et al., 2002; Ehlenfeldt and Prior, 2001). According to U.S. Department of Agriculture-National Agricultural Statistics Service (USDA, 2007), the United States ranks first in world production of blueberry, supplying 124,976 Mg of blueberry fruit on 21,376 ha in 2006.
Molecular genetic and genomic approaches will enable localization and isolation of genes controlling blueberry traits such as fruit size, fruit quality, disease resistance, and various environmental tolerances. Recently, several genes associated with cold hardiness have been identified and isolated from highbush blueberry using a large expressed sequence tag library, forward and reverse subtracted cDNA libraries, or microarrays (Dhanaraj et al., 2004, 2007; Naik et al., 2007). Once genes of interest are isolated, genetic engineering and transformation of blueberry will be a powerful approach to complement traditional breeding by rapidly introducing individual traits without changing the inherent desirable characteristics of existing cultivars (Ratnaparkhe, 2007; Song and Sink, 2005). To date, transformation methodologies for blueberry cultivars have been developed by several research groups (Cao et al., 1998, 2003; Graham et al., 1996; Rowland, 1990; Song and Sink, 2004, 2006; Song et al., 2007a).
There are some particularly persistent perennial weeds adapted to low pH soils of blueberry fields. To control weeds, nonselective and broad-spectrum herbicides, such as glyphosate and glufosinate ammonium, are preferable; however, they can be applied only as a directed spray under the bushes to avoid contact with green tissues. Blueberry plants withstanding broad-spectrum herbicides would add flexibility to weed control programs while providing a simple and effective management tool. Broad-spectrum herbicide resistance genes have been introduced into many crop species (Castle et al., 2004; James, 2007). In 2006, herbicide tolerance, deployed in soybean [Glycine max (L.) Merr.], maize (Zea mays L.), canola (Brassica napus L.), cotton (Gossypium L.), and alfalfa (Medicago sativa L.) continued to be the most dominant trait, occupying 68% (69.9 million hectares) of global biotechnology crops (James, 2007). In Vaccinium species, transformed cranberry (Vaccinium macrocarpon Ait.) plants with the bialaphos resistance gene (bar) showed resistance to 1000 mg·L−1 GS (Polashock and Vorsa, 2002). Using our established transformation protocol (Song and Sink, 2005), four constructs, including nos∷bar, 35S∷bar, (Aocs)3AmasPmas∷bar, and 34S∷bar were previously used to produce herbicide-resistant blueberry plants. Our preliminary results demonstrated that the nos∷bar gene can serve as a selectable marker for transformation, as well as being a source for herbicide resistance in blueberry plants (Song et al., 2007a).
Expression levels of a transgene are known to be greatly influenced by promoters and the host plant species (Ni et al., 1995; Song et al., 2007b; Wilmink et al., 1995). Evaluation of different promoters for genetic engineering blueberry plants has not been reported. The aims of this study were to evaluate four promoters for directing bar expression in ‘Legacy’ blueberry plants and to test their effect on herbicide resistance in the laboratory and field.
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